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Summary
The goal of this symposium is to congregate geoscientists from Taiwan and Russia for
establishing new as well as strengthening existing contacts. Geoscientists from Russia with
long‐ standing experience in the studies of Central Asia Orogenic belt (CAOB) and of
sedimentary records in Lake Baikal will share their expertise on different aspects of mountain
building in the CAOB and environmental and climatic changes to the geoscience community
of Taiwan. In turn Taiwan provides a young and still actively deforming analogue for the
CAOB, which are themselves still deforming although at much lower strain rates. This
comparison will permit us to better understand ancient processes by studying the modern ones.
The seminar will consist of a 2‐ day symposium held in Taipei and a subsequent 4‐ day
fieldtrip, offering a traverse of the mountain belt and the suture zone between Eurasia and
Philippine Sea plates. It provides an attractive opportunity for exchanging ideas and for
launching long‐ lasting bilateral collaborations.
Objective
Studies of environmental and climatic changes are urgent at present. It is well obvious as
the increase of annual temperature by two degrees can lead to the increase of natural
catastrophes (overfloods, landslides, hurricanes, aridity of climate, bush fires, etc.) by one
order. In order to know the climate in the future we have to decipher the climate record of the
past. A number of long-term continuous climatic records have been obtained from the ocean.
Such records are difficult to obtain for continents as the climatic archive on continents is
compiled from a series of short records, obtained from different sites. It hinders their
comparison. The lack of a continuous continental paleoclimatic record does not allow the
creation of a common model of the Earth’s climate and correspondingly the forecast of future
climatic changes. In this respect Lake Baikal depression with 8 km sedimentary record,
disclosing the history for 25-30 Ma, is a promising site for paleoclimatic research on the
continent. The location of Lake Baikal at high latitudes of the Northern Hemisphere with
clear variations in solar radiation and the absence of cover glacial make it an only site in the
Northern Hemisphere where a continuous long-term continental climatic record is possible to
be obtained. Lake Baikal Region is unique not only for studying paleoclimate but it is
characterized by a long history of geologic, volcanic and orogenic events. The association of
1
tectonic movements with climatic changes is one of significant goals of geology. On the
example of Baikal Rift Zone we can study the interaction of endogenous (volcanic) and
exogenous (glaciation) processes. Our studies showed the comparison of climatic record
obtained from the sedimentary core with the paleoclimatic reconstructions obtained from
lavas, having different textures resulting from eruptions in glacial and interglacial periods.
The lithospheric mantle is one of the major geochemical reservoirs in the Earth, and due
to its non-convecting character, it records cumulative geochemical effects of large-scale
tectonic events. However, the lithospheric mantle remains enigmatic due to its complex
history; this symposium will discuss updated information on its formation, behaviour and
modification associated with continental rifting. On the regional scale, the issue of which
mechanism(s) caused the Cenozoic continental extension in Central Asia remained
controversial for decades. An answer to this question will help to understand the whole
Cenozoic tectonic framework of Central Asia. The petrology and geochemistry studies can
integrate with available geophysical data to help define the connection between mantle
terranes and the major crustal terranes identified by geology and geophysics
The Symposium will discuss issues of association between climate and environment
changes with volcanic and orogenic processes in the Central Asia.
Topics of the Symposium:
Magmatism and geodynamics
Sedimentary records and paleoclimate
Gas hydrate problems
2
3
Brief schedule for the 2nd Taiwan-Russia Joint Symposium
September 25, 2011 9:00-9:30 Registration 9:40-9:50 Welcome address by Deputy Minsiter of NSC: Prof. Chen, Cheng-Hong 9:50-10:00 Opening address by Director of IES: Prof. Chao, Fong 10:00-11:00 Thematic Sessions I-1: Magmatism and geodynamics 11:00-11:20 Coffee break 11:20-12:00 Thematic Sessions I-2: Magmatism and geodynamics
12:00-14:00 Lunch break 14:00-15:30 Thematic Sessions I-3: Magmatism and geodynamics 15:30-15:50 Coffee break 15:50-16:50 Thematic Sessions I-4: Magmatism and geodynamics 16:50-18:00 Poster session 18:30 Ice-break party
September 26, 2011 9:00-11:00 Thematic Sessions I-5: Magmatism and geodynamics 11:00-11:20 Coffee break 11:20-12:00 Thematic Sessions I-6: Magmatism and geodynamics
12:00-14:00 Lunch break 14:00-15:00 Thematic Sessions II: Sedimentary records and paleoclimate 15:00-15:20 Coffee break 15:20-16:00 Thematic Sessions III: Gas hydrate problems and tectonics 16:00-16:30 Field excursion guiding (Prof. J. Bruce H. Shyu) 16:30-16:50 Coffee break 16:50-18:00 Free discussion
18:30 Dinner September 27-29, 2011 Field excursion to Eastern Taiwan Ophiolites
September 25, 2011
Thematic Sessions I-1: Magmatism and geodynamics Chair: Der-Chuen Lee
Time Presenter Title Page No.
10:00~10:30 M.I. Kuzmin* , V.V. Yarmolyuk
(Invited)
Plume magmatism of the Siberian craton: its
evolution in time and the significance in
orogeny
6
10:30~11:00 Louis S. Teng (Keynote) Rise and Fall of a Collisional Orogen –
Insights from Taiwan
7
Thematic Sessions I-2: Magmatism and geodynamics Chair: Sergey Dril
Time Presenter Title Page No.
11:20~11:40 Ivanov A.V.*, Demonterova
E.I.
Origin of basaltic melts in the Baikal rift due
to combination of two effects; active mantle
diapirism and passive decompression
8
11:40~12:00 Kovach V.P*., Rytsk E.Yu.,
Yarmolyuk V.V., Kotov A.B.
Isotope structure and continental crust
evolution in the eastern Transbaikalia, Central
Asian Orogenic Belt
11
Thematic Sessions I-3: Magmatism and geodynamics Chair: Shing-Ka’i Ma
Time Presenter Title Page No.
14:00~14:30 Alexander Kotov (Keynote) Timing of the main crust-forming events in
the eastern parts of the Central Asian
Orogenic Belt
14:30~14:50 Demonterova E.I.*, Ivanov A.V.,
Reznitskii L.Z., Belichenko V.G.,
Hung C.-H., Chung S.-L., Iizuka
Y., Wang K.-L.
History of the Tuva-Mongolian microcontinent
gglomeration in the Neoproterozoic: U-Pb
LA-ICP-MS dating of detrial zircons from a
sandstone of the Darkhat series, Western Hovsgol,
Northern Mongolia
13
14:50~15:10 Dril S.I.*, Antipin V.S.,
Chukonova V.S., Il’ina N.N.
Sr-Nd-Pb isotope systematics of rifting
basalts of the Kamar Ridge (Baikal Rift
System)
16
15:10~15:30 Perepelov A.B.*, Tsypukova
S.S.
Late-Cenozoic hawaiite volcanism of
Kamchnata and Baikal Area – indicator of the
composition and mantle metasomatism in the
setting of convergent lithosphere plates and
plume-lithosphere interaction
18
Thematic Sessions I-4: Magmatism and geodynamics Chair: Alexei Ivanov
Time Presenter Title Page No.
15:50~16:10 M.A. Gornova*, V.A. Belyaev,
O.Yu. Belozerova
Textures and geochemistry of the Saramta
peridotites (Siberian craton): melting and
refertilization on early stage of the continental
lithosphere mantle formation.
20
16:10~16:30 Khomutova M.Yu.*, Kuzmin
M.I., Yarmolyuk V.V.,
Voronstov A. A.
Convergent boundaries of the West Pacific
type and their significance for the origin of
magmatic rocks of Central Asian folded
complexes
21
16:30~16:50 Kuo-Lung Wang*, Mikhail Kuzmin,
V. Kovach, V. Yarmolyuk, S.Y.
O’Reilly, W.L. Griffin, N.J. Pearson
Existence of Microcontinent in the Central
Asia Orogenic Belt: In Situ Re-Os Evidence
from the Lithospheric Mantle
22
Poster session
16:50~18:00 Hsin-Ting Liu*, Ein-Fen Yu
and Der-Chuen Lee
Molybdenum isotopes for Lake Baikal
sediments
37
4
September 26, 2011
Thematic Sessions I-5: Magmatism and geodynamics Chair: Kuo-Lung Wang
Time Presenter Title Page No.
09:30~10:00 Bor-Ming Jahn (Invited) Distinct Crustal Development of SW and NE
Japan – Sr-Nd Isotopic Evidence and Tectonic
Implications
24
10:00~10:30 Vadim Kamenetsky*, Maya B.
Kamenetsky, Sun-Lin Chung,
Georg F. Zellmer, Dmitry V.
Kuzmin (Keynote)
Primary melts of the Emeishan and Karoo
flood basalts: Linking Ni and Ti potential of
the magmas and their mantle
25
10:30~11:00 Sun-Lin Chung (Keynote) A comparative study of the Caucasus/Iran and
Tibet/Himalaya orogenic belts
28
Thematic Sessions I-6: Magmatism and geodynamics Chair: Marina Gornova
Time Presenter Title Page No.
