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Estimation of the potential temperature of the Martian mantle from olivine P-zoning in the Tissint meteorite

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Introduction: Geochemical and geophysical data suggest that the martian mantle retained heat from Mars formation, and that it could still be actively convecting [e.g., 1]. Here we try to infer thermodynamical information related to martian mantle activity via mineralogical data acquired from olivine and pyroxene in the Tissint martian meteorite. Tissint is an exceptional martian sample, because it is an unusually depleted olivine-phyric shergottite that could testify to the composition of melts derived from the martian upper mantle. The rock's crystallization age is ~574 ± 20 Ma [2]. We here report major element core-to-rim evolution of clinopyroxene and P-zoning in olivine megacrysts and phenocrysts in Tissint in order to provide new insights into the dynamics of its parental magma chamber. Methods: Major element chemical data and X-ray elemental maps were collected on four polished thin sections of Tissint at the Imaging Spectroscopy and Analysis Center of the University of Glasgow (UoG) using a Zeiss-Sigma SEM equipped with an Oxford Instruments INCA system. Operating conditions include: working distance of 8.5 mm, beam current of 2.15 nA, and accelerating voltage of 20 kV. Acquired X-ray spectra were calibrated using mineral standards. Minor and trace elements were collected with an LA-ICP-MS at UoG. Results: Olivine in Tissint has a compositional range from Fo73 in cores to Fo22 in rims, with an average of Fo43-61. New data for P-zoning in Tissint olivine is comparable to previously published data from the shergottites [3]. Phosphorus was chosen as a recorder of large and fast variations in temperature of the magmatic system due to its chemical kinetic effect during olivine crystallization [4]. Specifically, the largest olivine grains (~2 mm) have very low (~0.02 wt.%) or absent P in their cores but P2O5 concentrations increase toward their rims (~0.20 wt.%). The smallest olivine grains (~100 µm) have higher concentrations of P2O5 independently of their cores and rims (ranging from ~0.05 to 0.25 wt.%) (Fig. 1). The average equilibration temperature calculated in these olivine grains is ~1577° C. Discussion: Oscillatory P-zoning in olivine can be a product of open-system processes [5]. We tested this possibility for the Tissint magmatic system by using information on pyroxene zoning and pyroxene/olivine equilibration temperatures. We found no evidence for magma mixing and other disequilibrium conditions. In light of this, we linked the marked differences in P-zoning between large and small olivine grains to vigorous convective activity in the Tissint magma chamber-with the entrapment of phosphorus in olivine due to convection-induced differential cooling in the magma chamber [4, 6]. Equilibration temperatures from olivine that were in equilibrium with the liquid were then used to infer a mantle potential temperature of ∼1720° C for the parcel of mantle that erupted the Tissint lava at ∼570 Ma, suggesting an active martian mantle was still in place until at least the late Amazonian.
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ESTIMATION OF THE POTENTIAL TEMPERATURE OF THE MARTIAN MANTLE
FROM OLIVINE P-ZONING IN THE TISSINT METEORITE
N. Mari1, L. J. Hallis1, A. J. V. Riches2, M. R. Lee1 1School of Geographical and Earth Sciences, University of Glasgow,
Glasgow, UK. Email: n.mari.1@research.gla.ac.uk 2Department of Earth Sciences, Durham University, Durham, UK.
Introduction: Geochemical and geophysical data suggest that the martian mantle retained heat from Mars formation,
and that it could still be actively convecting [e.g., 1]. Here we try to infer thermodynamical information related to martian
mantle activity via mineralogical data acquired from olivine and pyroxene in the Tissint martian meteorite. Tissint is an
exceptional martian sample, because it is an unusually depleted olivine-phyric shergottite that could testify to the
composition of melts derived from the martian upper mantle. The rock’s crystallization age is ~574 ± 20 Ma [2]. We here
report major element core-to-rim evolution of clinopyroxene and P-zoning in olivine megacrysts and phenocrysts in Tissint
in order to provide new insights into the dynamics of its parental magma chamber.
Methods: Major element chemical data and X-ray elemental maps were collected on four polished thin sections of
Tissint at the Imaging Spectroscopy and Analysis Center of the University of Glasgow (UoG) using a Zeiss-Sigma SEM
equipped with an Oxford Instruments INCA system. Operating conditions include: working distance of 8.5 mm, beam
current of 2.15 nA, and accelerating voltage of 20 kV. Acquired X-ray spectra were calibrated using mineral standards.
Minor and trace elements were collected with an LA-ICP-MS at UoG.
Results: Olivine in Tissint has a compositional range from Fo73 in cores to Fo22 in rims, with an average of Fo43-61. New
data for P-zoning in Tissint olivine is comparable to previously published data from the shergottites [3]. Phosphorus was
chosen as a recorder of large and fast variations in temperature of the magmatic system due to its chemical kinetic effect
during olivine crystallization [4]. Specifically, the largest olivine grains (~2 mm) have very low (~0.02 wt.%) or absent P in
their cores but P2O5 concentrations increase toward their rims (~0.20 wt.%). The smallest olivine grains (~100 µm) have
higher concentrations of P2O5 independently of their cores and rims (ranging from ~0.05 to 0.25 wt.%) (Fig. 1). The average
equilibration temperature calculated in these olivine grains is ~1577° C.
Discussion: Oscillatory P-zoning in olivine can be a product of open-system processes [5]. We tested this possibility for
the Tissint magmatic system by using information on pyroxene zoning and pyroxene/olivine equilibration temperatures. We
found no evidence for magma mixing and other disequilibrium conditions. In light of this, we linked the marked differences
in P-zoning between large and small olivine grains to vigorous convective activity in the Tissint magma chamber with the
entrapment of phosphorus in olivine due to convection-induced differential cooling in the magma chamber [4, 6].
Equilibration temperatures from olivine that were in equilibrium with the liquid were then used to infer a mantle potential
temperature of 1720° C for the parcel of mantle that erupted the Tissint lava at 570 Ma, suggesting an active martian
mantle was still in place until at least the late Amazonian.
References: [1] Hauck S. A. and Phillips R. J. (2002) J. geophys. Res. 107:E7. [2] Brennecka G. A., Borg L. E. and
Wadhwa M. (2014) Meteoritics & Planetary Science 49:412418. [3] Jean M. M., et al. (2017) LPS XLVIII, Abstract #2067.
[4] Milman-Barris M. L., et al. (2008) Contributions to Mineralogy and Petrology 155:739765. [5] Pearce T. H. (1994)
Feldspars and their reactions, 421:313-349. [6] Elardo S. M. and Shearer C. K. (2014) American Mineralogist 99:355-368.
Fig. 1 - Phosphorus concentration in Tissint olivines. A) P2O5 wt.% in the coarse olivine grains is dependent on the location
in the crystal, more abundant at the rim than the core. The olivine megacryst (inset) is 1.8 x 1 mm in size; B) P2O5 wt.% in
the small olivine grains (<< 1 mm) is independent of the location in the crystal, and these olivines are, on average, slightly
more enriched in phosphorus than the larger grains.
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