Topic
Phenocryst
About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.
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TL;DR: Olivine phenocryst compositions and whole-rock chemical compositions are used to identify primitive picrite basalts from widely separated parts of the Deccan flood basalt province as discussed by the authors.
Abstract: Olivine phenocryst compositions and whole-rock chemical compositions are used to identify primitive picrite basalts from widely separated parts of the Deccan flood basalt province. Overall, primitive picrites constitute a significant volume of rocks within the province. Most were probably emplaced along deep faults in the Cambay graben and Narmada rift regions. We combine mineral composition data on previously described samples from boreholes at Dhandhuka, Wadhwan and Botad with information on new finds of picritic basalts at Paliad, Anila, Pawagarh, Kawant and Ambadongar to help delineate the petrogenesis of these mafic rocks, and we also examine the nature and probable origin of picrite basalts from other regions of the Deccan, such as the Western Ghats. The combined data suggest that the incidence of high-MgO lavas decreased with time during the Deccan volcanic cycle.
65 citations
TL;DR: In this paper, the authors used petrology and 40 Ar/39 Ar dating of biotite, sanidine, hornblende, and plagioclase from the Toba Tuff to investigate the petrogenesis of the 2800 km 3 of the youngest Toba tuff.
Abstract: The petrogenesis of the 2800 km 3 of magma erupted as the youngest Toba Tuff has been investigated using experimental petrology and 40 Ar/ 39 Ar dating of biotite, sanidine, hornblende, and plagioclase from the tuff. We find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage. Hornblende is also not in isotopic equilibrium with biotite and sanidine, both of which grew experimentally. Hornblende thus appears xenocrystic, despite being a major phase in the tuff. Some plagioclase is also xenocrystic, on the basis of Ar isotopes, but others are probably phenocrystic, because plagioclase grew experimentally. Crystal clots of hornblende + plagioclase observed in the tuff suggest that the xenocrysts came from a common source, which was at least 1.5 Ma (the oldest hornblende 40 Ar/ 39 Ar age). Our results suggest that the Toba Tuff magma resided at nearly water-saturated pressures of 100–150 MPa and that xenocrysts were entrained as recently as 10 yr before the eruption. The ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 km 3 of magma.
65 citations
TL;DR: In this paper, the authors used mineral chemistry to determine processes occurring during early stages of the evolution of the Egmont volcano to a high-alumina basalt composition, where the amphibole stability field was intersected and these melts evolved along a calc-alkaline trend to dacite.
65 citations
TL;DR: In this article, anhydrite and numerous ultramafic inclusions in the lavas of Mount Lamington, together with reports of similar inclusions and anomalously high Ni and Cr contents in other volcanics of southeastern Papua are reported.
65 citations
TL;DR: Melt inclusions from Oligocene continental flood basalts (CFB) erupted in Yemen provide unique insight into the timing and nature of the processes that lead to crustal contamination and melt aggregation in CFB magmas.
65 citations