Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v14a..08f&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V14A-08
Physics
[1036] Geochemistry / Magma Chamber Processes, [1036] Geochemistry / Magma Chamber Processes, [1065] Geochemistry / Major And Trace Element Geochemistry, [3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology
Scientific paper
The Martian Rover Spirit has now collected some 60 whole-rock chemical analyses on rock outcrops and boulders on its traverse from its landing site on the Gusev crater plains into the nearby Columbia Hills. Although Fe-rich “basaltic” compositions dominate the data set, there is a relatively wide range of mafic compositions that define a striking trend of increasing Fe with increasing Mg, and a small subset of ultramafic compositions at higher Mg. Although many of the analyzed samples have been shown to have experienced the effects of acidic chemical weathering, only the minor elements S, P, and K appear to have been seriously disturbed. Comparisons of analyses of rocks before and after abrasion suggests that it is unlikely that the co-variation between Fe and Mg in the Spirit whole rock analyses is a weathering effect. Despite the coherence of the array of Spirit data in Fe-Mg space, however, no plausible crystal fractionation model can explain the decrease in Fe with Mg within the mafic samples, nor relate the ultramafic samples to the mafic ones, if the samples of the data set are assumed to represent volcanic compositions defining a liquid line of descent. This reflects a fundamental inconsistency between the compositions of the olivine present (Fo~50-60), as estimated by Mössbauer analysis, and that predicted on the basis of Fe/Mg partitioning (Fo ~ 80). Positively correlated linear arrays in Fe - Mg space are, however, a common feature of whole rock data collected across cyclic feldspar-rich and mafic-rich cumulates in layered igneous intrusions. In such cyclically layered sequences, the whole rock compositions reflect magmatic crystal sorting, and scatter along mixing lines between the compositions of dominant cumulate phases. There is in fact a striking correspondence between the Fe-Mg array of Spirit data and those of dunite to troctolite cumulate layers in the Fe-rich Lac Raudot Troctolite Complex of the Grenville Province of Québec. If the Spirit rocks are assumed to represent cumulate layers whose compositions reflect magmatic sorting, then the estimated olivine compositions (~ Fo 50-60) closely match those indicated to be present by Mössbauer analysis. This correspondence suggests that the rocks analyzed by Spirit may represent crystal cumulates, rather than volcanic liquid compositions, leading to the possibility that the stratification seen at number of localities along Spirit’s traverse represents magmatic sedimentation in a layered igneous intrusion, rather than aqueous, aeolian, or even volcanoclastic sedimentary processes.
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