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Expanding the application of the Eu‐oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior
Author(s) -
McCANTA Molly C.,
ELKINSTANTON Linda,
RUTHERFORD Malcolm J.
Publication year - 2009
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2009.tb00765.x
Subject(s) - martian , mineral redox buffer , meteorite , geology , pyroxene , mineral , crystallization , mineralogy , geochemistry , mantle (geology) , mars exploration program , olivine , materials science , astrobiology , thermodynamics , metallurgy , physics
— Experimentally rehomogenized melt inclusions from the nakhlite Miller Range 03346 (MIL 03346) and the lherzolitic shergottite Allan Hills 77005 (ALH 77005) have been analyzed for their rare earth element (REE) concentrations in order to characterize the early melt compositions of these Martian meteorites and to calculate the oxygen fugacity conditions they crystallized under. D(Eu/Sm) pyroxene/melt values were measured at 0.77 and 1.05 for ALH 77005 and MIL 03346, respectively. These melts and their associated whole rock compositions have similar REE patterns, suggesting that whole rock REE values are representative of those of the early melts and can be used as input into the pyroxene Eu‐oxybarometer for the nakhlites and lherzolitic shergottites. Crystallization f O 2 values of IW + 1.1 (ALH 77005) and IW + 3.2 (MIL 03346) were calculated. Whole rock data from other nakhlites and lherzolitic shergottites was input into the Eu‐oxybarometer to determine their crystallization f O 2 values. The lherzolitic shergottites and nakhlites have f O 2 values that range from IW + 0.4 to 1.6 and from IW + 1.1 to 3.2, respectively. These values are consistent with some previously determined f O 2 estimates and expand the known range of f O 2 values of the Martian interior to four orders of magnitude. The origins of this range are not well constrained. Possible mechanisms for producing this spread in f O 2 values include mineral/melt fractionation, assimilation, shock effects, and magma ocean crystallization processes. Mineral/melt partitioning can result in changes in f O 2 from the start to the finish of crystallization of 2 orders of magnitude. In addition, crystallization of a Martian magma ocean with reasonable initial water content results in oxidized, water‐rich, late‐stage cumulates. Sampling of these oxidized cumulates or interactions between reduced melts and the oxidized material can potentially account for the range of f O 2 values observed in the Martian meteorites.