Linking the Chassigny Meteorite and the Martian Surface Rock Backstay: Insights Into Igneous Crustal Differentiation Processes on Mars

Meteoritics & Planetary Science 44, Nr 6, 853–869 (2009) Abstract available online at http://meteoritics.org Linking the Chassigny meteorite and the Martian surface rock Backstay: Insights into igneous crustal differentiation processes on Mars Hanna NEKVASIL*, Francis M. MCCUBBIN, Andrea HARRINGTON, Stephen ELARDO, and Donald H. LINDSLEY Department of Geosciences, Stony Brook University, Stony Brook, New York 11794–2100, USA *Corresponding author. E-mail: [email protected] (Received 05 August 2008; revision accepted 18 March 2009) Abstract–In order to use igneous surface lithologies to constrain Martian mantle characteristics, secondary processes that lead to compositional modification of primary mantle melts must be considered. Crystal fractionation of a mantle-derived magma at the base of the crust followed by separation and ascent of residual liquids to the surface is common in continental hotspot regions on Earth. The possibility that this process also takes place on Mars was investigated by experimentally determining whether a surface rock, specifically the hawaiite Backstay analyzed by the MER Spirit could produce a known cumulate lithology with a deep origin (namely the assemblages of the Chassigny meteorite) if trapped at the base of the Martian crust. Both the major cumulus and melt inclusion mineral assemblages of the Chassigny meteorite were produced experimentally by a liquid of Backstay composition within the pressure range 9.3 to 6.8 kbar with bulk water contents between 1.5 and 2.6 wt%. Experiments at 4.3 and 2.8 kbar did not produce the requisite assemblages. This agreement suggests that just as on Earth, Martian mantle-derived melts may rise to the surface or remain trapped at the base of the crust, fractionate, and lose their residual liquids. Efficient removal of these residual liquids at depth would yield a deep low-silica cumulate layer for higher magmatic water content; at lower magmatic water content this cumulate layer would be basaltic with shergottitic affinity. INTRODUCTION to reach the surface or near-surface without compositional modification, they must have retained all minerals Igneous rocks on the Martian surface contain invaluable crystallizing during any protracted ponding stage. Secondary information on the compositional and thermal structure of igneous processes such as fractionation, in which residual the planet and on the changes in these over time. They provide melt separates from the crystallizing minerals, can induce information on the nature of the volatiles transported from the major changes in composition and greatly obscure mantle and contributed to the surface though volcanic information on the mantle source regions. emissions, the production of alteration assemblages, and the In continental intra-plate regions on Earth, secondary formation of hydrothermal deposits. The recent lander and fractionation processes produce compositionally diverse orbiter missions have greatly expanded our understanding of derivative magmas that are reflected in rocks ranging from igneous diversity on the Martian surface and, through this, gabbros and anorthosites to rhyolites and phonolites. expanded the potential for gaining new insights into igneous Experimental investigations (e.g., Thompson 1975; Scoates processes on Mars. et al. 1999; Filiberto and Nekvasil 2003; Nekvasil et al. 2004; Use of the varied compositional data on Martian igneous Whitaker et al. 2007) have shown that many of these diverse rocks to extract information on the mantle source regions and lithologies can be obtained by crystallization of associated volatile budgets is of major importance to the understanding continental tholeiitic magma with different bulk water of the evolutionary history of the planet. This, however, first contents at the base of a thick continental crust, and separation requires assessment of whether the rocks represent mantle- and ascent of these residual magmas to shallower levels derived magmas that are primary (that is, reflect the within the crust. These residual magmas on Earth are often composition of the source region), or represent compositions seen as surface lavas where they are exposed along with rocks modified by secondary processes. In order for primary melts of the parental tholeiite composition. The dominance of a 853 © The Meteoritical Society, 2009. Printed in USA. 854 H. Nekvasil et al. deep fractionation signature in such evolved intra-plate follows the Coombs trend of Miyashiro (1978). Furthermore, terrestrial lavas suggests that the terrestrial continental crust they inferred that this melt was trapped at pressures above provides a density barrier to ascending mantle-derived 4.3 kbar for several reasons. First, the Al/Ti ratios