11:20~11:40 Kwan-Nang Pang*, Sun-Lin
Chung, Mohammad Hossein
Zarrinkoub, Seyed Saeid
Mohammadi, Richard Walker,
Hsiao-Ming Yang, Hao-Yang
Lee, Chiu-Hong Chu, I-Jhen
Lin Bor-Ming Jahn (Invited)
40Ar-
39Ar age, geochemical and Sr-Nd
isotopic constraints on Late Cenozoic
intraplate volcanism, eastern Iran
29
11:40~12:00 Shing-Ka’i Ma*, John Malpas,
Costas Xenophontos,
Katsuhiko Suzuki and
Ching-Hua Lo
Mantle metasomatism and formation of alkali
basaltic magmas: implications from the
geochemistry of the Early Cretaceous Coastal
Ranges basalts, NW Syri
31
Thematic Sessions II: Sedimentary records and paleoclimate Chair: Kwan-Nang Pang
Time Presenter Title Page No.
14:00~14:20 Der-Chuen Lee* and
Shun-Chun Yang
Zinc and Cadmium isotopes for Lake Baikal
sediments
32
14:20~14:40 Mayuko Fukuyama* and
Alexander M. Spiridonov
Silver isotope measurements of Au-Ag ore
deposits in Russia by MC-ICP-MS
33
14:40~15:00 Mei-Fei Chu*, Norman J.
Pearson, Suzanne Y. O’Reilly,
Williams L. Griffin, Peter
Wieland
Lithium Isotopic Compositions in Ultramafic
Rocks: An Improved Methodology Using
MC-ICP-MS
34
Thematic Sessions III: Gas hydrate problems and tectonics Chair: Mayuko Fukuyama
Time Presenter Title Page No.
15:20~15:40 Saulwood Lin*, Genady V.
Kalmychkov, Tsanyao F. Yang,
Chieh-Wei Hsu, Yeecheng Lim,
Wangyen Cheng, and Tatyana V.
Pogodaeva
Climatic Changes and Sources of Sulfur in
Lake Baikal Methane Dominated Freshwater
Sedimentary Environment
35
15:40~16:00 V.S. Antipin*, S.I. Dril Patom crater (Eastern Siberia): structure,
geochemical features and origin
36
5
Plume magmatism of the Siberian craton: its evolution in time and the
significance in orogeny
M.I. Kuzmin1
, V.V. Yarmolyuk2,
1Institute of Geochemistry, SB RAS, Irkutsk, Russia
2IGEM RAS, Moscow, Russia
The present contribution will consider some problems of the intraplate magmatism
(magmatism of mantle plumes) based on the recent geophysical and in particular geochemical
data. The basis for such consideration is magmatic rocks, associated with the intraplate
occurrences of magmatic rocks (from the Ordovician up to now) within the Central Asia, i.e.
associated mainly with the central Asian magmatic plume.
We describe several large igneous provinces (LIPs) and the intervals in which they
were continuously formed within the limits of the Siberian continent: the Altay-Sayan Early
Paleozoic magmatic area (598-466 Ma), the Altay-Sayan LIP (408-393 Ma), the Vilyu LIP
(380-350 Ma), the Barguzin-Vitim LIP (310-275 Ma), the Late Paleozoic rift system of
Central Asia (318-242), the Siberian traps and West Siberian rift system (250-249 Ma), the
East-Mongolian and West-Trans Baikal LIP (228-195 Ma), and a number of various aged
Late Mesozoic and Cenozoic rift zones and magmatic areas (from 160 Ma to the present day).
Available rare element and isotopic characteristics of the intraplate magmatic rocks of
Siberia enable us to determine three primary sources –moderately depleted mantle (PREMA0,
enriched mantle (EM-I and EM-II) of the mantle origin magma. We propose that the model
explains the interaction of the hot mantle plume including hot spots (plume tails) with the
Siberian intraplate magmatism areas throughout the Phanerozoic eon.
Prime magmas from the mantle formed intraplate magmatism areas in different time
periods. The magmas originated from the three types of the mantle sources: PREMA, EM-II
and EM-I. Magma from the PREMA source formed magmatic areas in Siberia from
Ordovician to Early Carboniferous. Type EM-II acted in the combination with the PREMA to
form magmatic areas from Late Carboniferous to Early Jurassic. The EM-II type played a
larger role during the formation of the Permian-Triassic traps and West Siberian rift basalts.
Starting from Late Jurassic until Early Cenozoic, the PREMA source became the main source
again. In the Late Cenozoic, EM-I and PREMA sources dominated in forming the magmatic
areas in Siberia.
6
Rise and Fall of a Collisional Orogen – Insights from Taiwan
Louis S. Teng
Institute of Geosciences, National Taiwan University, Taipei, Taiwan, R.O.C.
e-mail: [email protected]
Mountain belts worldwide are tectonic collages made up of crustal blocks and
accretionary complexes assembled through multiple episodes of subduction, collision, and
terrain amalgamation. These tectonic processes, which are hard to envision in ancient
mountain belts, can be vividly illustrated in the present Western Pacific area, where island
arcs and microcontinents have recently been accreted to the Eurasian margin as collisional
orogens. Taiwan, comprising one of the most dynamic and short-lived mountain belts in the
Western Pacific, provides an actualistic example for the fast buildup and collapse of a
collisional orogen.
Located at the boundary between the Philippine Sea plate and the Eurasian plate, the
Taiwan mountain belt is formed by the collision of the Luzon Arc with the China continent.
The Longitudinal Valley of eastern Taiwan, which marks the onshore suture of the coalesced
Philippine Sea and Eurasian plates, separates the island into two geologic provinces. The
Coastal Range to the east comprises volcanic and siliciclastic sequences of the accreted Luzon
Arc system; whereas the Central Range and the Western Foothills to the west consist of
metamorphic and sedimentary rocks of the deformed China continental margin. Lying to the
west of the mountain belt, the coastal plain and the Taiwan Strait belong to the China
continental margin that has, not yet, been involved in the collision.
The collisional orogeny of Taiwan commenced in late Miocene time when the
north-trending Luzon Arc began to override the northeast-striking continental margin and to
shovel the continental margin strata into the accretionary wedge. At around 5 Ma, the
accretionary wedge was pushed upon the outer continental shelf and emerged as a small
island. Then the Luzon arc shifted its direction of motion to west-northwest and sped up its
impingement on the continent, which resulted in drastic collision that raised the accretionary
wedge into high mountain ranges around 3 Ma. As the oblique collision propagated from
northeast to southwest, the mountain belt was incrementally pushed to the southwest until it
has finally reached its present location. Meanwhile in northeastern part of the mountain belt,
the collision has ceased as the subduction polarity flipped and the Ryukyu arc volcanism set
in. Having lost the compressional support of collision, the mountain belt is currently in the
process of collapsing under the influence of lithospheric extension.
7
Origin of basaltic melts in the Baikal rift due to combination of two
effects; active mantle diapirism and passive decompression
Ivanov A.V., Demonterova E.I.
Institute of the Earth’s Crust SB RAS, Lermontov street, 128, Irkutsk, Russia
The Baikal rift system is located at the boundary between Siberian Craton and Amurian
microplate (Fig. 1). The Amurian plate is rotating counter-clockwise with Euler pole close to
Chara basin at the northeastern termination of the rift system. According to gravity data,
degree of extension increases linearly southwestward from the pole of rotation along the rift
axis (Fig. 2) [1]. Relation between basaltic magma generation and extension is an important
question for the understanding of the rifting. In north-eastern part of the rift system there are
two relatively small volcanic fields of size about 100 km across, the Udokan and Vitim
volcanic fields. The former is located at shoulder of the Chara rift basin and the latter in area
of small depressions aside of the axial zone of the rift system. In south-western part of the rift
system volcanism appeared over 450 km across rift basins (e.g. Tunka and Khubsugul), their
shoulders (e.g. Khamar-Daban ridge) and aside axial zone of rifting (e.g. Oka and Eastern
Tuva volcanic fields in the East Sayan Mountains). We calculated pressure (and hence depth)
of basaltic magma generation of this volcanic regions from major element chemistry of
erupted magma according to the procedure of [2] with minor modifications. The used
equation was: P(kbar) = - [0.85FeO(wt.%) - SiO2 (wt.%) - 48.93] / 0.33 [3]. Depth was
calculated from the assumption of crustal thickness of 50 km and crustal and mantle density
of 2.7 g/cm3 and 3.2 g/cm
3, respectively. Calculated depths for various volcanic fields are the
following: Udokan (117±25 km), Vitim (101±31 km), Tunka including northern
Hamar-Daban (102±16 km), Dzhida (95±18 km), Oka (79±18 km), Khubsugul (78±24 km)
and Tuva (78±15 km) [3]. It appears that depth of melting decreased linearly with distance
from the Euler pole of rotation along the rift axis. We interpret that depth of melting marks
lithospheric-asthenospheric boundary. Lithospheric thickness is gradually decreasing with
gradual increasing of the crustal extension (Fig. 2). However, comparing calculated pressure
(this work) and dry-xenoliths based geotherm [4] for the Oka volcanic field suggests that
passive melting of a dry mantle is impossible at calculated pressure (depth). Thus, upwelling
of hotter mantle is required. Gravity data suggests that upwelling of hot mantle diapirs was
sourced from mantle transition zone [5]. Extension in the Baikal rift did not cause melting,
but rather controlled magma production within rising diapirs. The work is supported by the
Russian Foundation for Basic Research (11-05-00425-а), Program no. 10.3 of the SB RAS
and the Integration Project of SB RAS_NNC Taiwan no. 142.
[1]. Zorin, Yu.A. and Cordell, L. (1991). Crustal extension in the Baikal rift zone.
Tectonophysics, 198, 117-121.
8
[2]. Demonterova, E.I., Ivanov, A.V., Rasskazov, S.V., Markova, M.E., Yasnygina, T.A. and
Malykh, Yu.M. (2007). Lithospheric control on Late Cenozoic magmatism at the boundary of
the Tuva-Mongolian massif, Khubsugul area, Northern Mongolia. Petrology, 15, 90-107.
