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To gain insights into the composition and heterogeneity of Earth’s interior, the partial pressure of oxygen (oxygen fugacity, orfO₂) in igneous rocks is characterized. A surprising observation is that relative to reference buffers,fO₂s of mantle melts (mid-ocean ridge basalts, or MORBs) and their presumed mantle sources (abyssal peridotites) differ. Globally, MORBs have near-uniformfO₂s, whereas abyssal peridotites vary by about three orders of magnitude, suggesting these intimately related geologic reservoirs are out of equilibrium. Here, we characterizefO₂s of mantle melting increments represented by plagioclase-hosted melt inclusions, which were entrapped as basaltic melts migrated from their sources toward the seafloor. At temperatures andfO₂s constrained by rare earth element distributions, a range offO₂s consistent with the abyssal peridotites is recovered. ThefO₂s are correlated with geochemical proxies for mantle melting, suggesting partial melting of Earth’s mantle decreases itsfO₂, and that the uniformity of MORBfO₂s is a consequence of the melting process and plate tectonic cycling.

The surface of Mars is enriched in Cl and S which is linked to volcanic activity and degassing. Similarly, elevated Ge and Zn levels in Gale crater sedimentary bedrock indicate a magmatic source for these elements. To constrain the relative effects of Cl and S on the outgassing of these trace metals and chemical characteristics of primary magmatic vapor deposits incorporated to Martian surface, we conducted a set of degassing and fumarolic alteration experiments. Ge is found to be more volatile than Zn in all experiments. In S‐bearing runs, the loss of Ge and Zn was less than any other experiments. In Cl‐only runs, degassing of Zn was more than twice that of Ge within the first 10 min and percent loss increased for both elements with increasing time. In Cl + S runs, S‐induced reduction of GeO₂and ZnO to metallic Ge and Zn switches the preference of chloride formation from Zn to Ge. Up to 90% of Ge and Zn loss in the 1‐h no volatile‐added (NVA) experiments might be due to the small amounts of Cl contamination in NVA mixes via other oxides used for synthesis. Alteration experiments show different phases between 1‐h and 24‐/72‐h runs. In 1‐h runs, anhydrite and langbeinite dominate while in 24‐/72‐h runs halite and sylvite dominate the condensate assemblages. S‐bearing phases form as the intermediate products of fumarolic deposition, while chlorides are common when the system is allowed to cool gradually. One‐hour exposure was sufficient to form alteration phases and vapor deposits such as NaCl, KCl, CaSO₄, and langbeinites on the Martian analog minerals. These salts were identified in Martian meteorites and in situ measurements. Our results provide evidence that volcanic degassing along with fumarolic alteration could be a potential source for the enrichment and varying abundances of Cl, S, Fe, Zn, Ge in Martian surface, as well as a cause for Ge depletion in shergottites.

Plagioclase ultraphyric basalts (PUBs) are a class of mid‐ocean ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15–40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO₂contents in plagioclase‐hosted inclusions, many of the megacrysts formed at upper mantle pressures (∼3–7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using mid‐ocean ridge basalts glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering the solidus for upper mantle conditions, and driving evolving magmas toward higher Ca. As magmas rise and pressure drops, the pseudoazeotrope disappears, and the feldspar loop at the high‐An end rises, causing those magmas to undergo decompression crystallization of plagioclase and resorption of olivine. Therefore, the conditions which generated the magmas from which the megacrysts form disappear as the magmas rise and magmas evolve toward lower Ca, Mg (as we normally assume during plagioclase + olivine crystallization). In effect, the phase equilibria conditions that allow for the generation of such liquids also prevent them from being erupted as lavas.