Large‐ion lithophile element fractionation during the early differentiation of Mars and the composition of the martian primitive mantle

Abstract

Abstract-Basaltic shergottites display a systematic decrease in K/Th, K/U, and K/La ratios with increasing K content. These trends are interpreted as mixing lines between relatively young martian magmas derived from highly depleted mantle sources and an ancient large-ion lithophile (LIL) element-enriched crustal component. One implication of this is that a substantial fractionation of these ratios occurs during the early crustal differentiation on Mars. Isotopic evidence from SNC meteorites and compositional data from Pathfinder and orbital gamma ray spectroscopy suggest that in excess of 50% of the LIL element complement of Mars resides in the crustal reservoir. If so, the primitive mantle of Mars is significantly more volatile-depleted (i.e., lower K/Th, K/U, K/La) than previously thought but probably (though not necessarily) still less volatile-depleted than the primitive mantle of the Earth. The La/Th ratios of virtually all SNC meteorites are subchondritic, including those with the most severe LREE-depletion. Extrapolation of the basaltic shergottite trend suggests that both the depleted mantle end member and the enriched crustal end member have subchondritic La/Th ratios. This is in contrast with the Earth where basalts from LIL element-depleted sources such as MORB have superchondritic La/Th ratios, complementary to the subchondritic ratios of the continental crust. Accordingly, assuming that the refractory elements are in chondritic proportions for the Mars primitive mantle, an additional major geochemical reservoir must exist on Mars that may not yet have been sampled.

Constraints on the structure of the martian interior determined from the chemical and isotopic systematics of SNC meteorites

“…Another possible problem with this model is that the shergottite suite has sub-chondritic La/Th ratios (McLennan 2003). I expect that bulk Mars should have a chondritic La/Th ratio and, therefore, should have a complementary reservoir that has La/Th greater than chondritic.…”

“…Finally, as suggested by McLennan (2003), another martian reservoir may exist that we have yet to sample. This is certainly possible.…”

Ancient heat flow, crustal thickness, and lithospheric mantle rheology in the Amenthes region, Mars

“…In this paper, we use crustal potassium and thorium abundances of 3630 ± 20 and 0.70 ± 0.02 ppm respectively, average values deduced from GRS data for the ancient southern highlands (Taylor et al, 2006). For uranium abundance, we use a Th/U ratio of 3.6 deduced from SNC meteorite geochemistry (McLennan, 2003). Absolute age for the beginning of the Late Noachian and the end of the Early Hesperian are 3.8 Ga and 3.6 Ga, respectively, following the cratering chronology of Hartmann and Neukum (2001).…”

The Petrochemistry of Jake_M: A Martian Mugearite

“…have been used to infer that the primitive martian mantle was richer in Na and K than the terrestrial mantle by as much as a factor of two (e.g., 65,[66][67][68][69]. On this basis alone, although few alkaline martian rocks have been documented thus far, it would not be surprising if alkaline magmas derived from relatively alkali-rich sources (either primitive martian mantle or mantle that has been metasomatized by low-degree melts of relatively primitive mantle) were more common on Mars than they are on Earth [on Earth, alkaline lavas are rare from a planetary perspective, representing an estimated < 1 vol.…”