[3]. Ivanov, A.V. and Demonterova, E.I. (2010) Extension in the Baikal rift and the Depth of
Basalt magma generation. Doklady Earth Sciences, 435, 1564-1568.
[4] Ivanov, A.V., Palesskii, S.V., Demonterova, E.I., Nikolaeva, I.V., Ashchepkov, I.V. and
Rasskazov, S.V. (2008). Platinum group elements and rhenium in mantle xenoliths from the
East Sayan volcanic field (Siberia, Russia) : evaluation of melt extraction and refertillization
processes in the lithospheric mantle of the Tuva-Mongolian massif. Terra Nova, 20, 504-511.
[5]. Zorin, Y.A., Turutanov, E.Kh., Kozhevnikov, V.M., Rasskazov, S.V. and Ivanov, A.V.
(2006). The nature of Cenozoic upper mantle plumes in East Siberia (Russia) and Central
Mongolia. Russian Geology and Geophysics, 47, 1056-1070.
Figure 1. Distribution of volcanic rocks within the Baikal Rift.
9
Figure. 2. The correlation of the crustal extension value (A) and the 0.85SiO2-FeOt parameter
used for the depth calculation of generation of primary basalt melts (B). The linear regression
equations and the correlation coefficients are given. In the upper figure the data for
gravimetric profiles after [1] are shown. of I–IV [2]
10
ISOTOPE STRUCTURE AND CONTINENTAL CRUST EVOLUTION IN
THE EASTERN TRANSBAIKALIA, CENTRAL ASIAN OROGENIC
BELT
V.P. Kovach1, E.Yu. Rytsk
1, V.V. Yarmolyuk
2, A.B. Kotov
1
1Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Makarova emb., 2,
St-Petersburg, 199034, Russia
2Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy
of Sciences, Staromonetnyi per. 35, Moscow, 109017 Russia
New and published geochronological and Nd isotopic data for rock complexes of the
eastern Transbaikalia segment of the Central Asian Orogenic Belt (CAOB) presented.
Geological structure of the eastern Transbaikalia defined by Baikal-Patom and Baikal-Muya
belts, and Barguzin-Vitim super-terrain.
Nd isotopic data indicate that magmatic and sedimentary rocks of the Baikal-Patom belt
were formed from mainly Achaean and Paleoproterozoic crustal sources with limited addition
of juvenile material related to Neoproterozoic rifting in the Baikal-Patom belt. The
Baikal-Patom belt represent an example of the early Precambrian isotope province of the
eastern Transbaikalia.
Early Baikalian (ca. 1.0-0.8 Ga) crust-forming events distinguished in the Kichera and
Param-Shaman zones of the BMP and probably in the Amalat metamorphic terrain of the
Barguzin-Vitim super-terrain. Late Baikalian (ca. 0.7-0.62 Ga) crust-forming event and
reworking of late Neoproterozoic juvenile crust are characteristic for the Karalon-Mamakan,
Yana and Katera-Uakit zones of the Baikal-Muya belt. Nd data for granitoids and sediments
of the Anamakit-Muya zone indicate their formation in result of early Precambrian crust
reworking during early Baikalian crust-forming processes. Paleozoic granitoids of the
Baikal-Muya belt formed by reworking of early Precambrian and Neoproterozoic crustal
sources. Early Precambrian and Meso- and Neoproterozoic isotopic provinces juxtaposed in
the Kichera, Anamakit-Muya zones, Yana and Katera-Uakit zones, whereas Param-Sham and
Karalon-Mamakan of the Baikal-Muya belt represent the Neoproterozoic isotope provinces.
Nd isotopic data suggest presence of early Precambrian as well as Neoproterozoic
crustal sources in structure of the Barguzin-Vitim super-terrain. Mixing of such sources
during sedimentation and/or tectonic superposition of rock complexes of different ages and
their reworking led to formation of Paleozoic granitoids with Mesoproterozoic Nd model
ages.
Early Precambrian and Mesoproterozoic isotope provinces superimposed in the Amalat,
Vitimkan-Tsypa, Itantsa-Turka and Konda terrains of the Barguzin-Vitim super-terrain.
Uda-Vitim and Malkhan-West-Stanovoy zone represent Mesoproterozoic isotope provinces.
11
Barguzin and Khilok-Vitim terrains additionally include Neoproterozoic isotope provinces.
In summary, early Baikalian crust-forming events are manifested in the narrow trough
zones of the Baikal-Muya belt whereas the Late Baikalian events, in the Karalon-Mamakan,
Yana, and Katera-Uakit zones. The remobilization of the Early Precambrian continental crust
and contribution of the Neoproterozoic juvenile sources are typical of most of the
Anamakit-Muya zone and Baikal-Patom belt. Reworking of mixed early Precambrian and
Neoproterozoic crustal sources is characteristic for the Barguzin-Vitim super-terrain.
12
History of the Tuva-Mongolian microcontinent agglomeration in the
Neoproterozoic: U-Pb LA-ICP-MS dating of detrital zircons from a
sandstone of the Darkhat series, Western Hovsgol, Northern Mongolia
Demonterova E.I., Ivanov A.V., Reznitskii L.Z., Belichenko V.G., Hung C.-H.2, Chung S.-L.
2,
Iizuka Y.3, Wang K.-L.
1 Institute of the Earth’s Crust SB RAS, Irkutsk, Russia
2 Department of Geosciences, National Taiwan University, Taipei, Taiwan
3 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
Tuva-Mongolian microcontinent is one of the key structures of the Central-Asian
Orogenic Belt. It has two major members; Vendian-Cambrian platform-type carbonate cover
and older basement. The basement in its turn is subdivided into the Early Precambrian
granites of the Gargan block and volcanics and terrigenous sediments of the Riphean arc
system (Fig. 1). The highly debated question is if the Early Precambrian rocks of the
Tuva-Mongolian microcontinent were derived from Siberia or any other continent. To answer
this question we dated detrital zircons collected from Riphean sandstone of the Darkhat series
(Western Hovsgol, Northern Mongolia) by U-Pb LA-ICP-MS.
The detrital zircons of three age generation were recovered; 750-850 Ma, 900-940 Ma
and 2660–2720 Ma (Fig. 2). The former age generation is represented by zircons from
volcanics of the Riphean arcs and related complexes. Basic sills, which intrude the Oka
accretionary prism, were dated by U-Pb on zircons as 753 ± 16 Ma [1]. Ignimbrites of the
Sarkhoi arc, which is analogue of the Darkhat series but at the territory of Russia, were dated
by U-Pb on zircons as 781 ± 11 Ma [2]. Rhyolites of the Shish-Khid-Gol arc were dated by
U-Pb on zircons as 800 ± 2.6 Ma [3], and tonalites of the Sumsunur complex yielded U-Pb
ages of 785 ± 11 Ma [4]. In other words, practically all detrital zircons with ages between 750
Ma and 850 Ma have their companions in form of dated geological complex of the
Tuva-Mongolian microcontinent.
Within the Tuva-Mongolian microcontinent there were not found yet complexes with
ages between 900 Ma and 940 Ma. This could be due to either incomplete geochronologic
studies or detrital zircons in the Darkhat sandstone were redeposited from the Oka
sedimentary prism.
Detrital zircons of the oldest age group have been derived from the Early Precambrian
granites of the Gargan block whose age according to SHRIMP U-Pb dating is about 2.7 Ga
[5].
Notable feature is that there are no detrital zircons with typical ages for the Siberian
craton, particularly the age of about 1.8 Ga [6]. This allows us to conclude that the
13
Tuva-Mongolian microcontinent was not in vicinity of the Siberia at the time of its
agglomeration in Neoproterozoic (Riphean).
The work is supported by Program no. 10.3 of the SB RAS, the Integration Project of
SB RAS_NNC Taiwan no. 142 and SB RAS-MAS 2011-2012.
[1] Kuzmichev A.B., Larionov A.N. In: Geodynamic evolution of lithosphere The Central
Asian Orogenic Belt, Proceedings, 2010, v. 1, p. 159–160. (in Russian)
[2] Kuzmichev, A.B., Sklyarov E.V., Postnikov A. et al. Island Arc, 2007, v. 16, p. 224–242.
[3] Kuzmichev A., Kroner A., Hegner E. et al. // Precambrian Research, 2005, v. 138, p.
125–150.
[4] Kuzmichev A.B. Tectonic History of the Tuva-Mongolian Massif: Early Baikalian, Late
Baikalian and Early Caledonian Stage. Probel, Moscow, 2004, 192 p. (in Russian)
[5] Kovach V.P., Matukov D.I., Berezhnaya N.G. et al. In: 32th IGC – Florence. Session:
_T31.01 – Tectonics of Precambrian mobile belts, 2004, Pt. 2, p. 1263.
[6] Salnikova, E.B. Kotov, A.B., Levitskii V.I. et al. Stratigraphy. Geol. Correlation, 2007, v.
15, p. 3-19. (in Russian)
Рис. 1. Structure of the Tuva-Mongolian and Dzabkhan microcontinents [2]. 1 – Cenozoic
basins, 2 – Vend-Cambrian carbonate cover, 3 – Riphean ophiolites and arcs, 4 – Riphean
terrigenous sediments, 4 – Early Cambrian structures, 6 – other Early Paleozoic structures.
14
Рис. 2. U–Pb-concordia diagram for detrital zircons from the sandstone of the Darkhat series,
Western Hovsgol, Northern Mongolia.
