ALBERT

Description: Spinel is a very important rock-forming mineral that is found in basalts from Earth, Mars, the Earth’s Moon, and basaltic meteorites. Spinel can be used as a sensitive indicator of petrologic and geochemical processes that occur in its host rock. This paper highlights the role of increasing f O 2 (from IW-1 to FMQ+2) in converting a 〉90% normal spinel to an ~25% magnetite (inverse) spinel, the trajectory of D V spinel/melt as it relates to the ratio of V 3+ /V 4+ in the melt, and the crystal chemical attributes of the spinel that control the intrinsic compatibility of both V 3+ and V 4+ . This work examines the nuances of the V partitioning and provides a crystal chemical basis for understanding Fe 3+ , Cr, and V substitution into the octahedral sites of spinel. Understanding this interplay is critical for using spinels as both indicators of planetary parentage and reconstructing the redox history of magmatic systems on the terrestrial planets. Three potential examples for this use are provided. In addition, this work helps explain the ubiquitous miscibility gap between spinels with changing ülvospinel contents.

Description: A revised diagram plotting the Fe/Mn ratio of pyroxene and olivine verses the anorthite content of plagioclase indicates that the Angrite Parent Body originated from a solar system reservoir with a similar Mn-Fe signature to that from which the Earth was derived. The major difference in terrestrial and angritic basalts is the extreme volatile depletion (particularly Na and K) in the latter. Considerable research and publication has been focused on angrite meteorites, which are among the oldest objects in the solar system (~4.56 Ga; Keil 2012 ). These meteorites include a silica-undersaturated mafic mineral assemblage. The identity and location of their parent body is still unknown and widely debated. Our new work on angrites SAH 99555, LEW 86010, NWA 10463, LEW 87051, and Angra dos Reis focused on olivine and is interpreted in the context of existing pyroxene and plagioclase data sets. This paper improves the "Mn-Fe in olivine and anorthite content of plagioclase tool for planetary parentage" specifically targeted at finding members of the angrite group of meteorites.

Description: A martian basalt (Yamato 980459) composition was used to synthesize olivine, spinel, and pyroxene at 1200 °C at five oxygen fugacities: IW-1, IW, IW+1, IW+2, and QFM. The goal of this study is to examine the significant variation in the value of D V pyroxene/melt with changing Wo content in pyroxene. While most literature on this subject relies on electron microprobe data that assumes that if the Wo component (CaSiO 3 ) is 〈4 mol%, the pyroxene is in fact orthopyroxene, we’ve made a more robust identification of orthopyroxene using appropriate Kikuchi diffraction lines collected during electron backscatter diffraction analysis. We compare augite (Wo ~ 33), pigeonite (Wo ~ 13), orthopyroxene (Wo 〈4), and olivine. In augite (Wo ~ 33), the M2 site is 8-coordinated, while in pigeonite (Wo ~ 13), the site is 6-coordinated. The larger (8-coordinated) M2 site in augite requires structural expansion along the chain direction. The longer chain is enabled by the substitution of the larger Al for Si. The Al 3+ substitution for Si 4+ causes a charge deficiency that is made up, in part, by the substitution of V 4+ and V 3+ in the pyroxene M1 site. This rationale does not fully explain the dramatic decrease in D V orthopyroxene/melt . In monoclinic pyroxenes, the TOT stacking is characterized by + + + + (indicating the direction), a stacking pattern that produces a monoclinic offset. In orthopyroxene, the stacking is + + – –, which produces an orthorhombic structure. The M2 site is located between the reversed TOT units and is highly constrained to 6-coordination and thus cannot contain significant Ca that requires 8-coordination. Because the M2 site in orthopyroxene is small and constrained, it accommodates less Al in the tetrahedral chains and thus less V in the pyroxene M1 site.