Genesis of the Mars Pathfinder “sulfur-free” rock from SNC parental liquids

• Published in: Geochimica et cosmochimica acta Vol. 64; no. 14; pp. 2535 - 2547
• Main Authors: Minitti, M.E, Rutherford, M.J
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2000
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(00)00366-5

Combined rock and soil measurements from the Mars Pathfinder APXS established the composition of the “sulfur-free” rock whose chemical characteristics suggest it is an igneous andesite. We experimentally investigated the formation of the sulfur-free rock through equilibrium crystallization of a primitive SNC basalt under hydrous (1.0–1.5 wt% H 2O) and dry conditions at the QFM oxygen buffer. The experiments determined crystallization sequences and liquid lines of descent for a basalt with a high-FeO, low-Al 2O 3 (relative to tholeiitic magma) composition characteristic of SNC magmas. In the hydrous experiments, the crystallization sequence between ∼1090°C and 950°C is pigeonite → sub-calcic augite → Ti-magnetite → plagioclase → ilmenite + fayalite. The crystallization sequence of dry experiments between ∼1130°C and 980°C resembles that of the hydrous experiments except for the appearance of plagioclase and fayalite before Ti-magnetite. The liquid lines of descent reveal that fractionation of a hydrous, primitive SNC basalt can produce a melt equivalent to the sulfur-free rock composition whereas fractionation of a dry SNC basalt fails to produce such a melt. The effect of water on the timing and amount of phenocryst growth, particularly Fe-Ti oxide crystallization, is critical in achieving the observed SiO 2 enrichment in the hydrous experiments. The hydrous basalt reaches andesitic residual melt compositions at ∼40% crystallization and the dry basalt eventually produces a minimum andesitic composition melt at ∼90% crystallization. The low degree of crystallization necessary to reach andesitic residual melt compositions in the hydrous experiments would greatly facilitate the extraction of this andesitic melt (and more evolved melts) from associated crystals. Further experiments at other oxygen fugacities reveal that oxidizing fO 2’s (MnO-Mn 3O 4) encourage silicic melt formation by enhancing oxide crystallization; conversely, reducing fO 2’s (GCH) hinder silicic melt formation. By analogy with terrestrial magmatic processes, water could enter basaltic melt on Mars through one or more pathways: partial melting of water-bearing mantle and concentration of water by fractional crystallization; stoping or assimilation of hydrothermally altered crust; direct introduction of hydrothermal fluids into a magma during caldera collapse.