15
Sr-Nd-Pb isotope systematics of rifting basalts of the Kamar Ridge (Baikal
Rift System)
Dril S.I., Antipin V.S., Chukonova V.S., Il’ina N.N.
Institute of Geochemistry, SB RAS, Irkutsk, Russia
Basalt volcanism which is spatially distributed in the Baikal Rift System (BRS) is of
particular interest for researchers. By now there is a big database concerning the composition
of basalts of the largest volcanic areas: Vitims plateau, Udokan and Khamar-Daban Ridges,
Tunka valley, East Sayan Mountains [1-2]. Wide variations of chemical composition of rocks,
a multi-stage and prolonged volcanic process, resulting in spatial correlations and
comparisons for revealing a general pattern of petrogenesis within the BRS require further
studies of the basalt composition and primarily their isotope-geochemical features.
The ―top‖ Neogenic basaltic covers are abundant within the Kamar Ridge in 10 -
20 km southwestward of Slyudyanka city [2]. Two groups of rocks are distinguished
amongst the studied basalts: 1) the varieties containing lherzolite nodules, inclusions of black
pyroxenites, orthopyroxene megacrysts, which compose the neck in the Sukhaya River upper
reaches [1]; and 2) the varieties, which do not contain deep-seated inclusions or mineral
megacryst associations. The first group of basalts belongs to the rocks of alkaline K-Na-series
(AB), whereas the second group is a member of K-Na-subalkaline series (SAB).
By the ratio of normative minerals ABs are olivine basanites and contain from 3.5 up
to 11.2 % of normative nepheline, and both olivine basanites (0,2 - 4,1 % of normative
nepheline), and olivine tholeiites with normative hyperstene (2,6 - 20,2 %) are distinguished
among SAB. By the geochemical features both considered groups of basalts are quite
typical of volcanics occurring in the BRS and are characterized by high contents of large-ion
lithophyle and high-charge elements, as well as Sn, F. Nevertheless, ABs turn out to be
enriched by these elements in 1.5 - 2 times in comparison with SAB. It is verified by
noticeably different concentrations of rare earth elements in rocks, as well as various degree
of fractionation of their rare earth spectra (La/Yb 15 - 24,7 in AB and La/Yb 10 - 13,7 in
SAB). Alkaline basalts are more differentiated (Mg# = 0,44 - 0,57) in comparison with
subalkaline varieties (Mg# = 0,57 - 0,61), that results in lower Ni Co, Сг, V contents in
the first groups as compared with the second one.
We calculated the models of equilibrium non-modal melting of primitive spinel,
metasomatically enriched (amphibolized) spinel and primitive garnet lherzolites to specify the
nature of magma-generating substrata for AB and SAB. The compositions of both AB and
SAB lie in the field of melting of metasomatically enriched spinel lherzolites, approaching to
the trend of melting of moderately enriched substratum containing 1 - 2 % of modal
amphibole.
16
Variations of 87Sr/86Sr (0) and Ba/Nb values in AB and SAB are similar and coincide
with the field of variations of compositions of intraplate basalts distributed within the
Central-Asian folded belt (CAFB) in the Neogene-Quaternary. All these types of basalts
demonstrate insignificant variations of Ba/Nb values with noticeable variations in
strontium isotope composition that is related to the interaction of the substance of depleted
mantle domains and enriched mantle EM I source [7].
There is an opposite tendency (Fig.2) for the field of Paleozoic-Mesozoic intraplate
basalts the CAFB: wide variations of Ba/Nb values and a moderate variability of 87Sr/86Sr
(0) values. A similar tendency is common to the island arc basalts formed during the melting
of the depleted mantle substratum, metasomatized by the influence of the subducted fluid
component. It is impossible to exclude, that melting of the Paleozoic-Mesozoic basaltic
magmas in the CAFB took place with the contribution of the mantle modified by the ancient
subduction processes. Thus, two types of mantle sources defining the composition of
intraplate basaltoids in the CAFB can be distinguished. The mantle substratum,
metasomatized by the ancient subduction processes was important in the source of melts
during the Paleozoic and Mesozoic, while the melts including the volcanic area of the
Kamar Ridge generated by the depleted and enriched (EM I) mantle sources in the Neogene
and Quaternary.
The researches were supported by Integration project SB –RAS # 142, Integration
project ONZ - 9.3, the Integration project of SB RAAS- Far East Branch of Russian
Academy of Sciences №13.
17
Late-Cenozoic hawaiite volcanism of Kamchatka and Baikal Area –
indicator of the composition and mantle metasomatism in the setting of
convergent lithosphere plates and plume-lithosphere interaction
Perepelov A.B., Tsypukova S.S.
Institute of Geochemistry, SB RAS, Irkutsk, Russia
Hawaiites are found in volcanogenic formations of oceanic islands and uplifts,
intracontinental rift zones and igneous provinces, active continental margins and island arc
systems. The comparison of mineralogical and compositional characteristics of hawaiites as
indicator geochemical type of volcanic rocks is perspective for revealing the features of
magma formation and composition of sources of the same magmas in various geodynamic
settings. From the viewpoint of compilation of known and suggested parameters of
hawaiites classification the given report makes an attempt of the comparative analysis of
isotope-geochemical features of this type of rocks for oceanic islands, active continental
margins and intracontinental rift zones.
We compared the hawaiites based on original (Kamchatka, Northern Mongolia) and
literature data (Hawaiian islands). Within the island arc system of Kamchatka the hawaiitic
lavas were found and investigated in is back zone, in large Pliocene-Quaternary volcanic
centers of the Mid oceanic Ridge (Belogolovskyi, Big) and Pleistocene-Holocene zones of the
area volcanism (Sedakinskaya, Dola Geologov). In northern Mongolia hawaiites were studied
on the example of the Early Miocene volcanic plateau Kheven Zalu Uriin Sar’dag on
southwest margin of the Baikal Rift Zone (BRZ).
Petrochemical characteristics of hawaiites of all investigated geodynamic settings are
considerably different. Hawaiites of the oceanic islands show the highest Ti, P, Na, Th, U,
REE, Zr, Hf, Y concentrations. Rocks of this type in island arc system and intracontinental
rift zone, on the contrary, demonstrate higher K and Pb concentrations. A relative distribution
of magmatophyle elements shows, that hawaiites of Kamchatka and BRZ, as compared with
those from Hawaiian islands, are also different in positive anomalies of Ba, K, Pb and Sr
spectra. On the other hand, the degree REE fractionation in hawaiites increases in a series:
island arc system (La/Yb=6-14), oceanic islands (8-17) and intracontinental rift (16-25).
The comparison of isotope characteristics of hawaiites from Kamchatka, Northern
Mongolia and Hawaiian islands shows that hawaiites of island arc system of Kamchatka have
similar isotope values with the same rocks from the Hawaiian islands. On the other hand,
hawaiites of the Baikal Rift system (Northern Mongolia) differ from them in the most
radiogenic Sr, Nd and less radiogenic Pb compositions.
When analyzing the isotope-geochemical characteristics of hawaiites of the Northern
Mongolia (Kheven plateau) we came to the conclusion concerning the origin of initial
magmas for them with the contribution of PREMA and EM I mantle sources. Taking into
18
account that the mantle reservoir PREMA is considered as prevailing weakly-depleted mantle
and is defined by the average composition of various mantle sources, it is necessary to
conclude, that the enriched EM- I mantle was the major source for the origin of hawaiitic
magmas of the Northern Mongolia. The origin of hawaiitic magmas in this case can be
considered from the viewpoint of plume magma genesis and recycling of the substance of the
ancient continental crust. The formation of magmas of such type in the geodynamic setting of
island arc, in this case, can be connected with the occurrence of the asthenosphere mantle
magmas at a stage of change of geodynamic settings with the contribution of the substance of
the depleted magmas and the substance of the metasomatized supra-subduction mantle wedge.
Magmatism of oceanic islands including hawaiitic is usually considered from the viewpoint of
hot spots. In conformity with the enrichment by HFSE and LILE components and
sufficiently low radiogenic isotope values, the hawaiites of the oceanic islands can be
derivatives of magmas of the lower mantle weakly-depleted(enriched) reservoir. The
similarity of their isotope characteristics with hawaiites of the island arc system within such a
model can be defined by the absence of influence of the ancient continental crust recycling.
19
Textures and geochemistry of the Saramta peridotites (Siberian craton):
melting and refertilization on early stage of the continental lithosphere
mantle formation.
M.A. Gornova, V.A. Belyaev, O.Yu. Belozerova
Institute of Geochemistry, SB RAS, Irkutsk, Russia
Saramta harzburgites are highly refractory in terms of lack of residual clinopyroxene,
olivine Mg# (up to 0.937) and spinel Cr# (~0.5), suggesting high degree of partial melting
(>30%). Detailed studies of their microstructures show that they have extensively reacted
with a high- SiO2 melt, leading to crystallization of orthopyroxene, clinopyroxene, amphibole
and spinel at the expense of olivine. The major element composition of least reacted
harzburgites are mostly coincident with residues of refractory peridotite produced by
fractional melting (initial melting pressures > 3 GPa and melt fractions ~40%). Further
non-residual clinopyroxenes are highly depleted in Yb, Zr and Ti but highly LREE-enriched.
A two-stage partial melting/melt–rock reaction history is proposed: 1) primitive mantle
underwent depletion in the garnet stability field followed by melting in the spinel stability
field; 2) refractory peridotites underwent refertilization by high-SiO2 melt in supra-subduction
zone. Rare Saramta lherzolites probably formed from more refractory harzburgites as a result
of this melt–rock reaction.
