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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 fO₂ (from IW-1 to FMQ+2) in converting a >90% normal spinel to an ~25% magnetite (inverse) spinel, the trajectory of DVspinel/melt as it relates to the ratio of V3+/V4+ in the melt, and the crystal chemical attributes of the spinel that control the intrinsic compatibility of both V3+ and V4+. This work examines the nuances of the V partitioning and provides a crystal chemical basis for understanding Fe3+, 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. [ABSTRACT FROM AUTHOR]

The milky appearance shown by certain type IaB diamonds has been subjected to several recent studies, but the origin of this feature is not fully understood. Here several type IaB diamonds with a milky appearance have been studied by cathodoluminescence (CL), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). CL of several hazy type IaB diamonds shows scattered or orientated micro-sized spots or short linear luminescence features. TEM observation revealed that those spots and linear features are caused by dislocation loops that are likely responsible for the hazy appearance of the host diamonds. It is also shown that type IaB diamonds with a cloudy appearance contain nano-sized inclusions with negative crystals of octahedral shape. Some of these negative crystals contain a precipitate that can be explained by a compressed disordered cubic δ-N2 phase observed by high-resolution TEM. In one of the milky IaB diamonds with platelet defects, polycrystalline areas composed of columnar diamond crystals elongated radially in [110], similar to ballas diamond, were revealed by EBSD. Taking into account these observations, it is suggested that the dislocation loops, nano-sized inclusions (negative crystals) and/or characteristic grain boundaries of the radiating fibrous crystals would be the origins for the milky appearance of the type IaB diamonds studied here. Those results add a complementary explanation that accounts for the milkiness of type IaB diamonds studied before. [ABSTRACT FROM AUTHOR]

Highly restitic garnet-kyanite-phlogopite metapelitic schists from the Goshen Dome of western Massachusetts contain: a population of prograde monocrystalline, megacrystic garnet, some with significant P in substitution for Si; precipitates of hydroxylapatite and rutile; and <1 μm zircon crystals of undetermined origin and abundance on the order of 105/mm3. The unusual P content and the abundant internal precipitate suite are similar to features reported in garnet from ultrahigh-pressure (UHP) and mantle settings, suggesting a potential (U)HP origin for the garnet megacrysts. Zircon included in megacrysts is surrounded by radial fractures, indicating in situ volumetric expansion or new growth. Cores display rare earth element (REE) profiles and cathodoluminescence (CL) zoning consistent with magmatic growth, and yield only Paleozoic dates (447-404 Ma). The embayed core-rim boundary is marked by a several micrometers wide band of CL-dark zircon enriched in Y, P, U, and Th that is interpreted as the accumulation of redistributed xenotime component from the original zircon rim during metamorphism. Outside of this band, the rim has elevated Hf, Th/U << 1, and steep heavy REE profiles. The metamorphic rims yield concordant dates from 400 to 381 Ma. Matrix zircon grains have magmatic cores (1726-415 Ma) with similar core-rim boundaries enriched in Y, P, U, and Th. Metamorphic rims on matrix zircon yield slightly younger dates (393-365 Ma) and are compositionally heterogeneous. The difference between the youngest core and oldest rim indicates a short interval (ca. 4 Ma) between deposition of detrital zircon and the onset of metamorphism in the earliest Acadian. The oldest zircon rim dates are found within phosphatic garnet megacrysts of possible very high-pressure origin. The compositional uniformity of these rims indicates equilibrium with a single source; the anomalous composition suggests a combination of dissolution-reprecipitation and new growth of zircon that is derived from garnet. The range in both composition and dates indicates that matrix zircon rims formed in response to local changes in mineralogy and fluid/melt composition and/or availability. New growth of zircon on these grains cannot be confirmed, suggesting that dissolution-reprecipitation reactions during continued metamorphism may be the dominant mechanism that formed these rims. The data collectively suggest that dissolution-reprecipitation may be a common mechanism for producing metamorphic rims on zircon that does not require additional Zr and Hf, which are limited within most metamorphic settings. [ABSTRACT FROM AUTHOR]

The article presents the views of Palke and Breeding related to the origin of needle-like rutile inclusions in natural gem corundum using electron probe micro-analyzer data, laser ablation inductively coupled plasma mass spectrometry; and nano-secondary ion mass spectrometry image.