Fundamental Importance of Returned Samples to Understanding the Martian Interior

Transcription

1 Fundamental Importance of Returned Samples to Understanding the Martian Interior David S. Draper and Carl B. Agee Institute of Meteoritics Department of Earth and Planetary Sciences University of New Mexico Ground Truth 2008 Draper 3/06

2 Outline Introduction: Widely believed that magma ocean is best explanation for martian basalt compositional ranges Rapid differentiation of initial MO produced distinctive reservoirs Magma Ocean models based on martian basalt compositions: All martian basaltic meteorites have substantially superchondritic ic CaO/Al 2 O 3 likely produced by early garnet fractionation Minimum depth of MO ~ km Are shergottites representative of Mars basalts? Probably only a few volcanic centers sampled Basalts with chondritic CaO/Al 2 O 3 would negate global MO model Alternatives: magma lake, deep plume Determining most fundamental event in martian history depends on acquiring reliable surface basalt compositions

3 cpx, ilm cpx, opx opx, olivine oliv, majoritic garnet majoritic garnet Introduction Mars Magma Ocean Accounts for formation of discrete geochemical reservoirs within first tens of Ma after accretion 1: Strongly depleted, lower-fo 2 2: Much more enriched, higher-fo 2 Superchondritic CaO/Al 2 O 3 imparted by early garnet crystallization Subsequent processes of crystallization, overturn, and cumulate melting (ol( ol,, opx controlled) will not perturb this ratio

4 cpx, ilm cpx, opx opx, olivine oliv, majoritic garnet majoritic garnet Introduction Mars Magma Ocean Garnet becomes majoritic at elevated P; can be used to infer minimum MO depths ~ km Basalts: Probes of their source rocks Primitive basalts record the CaO/Al 2 O 3 of their sources (ol( ol,, opx don t perturb) Thus they recover the fingerprint of earliest MO garnet crystallization

5 MO based on basaltic meteorites Martian basaltic meteorites Significantly superchondritic in CaO/Al 2 O 3 Sequestration of Al in aluminous phase Early garnet removal imparts this feature (plag( not likely in these bulk comps) Garnet becomes majoritic at elevated P; can be used to infer minimum MO depths ~ km CaO/Al 2 O Meteorites Calc. parents Y Gusev QUE94201 chondritic SM Shergotty LEW-ol+pig LEW-ol LEW88516 EET79001A ALH Eg Mg# Am

6 Gusev basalts have chondritic CaO/Al 2 O 3 MER Gusev compositions: Essentially chondritic in CaO/Al 2 O 3 Not clear whether these are bona fide basalts; likely significantly altered Not yet demonstrated that these represent liquids CaO/Al 2 O Meteorites Calc. parents Y Gusev QUE94201 chondritic SM Shergotty LEW-ol+pig LEW-ol LEW88516 EET79001A ALH Eg Mg# Am

7 cpx, ilm cpx, opx Key Point: : If there are primitive basalts with genuinely chondritic CaO/Al 2 O 3, there cannot have been a GLOBAL magma ocean 2.0 Meteorites Calc. parents Y Gusev opx, olivine CaO/Al 2 O QUE94201 SM Shergotty LEW-ol+pig Eg LEW-ol LEW88516 EET79001A ALH77005 Am oliv, majoritic garnet 0.5 garnet majoritic garnet chondritic Mg#

8 SNCs are probably a biased sample set Basaltic meteorites may well sample only a few volcanic centers No reason to expect that they are representative of global martian basalt compositions Perfectly reasonable to suggest that there are broad regions whose basalts are unsampled Ta/Tb ~180 ~ Initial 87 Sr/ 86 Sr

9 Alternatives to global MO? Magma lake: localized pond of sufficient depth Deep-seated plume Something else: Never was a magma ocean?? 1500 km deep MO

10 Alternatives to global MO? Reese & Solomatov Icarus 2006 Impact-generated melt regions: Will they spread to global extent before they cool and crystallize? Something large enough to form km deep pond would probably spread out such that base of MO not deep enough for garnet crystallization

11 Alternatives to global MO? Time for crystallization likely << time for isostatic adjustment Thus if correct factors prevail of initial melt size and solid rheology of proto-mars, extensive region can result that retains magma ocean signature but is not a global magma ocean Reese & Solomatov Icarus 2006

12 Alternatives to global MO? Deep-seated mantle plume in ancient Mars: Same physical processes, much more restricted lateral extent Simple to envisage as reaching sufficient depth such that garnet removal can impart superchondritic CaO/Al 2 O 3 Li & Kiefer 2007

13 Conclusions Whether Mars ever experienced a global magma ocean is among the most fundamentally important events in the planet s history Unless a shergottite with chondritic CaO/Al 2 O 3 is discovered, only (unaltered) surface basalts will provide the key data for making this determination Thus it is of critical importance that MSR materials include surface basalts

14

15 Introduction Martian basaltic meteorites Significantly superchondritic in CaO/Al 2 O 3 Sequestration of Al in aluminous phase Early garnet removal imparts this feature (plag not likely in these bulk comps) CaO/Al 2 O Meteorites Calc. parents Y Gusev QUE94201 SM Shergotty LEW-ol+pig LEW-ol LEW88516 EET79001A ALH Eg Mg# Am

16 Garnet Composition with Pressure Al pfu P, GPa Mg-rich Fe-rich Majorite Can be viewed as pyrope-enstatite enstatite solid solution With P, Si per formula unit increases, Al pfu decreases Mg-rich vs. Fe-rich Shift to majorite more sudden in peridotitic systems Shift much more gradual in Fe-rich systems pertinent to martian mantle

17 Depth of Garnet Crystallization Al pfu P, GPa Mg-rich Fe-rich Al pfu corresponds to up to 15+ GPa (~ km) Roughly in the middle of range inferred from CaO/Al 2 O 3 alone

18 Predicting Garnet REE partitioning Hold garnet composition fixed except for varying majorite content Assume constant melt composition Reasonable: Only small amount of crystallization being modeled Calculate REE in post-garnet MO liquids and compare to values used in Borg & Draper 03 model Not a unique solution but one that has been shown to provide matches with SNC comps

19 Effect of majorite content on liquid CaO/Al 2 O CaO/Al 2 O 3 liq Shergottites Calc. Parents 10% 8% 7% 6% 5% 3% Possible liquids: QUE94201, Y Chondritic Al pfu in garnet

20 Predicting Garnet REE partitioning 10 REEs in MO: Test Cases REE/Chondrite %/XAl = 2.0 5%/XAl = 1.6 6%/XAl = 1.4 7%/XAl = 1.2 8%/XAl = 1.1 BD03 Comparison with model of Borg & Draper 2003 La Ce Nd Sm Eu Gd Tb Dy Er Tm Yb Lu