20
Convergent boundaries of the West Pacific type and their significance for
the origin of magmatic rocks of Central Asian folded complexes
Khomutova M.Yu.1,
Kuzmin M.I.1, Yarmolyuk V.V.
2, Voronstov A. A.
1
1Institute of Geochemistry, SB RAS, Irkutsk, Russia
2IGEM RAS, Moscow, Russia
Amongst convergent boundaries of lithosphere plates we can distinguish three types of
boundaries. One type includes zones of continental collision, the second one involves active
continental margins and the third one corresponds to island arcs. The latter is the most
abundant in the west of the Pacific ocean and thus is distinguished as West-Pacific type of
convergent boundaries. Specific feature of this type is a combination of island arcs (Kurile –
Japanese-Idzu-Bonin – Mariana – Philippine), rift structures occurring in the back
surrounding and including back-arc seas (Okhotsk-Japanese- Philippine) complicated by
grabens (grabens of the Japanese sea, Sumizu rift, Okinawa, Palavan troughs, etc) and
systems of faults superimposed on the continental margin of the Eastern Asia and marked
by recent volcanic occurrences. Another specific feature of boundaries of this type results
from the composition of rocks occurring within these boundaries.
Systematic seismic-tomographic investigations of convergent zone between the Asian
continent and Pacific plate indicate that the region is characterized by a combination of
low-velocity ―hot‖ and high-velocity ―cold‖ mantle. A ― cold mantle‖ corresponds to the
subducted oceanic lithosphere, which is continuously observed up to the transitional zone at
the lower part of upper mantle, forming extent ―stagnated‖ sites along upper mantle; while
in lower layers of mantle it occurs as bands of lenses, subsided up to the mantle’s bottom. A
―hot‖ mantle is found throughout the whole depth as large lenses, including those cutting by
the subducted slabs. An accumulation of the hot mantle in this part of the west margin of the
Pacific ocean suggests a link between the mantle and a number of mantle plumes, being most
likely components of the Pacific super-plume. We believe that combination of subduction
zones and mantle plumes on the convergent boundaries of the West-Pacific type is
predetermined by specific geodynamic setting. We propose the following mechanism for its
origin. Decompressed (hot) mantle at the bottom of cold oceanic lithosphere served as a
trigger for immersing the latter into the mantle. Flexure arisen in the lithosphere was
accompanied by its fracture and subsiding of hanging margin of the cold oceanic slab into the
hot mantle of the plume and thus gave rise to subduction zone. Immersion of lithosphere into
mantle was associated with compensation and uplifting of hot substance from deep mantle
horizons towards surface giving rise to upper mantle plumes. As specific feature of
subduction on these boundaries is origin of horizontal flows of subducted lithosphere in
transitional mantle zone, compensated upper mantle plumes originate throughout entire
mantle being under influence of these flows, thus covering vast areas in back part of island
arc. The model proposed can explain specific features of structure and magmatism of
convergent zones of this type.
21
Existence of Microcontinent in the Central Asia Orogenic Belt:
In Situ Re-Os Evidence from the Lithospheric Mantle
Kuo-Lung Wang1, Mikhail Kuzmin
2, V. Kovach
3, V. Yarmolyuk
4, S.Y. O’Reilly
5,
W.L. Griffin5, N.J. Pearson
5
1Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan, R.O.C.
2Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Irkutsk, Russia
3Inst. Precamb. Geology and Geochron., Russian Academy of Sciences, St. Petersburg, Russia
4IGEM, Russian Academy of Sciences, Moscow, Russia
5GEMOC Key Centre, Macquarie University, Sydney, NSW 2109, Australia
E-mail: [email protected]
Recent studies have noted that volumes of ancient depleted material can survive in the
convecting asthenospheric mantle for long periods so that the use of Os model ages of mantle
xenoliths to constrain the age of lithospheric mantle events should be approached with caution.
In this study, we demonstrate the in situ Os dating work on sulfides in the peridotitic xenoliths
from cratonic (Tok, Russia) and off-cratonic (Tariat and Dariganga, Mongolia) setting of the
Neoproterozoic-Phanerozoic Central Asia Orogenic Belt (CAOB) to examine lithospheric
formation. One least-disturbed sulfide from Tok region, with 187
Re/188
Os=0.063, yield TMA
model age of 1.2 Ga. A few Tok sulfides yield an apparent isochron indicating an age of 3.2
Ga. The initial 187
Os/188
Os ratio (0.117) of depleted component in the Re-Os mixing line
shows similar value with the lowest whole-rock 187
Os/188
Os ratio reported by Ionov et al.
(2006; 0.116). All of the above indicate the presence of an Archean domain beneath the Tok
region, which was affected by Neoproterozoic CAOB event.
Both TMA from the least-disturbed sulfides (187
Re/188
Os<0.07) and TRD from higher
Re/Os sulfides without later introduction/loss of Os, yield model ages ranging from 0.5 to 3.0
Ga, with peaks around 1.7-1.5, 1.2 and 0.7-0.5 Ga. These ages suggest that the
sub-continental lithospheric mantle (SCLM) beneath the Tariat region formed at least by the
Proterozoic time, and that some domains are Archean. The oldest age reported on the
Precambrian Tarvagatay Terrane, where is underlain by Tariat volcanic field, is ca 3.05 Ga by
Pb-Pb zircon dating in anorthosite (Mitrofanov et al., 1985). Other zircon U-Pb ages from
nearby anorthosites are 1.78 and 1.7 Ga (I. Kozakov unpubl. data). The sulfide Os ages are
consistent with these formation events recorded in the overlying crust. Younger sulfide Os
ages (1.2 and 0.7~0.5 Ga) may mark the commencement of the Central Asia Orogeny since
the Neoproterozoic and involvement of the mantle as suggested by Jahn (2004).
It would be a remarkable coincidence if sulfides derived from randomly selected
fragments of refractory materials in the convecting asthenospheric mantle would combine to
give such a systematic correlation. Moreover, some of the ancient Os model ages are from
apparently residual sulfide phases with subchondritic 187
Re/188
Os and 187
Os/188
Os. To interpret
22
these sulfides as derived from depleted material residing within the asthenospheric mantle, it
would be necessary to quantitatively melt the older sulfides, transport them into the SCLM
and deposit them again without modifying their isotopic systematics. This seems to be an
unlikely scenario. We therefore prefer the simplest interpretation of these data: the sulfide Os
ages in the Tariat peridotites record major events (i.e., melt extraction) that affected the
underlain SCLM.
Sulfides in Dariganga peridotites also have Mesoproterozoic Os model ages (two TMA
of 2.0, 1.4 Ga and two TRD of 1.8, 1.2 Ga). Although Proterozoic crustal events have not been
reported in this region so far, Proterozoic Nd model ages for basement rocks around the
Xilinhot region in the vicinity of the Dariganga Plateau (B. Chen, pers. comm.) suggest that a
Precambrian crustal terrain, a counterpart of the underlying Mesoproterozoic lithospheric
mantle, should be expected and might be found by studies of deep-crustal xenoliths in the
Dariganga region.
23
Distinct Crustal Development of SW and NE Japan – Sr-Nd Isotopic Evidence and Tectonic Implications
BOR-MING JAHN
Department of Geosciences, National Taiwan University, Taipei 116, Taiwan
(e-mail: [email protected])
The Japanese Islands represent a Phanerozoic subduction-accretion orogen developed
along the western Pacific convergent margin. The formation of the Japanese Islands,
particularly SW Japan, has been taken as a classic model of accretionary orogeny. According
to Maruyama (1997), the most important cause of the orogeny is the subduction of an oceanic
ridge, by which the continental mass increases through the transfer of granitic melt from the
subducting oceanic crust to the orogenic belt. Sengor and Natal’in (1996) named the orogenic
complex the "Nipponides", consisting predominantly of Permian to Recent
subduction-accretion complexes with very few fragments of older continental crust. These
authors pointed out the resemblance in orogenic style between SW Japan and the Altaids or
Central Asian Orogenic Belt (CAOB). However, the granitoids from SW Japan have high
initial 87
Sr/86
Sr ratios (0.705-0.713), negative εNd(T) values and Proterozoic Sm-Nd model
ages (1000-2000 Ma). These data are in strong contrast with those of two celebrated
accretionary orogens, the CAOB and Arabian-Nubian Shield (ANS), but are comparable with
those observed in SE China and Taiwan, or in classical collisional orogens in the European
Hercynides and Caledonides. Despite the well-documented subduction-accretion complexes
in Japan, the isotopic data dictate that the granitoids were mainly produced by remelting of
Proterozoic rocks, overlain by the accretionary complexes in SW Japan. However, the
isotopic data support the idea of Isozaki (1996) and Maruyama et al. (1997) that the
proto-Japan (SW Japan) was initially developed along the southeastern margin of the South
China Block.
On the other hand, the crustal and tectonic development of NE Japan (and Hokkaido)
appears to be highly distinct from SW Japan as inferred from the highly contrasting Sr-Nd
isotopic characteristics of granitoids. The low 87
Sr/86
Sr ratios (< 0.705), positive εNd(T) values
and young model ages (400-1000 Ma) in the granitoids of NE Japan indicate that its bulk
crust is much more ―juvenile‖ than that of SW Japan. The granitoids from NE Japan are
mainly of TTG (tonalite-trondjemite-granodiorite) or adakitic composition. The broad
similarity in the age distribution (Cretaceous-Paleogene) of felsic magmatism, the occurrence
of Jurassic accretionary complexes and the presence of local Paleozoic continental fragments
(S. Kitakami) in NE Japan and the Sikhota-Alin belt suggests a possible link between them. In
this scenario, NE Japan was not related to SW Japan in the initial development of proto-Japan.
