Crustal magnetization and magnetic petrology from the IDDP-drilling

Carsten Vahle1, Agnes Kontny1, Frank Dietze1, H. Audunsson2

1 Geologisch-Paläontologisches Institut, University Heidelberg, INF 234, 69120 Heidelberg, Germany2 Technical University of Iceland, Hofdabakka 9, 110 Reykjavik, Iceland [email protected]

AcknowledgementsWe would like to thank different people at ISOR for their kind helpfulness. We’re grateful to the DFG, which funds this research (Ko1514/3).

References:Fridleifsson, G.O. (2005): REYKJANES Well Report RN-17 & RN-17ST, Iceland Geosurvey internal report, Reykjavik, Iceland.Jakobsson, S.P., Jonsson, J., and F. Shido (1978): Petrology of the Western Reykjanes Peninsula, Iceland. Journal of Petrology, 19, 669-705.Karlsdottir, R. (2005): TEM-mælingar á Reykjanesi 2004, Iceland Geosurvey internal report ÍSOR-2005/002, Reykjavik, Iceland.Kiss, J., Szarka, L., and E. Prácser (2005): Second order magnetic phase transition in the Earth. Geophysical Research Letters, 32, L24310, doi:24310.21029/22005GL024199.Saemundsson, K. (1995): Svartsengi, Eldvörp and Reykjanes geological map (bedrock) 1:25000. Orkustofnun, Hitaveita Sudurnesja and Landmaelingar Iceland.Sigurgeirsson, Th. (1975): Unpublished aeromagnetic maps.

Introduction

Results

Kraf laKraf la

StardalurStardalur

ReykjanesReykjanes

dri l l s i tedri l l s i te

http://members.tripod.com/~AntonBerger/is_maps.htm

profile 1

profile 2

profile 3

the field magnetic measurements show similarities with the aeromagnetic survey butreveal local anomalies of smaller scales

especially the younger lava flows, where the marginal scoriaceous parts of the flowsare not eroded yet, contribute to high magnetic field intensities due to their highremanent magnetization

at deeper crustal levels or in lava flows with elevated temperatures (e.g. near geothermalfields), respectively, the induced component of total field magnetization could increaseseverely as temperatures near the susceptibility peak associated with Ti-richtitanomagnetite or titanomaghemite are reached (e.g. Kiss et al. 2005)

the contribution of intrusions like the dolerite dike to the total magnetic field seemsto affect mainly the induced component

the different lava origins (picrite basalt lava shields, olivine tholeiite lava shields andtholeiite fissure lavas) show distinctly different magnetic properties, which could berelated to magma composition; differences with regard to extrusion conditions andcooling history are the subject of further studies...

Conclusions

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NR

M [A

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Stampahraun 4Lava north of gunnaSyrfell fissureSkalafellHaleyjabunga picritepillow lava, hyalocl.RN-19 drilling

R N 7R N 7

R NR N 1313

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Q


R N 7R N 7R NR N 1313

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R N 7R N 7

R NR N 1313highly vesicular,lower abun-dance but larger(2 - 8 mm diam.)

extremely vesicle-rich, 1 - 3 mm diam.,almost glassy matrix

Crustal magnetization and magnetic petrology from the IDDP-drilling RN-19 and its surroundingon the Reykjanes peninsula (Iceland) - discrimination of different volcanic rock units

The geothermal field at Reykjanes peninsula is located at theboundary where the submarine Reykjanes Ridge passes overinto the rift zone of southwestern Iceland. The geothermalfield coincides with a magnetic low in the aeromagneticanomaly map and is situated within a dense NE-SW fissureand fault zone. Surface geology is characterized by differenthistoric fissure eruptions (youngest from 1226 AD), shieldlava (12.5 – 14.5 ky) and intercalated pillow basalt – hyalo-clastite ridges probably formed during the last glacial episode(14.5 – 20 ky).During a field magnetic study in the vicinity of the geothermalfield in summer 2005 different volcanic rock units have beensampled to correlate rock magnetic and magneto-mineralogicalpropert ies with magnetic f ield intensity. Addit ional ly,measurements on a dense dolerite intrusion, recovered fromthe RN-19 borehole (2245 – 2248 m depth) in May 2005 withinthe frame of IDDP, should shed light on the influence ofcrustal rocks on the total magnetic field intensity.

Generally, the natural remanent magnetization andmagnetic susceptibi l i ty, measured on rockspecimen, is high. The high NRM coincides withthe magnetic high outside the geothermal field. TheKoenigsberger ratios (Q) are also high for all surfacesamples, indicating the predominance of remanentmagnetization.

