Workshop on Unmixing the SNCs (2002) 6025.pdf

GEOCHEMISTRY OF MARTIAN AND THE PETROLOGIC EVOLUTION OF MARS. D. W. Mittlefehldt, Mail Code SR, NASA/Johnson Space Center, Houston, TX 77058, USA ([email protected]).

Introduction: Mafic igneous rocks serve as probes Mars Earth M oon of the interiors of their parent bodies – the composi- tions of the contain an imprint of the source 10 region composition and , the melting and 1 crystallization processes, and mixing and assimilation.

Although complicated by their multifarious history, it (mg/g) P 0.1 is possible to constrain the petrologic evolution of an igneous province through compositional study of the 0.01 rocks. Incompatible trace elements provide one means 0.1 1 10 100 of doing this. I will use incompatible element ratios of La (µg/g) martian meteorites [1] to constrain the early petrologic evolution of Mars. Incompatible elements are strongly Mars Earth M oon

partitioned into the melt phase during igneous proc- 10 esses. The degree of incompatibility will differ de- pending on the phases in equilibrium with the 1 melt. Most martian meteorites contain some cumulus grains, but nevertheless, incompatible element ratios of (mg/g) P 0.1 bulk meteorites will be close to those of their parent 0.01 magmas. ALH 84001 is an exception, and it will not 0.01 0.1 1 10 100

be discussed. The martian meteorites will be consid- Yb (µg/g) ered in two groups; a 1.3 Ga group composed of the clinopyroxenites and dunite, and a younger group Figure 1. P-La and P-Yb plots for mafic igneous rocks composed of all others. from Mars, the Earth and Moon. Planetary Comparisons: On Earth, P and La are Crustal Assimilation: Recently, a number of geo- highly correlated in igneous rock suites, resulting in a chemical characteristics of a subset of martian meteor- small range in P/La ratios for terrestrial mafic magmas ites have been ascribed to assimilation of crustal mate- (Fig. 1). Magmas with higher incompatible element rial by mafic magmas [e.g. 3]. The P/La ratios for mar- contents have lower P/La, indicating La is more in- tian meteorites <1 Ga in age are positively correlated 143 compatible than P. This is also supported by various with their ε Nd [1] (Fig. 2), a presumed measure of estimates of the composition of the bulk continental crustal assimilation/contamination, indicating that this crust [2], which is more highly enriched in La than P process may have contributed to the scatter in P/La. compared to an estimated bulk silicate Earth [2]. Ma- This is in general in accord with terrestrial geochemis- fic rocks from the Moon follow the terrestrial example try – the P/La ratio for the bulk continental crust is – P/La ratios exhibit a narrow range (Fig. 1). In con- lower than that estimated for the bulk silicate Earth [2]. trast, martian meteorites show a wide range in P/La However, P and La are enriched in the continental crust (Fig. 1). The planetary distinctions are also clearly by roughly 10× and 30×, respectively [2]. QUE 94201 143 shown on a P vs. Yb diagram (Fig. 1). The terrestrial has the highest ε Nd (Fig. 2) and the highest P con- mafic rocks show no correlation between P and Yb, tent (Fig. 1). This rock is considered to have suffered lunar rocks occupy a band of increasing P and Yb, minimal crustal contamination [see 3]. Hence, if ba- while in martian meteorites these elements are strongly salts like Shergotty were formed by contamination of correlated. This suggests a fundamental difference in primary or evolved magmas, then these magmas, and the petrologic evolution of Mars as compared to the possibly their source regions, must have had incom- Earth or Moon. The constancy of Yb/P implies that the patible element contents different in detail from those partitioning of these elements is governed by a single of QUE 94201. Thus, for example, their Sm/Nd ratios phase, or two phases either in constant proportions or may have been lower, and the source region would then with similar P/Yb partition coefficient ratios. The have had lower ε143Nd. Regardless of complications phase or phases would have a much lower partition from possible assimilation of crust, the P/La ratios of coefficient for La, such that P and La are decoupled. martian mafic igneous rocks show wide ranges, while