It is speculated that NE Japan and the Sikhota-Alin belt constituted the same tectonic unit
from the Jurassic to Paleogene.
Keywords: SW Japan, NE Japan, Sikhota-Alin belt, accretionary orogen, granitoids, Sr-Nd isotopes,
accretionary complex.
24
Primary melts of the Emeishan and Karoo flood basalts: Linking Ni and Ti
potential of the magmas and their mantle sources
Vadim S. Kamenetsky1, Maya B. Kamenetsky
1, Sun-Lin Chung
2, Georg F. Zellmer
3, Dmitry
V. Kuzmin4
1ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tas, Australia
2Department of Geosciences, National Taiwan University, Taipei, Taiwan;
3Institute of Earth Sciences-Academia Sinica, Taipei, Taiwan;
4Max Planck Institute for Chemistry, D-6500 Mainz, Germany
Introduction
Throughout geological history all continents have been periodically flooded by
enormous amounts (> 106 km
3) of basaltic and rare picritic magma within time-scales of only
a few millions of years, so-called Large Igneous Provinces (LIP) The contributions of mantle
peridotite and recycled crust to flood magmas are poorly understood. In theory, siliceous
melts formed from eclogite should react with peridotite, converting it to olivine-free
pyroxenite. Further melting of this hybrid lithology in the absence of residual olivine is
characteristically more voluminous than the melting of peridotite (at a given pressure and
temperature), and thus can be linked to extensive continental magmatism. Partial melts of the
hybrid pyroxenite are enriched in Si and Ni but poorer in Mg, Ca, and Mn than melts of
peridotite. This study of primitive olivine phenocrysts and olivine-hosted melt inclusions is
aimed at recognising compositions of flood basalt primary melts from the Emeishan (SW
China) and Karoo (S Africa) LIPs and their respective mantle sources, with insights into the
potential of magmas to supply Ni and Cu to immiscible sulphide melts.
Picrites in Large Igneous Provinces
Magmatism of Large Igneous Provinces is characteristically basaltic with scarce
presence of olivine phenocrysts that have evolved compositions (< 85 mol% Fo), whereas
picrite lavas with high-Fo olivine are either unknown or very rare in many localities. As
picrites may provide valuable information about the composition and temperature of primary
melts and their evolutionary paths, this study has been focussed at two magmatic provinces,
where picrites are relatively common. Picrites (16-26 wt% MgO) from poorly known Late
Permian (~250 Ma) Emeishan LIP (SW China) were sampled from a single lava flows in
Yongsheng and Binchuan areas. We also examined a picrite sample (16 wt% MgO) from the
Late Jurassic (~180 Ma) Karoo province (S Africa) in Nuanetsi area. All samples are massive,
moderately fresh rocks with abundant large olivine and clinopyroxene phenocrysts, and
Cr-spinel microphenocrysts.
Emeishan (SW China). Picrites and basalts in the Emeishan LIP have a wide range of
compositions that are traditionally subdivided into low-Ti and high-Ti series. The samples in
25
this study belong to high-Ti (Yongsheng) and high-Ti (Binchuan) endmembers, with all other
published rock compositions ranging in between. Compositions of primitive olivine
phenocrysts (79-91.6 mol% Fo in Yongsheng and 83-91 mol% Fo in Binchuan) are also
representative of the petrological endmembers in terms of abundances of trace elements (Ni
and Mn). The Yongsheng olivine contrasts with the olivine from Binchuan and other areas in
having the highest Ni and lowest Mn at a given Fo (Fig. 1). The olivine compositions,
interpreted using the method of Sobolev et al. (2007), suggest hybrid mantle source (60%
pyroxenite) for the Yongsheng picrite and peridotite source for Binchuan picrite. Similarly,
contrasting compositions of Cr-spinel (2-7 and 0.5-0.7 wt% TiO2, 6-12 and 13-18 wt% Al2O3
in Yongsheng and Binchuan, respectively) argue for a wide range of parental melt
compositions.
Heated and homogenised melt inclusions in olivine (Yongsheng) and Cr-spinel
(Binchuan) accentuate differences between magmas in spatially distinct (but still close) areas.
They have broad geochemical similarities to the whole rock compositions, however, their
variations in trace elements at a given MgO suggest a range of parental magma compositions
for a given sample. The main differences are the abundances of trace elements (much more
enriched in Yongsheng compared to Binchuan) and presence/absence of garnet in the source
(Gd/Yb = 4.1 and 1.6, respectively). A difference in the isotope composition would be
expected, however, the picrites from both area have almost identical 87
Sr/86
Sr (0.7045) and
εNd (1.7) initial values.
0.2
0.3
0.4
0.5
0.6
79 81 83 85 87 89 91 93
0.1
0.2
0.3
MnO, wt%
NiO, wt%
Fo, mol%
Fig.1. Composition of olivine from Emeishan (blue
– Dukou, red – Binchuan) and Karoo (black)
picrites compared to olivine from other intraplate
magmas (field after Sobolev et al., 2007).
0.2
0.3
0.4
0.5
0.6
79 81 83 85 87 89 91 93
0.1
0.2
0.3
MnO, wt%
NiO, wt%
Fo, mol%
Fig.1. Composition of olivine from Emeishan (blue
– Dukou, red – Binchuan) and Karoo (black)
picrites compared to olivine from other intraplate
magmas (field after Sobolev et al., 2007).
0.2
0.3
0.4
0.5
0.6
79 81 83 85 87 89 91 93
0.1
0.2
0.3
MnO, wt%
NiO, wt%
Fo, mol%
Fig.1. Composition of olivine from Emeishan (blue
– Dukou, red – Binchuan) and Karoo (black)
picrites compared to olivine from other intraplate
magmas (field after Sobolev et al., 2007).
Fo, mol%
YongshengBinchuanNuanetsi
intraplatemagmas
Karoo (S Africa). The Nuanetsi picrite belongs to the high-Ti suite, and thus has
mineralogical and geochemical similarities to the Yongsheng picrite. Primitive olivine
phenocrysts (84-91 mol% Fo) have significantly higher NiO (up to 0.57 wt%), NiO/MgO
(0.008-0.012) and FeO/MnO (74-84) than in other continental flood basalts (Fig. 1),
suggesting up to 80% of the ―pyroxenite‖ in the hybrid mantle source. Compositions of
olivine-hosted melt inclusions strongly argue for non-peridotitic mantle source for the
primary (15 wt% MgO) melts: high-SiO2 (52-54 wt%), high-Ni (~660 ppm), low-CaO (5-6
wt%), enrichment in K2O, P2O5, TiO2 and incompatible trace elements (La/Sm = 5.6), and
strong depletion in HREE (―garnet‖ signature; Gd/Yb = 5.3, Fig. 2). Unusual source
26
compositions are also reflected in initial Pb-Sr-Nd isotope ratios (206
Pb/204
Pb = 17.262, 87
Sr/86
Sr = 0.70588, εNd = -9).
Discussion
Mineralogical, geochemical and melt inclusion data testify for significant diversity
among parental magmas of picrites and basalts in the Emeishan and Karoo LIPs. A variety of
primary melts in these settings reflect multiple mantle mantle sources, ranging from a
garnet-free peridotite (Binchuan) to a ―hybrid‖ garnet-pyroxenite or eclogite (Yongsheng,
Nuanetsi). All sources are probably located in the continental lithosphere and have different
residence times, as suggested by the radiogenic isotopes.
Petrological characteristics, similar to those of the studied LIP magmas, have been found in a
zero-age primitive (8.5 wt% MgO) Ni-rich (290 ppm) glass from the Mid-Atlantic ridge near
the Bouvet Triple Junction. This glass (206
Pb/ 204
Pb = 17.188, 87
Sr/86
Sr = 0.71209, εNd = -18)
records melting of a pure garnet pyroxenite source with an ancient lithospheric (lower
crustal ?) LOMU isotopic flavour. Based on tectonic reconstructions, the Karoo LIP and
Bouvet TJ were closely spaced at the time of opening of the South Atlantic. We argue that
pyroxenite-derived primitive melts were not only an important component during the
initiation and development of LIPs, but continue to supply melts to the modern ridge system.
The Bouvet TJ glass contains globules of Cu-poor, but Ni-rich (23-25 wt%) sulphide, which
composition is consistent with strong enrichment of the melt and liquidus olivine in Ni. We
conclude that LIP primary melts derived from pyroxenite/eclogite lithologies in the
subcontinental lithospheric mantle are exceptionally enriched in Ni, and, before Ni is
consumed by crystallising olivine, are capable of generating immiscible sulphide liquids at
earlier stages of fractionation.
27
A Comparative Study of the Caucasus/Iran and Tibet/Himalaya Orogenic
Belts: Magmatic Perspectives
Sun-Lin Chung
Department of Geosciences, National Taiwan University
This report is based on a newly approved project for conducting a comparative
study, from magmatic perspectives, of the Caucasus/Iran and Tibet/Himalaya orogens,
two most important collision zones that take place in East and West Asia, respectively,
in the Alpine-Zagros-Himalayan mountain range or so-called Tethyan orogenic belt.
This study aims at a more comprehensive understanding of the magmatic and tectonic
evolution in continent-continent collision, with the Tibet/Himalaya exemplifying a
well-established or mature stage of collisional orogen and the Caucasus/Iran an initial
to intermediate stage, to depict the whole spectrum of geodynamic processes through
which collisional orogeny evolves.
