Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

Dynamic deformation of volcanic ejecta from the Toba caldera: Possible relevance to Cretaceous/Tertiary boundary phenomena

Abstract: Plagioclase and biotite phenocrysts in ignimbrites erupted from the Toba caldera, Sumatra, show microstructures and textures indicative of shock stress levels higher than 10 GPa. Strong dynamic deformation has resulted in intense kinking in biotite and, with increasing shock intensity, the development of plagioclase of planar features, shock mosaicism, incipient recrystallization, and possible partial melting. Microstructures in quartz indicative of strong shock deformation are rare, however, and many shock lamellae, if formed, may have healed during post-shock residence in the hot ignimbrite; they might be preserved in ash falls. Peak shock stresses from explosive silicic volcanism and other endogenous processes may be high and if so would obviate the need for extraterrestrial impacts to produce all dynamically deformed structures, possibly including shock features observed near the Cretaceous/Tertiary boundary. 38 references, 3 figures.

Volatile and Trace Element Composition of Melt Inclusions From the Lower Bandelier Tuff: Implications for Magma Chamber Processes and Eruptive Style

Abstract: The Lower Bandelier Tuff, erupted from the Valles Caldera at 1.51 Ma, is composed of a Plinian tephra and associated ignimbrite. Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz, sanidine, and pyroxene phenocrysts, a strong volatile gradient was present in the upper portion of the magma chamber. In the 20 km3 of magma that produced the Plinian tephra, most H2O and F contents of MI are between 4 to 5 wt % and 0.18 to 0.20 wt%, respectively, and between 3 to 4 wt% and 0.12 and 0.14 wt% in the first-deposited basal ignimbrite. Measurements of the H2O and F contents of the 380 km3 of magma which produced the bulk of the ignimbrite are concentrated between 2 to 3 wt% and 0.05 to 0.14 wt% respectively. The invariance of H2O (and possibly Cl and B) relative to F in the magma which formed the Plinian tephra suggests that this portion of the magma chamber was saturated with respect to an H2O-rich vapor phase. The trace element composition of MI are varied, and overlap with composition of pumice lumps. A group of MI are rich in Ti, Sr, Ba and low in B, Cl, Rb, Y, Nb, and Th, and are compositionally similar to analyses of matrix glass from some ignimbrite pumice lumps. These represent the mixing of a second rhyolite into the base of the Lower Bandelier magma chamber. MI influenced by the second magma have not been found in Plinian samples, but occur in ignimbrite samples from base to top of the deposit. When these MI are removed from the data set, the remainder show strong linear correlations between Nb and Rb, Y, Zr, and Th. These correlations can be most easily explained by fractionation of approximately ∼40% quartz and alkali feldspar (with trace amounts of chevkinite). MI from the Plinian tephra are similar to bulk Plinian pumice composition, suggesting that magmatic evolution was well-progressed at the time that the MI were trapped. However, fractional crystallization is difficult to reconcile with the observed distribution of some other elements, including H, B, Li, F and Cl. The enrichment of H2O and F in the upper portion of the magma chamber, along with their lack of correlation with Nb, Rb, Zr, Y, and Th, suggest that these sets of elements were decoupled during magmatic evolution. The upward enrichment of H2O and F must have occurred at a faster rate than other trace elements. The observed volatile gradient may have influenced the eruption dynamics of the magma, initially causing discrete layers of magma to be removed when the gradient was large, followed by chaotic eruption of the bulk of the ignimbrite when the gradient was small. Comparison with the Bishop Tuff eruption, however, suggests that the chaotic eruption style of the LBT may be related to vent geometry and conduit evolution, promoting high discharge rates, not differences in the absolute concentration of volatiles.

Origin of bimodal volcanism, southern Basin and Range province, west-central Arizona

Abstract: Miocene volcanic rocks in the Castaneda Hills area, west-central Arizona, are interbedded with continental clastic sedimentary rocks, minor limestone, gravity glide blocks of Precambrian(?) and Paleozoic(?) rocks, and monolithologic megabreccia. The sedimentary and volcanic units dip to the southwest and are offset by northwest-trending listric and high-angle normal faults. The listric faults coalesce at the Rawhide detachment fault, which overlies mylonitic gneiss. The volcanic suite is strongly bimodal; rocks with 55 to 71 wt % SiO 2 are rare. On the basis of age, geomorphic position, and petrography, five volcanic units can be distinguished: older basalts (18.7 and 16.5 m.y. old), quartz-bearing basalts (13.7 and 12.4 m.y. old), rhyolite lavas and tuffs (15.1 to 10.3 m.y. old), mesa-forming basalts (13.1 to 9.2 m.y. old), and megacryst-bearing basalts (8.6 to 6.8 m.y. old). Most of the basalts contain groundmass olivine and titanaugite phenocrysts and are alkali-olivine basalts. Many rhyolites contain more than 75 wt % SiO 2 . The initial whole-rock Sr isotopic composition of the basalts indicates that they are partial melts of an isotopically vertically heterogenous mantle. The chemical composition of some of the megacrysts in megacryst-bearing basalts with 87 Sr/ 86 Sr i equal to .7035 and .7038 supports a high-pressure mantle origin. The low (.7034) Sr ratio and lack of evidence for mixing with young rocks indicate that the quartz-bearing basalts were also derived from the mantle. Other basalts with 87 Sr/ 86 Sr i > 0.705 probably were derived from old, lithospheric mantle with a high Rb/Sr ratio and do not appear to be contaminated with old, upper-crustal material. The rhyolites have initial Sr isotopic ratios of 0.7093 and 0.7141. These ratios indicate that the rhyolites were not differentiated from the basalts. Partial melting of 1.3-b.y.-old lower-crustal material with Rb/Sr = 0.10 to 0.19 satisfactorily explains the isotopic ratios of the rhyolites. Granulite, which may constitute the lower crust in this part of Arizona, has Rb/Sr ratios similar to those required to produce the rhyolites. K substituted for Na during cooling and devitrification in some of the rhyolites. Partial melting of upper-mantle peridotite and old lower-crustal granulite from 19 to 7 m.y. ago in the Castaneda Hills area produced the bimodal volcanic suite. The nearly contemporaneous production of basaltic and rhyolitic magma from the Earth9s crust and mantle requires extremely heterogenous source regions. Asthenospheric upwelling associated with basin-range extensional tectonism probably produced the heating event that caused partial melting and basaltic magma generation at different levels in the mantle. Partial melting in the lower crust to produce rhyolitic magmas probably was caused by the intrusion of basalt magma. The basaltic and rhyolitic magmas formed in separate source regions, rose independently, and erupted at the same time and place.