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CONFERENCE SUMMARY
Compiled by David Beaty, Lars Borg, David Draper, Walter Kiefer, Jim Papike, Kevin Righter, Chip Shearer, Sue Smrekar, Jeff
Taylor
Correspondence author: Dave Beaty (JPL/Caltech), [email protected], 818-354-7968
CL#12-4839
Top things we know about the martian mantle
1. Petrologic Character.
a) The martian mantle is richer in Fe (Mg# 75-80) and Na (0.5-
0.9 wt. %) than Earth’s mantle (Mg# 89; 0.35 wt %,
respectively). The martian mantle is oxidized (mostly
IW±0.5 or so), with a restricted range in oxidation state
(~IW-1 to ~IW+3) compared to early Earth (IW-2) and its
later range of oxidation state (IW to IW+9).
b) The martian mantle has undergone a much more severe
geochemical depletion than has the mantle of Earth. Key
measure: The highest epsilon neodymium for Earth rocks is
a little over +10; for Mars, it's +50 or more, representing a
couple orders of magnitude of stronger depletion.
Top things we know about the martian mantle
2. Origin.
• Differentiation of the planet to form the mantle, and differentiation of
the mantle itself, occurred as a result of fractional crystallization of a
magma ocean.
• The martian mantle is old: 4520 Ma.
• Magma ocean differentiated very rapidly, tens of Ma.
3. Heterogeneity. The martian mantle is chemically extremely heterogeneous
(by comparison to Earth’s mantle). Once the mantle formed it did not
homogenize through mixing.
4. Thermal history. Due to its size, the martian mantle cooled more rapidly
than Earth. The effects of the cooler temperature on volcanism were
partially offset by a mantle composition that melts at lower temperature
than Earth’s mantle.
5. Mantle Convection. Volcanism has been an important process
throughout martian history, extending essentially to the present day. The
majority of martian volcanism is likely due to adiabatic decompression
melting driven by mantle convection, possibly up to the present.
0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29
“Depleted” Subgroup(N=5)
Nahklites/Chassigny
Data from Brennecka et al., 2012; Borg et al., 19972003, Caro et al. 2009; Harper et al., 1995; Symes & Borg, unpublished)
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.14194
1.14192
1.14190
1.14182
1.14180
1.14196
“Intermediate” Subgroup(N=3)
MSWD = 3.8
Age = 4516 +15/-16 Ma(εNd)Terr. = -0.05 ± 0.06(146Sm/144Sm)Ref = 0.0085Slope =0.00124 ± 13MSWD = 3.8
147Sm/144Nd (source)
142Nd/1
44Nd w
hole roc
k
ε142Nd w
hole roc
k
“Enriched” Subgroup(N=6)
1.14188
1.14186
1.14184
1. Origin, Age
Tissint(Brennecka et al., 2012)
LA1, Dhf 378, NWA1068, NWA4468(Symes & Borg, unpublished)
From L. E. Borg, S. J. Symes, N. Marks, A. M. Gaffney, and C. K. Shearer
Depleted Mantle / “Garnet Peridotite” Zone
Intermediate Source
Enriched Source / “Eclogite” Zone
Incr
easi
ng P
hosp
horu
s +
RE
E
ALHA77005, LEW 88516
Shergotty, LA
Y98, QUE 94201
Olympus Mons
21
From J.J. Papike, P.V. Burger, C.K. Shearer and F.M. McCubbin
2. Mantle Petrology
Magmas as mantle probes
Bulk Chemistry Considerations
• Wänke and Dreibus bulk Mars composition model still quite good
• Mars clearly enriched in FeO compared to Earth
• K and other moderately volatile elements depleted by factors of a few cf CI chondrites
• Highly volatile elements have roughly uniform depletions cf CI chondrites, ∼0.03
• Water content about the same as Earth (D/H also seems to be the same)Discussion: However, some major parts of the Earth’s mantle are wetter and in fact water is called upon as the main flux for melting
From Jeff Taylor
Shergotty
ZagamiLos Angeles
RBT 04261 (ol-phy)
LAR 06319 (ol-phy)
EET (A) (ol-phy)
EET (B)DaG 476
QUE 94201
fO2 estimates based on the pyroxene Eu oxybarometer indicate a range of mantle redox (~IW-1 to ~IW+2).