In the Tibet/Himalaya orogenic belt, the Neotethyan subduction related arc
magmatism started from the Jurassic and lasted until the Eocene. In southern Tibet, the
volcanic records are characterized with southward migration and intensification at ca.
50 Ma, suggesting rollback and breakoff of the subducted Neotethyan slab at the early
stage of the India-Eurasia collision that may have begun by 55 Ma. Post-collisional
magmatism, showing either ultrapotassic or adakitic geochemical features, started ca.
30 Ma as a result of removal of collision-thickened lithospheric root. In contrast to
other active collision zones that involve broad, diffuse regions of crustal deformation,
the Caucasus/Iran orogenic belts has an intriguingly sharp northern boundary defined
by the linear trend from the Great Caucasus to Kopeh Dagh, a feature lacking in the
mature India-Eurasia collision zone. Although the true continent-continent contact
between Arabia and Eurasia appears to have initiated no later than the early Miocene,
some large-scale ocean basins such as those in the Black and Caspian Seas are yet
closed. However, our new data from Georgia, Armenia and Iran indicate a
southeastward younging of termination of the Neotethyan subduction related
calc-alkaline magmatism, from the Oligocene in Armenia to the middle Miocene in
Esfahan and the late Miocene in Kerman areas along the Urumieh-Dokhtar magmatic
arc. These data are consistent with the argument for an oblique Arabia-Eurasia collision
and diachronous contact between the two continents along the Bitlis-Zagros suture
zone. This report will also discuss new results on three specific issues: (1) the unusual
occurrence of widespread post-mid-Miocene (≤11 Ma) igneous activity in the
Caucasus/Iran/Anatolia (―CIA‖) zone, a domain of active contraction owing to the
28
Arabia-Eurasia collision, (2) the temporal and spatial evolution of the Urumieh
Dokhtar Magmatic Arc, which extends from Armenia to the entire Iran, resulting from
the Neotethyan subduction that was prevailing in the region before the collision began,
and (3) the nascent volcanism in Bazman and Taftan areas, SE Iran related possibly to
the active Makran subduction that may have initiated after the closure of the Neotethys.
29
40Ar-
39Ar age, geochemical and Sr-Nd isotopic constraints on Late Cenozoic
intraplate volcanism, eastern Iran
Kwan-Nang Pang1, Sun-Lin Chung
1*, Mohammad Hossein Zarrinkoub
2, Seyed Saeid
Mohammadi2, Richard Walker
3, Hsiao-Ming Yang
1, Hao-Yang Lee
1, Chiu-Hong Chu
1, I-Jhen
Lin1
1Department of Geosciences, National Taiwan University, Taipei P.O. Box 13-318, Taipei 10699, Taiwan
2Department of Geology, Birjand University, Birjand, Iran
3Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK
Miocene to Quaternary alkali basalts occur as small volcanic fields or scattered outcrops
in the Lut-Sistan region, eastern Iran. They were spatially associated with two active,
north-south-trending dextral strike-slip fault systems in the region, i.e. the Neh faults in the
Sistan suture zone and the Nayband faults ~200 km further to the west. Here, we present new 40
Ar-39
Ar age, geochemical and Sr-Nd isotopic data for a suite of Lut-Sistan alkali basalts to
decipher the petrogenetic processes responsible for their formation and to study their regional
tectonic implications. The new ages indicate that the volcanism commenced since ~14 Ma,
approximately 9 m.y. earlier than other published ages. The rocks are dominated by hawaiite
and mugearite with alkalinity index between +0.87 and +2.67. Petrographic observations and
major and trace elemental variations suggest that the alkali basalts underwent variable
fractionation of olivine, clinopyroxene, plagioclase and Fe-Ti oxides. Chondrite-normalized
rare earth element and mantle-normalized trace element patterns of these rocks resemble that
of average ocean island basalt showing enrichments in all incompatible trace elements. Low
to moderate initial Sr isotopic ratios (0.7047–0.7065), high Nd(t) (+1.4 to +3.6) and trace
element ratios indicate that crustal contamination was not significant. The rocks have neither
geochemical features pointing to residual amphibole or phlogopite, nor arc magmatic
signatures characteristic of the Iranian sub-continental lithospheric mantle. Thus, the alkali
basalts most likely have asthenospheric origin. Modeling of REE suggests that they could
have formed by low degrees of partial melting (~3–10%) of garnet peridotites that are more
REE-rich than the primitive mantle. We propose that the east Iranian alkali basaltic volcanism
was triggered by asthenospheric upwelling, presumably caused by delamination of thickened
lithospheric root following the collision between the Lut and Afghan continental blocks
during Late Cretaceous. Our results imply that two contrasting tectonic regimes might coexist
in Iran since Miocene, i.e. extensional in eastern and compressional in southwestern Iran.
30
Mantle metasomatism and formation of alkali basaltic magmas:
implications from the geochemistry of the Early Cretaceous Coastal Ranges
basalts, NW Syria
George S.-K. Maa,b
, John Malpasa, Costas Xenophontos
a, Katsuhiko Suzuki
c and Ching-Hua
Lod
aDepartment of Earth Sciences, The University of Hong Kong, Hong Kong
bIES, Academia Sinica, Taipei, Taiwan
cIFREE, JAMSTEC, Natsushima, Yokosuka, Japan
dDepartment of Geosciences, National Taiwan University, Taipei, Taiwan
Abstract
The origin of alkaline volcanism has long been the subject of debate in petrology.
Although recent consensus has arrived at the line that volatile-free peridotite is not capable of
generating silica deficient (alkaline) magmas [1], the exact source of this magma type remains
controversial. At present, the two most plausible models appear to be — (i) melting of
hydrous metasomatic veins ± trapped incipient melts within the lithospheric mantle [2, 3] and
(ii) melting of an incompatible-element enriched peridotite source ± eclogites in the presence
of CO2 [1, 4].
The Mesozoic alkaline volcanism (ankaramites and transitional basalts) of the Coastal
Ranges, NW Syria investigated in this study was erupted in Arabia during times of passive
continental margin rifting associated with the evolution of the Neotethys and Eastern
Mediterranean. Isotopic and geochemical analysis reveals distinct compositions between the
two lava series (ankaramites: Nd(t) = 5.1–5.6, 87
Sr/87
Sr(t) = 0.70293–0.70302, 187
Os/188
Os(t) =
0.227–0.242; transitional basalts: Nd(t) = 4.0–4.6, 87
Sr/87
Sr(t) = 0.70320–0.70424, 187
Os/188
Os(t) = 0.392; and lower SiO2, higher TiO2, Nb/U, Nb/Th, Nb/La and Ce/Pb in the
ankaramites). Except the Os isotopic compositions which can be explained by subtle crustal
contamination, most of the compositional characteristics of the lavas are thought to be
inherited from the mantle source that contained an ―enriched‖ component, likely derived from
melting of or reaction with amphibole-rich metasomatic veins within the lithosphere.
Accordingly the compositional distinctions between the two lava series can be explained by
melting/reacting varying amounts of the metasomatic veins, with the ankaramites having
more of this metasomatic component. A contribution from deeper mantle sources or an
anomalously hot mantle (e.g. arrival of a mantle plume) is not required to explain the data,
which are more likely the result of melting of a metasomatically hydrated lithospheric mantle
at relatively low temperatures during times of Levant regional extension.
[1] Dasgupta et al. (2010) EPSL 289, 377.
[2] Pilet et al. (2008) Science 320, 916;
[3] Pilet et al. (2011) J. Petrol. 52, 1415;
[4] Dasgupta et al. (2007) J. Petrol. 48, 2093.
31
Zinc and Cadmium isotopes for Lake Baikal sediments
Der-Chuen Lee and Shun-Chun Yang
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
The distributions of Zn and Cd in the open oceans are quite similar to that of typical
nutrients, e.g., P, and this is because both Zn and Cd are essential nutrients for phytoplankton
to undergo photosynthetic reaction, fixation of carbon (primary productivity), and as a whole
marine phytoplankton is responsible for half of the primary productivity on Earth. Since the
marine primary productivity is directly linked to the pCO2 in the atmosphere, hence it is also
linked to the global climate changes. Furthermore, both Zn and Cd are moderately volatile, it
is easy to vaporize and fractionate Zn and Cd isotopes during metal smelting, incineration,
and agricultural burning. Consequently, Zn and Cd isotopes have also been used to study the
relative inputs between lithogenic components and pollution in aerosols and marine system.
Both Zn and Cd isotopic studies, Zn in particular, are still in the early stages, and are
mostly focused on modern marine system. In order to test if Zn and Cd isotopes can be used
as proxies for the bio-productivity and climate changes during the last glacial inter-glacial
period for Lake Baikal, a ~ 3 meters long gravity core (GC-99; sample locality: 52°05’23‖N,
105°15’24‖E) sampled near the bore hole of BDP-99 in Lake Baikal is used in this study.
Samples are taken continuously every cm interval for the entire core. For the initial test, 6
samples, 0-1cm, 9-10cm, 19-20cm 29-30cm 39-40cm and 49-50cm, are selected. In order to
extract the authigenic portions, which were precipitated along with Fe-Mn oxides directly out
of the lake water onto the sediments, a series of leaching procedures were performed aimed at
removing the carbonate fractions, and to collect the authigenic fractions, while leaving the
lithogenic sediments unaffected, for the Zn and Cd isotopic measurements. In order to
minimize the potential isotopic fractionations introduced by non-quantitative yield from
column chemistry and also during isotopic measurement, the ―double spike‖ technique is used
in this study. The pre-mixed and calibrated 67-70
Zn and 110-111
Cd double spikes were added to
the leachate prior to the column chemistry. Two columns were used to separate and purify Zn
and Cd. Initial results showed that there are significant Zn and Cd isotopic variations, and Zn
and Cd isotopic data are decoupled among the initial 6 samples. Although preliminary, the
results seem to indicate evidence of biological and human activities in the sediment core of
Lake Baikal. More data and, in particular, the chronology of the sediment core are needed in
order to better constrain the relationship between the Lake Baikal and global climate changes
in the past.
