Abstract Olivine‐phyric shergottites represent primitive basaltic to picritic rocks, spanning a large range of Mg# and olivine abundances. As primitive olivine‐bearing magmas are commonly representative of their mantle source on Earth, understanding the petrology and evolution of olivine‐phyric shergottites is critical in our understanding of Martian mantle compositions. We present data for the olivine‐phyric shergottite Northwest Africa ( NWA ) 10170 to constrain the petrology with specific implications for magma plumbing‐system dynamics. The calculated oxygen fugacity and bulk‐rock REE concentrations (based on modal abundance) are consistent with a geochemically intermediate classification for NWA 10170, and overall similarity with NWA 6234. In addition, we present trace element data using laser ablation ICP ‐ MS for coarse‐grained olivine cores, and compare these data with terrestrial and Martian data sets. The olivines in NWA 10170 contain cores with compositions of Fo 77 that evolve to rims with composition of Fo 58 , and are characterized by cores with low Ni contents (400–600 ppm). Nickel is compatible in olivine and such low Ni content for olivine cores in NWA 10170 suggests either early‐stage fractionation and loss of olivine from the magma in a staging chamber at depth, or that Martian magmas have lower Ni than terrestrial magmas. We suggest that both are true in this case. Therefore, the magma does not represent a primary mantle melt, but rather has undergone 10–15% fractionation in a staging chamber prior to extrusion/intrusion at the surface of Mars. This further implies that careful evaluation of not only the Mg# but also the trace element concentrations of olivine needs to be conducted to evaluate pristine mantle melts versus those that have fractionated olivine (±pyroxene and oxide minerals) in staging chambers.
Abstract Apatite is the major volatile‐bearing phase in Martian meteorites, containing structurally bound fluorine, chlorine, and hydroxyl ions. In apatite, F is more compatible than Cl, which in turn is more compatible than OH . During degassing, Cl strongly partitions into the exsolved phase, whereas F remains in the melt. For these reasons, the volatile concentrations within apatite are predictable during magmatic differentiation and degassing. Here, we present compositional data for apatite and merrillite in the paired enriched, olivine‐phyric shergottites LAR 12011 and LAR 06319. In addition, we calculate the relative volatile fugacities of the parental melts at the time of apatite formation. The apatites are dominantly OH ‐rich (calculated by stoichiometry) with variable yet high Cl contents. Although several other studies have found evidence for degassing in the late‐stage mineral assemblage of LAR 06319, the apatite evolutionary trends cannot be reconciled with this interpretation. The variable Cl contents and high OH contents measured in apatites are not consistent with fractionation either. Volatile fugacity calculations indicate that water and fluorine activities remain relatively constant, whereas there is a large variation in the chlorine activity. The Martian crust is Cl‐rich indicating that changes in Cl contents in the apatites may be related to an external crustal source. We suggest that the high and variable Cl contents and high OH contents of the apatite are the results of postcrystallization interaction with Cl‐rich, and possibly water‐rich, crustal fluids circulating in the Martian crust.
Crustal assimilation is a common processes affecting basaltic magmas that traverse thick crustal sequences.Discriminating crustal assimilation from mantle sources, however, is not straightforward.We present Sr-Nd isotope and incompatible trace element (ITE) data from a 538m deep borehole intersecting the transition from picrite to basalt at the base of the Tuli basin high-Ti lavas, Karoo CFB, in order to assess crustal assimilation relative to mantle source heterogeneity.From 538m to 300m, picrites display gradual decrease in ITE concentrations (e.g., Ba 864 to 476 ppm), decrease in Zr/Y (16.7 to 10.7), decrease in Th/Nb (0.17 to 0.14), decrease in ( 87 Sr/ 86 Sr) i (0.7055 to 0.7050), increase in ɛNd i (-8.5 to -5.3).At ±300m, picrites transition to basalts and a large change in the chemistry is observed with a rapid increase in ( 87 Sr/ 86 Sr) i (up to 0.7060), increase in Th/Nb (up to 0.23), and decrease in ɛNd i (down to -8.9); at constant Zr/Y ratio of ±9.5.The range in Zr/Y is interpreted to reflect a gradual increase in the degree of partial melting in the source, in the presence of residual garnet, over time.The low and constant Zr/Y above ±300m reflects the highest degree of partial melting obtained in the source.At this point, crustal chambers developed and picrite parent melt evolved to basalt.The ITE enriched picrites and the range in ( 87 Sr/ 86 Sr) i and ɛNd i between 538m and 300m is interpreted to reflect a heterogeneous mantle source and cannot be reproduced by assimilation-fractional-crystallisation (AFC) modelling.In contrast, the major change in chemistry at ±300m likely reflects crustal assimilation during the initial formation of crustal staging chambers and the evolution to basaltic magmas.We show that picrites are derived directly from a heterogeneous mantle source can have similar initial Sr-Nd isotopes to those affected by crustal assimilation.The major difference is in the degree of ITE enrichment.Highly ITE enriched low-degree partial melts do not show the effects of crustal assimilation, however, later stage, higher degree, partial melts that have lower ITE content may have undergone a stage of crustal assimilation during the development of crustal staging chambers and the evolution to basaltic magmas.
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