Phenocryst

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

Temperatures and H 2 O contents of low-MgO high-alumina basalts

Abstract: Experimental evidence is used to estimate H2O contents in low-MgO high-alumina basalts (HABs) ( 8wt.% MgO) could have H2O contents reaching no more than 1–2wt.%. The more common low-MgO HABs could have existed as liquids within the crust with H2O contents of 4 wt.% or higher at temperatures<1100°C. Magmas with these high H2O contents will saturate with and exsolve aqueous fluid upon approaching the surface. They cannot erupt as liquids and must grow crystals at shallow depths, thus accounting for the abundant phenocrysts in low-MgO HABs and BAs.

Solidification fronts and magmatic evolution

Abstract: From G. F. Becker's and L. V. Pirsson's early enunciations linking the dynamics of magma chambers to the rock records of sills and plutons to this day, two features stand at the centre of nearly every magmatic process: solidification fronts and phenocrysts. The structure and behaviour of the envisioned solidification front, however, has been mostly that akin to non-silicate, non-multiply-saturated systems, which has led to confusion in appreciating its role in magmatic evolution. The common habit of intruding magmas to carry significant amounts of phenocrysts, which can lead to efficient fractionation, layering, and interstitial melt flow within extensive mush piles, when coupled with solidification fronts, allows a broad understanding of the processes leading to the rock records of sills and lava lakes. These same processes are fundamental to understanding all magmas. The spatial manifestation of the liquidus and solidus is the Solidification Front (SF); all magmas, stationary or in transit, are encased by SFs. In the ideal case of an initially crystal-free, cooling magma, crystallinity increases from nucleation on the leading liquidus edge to a holocrystalline rock at the trailing solidus. The package of SF isotherms advances inward, thickening with time and, depending on location — roof, floor, or walls — and the initial crystallinity of the magma, is instrumental in controlling magmatic evolution. Bimodal volcanism as well as much of the structure of the oceanic crust may arise from the behaviour of SFs. In mafic magmas, somewhere near a crystallinity (N) of 55% (vol), depending on the phase assemblage, the SF changes from a viscous fluid (suspension (0

Ultra-depleted primary melt included in an olivine from the Mid-Atlantic Ridge

Abstract: MODELS of magma genesis at mid-ocean ridges1, together with recent experimental data2 and observations of trace element abundances in clinopyroxenes from abyssal peridotites3, suggest that small-volume melt fractions can be efficiently extracted from the melting mantle. As shown in ref. 3, residues of this type of melting (fractional melting) display extremely low abundances of incompatible trace elements and extreme fractionation amongst them, especially at advanced stages of the process because of the compounded effects of differences in the partition coefficients. If this process operates beneath mid-ocean ridges, one would expect to sample melts that are correspondingly depleted and fractionated in trace elements. Indeed, the existence of very depleted melts in mid-ocean ridges was predicted previously4–6 in order to explain the presence of magnesian pyroxene and calcic plagioclase in mid-ocean-ridge basalts. We report here the discovery of melt that has major and trace element characteristics consistent with these predictions4–6, occurring as an inclusion in an olivine phenocryst in a typical mid-ocean-ridge basalt from the Mid-Atlantic Ridge. Although our preferred model for the origin of this 'ultra-depleted' melt is critical (continuous) melting, we cannot at this stage rule out other models. Our results underscore the importance of trapped melt inclusions as recorders of the processes involved in melting and melt extraction, and also as pointers to primary melt compositions.