Abstract
Phosphates from the Martian shergottite NWA 2975 were used to obtain insights into the source and subsequence differentiation of the melt/melts. The crystallization of two generations of fluorapatite (F > Cl~OH and F-rich), chlorapatite and ferromerrillite-merrillite were reconstructed from TEM (Transmission Electron Microscopy) and geochemical analyses. The research results indicated that the recognized volatiles budget of the two generations of fluorapatite was related to their magmatic origin. The apatite crystals crystallized from an evolved magma during its final differentiation and degassing stage. In turn, chlorapatite replaced ferromerrillite-merrillite and was not related to, mantle-derived shergottite magma. The relationship between merrillite and apatite indicates that apatite is most probably a product of merrillite reacting with fluids. REE (rare earth elements) pattern of Cl-apatite might point to an origin associated with exogenous fluids mixed with fluids exsolved from evolved magma. The study shows that, among the three types of apatite, only the fluorapatite (F > Cl~OH) is a reliable source for assessing the degree of Martian mantle hydration. The occurrence of apatite with merrillite requires detailed recognition of their relationship. Consequently, the automatic use of apatite to assess the water content of the magma source can lead to false assumptions if the origin of the apatite is not precisely determined.
Highlights
The present study shows three generations of apatite coexisting with merrillite in the evolved shergottite NWA 2975 [30,41,42]
The contact between merrillite and chlorapatite is poorly visible in backscattered electrons (BSE) images (Figure 2), but it is defined on Clon elemental maps maps
The present study provides data on ferromerrillite and three types of apatite
Summary
Phosphates that occur in extra-terrestrial mafic rocks may provide information regarding water (volatiles) content in the environment leading to their formation and transformation (e.g., [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]). Volatile-bearing apatite (Ca10 (PO4 ) (Cl, F, OH)2 ), volatile-free merrillite (Ca18 Na2 Mg2 (PO4 )14 ) and the high-pressure phase tuite (γ-Ca3 (PO4 )2 ) are frequently observed in extra-terrestrial bodies ([1,3,5,6,12,22,23,24,25] and references therein) These minerals are thought to be of different origins due to their joint or separate occurrence and their chemical and structural transformation processes. The aim was to determine the phosphate compositions and structures to gain insights into the processes contributing to their formation and transformation Previous studies on this meteorite have reported the joint occurrence of two phosphate species, merrillite and chlorapatite [29,30,41,42], with zoned pyroxene, maskelynite (a glass phase with a plagioclase composition), other accessory minerals and glass veins/pockets. It was concluded that the water content in the shergottite magma source was comparable with that of the terrestrial Mid-Ocean Ridge Basalts (MORB) environment
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