Phenocryst

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

Changes in magma composition at Arenal volcano, Costa Rica, 1968 1985: Real-time monitoring of open-system differentiation

Abstract: Arenal volcano in Costa Rica has been erupting nearly continuously, but at a diminishing rate, since 1968, producing approximately 0.35 km3 of lavas and tephras that have shown consistent variations in chemistry and mineralogy. From the beginning of the eruption in July 1968 to early 1970 (stage 1, vol.=0.12 km3) tephras and lavas became richer in Ca, Mg, Ni, Cr, Fe, Ti, V, and Sc and poorer in Al2O3 and SiO2. Concentrations of incompatible trace elements (including Sr) decreased by 5%–20%. Phenocryst contents increased 20–50 vol%. During stage 2 (1970–1973, vol. = 0.13 km3) concentrations of compatible trace elements rose, and concentrations of incompatible trace elements either remained constant or also rose. Al2O3 contents decreased by 1 wt%. Phenocryst content increased slightly, principally due to increased orthopyroxene. During stage 3 (mid-1974 to the present, vol.= 0.10 km3) concentrations of SiO2 increased by 1 wt%, compatible trace elements decreased slightly, and incompatible trace element concentrations increased by 5% to 10%. Although crystals increased in size during stage 3, their overall abundance stayed roughly constant. Our modeling suggests that early stage-1 magmas were produced by boundary layer fractionation under high-p H2O conditions of an unseen basaltic andesitic magma that intruded into the Arenal system after approximately 500 B.P. Changes in composition during stage 2 resulted from mixing of this more mafic original magma with new magma that had a similar SiO2 content, but higher compatible and incompatible element concentrations. The changes during stage 3 resulted from continued influx of the same magma plus crystal removal. We conclude that the eruption proceeded in the following way. Before 1968 zoned stage-1 magma resided in the deep crust below Arenal. A new magma intruded into this chamber in July 1968 causing ejection of the stage-1 magmas. The intruding magma mixed with mafic portions of the original chamber producing the mixed lavas of stage 2. Continued mixing plus crystal fractionation along the chamber and conduit walls produced stage-3 lavas. The time scales of crustal level magmatic processes at Arenal range 100–103 years, which are 3–6 orders of magnitude shorter than those of larger, more silicic systems.

On some fundamentals of igneous petrology

Abstract: The age-old process of crystal fractionation leading to the diversity of the igneous rocks and Earth itself is an exceedingly well-understood chemical process in magmatism and physical chemistry. But the broader physical aspects of this and related processes have proven elusive on many fronts, especially in its relation to the spatial variations in rock composition, texture, and macroscopic features like layering. Magmatic systems, be they volcanic, dikes, sills, or plutons, are generally analyzed with a problem at hand and an end result in mind. The processes invoked to solve these problems, which are most often purely chemical, are often unique to each problem with few if any general principles emerging that are central to understanding the wider perspective of magmatic processes and problems. An attempt is made at the outset to provide a list of inviolate Magmatic First Principles that are relevant to analyzing most magmatic problems. These involve: initial conditions; critical crystallinity; solidification fronts; transport and emplacement fluxes; phenocrysts, xenocrysts, primocrysts; crystal size; layering and crystal sorting; thermal convection; magmatic processes are physical. Along with these principles, two reference magmatic systems are suggested where the initial conditions and outcome are unequivocal: the Sudbury impact melt sheet and the Hawaiian lava lakes. Sudbury formed in ~5 min by superheated magma crystallized to a near uniform sequence, while the tiny lava lakes, formed of crystal-laden slurries, form a highly differentiated layered sequence. The major difference is in the initial conditions of formation, especially the nature of the input materials. The challenge is to construct and analyze magmatic systems (i.e., magma chambers, sills, dikes, and lavas) using these reference end members and the suggested principles. The Hawaiian 500,000 year volcanic record exhibits what can be expected as input materials, namely a highly varied output of magma of an overall composition reflecting the abundance of entrained olivine primocrysts. The provenance of these crystals is varied, and within any single sample, the population may be highly heterogeneous in composition from crystal to crystal, yet the overall pattern of chemical fractionation is exceedingly regular and well defined. If similar inputs go to form large intrusions, these systems will undoubtedly be dominated by crystal-rich slurries, which provide a vast set of physical processes promoting exotic layering and, at the same time, given the effects of annealing and continued crystal growth, a final chemical record adhering to all the time-honored effects of crystal fractionation. The long assumed initial condition of instantaneously emplaced crystal-free magmas cannot reasonably produce the observed rock records.

The giant Zaozigou Au-Sb deposit in West Qinling, China: magmatic- or metamorphic-hydrothermal origin?

Abstract: Understanding the relationship between mineral occurrences and host granitic rocks can be controversial. The Zaozigou Au-Sb deposit (118 t Au, 0.12 Mt Sb), hosted in metasedimentary rocks and dacitic to granodioritic sills and dikes, is one such example of a large gold deposit argued to have formed from either magmatic or metamorphic hydrothermal processes. Two populations of monazite are identified within a mineralized dacite located along a major shear zone. Magmatic monazite commonly occurs within magmatic biotite and quartz phenocrysts and is characterized by uniform and high Th concentrations. It has a crystallization age of 238.3 ± 2.6 Ma, consistent with the zircon U-Pb age of 238.0 ± 1.8 Ma from the same dacite. Hydrothermal monazite is associated with sulfides and sericite, and has a 207Pb-corrected 206Pb/238U age of 211.1 ± 3.0 Ma. The amount of Th in hydrothermal monazite is widely variable. The low Th content of some monazite grains reflects direct precipitation from a metamorphic hydrothermal fluid. Furthermore, the elevated Th content in other hydrothermal monazite grains is likely due to the release of Th (and U) into hydrothermal fluids by dissolution of pre-existing Th-rich minerals in the country rock during ore-related alteration events. The magmatism, which overlaps Middle-Late Triassic terrane subduction-accretion in the West Qinling orogen, thus pre-dates the ore-forming event by about 30 m.y. The δ34S values of pyrite, arsenopyrite, stibnite, marcasite, and chalcopyrite from disseminated- and vein-type ores range from − 12.0 to − 5.5‰. Such negative values are distinct from those measured for other deposits in the northwestern part of the orogen that are genetically related to Triassic magmatism, including the Xiekeng-Jiangligou-Shuangpengxi Cu-Au-Fe-Mo skarn, Laodou reduced intrusion-related Au, and Gangcha epithermal Au ores. The Zaozigou deposit is best classified as an epizonal orogenic Au-Sb deposit. Our results demonstrate the usefulness of high-precision in situ geochronology on monazite for deciphering age relationships in ore deposits that have spatial associations with granitic rocks, thus aiding in the testing of the veracity of ore formation models.