Temperature and composition of the mantle sources of martian basalts as
constrained by MAGMARS, a new melting model for FeO-rich peridotite
Abstract
The martian surface is predominantly covered by FeO-rich basalts and
their alteration products. Several samples, either analyzed in
situ by rovers or recovered as meteorites, might represent primitive
(i.e. near-primary) basaltic melts that can shed light on the
mineralogy, the bulk composition, and the temperature of their mantle
sources. We recently developed a new melting model, called MAGMARS, that
can predict the melt compositions of FeO-rich mantles and the martian
mantle in particular (Collinet et al., submitted to JGR:P). It
represents a more accurate alternative to pMELTS (Ghiorso et al., 2002,
G3), which systematically overestimates the FeO and MgO content of
martian melts and underestimates the SiO2 content (by up to 8 wt.%).
MAGMARS can simulate near-fractional and batch melting of various mantle
compositions. For example, MAGMARS can produce melts identical to the
Adirondack-class basalts by near-fractional melting, between 2.3 and 1.7
GPa, of a depleted mantle with a potential temperature (Tp) of 1390°C
(~7 wt.% melt fraction). For this study, MAGMARS is
applied to all other martian basalts from which the primary melt
compositions can be inferred in order to constrain their mantle sources:
the Columbia hills basalts, igneous rocks from Gale crater,
shergottites, nakhlites and Northwest Africa (NWA) 7034/7533. We find
that a few basaltic clasts in the pre-Noachian polymict regolith breccia
NWA 7034/7533 are the only samples with bulk compositions that could
represent melts derived from a primitive mantle. The Columbia hills
basalts (Gusev crater), alkali-rich rocks from Gale crater, nakhlites
and enriched shergottites are most easily reproduced by melting depleted
mantle reservoirs that were re-fertilized to different degrees in
alkalis by fluids or melts (i.e. metasomatized sources). Most martian
basalts, with the exception of depleted shergottites, can be produced
from martian mantle reservoirs with Mg# comprised between 75 and 81.
From this sample set, the melting conditions of the martian mantle seem
to remain relatively stable through time (Tp = 1400 ± 100 ºC and P = 2 ±
0.5 GPa) but the depleted nature of all mantle sources sampled after the
pre-Noachian points towards an early crust-mantle differentiation.