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Chemical Compositions of Martian Basalts (Shergottites): Some Inferences on Basalt Formation, Mantle Metasomatism, and Differentiation in Mars

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Chemical Compositions of Martian Basalts (Shergottites): Some Inferences on Basalt Formation, Mantle Metasomatism, and Differentiation in Mars Meteoritics & Planetary Science 38, 12, 1849–1864 (2003) Abstract available online at http://meteoritics.org Chemical compositions of martian basalts (shergottites): Some inferences on basalt formation, mantle metasomatism, and differentiation in Mars Allan H. TREIMAN Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, USA E-mail: [email protected] (Received 31 January 2003; revision accepted 13 November 2003) Abstract–Bulk chemical compositions of the shergottite basalts provide important constraints on magma genesis and mantle processes in Mars. Abundances of many major and trace elements in the shergottites covary in 2 distinct groups: Group 1 (G1) includes mostly highly incompatible elements (e.g., La, Th), and Group 2 (G2) includes mostly moderately incompatible elements (e.g., Ti, Lu, Al, Hf). Covariations of G2 elements (not necessarily linear) are consistent with partitioning between basalt magma and orthopyroxene + olivine. This fractionation represents partial melting to form the shergottites and their crystallization; the restite minerals cannot include aluminous phase(s), phosphate, ilmenite, zircon, or sulfides. Overall, abundances of G1 elements are decoupled from those of G2. In graphing abundances of a G1 element against those of a G2 element, G1/G2 abundance ratios do not appear to be random but are restricted to 4 values. Shergottites with a given G1/G2 value need not have the same crystallization age and need not fall on a single fractionation trajectory involving compatible elements (e.g., Ti versus Fe*). These observations imply that the G1/G2 families were established before basalt formation and suggest metasomatic enrichment of their source region (major carrier of G2 elements) by a component rich in G1 elements. Group 1 elements were efficiently separated from G2 elements very early in Mars’ history. Such efficient fractionation is not consistent with simple petrogenesis; it requires multiple fractionations, “complex” petrogenetic processes, or minerals with unusual geochemistry. The behavior of phosphorus in this early fractionation event is inexplicable by normal petrogenetic processes and minerals. Several explanations are possible, including significant compatibility of P in majoritic garnet and the presence of P-bearing iron metal (or a phosphide phase) in the residual solid assemblage (carrier of G2 elements). If the latter, Mars’ mantle is more oxidized now than during the ancient fractionation event. INTRODUCTION Two recent developments make revisiting older inquiries into the bulk compositions of shergottites worthwhile. First, Twenty years ago, Bogard and Johnson (1983) presented many new and varied shergottites have been discovered, the first solid evidence that the “SNC” meteorites, including including the critical samples Queen Alexandra Range (QUE) the shergottites, formed on Mars. Since then, the meteorites 94201, Los Angeles, and Dar al Gani (DaG) 476. These have been studied intensely for insights into martian meteorites, in particular, have increased the known chemical volcanism, mantle chemistry, and differentiation history (e.g., diversity of the shergottites and, thus, allow new insights into McSween 1994). One of the most valuable approaches has the processes responsible for that diversity. Second, new been interpretation of their bulk chemical compositions using radioisotopic data on the shergottites, especially on those methods established in studies of the Earth’s basalts and critical samples, have resolved several uncertainties in their mantle (e.g., Wänke 1981; Drake 2000). Applied to Mars, petrogenesis. Since the definitive works listed above, the those methods have yielded estimates of the bulk composition crystallization ages of the shergottites have been firmly of the planet, its mantle, and its core (e.g., Treiman et al. 1986, established: most at ~180 Ma, QUE 94201 at 327 Ma, and DaG 1987; Longhi et al. 1992) and constraints on its differentiation 476 at 474 Ma (Borg et al. 1997, 2003). Consideration of initial history (e.g., Treiman et al. 1986). radioisotopic ratios has led to the recognition that Mars 1849 © Meteoritical Society, 2003. Printed in USA. 1850 A. H. Treiman underwent an extreme geochemical fractionation very early in incompatible elements and in element abundance ratios if the its history and, thus, that the chemical compositions of the elements are carried in different minerals (Fig. 1). shergottites need not represent only recent fractionations. A third cause of uncertainty in the chemical data is In light of these new developments, I have revisited my