Most of the study area is covered by stronglymagnetic Stampahraun and Skalafell pahoehoeand block lava stemming from fissure eruptions.The rock magnetic characteristics of theses flowsare quite similar, whereas the older flow (Skalafell)shows stronger scattering. The pillow lava andespecially the picritic Haleyjabunga shield lavashow lower NRM intens i ty and magnet icsusceptibility, whereas especially for the shieldlavas this could be related to less Ti and total Fein the magma, therefore constricting crystallizationof Fe-Ti oxides. By contrast, the highly magneticfissure lavas have slightly higher Ti- and total Fe-contents (Jakobsson et al. 1978).

For the NRM intensity, a weak dependence on theweight (used as approximation for vesicularity) canbe observed. The highly vesicular, more scoriaceoussamples (lower weight) have higher NRM thanmassive lava. Therefore, the marginal parts of lavaflows, where the lava cools and solidifies fasterthan in the interior (resulting in abundant Fe-Tioxides with small grain sizes, few µm), excite highNRM and magnetic field intensities, respectively.

The NRM of the doleritic dike sample from RN-19drilling is rather low but susceptibility is high,indicating large grain sizes, formed during typicallyslow cooling of a intrusion.

Combination map (modified from Fridleifsson 2005) of surfacetopography, surface structural interpretations and geothermalmanifestations (from K. Sæmundsson, pers. com., revised maps),and an airborne magnetic survey from Th. Sigurgeirsson (1975)with an acquisition flight hight of 150 m above sea-level, and aninterpretation of the extents of the geothermal system at Reykjanesbased on a recent (2004) TEM survey (Karlsdóttir 2005).Additionally the magnetometer profiles and sample locations areshown.

Geologic map of Svartsengi andsurroundings; shown are the threeprofiles measured with a GSM-19TGproton precession magnetometerwith sample locations (modifiedSaemundsson, 1975).

First temperature dependent magnetic susceptibility data indicatehomogeneous Ti-rich (Tc = 60 - 240 °C) and Ti-poor titanomagnetite(Tc = 350 - 520 °C), and magnetite (Tc = 580 °C). Partly,titanomagnetite has been oxidized to titanomaghemite with Tc

ranging between 320 and 510 °C. The occurrence of magnetiteand the low-temperature behavior of k(T) curves below –150 °C

indicate exsolution texturestypically forming during high-temperature oxidation, e.g. atslowly cooling parts of a lavaflow or in an intrusion (seesample from RN-19 drilling).

The MDF (median destructivefield), derived from alternatingfield demagnetization of NRM,varies between 16 and 31 mT.This indicates small grainsizes (< 10 µm) and/or oxi-dized titanomagnetite, respec-tively.

Crustal magnetization and magnetic petrology from the IDDP-drilling RN-19 and its surroundingon the Reykjanes peninsula (Iceland) - discrimination of different volcanic rock units

Rock magnetic properties of Reykjanes surface samples andRock magnetic properties of Reykjanes surface samples andfrom the RN-19 drillingfrom the RN-19 drilling ::

1 km

N

63°49’ N

63°50’ N

63°51’ N

63°48’ N22°44’ W 22°42’ W 22°40’ W

Stampahraun 4, 13th cent.remnant of tuff cone

Stampahraun 3, 2000-3000 a

Stampshraun

Lava north of gunna

Stampahraun 1,2

lavas 8000-3000 aSyrfell fissure

Melshraun

Skalafell

lavas 11500-8000 aSandfellshodca. 12500 aHaleyjabunga picrite14500-12500 a

shield lava ridges 20000-14500 alava coverhyaloclastite breccia

pillow lava, hyalo. tuff

fissure

crater lava channel

lava margin

fault

steam vent

altered ground

sample

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Tc = 318°C/493°C

Tc = 560°C/576°C

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RN7-1Skalafell

temperature [°C]

Tc = 539°C/ 586°C

Tc = 50°C/154°C

Tc = 432°C/515°C

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RN13-1Syrfell fissure

Tc = 547°C/590°C

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RN-19 2246.16dolerite dike

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MDF = 24 mT

NRM = 12.0 A/m RN13-1Syrfell fissure

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NRM = 33.6 A/m

MDF = 17 mT

RN7-1Skalafell

strongly influenced by topographic features, causing big scattering, but also some local anomaliescan be observed, which are not displayed by the aeromagnetic survey (see map above)

Geomagnetic field intensityGeomagnetic field intensity relative to the regional field at Reykjanes relative to the regional field at Reykjanes(measured in field with proton precession magnetometer, correcte(measured in field with proton precession magnetometer, correcte d for daily secular variation and smoothed, IGRF10)d for daily secular variation and smoothed, IGRF10)

Reykjanes - profile 2

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pillow /hyaloclasite

ridge

Háleyjabungacrater

faultfault

NW SE


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Eldborg grynnnricrater

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NW SE


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fault

Skalafellcrater

discharge of steam

valley

pillowmount

older Flow

NW SE

local depression

east of the fissure at ca. 3200 m

fault in the eastern part at ca. 2700 m

tumulus in the eastern part at ca. 3200 m

Documents

Crustal magnetization and magnetic petrology from the IDDP-drilling