of the magmas. A significant density barrier would induce ponding augites are consistent with experimental data on augite of ascending magma at the base of the barrier. Only upon crystallizing from hawaiite above 4.3 kbar. Second, the crystallization of dense minerals would the residual liquids be cumulus mineral compositions, the main minerals within the able to overcome the density barrier and separate and ascend melt inclusion assemblage, and the residual rhyolitic glass towards the surface. Such a fractionation process could compositions, can all be produced experimentally from produce lavas (residual liquids) that are compositionally quite hawaiite only at pressures above 4.3 kbar (and specifically at distinct from the original parental magma. The retention of a 9.3 kbar). Third, the low temperatures of magmatic feldspar clear high-pressure signature in the terrestrial intraplate bulk pairs within the melt inclusions (McCubbin and Nekvasil lava compositions further suggests that if any additional 2008) attest to significant solidus temperature depression and ponding and crystallization occurs within the crust, it did not hence, high volatile solubility in the melt, attainable only at result in a sufficient loss of crystals to move the lavas away elevated pressure. The presence of kaersutite provides further from the high-pressure trend. The primary crust on Mars support for elevated pressures of crystallization of the melt (Norman 1999, 2002) may have similarly provided a density inclusion assemblage. Although fluor-kaersutite can form at barrier for Martian mantle derived magmas. But is there any 1 atm (McCubbin et al. 2007a), the low F contents of the melt evidence of deep ponding events that have produced inclusion kaersutite of the Chassigny meteorite are consistent derivative magmas on Mars? Ideally, such evidence would be with higher pressures. provided by demonstrating that a known surface lithology If the Chassigny dunite is the product of the representative of a liquid composition could, if ponded at accumulation of minerals crystallizing from a magma body at depth, give rise to a known cumulate lithology. This would be depth, then its highly cumulate character (with over 95% further strengthened if, as on Earth, examples of both the cumulus olivine) is a testament to efficient crystal parental magma and the high pressure derivatives were found fractionation through melt removal. Because of the density in close proximity. difference between the residual liquid and the cumulus Exposure of deep-seated cumulates residual to crystals, the melt residual to cumulus mineral crystallization fractionation at the base of a thick crust on the surface of a likely rose to some shallower level within the crust, or even to planet with no plate-margin tectonics and hence, no exposure the surface. However, is there any evidence on the Martian of deep layers during uplift and erosion is likely to be rare. surface of a magma that reflects the type of parental liquid For Mars, as on Earth in continental intra-plate and cratonic that could produce the Chassigny cumulus and melt inclusion regions (e.g., Féménias et al. 2003; Dessai et al. 2004), assemblages at depth? Is any liquid residual to cumulus evidence of deep lithologies is likely restricted to accidentally mineral formation reflected in any of the surface rocks included cognate mineral grains, inclusions brought up analyzed on Mars? by derivative liquids from depth, or xenoliths brought up by The paucity of compositional data on Martian igneous unrelated magma once extensive meteorite bombardment of lithologies severely limits the possibility that lavas the surface ceased. Recent mineralogical and experimental representing liquids parental to Chassigny-like dunites and data on potential liquids associated with the formation of the derivatives from deep fractionation were found. Rocks that Chassigny meteorite suggest that this meteorite may well unequivocally represent melt compositions are rare among have had a deep-seated origin (Nekvasil et al. 2007; the SNC meteorites. However, the MER Rover Spirit has McCubbin and Nekvasil 2008) and was brought to shallower provided information on fine-grained vesicular volcanic levels through one of such processes before excavation by lithologies that likely represent melt compositions (McSween meteorite impact. et al. 2006). Any lithology among these that could represent a The Chassigny meteorite is a dunite with cumulus Fo68 liquid parental to a cumulate of deep origin must satisfy olivine and chromite with interstitial pyroxenes, apatite, several criteria: (i) upon crystallization at the pressure of maskelynite, and oxides. The olivine hosts large polyphase formation of the cumulus minerals it must become saturated inclusions that have been interpreted

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