From Mini Wadhwa
Mantle Oxidation State
Estimates of P-T of basalt formation• Inverse Experimental Modeling Approach
– Meteorites:• Yamato 980459 (Musselwhite et al. 2006)
• NWA 1068 (Filiberto et al. 2010)
– Surface Basalts• Humphrey (Monders et al. 2007; Filiberto et al. 2008)
• Fastball (Filiberto et al. 2010)
• Geochemical Modeling– Ol-Melt Mg-exchange thermometry (Putirka 2005)
• Surface Basalts (Filiberto & Dasgupta 2010)
– Silica-activity in the melt barometery (Lee 2009)• Surface Basalts (Filiberto & Dasgupta 2010)
– pMELTS calculations• NWA 5789 (Gross et al. 2011)
• NWA 2990/5960/6234/6710 (Gross et al. submitted)
• Early Mars mantle ~200K cooler than Early Earth
From Justin Filiberto and Rajdeep Dasgupta
Volatile siderophile elements - Can be used to place
constraints on accretion models
From K. Righter and M. Humayun
Importance of water in the Martian interior
� Planetary accretion models
� Magmatism� Water lowers the solidus of mantle lithologies(Gaetani and Grove, 1998; Green, 1973; Hirose and Kawamoto, 1995; Médard and Grove, 2008 )
� Magma transfer and eruption style
� Rheology� Presence of water in olivine makes it weaker(Chopra and Paterson, 1984; Dixon et al., 2004; Drury, 1991; Hirth and Kohlstedt, 1996; Hirth et al., 2000;
Justice et al., 1982; Karato, 1993; 2010; Mackwell et al., 1985; Mei and Kohlstedt, 2000; Walker et al., 2007)
� Thermal evolution
� Seismic wave attenuation(Jung and Karato, 2001; Karato, 2004; 2006)
From Anne Peslier
Volatiles in the Mantle and Lower
Crust
• SNC sources contained
water
– Independent of
enrichment
• Mantle reduced, may be
graphite saturated
• Hydrous magmas may
have crystallized volatile-
bearing mineral
assemblages in crust
From McCubbin et al. (2012) Geology
From Francis M. McCubbin and Stephen M. Elardo
(Bertka and Fei, 1997)
She
rgot
tites Mantle Mineralogical Model
From V. Debaille &
A.D. Brandon
Debaille et al., 2008 + data from Blichert-Toft et al., 1999 and Bouvier et al., 2005
Shergottite mixing line
Depleted shergottites
Enriched shergottites
Intermediate shergottites
3. Heterogeneity
From V. Debaille & A.D. Brandon
4. Temperature: Earth vs. Mars
• Mars: Mg # ~ 75-80, alkalis ~ 0.5 weight %
• Earth: Mg # 89, alkalis 0.35 weight %
• Predicted solidus reduction on Mars– 1 GPa: Alkalis 15 °C, Mg # 11-17 °C
– 3 GPa: Alkalis 20 °C, Mg # 21-32 °C
• Combined effect suggests Mars primitive mantle dry solidus is ~ 30-50 °C lower than Earth solidus.
• Effect is most important on early Mars. Present-day effect will be smaller (Mg # effect persists, alkali effect decreases with time).
From Walter S. Kiefer, Justin Filiberto, and Constantin Sandu
5. Mantle Convection
< 100 Ma
180 Ma
• Geologic mapping and meteorite isotopes
both imply Mars was volcanically active from
its formation into the recent past.
• Implies adiabatic decompression melting
and hence mantle convection have been
important throughout martian history.
Discussion: stated too strongly?
• Preservation of large isotopic anomalies
requires long-term separation of at least 2
distinct mantle reservoirs.
• Perhaps Tharsis and Elysium are these
separate reservoirs (testable with sample
return).From Walter S. Kiefer, Justin Filiberto, and Constantin Sandu
Top things we don’t know about the martian mantle
1. Physical structure
2. Thermal State/History. What is the thermal state and history of
the martian mantle (and the interior in general)?
3. Process of magma generation. Where?, when?, how?
4. Heterogeneity. What is the extent of compositional
heterogeneities in the mantle?
5. Volatile content of the mantle (esp. C, H, O, S)
6. Mineralogy. What is the mineralogical structure of the mantle?
7. Convection. How has the vigor and pattern of mantle
convection varied with time?
Many details of our understanding of the petrologic character of the mantle are
dependent on interpretations of magmatic liquid compositions using rocks
whose bulk chemistry has been affected by crystal accumulation/loss,
weathering, and alteration. How will these conclusions be refined by the study
of rocks in the for which igneous liquid chemistry can be unambiguously
interpreted?
Backup Slides
Sulfur Concentration of Martian Magmas
1. How does S behave in
martian magmas?
2. What is the likely
fluxes of S volatiles
from martian interior
to the atmosphere?
3. How could Mars get
so much S?
What is the sulfur
solubility of martian
magmas?
From Shuo Ding and Rajdeep Dasgupta
Depth of Garnet Crystallization
– 1.2 – 1.4 Al pfu
corresponds to 11-12 up
to 15+ GPa (~1200 –
1500 km)
– Roughly in the middle of
range inferred from
CaO/Al2O3 alone
– Consistent with Righter’s
estimate of 14 GPa (this
morning)0 5 10 15 20 25
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Al pfu
P, GPa
Mg-rich
Fe-rich
From David S. Draper