sample variability. Several shergottites are not homogeneous: early work (Treiman et al. 1986) to see if the regularities Elephant Moraine (EET) A79001 has 2 lithologies, 1 with established then are still valid and to interpret them in terms of xenoliths; Zagami has multiple lithologies; Allan Hills (ALH) geochemical processes. In this work, the sources of data and A77005 and Lewis Cliff (LEW) 88516 have domains of their uncertainties are described first. Then, the incompatible different textures; and the 2 Los Angeles stones may be element groups of Treiman et al. (1986) are revisited and chemically distinct (Steele and Smith 1982; Treiman et al. revised. Abundances of the Group 2 elements (moderately 1994; McCoy et al. 1999; Rubin et al. 2000). In such cases, incompatible) are explained in terms of recent basalt/mineral which lithology is represented by a given analysis may not be fractionations. The separation of Group 1 (highly clear, nor whether it corresponds to a recognized lithology. incompatible) and Group 2 elements is shown to have occurred early in Mars’ history, and several elements (notably RATIONALE AND ELEMENT GROUPS phosphorus) behaved differently then than in more recent fractionations. In the last 20 years, bulk compositions of martian basalts have been used to constrain the composition of Mars’ mantle DATA SOURCES and the petrogenesis of its basalt (e.g., Treiman et al. 1986; Longhi et al. 1992). The composition of a basalt reflects (to Many chemical analyses of shergottite meteorites are greater or lesser extent) the composition of its mantle source, available (e.g., Treiman et al. 1986; Lodders 1998; Meyer as modified by element distributions during melting and 2003). Those used here, Table 1, emphasize average or “best crystallization. The basalt’s mantle source is related to the estimate” analyses and recent data. In several cases, duplicate bulk composition of the planet and of the solar nebula (i.e., CI analyses are included, e.g., 3 separate analyses of Los chondrites) by known processes of accretion, volatile loss, Angeles and 2 each of Shergotty and Zagami. Additional analyses not in the compilations are from Wang et al. (1998), Rubin et al. (2000), Barrat et al. (2001, 2002a, b), Jambon et al. (2002), and others listed in the figure captions. Uncertainties in these chemical data are considerable, derive from several causes, and cannot be adequately quantified. Thus, the following figures do not show “error bars,” and the quality of the correlations must be viewed skeptically. Analytical uncertainties for each individual data point depend on the usual factors (see original papers for discussions) and vary by element, analytical method, and element abundance. More critical is the issue of unrepresentative sampling— are the analyzed aliquots statistically representative of the bulk rock? Sample masses for chemical analyses of martian meteorites (when given) are typically 100–500 mg, and this is Fig. 1. Abundances of La, Th, and K (G1 elements, Table 1) from generally too small to yield representative analyses of individual analyses of shergottite meteorites LEW 88516, Shergotty, crystalline basalt rocks (Ryder and Schuraytz 2001). This and Los Angeles. Among the analyses of each meteorite, abundances problem is exacerbated for the most incompatible trace of an element can vary by a factor of ~1.5. Abundance ratios may be elements, which are concentrated in small, dispersed mineral relatively constant (e.g., K/La in Shergotty ranges only from ~600 to ~700) or highly discordant (Th/La in Los Angeles ranges from ~140 grains or melt pockets. The problem of nonrepresentative to ~225). The straight lines encompass all K/La ratios for these analyses is most acute where the elements are carried by analyses. The data are from Dreibus et al. (1982, 1992), Laul et al. different phases (e.g., La and K). Thus, analyses of small (1986), Treiman et al. (1986, 1994), Rubin et al. (2000), and Jambon samples tend to show wide ranges in abundances of et al. (2002). Table 1. Element Groups 1 and 2 in shergottites (modified from Treiman et al. [1986]). Group 1 elements Geochemistry Group 2 elements Geochemistry La, Ce, Nd Light REE Sm, Du, HREE Middle and heavy REE Th, U, Ta, Nb, W Hight field strength Ti, Sc, Zr, Hf, In, P High field strength K, Rb, Cs, Ba, Pb Large ion lithophile Al, Ga, Na, Eu, Sr “Plagiophile” Cl, Br Halogens Chemical compositions of martian basalts 1851 and core formation. Thus, if 2 elements have identical systems, these elements would dissolve completely in silicate behaviors in basalt melting and element partitioning, their melt, be excluded completely from silicate phases, and abundance ratio in the basalt should be identical to that in the behave as Group 1 (highly incompatible). In sulfur-rich source mantle (see Drake 2000; but, see Sims and DePaolo systems, these elements should enter sulfide phases and be 1997). Further, that mantle
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