32
Silver isotope measurements of Au-Ag ore deposits in Russia by
MC-ICP-MS
Mayuko Fukuyama1, Der-Chuen Lee
1 and Alexander M. Spiridonov
2
1Institute of Earth Sciences, Academia Sinica
2Institute of Geochemistry Siberian Branch, Russian Academy of Sciences
The isotopic composition of Ag has received special attention in the past because of its
ability to date and trace processes that took place very early in the solar system. This potential
is based on the decay of the extinct radionuclide 107
Pd to 107
Ag with a half-life of 6.5 My,
which can produce variation in the Ag isotopic composition. Silver is a moderately volatile
and mainly chalcophile element, while Pd is more refractory and siderophile. This renders the
system particularly useful for dating core formation processes and volatile loss. Early Ag
isotopic studies successfully provided evidence for the former presence of 107
Pd and
established a chronology for volatile depleted iron meteorites. These Ag isotopic analyses
were performed by thermal ionization mass spectrometry (TIMS) and reported a precision of
~1-2 permil. The precision of TIMS measurements is limited by the fact that Ag has only two
isotopes (107
Ag and 109
Ag) and thus no internal correction is possible for the mass
fractionation induced during thermal evaporation in mass spectrometers.
More recent studies have taken advantage of the development in the field of
multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) and have
been able to improve the analytical precision for Ag isotopic analyses by an order of
magnitude. The improvement in precision afforded by MC-ICPMS analysis has already led to
the discovery of Ag isotopic variation in native Ag ore samples, which are probably due to
stable Ag isotope fractionation during natural chemical processing (Hauri et al., 2000). This
opens up a whole new field of applications of Ag isotope variation in economic geology and
hydrothermal geochemistry studies.
In our study, we combined standard-sample bracketing and external normalization to Pd
or Cd for the Ag isotopic measurements. The external reproducibility for geological samples
based on replicate analyses of rocks is ±0.5 . We have first applied this new technique
to study the Au-Ag ore deposits in Japan, and have found Ag isotopic variations among ore
deposits, as well as different Ag bearing minerals from different geological settings. We are
now starting to investigate the Ag isotopic variations for a series of Ag-Au ore deposits in
Russia in order to better constrain the formation and evolution of these ore deposits.
Silver-bearing minerals were separated from samples, while Ag occurs as either native or
sulfide minerals, and the Ag isotopic variation for each mineral species and the difference of
Ag isotopic ratio between Au-Ag ore deposit between Russia and Japan will be presented.
References
Hauri et al. (2000) Lunar Planet. Sci. 31, 1812.
33
Lithium Isotopic Compositions in Ultramafic Rocks: An Improved
Methodology Using MC-ICP-MS
Mei-Fei Chu1,2
*, Norman J. Pearson1, Suzanne Y. O’Reilly
1, Williams L. Griffin
1, Peter
Wieland1
1 Australian Research Council National Key Centre for Geochemical Evolution and Metallogeny of Continents
(GEMOC), Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
2 Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan
Although the Li isotopic methodology using MC-ICPMS has become more and more
mature, measurements for rocks with low Li concentrations is still a challenge, particularly
for the ultramafic rocks with Li of ≤1 ppm. The main problem is, after column separation,
impurities/matrices in their Li cuts may show relatively higher proportions and cause
significant mass fractionation during instrumental analyses. Results of the Li chromatography
in this study, focusing on ultramafic rocks, reveal that Cr, though rarely mentioned in other Li
isotopic methods, is the main matrix in the Li cut of ultramafic rocks (Cr/Li: up to 30) as well
as Na (Na/Li: up to 20) that has been widely known as the main impurity since last century.
By analyses of Na- and Cr-doped, or mixing standards, our MC-ICPMS was demonstrated its
insensitivity to these matrices, and gives a long-term precision of 0.8 ‰ (2 S.D.) in Li
isotopic determination. As representatives of ultramafic rocks, Li isotopes of some reference
materials were also analysed in this study, and show consistent results with published values
(PCC-1: 7Li= +8.1 +/- 0.8, n= 4; UB-N:
7Li= -3.1 +/- 0.6, n= 13).
34
Climatic Changes and Sources of Sulfur in Lake Baikal Methane
Dominated Freshwater Sedimentary Environment
Saulwood Lin1, Genady V. Kalmychkov
2, Tsanyao F. Yang, Chieh-Wei Hsu
1, Yeecheng Lim
1,
Wangyen Cheng1, and Tatyana V. Pogodaeva
3
1Institute of Oceanography, National Taiwan University,
2Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences
3Limnological Institute, Siberian Branch of Russian Academy of Sciences
Climatic change is affecting size of the permafrost in the polar region, which may
dramatically increase the amount of methane flux entering the atmosphere. Melting of
ice may, however, increase the amount of seawater entering those areas originally cover
primarily by the permafrost ice. While melting of ice is increasing methane flux of the
polar region to the atmosphere, the increasing of seawater sulfate to the permafrost may
have an adverse effect on methane flux. Seawater sulfate play a very important role in
determining depth of sulfate/methane transition zone and subsequently oxidation and rate
of methane upward migration in marine sediments. Lake Baikal experienced past
glacial/interglacial changes and may have different degree of sulfate influx entering the
lake from surrounding environments during various climatic changes. Melting of glacial
ice may have increased amount of weathering products, which may enhance the amount
of sulfate entering the Lake. Records of these climatic changes are preserved in Lake
Baikal sediments. The objectives of this research are to study sources of sulfate and
amounts of sulfate preserved in Lake Baikal sediments. The results may serve as a
valuable proxy in unraveling the effect of increasing sulfate on the polar region gas
hydrate dissociation during the climatic warm period. Results of climatic changes in the
Lake Baikal sediments will be presented in this symposium.
Acknowledgement: The PIs thank financial supports from RFBR to GK
(10-05-92004-NNS-a) and NSC (NSC 99--‐ 2923--‐ M--‐ 002--‐ 005--‐ MY3) to SL in
making this endeavor and research possible.
35
Patom crater (Eastern Siberia): structure, geochemical features and origin
V.S. Antipin, S.I. Dril
Institute of Geochemistry, SB RAS, Irkutsk, Russia
It is established, that the origin of the Patom crater is connected with endogenous
processes. The leading contribution in which was played by supply of deep flow of the fluid
components which were responsible for the formation of gravel heap about 500 years ago. It
was verified by the zonal structure of the crater, petrographic as well as geochemical features
of carbonate and terrigenous rocks, resulting from a prolonged formation of separate zones.
We found that when the crater was formed silicate rocks (sandstones and slates), which
are contained in the eruptive breccia were subject to the influence of gas or fluid components
and intensively carbonatized. Within the crater they are essentially enriched with Ca and Sr
relative to the same rocks in the hosting sequence. However, the share of radiogenic strontium
isotope and accordingly 87Sr/86Sr values in sandstones and slates within the limits of a crater
sharply decrease. This fact indicates the influence of fluids, being initially depleted in
radiogenic strontium isotope on the given terrigenous rocks in the crater. It suggests the
existence of a deep magmatic source. However, these fluids were saturated by СО2 and
transferred a significant amount of Sr that led to the enrichment all terrigenous rocks of a
crater by this element.
36
Molybdenum isotopes for Lake Baikal sediments
Hsin-Ting Liu1,2
, Ein-Fen Yu2 and Der-Chuen Lee
1
1 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
2 Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan
The isotopic composition of molybdenum (Mo) is known to be a potential proxy for
redox condition. In the marine oxic environment, the Mo isotopic compositions are generally
enriched in light isotopes relative to the anoxic environment. As a result, most of the efforts
for Mo studies have been focused on reconstructing the paleo-redox environment.
In order to test if Mo isotopes can be applied to study the paleo-redox condition for lake
environment, a ~ three meters long gravity core (GC-99; 52°05’23‖N, 105°15’24‖E) sampled
near the bore hole of BDP-99 in Lake Baikal is used in this study. For the initial test, 6
samples are selected, and they are 0-1cm, 9-10cm, 19-20cm 29-30cm 39-40cm and 49-50cm.
Approximately 100 mg for each sample is used for the sequential leaching procedures
(modified from Xu and Marcantonio, 2004), to remove salt and carbonate fractions, and to
collect the reducible fractions, precipitated directly from the lake water, for the measurement.
The total Mo removed from each sample is ranging from 31 to 203 ng, while the blank is
negligible at ~ 0.3 ng. In order to minimize the potential mass dependent isotopic
fractionation introduced by non-quantitative yield for column chemistry and during mass
spectrometric measurement, the ―double spike‖ technique is used in this study. The pre-mixed
and calibrated double spike consists of two enriched 97
Mo and 94
Mo spikes, and is added into
the leachate prior to the column chemistry. Three columns are used to separate and purify Zn
and Cd, Fe, and Mo sequentially. Isotopic measurement will be analyzed using the Nu Plasma,
a MC-ICPMS, at the IES, and be presented during the meeting. The Mo data will be
combined with other isotopic data, e.g., Cd and Zn from the same samples, also will be
presented in this meeting, so that we can better constrain the relationship between the Lake
Baikal and global climate changes in the past.
37