Workshop on Unmixing the SNCs (2002) 6025.pdf

PETROLOGIC EVOLUTION OF MARS: duck

the P/Yb ratios are roughly constant, quite unlike the no case did either QUE 94201 or the 1.3 Ga martian cases for the Earth or Moon. meteorites fall within the envelop of possible melt compositions in any of the tested models. In short, <1 Ga 1.3 Ga sequential melting of a Dreibus-Wänke martian mantle 100 composition under any P-T regime will not yield melts Q with incompatible element ratios that come close to

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-3 those of either the 1.3 Ga martian meteorites, nor of the most pristine younger volcanic rock, QUE 94201. Al- b

P/La (x10 P/La 1 ternative models must be sought. High Pressure Cumulates: I have begun evaluat-

0.1 ing high pressure ocean cumulates as alterna- -10 0 10 20 30 40 50 tive source regions for the martian meteorites. The ε 143Nd -like REE pattern of QUE 94201 suggests that Figure 2. Correlation of P/La with ε143Nd in martian cumulus garnet may be required. However, garnet meteorites. Q = QUE 94201; b = martian basalts Los sensu stricto is not a near-liquidus phase in ultramafic Angeles, Shergotty and Zagami. melt compositions, and thus is not likely to be a cumu- Incompatible Element Ratios: Martian meteorites lus phase. At P >15 GPa, majorite does become a liq- 1.3 Ga in age do not follow the trend exhibited by the uidus or near liquidus phase in ultramafic melts [5]. younger rocks (Fig. 2). These rocks have by far the Limited partition coefficient data [1] indicate that ma- lowest P/La ratios, yet intermediate ε143Nd. They are jorite crystallizing from a martian magma ocean should also distinct in other trace element ratios (Fig. 3). have a garnet-like REE pattern, and thus could impose These distinctions are not due to crustal assimilation – a garnet-like signature to the cumulates. In addition, the terrestrial continental crust has CI-normalized very limited P data suggest that majorite/melt Kds for P Hf/Sm of ~1.3 and Ta/La of ~1.0 [2] and it is likely and Yb are similar at ~1. Thus, cumulus majorite that the martian crust is also unfractionated in these could define the P/Yb of the martian source ratios from the primitive Mars composition. Thus, it is regions, and P and La would be decoupled. Finally, unlikely that the low ratios observed for the 1.3 Ga measured Hf/Sm partition coefficient ratios for ma- martian meteorites could be a result of contamination jorite are >2, although Hf is incompatible in majorite by martian crustal rocks. The 1.3 Ga martian meteor- (kds ~0.5-0.9). Hence, it appears plausible that the ites and QUE 94201 have the lowest and highest P/La parent magma of QUE 94201 originated by remelting a ratios, respectively. magma-ocean-cumulate source region in which ma- jorite was a cumulus phase. However, a serious stum-

<1 Ga 1.3 Ga bling block might be Al. Martian basalts are noted for being Al-poor [see 3], while a majorite-rich source region should be Al-rich. 2 The 1.3 Ga martian meteorites must have come

CI Q from a different source in order to explain the different 1 P/La, Hf/Sm and Ta/La ratios (Figs 1, 3). Other possi-

(Ta/La) ble high P phases are Mg-perovskite, γ-VSLQHO -phase 0.5 and clinopyroxene. I will be evaluating these as possi- ble cumulus phases in addition to majorite to explain 0.5 1 2 the geochemical characteristics of the 1.3 Ga martian (Hf/Sm)CI Figure 3. Ta/La vs. Hf/Sm for martian meteorites meteorites. showing the distinction between the 1.3 Ga and References: [1] References for younger rocks. Q = QUE 94201. data, and high P partition coefficients can be found at Modeling Mars: Using the experimental high http://ares.jsc.nasa.gov/mittlefehldt/. pressure mineralogy [4] of a Dreibus-Wänke Mars [2] http://www.earthref.org/databases/GERMRD/reserv composition and a variety of partition coefficients [1], I oir.htm. [3] McSween (2002) & Planet. have attempted to model the martian meteorite trace Sci., 37, 7. [4] Bertka and Fei (1997) J. Geophys. Res., element signatures as arising from a sequentially de- 102, 5251. [5] Ohtani et al. (1989) Contrib. Mineral. pleted primitive mantle (i.e. fractional fusion). Models Petrol., 103, 263. were done for pressures in the garnet lherzolite and the majorite-clinopyroxene--phase stability regimes. In