Phenocryst

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

Potassic Mafic Lavas of the Bearpaw Mountains, Montana: Mineralogy, Chemistry, and Origin.

Abstract: The volcanic rocks of the Bearpaw Mountains are part of the Montana high-potassium province, emplaced through Archaean rocks of the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks, dominated by minettes and latites, have a volume of U 825 km3. The minettes range in composition from 20 to 6% MgO. The more magnesian varieties contain the phenocryst assemblage forsterite + Cr-spinel + diopside ± phlogopite. More evolved rocks are olivine-free, with an assemblage of either salite + phlogopite + pseudoleucite or salite + phlogopite + analcime. The analcime is thought to be secondary after leucite, produced by loss of potassium from the minettes. Mineral chemistry and textures, especially of clinopyroxenes, indicate that mixing between minette magmas of varying degrees of evolution was commonplace. Compositional variation was further extended by accumulation of olivine + spinel + clinopyroxene phenocrysts, and by the preservation of mantle xenocrysts in the minettes. The primary minette magmas are inferred to have had 12–14% MgO and to have been generated at 30 kb from an olivine + diopside + phlogopite-bearing source. The primary magmas evolved dominantly by fractionation of olivine + diopside. The minettes have high contents of large ion lithophile elements (LILE) and light rare earth elements (LREE), with K2O up to 6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0 ranges from 0.707 to 0.710 and £Nd varies from –10 to –16. These data, plus high LILE/HFSE (high field strength elements) values, are interpreted to show that the minettes contain at least three different mantle components. The lithosphere was initially depleted in Archaean times, but was metasomatically enriched in the Proterozoic and in the late Cretaceous and early Tertiary. The latites have many chemical features in common with the minettes, such as potassic character and high LILE/HFSE values. They formed by fractional crystallization of minette magma in combination with assimilation of crustal rocks; this process enriched the magmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemical data for phenocrysts and bulk rocks in minettes suggest mixing between minette and latite magmas.

Petrogenetic Linkages Among Martian Basalts. Implications Based on Trace Element Chemistry of Olivine

Abstract: The shergottites exhibit a range of major and trace element compositions, crystallization ages, and initial Sr, Nd, Hf, and Pb isotopic compositions. To constrain the physical mechanisms by which shergottites obtain their compositional characteristics, we examined the major and trace element record preserved in olivine in the more primitive shergottites. Based on such characteristics as the Mg#, V zoning, calculated DNi,Co, the olivine in Y-980459 are most likely phenocrysts. Many of these same characteristics indicate that the olivines in other shergottites are not in equilibrium with the adjacent melt. However, in most cases they are not xenocrystic, but additions of olivine from the same basaltic system. Elephant Moraine (EET) A79001 may be an exception with the olivine data suggesting that it is xenocrystic. In this case, the olivine crystallized from a reduced and LREEdepleted melt and was incorporated into an oxidized and enriched basalt. Vanadium and CaO in olivine appear to record the appearance of spinel and pyroxene on the liquidus of most of the shergottites. Most of the olivine shergottites represent basalts produced by melting of reduced (IW to IW + 1), depleted mantle sources. Olivine data indicate that many of the primary melts derived from this source had similar Ni, Co, and Mn. Shergottites such as Northwest Africa (NWA) 1110/1068 and perhaps Roberts Massif (RBT) 04261 that appear to be derived from more enriched sources have distinctly different olivine. In the case of NWA 1110/1068, the olivine data suggests that the enriched component was added to system prior to olivine crystallization.

Nd, Pb and Sr isotopic data from the Napak carbonatite-nephelinite centre, eastern Uganda: an example of open-system crystal fractionation

Abstract: Nd, Pb and Sr isotopic data from nephelinite lavas from the Tertiary nephelinite-carbonatite complex of Napak, eastern Uganda, show large isotopic variations that can only be attributed to open-system behaviour. Possible explanations of the data include mixing between nephelinitic melts derived from an isotopically heterogeneous mantle, or interaction between a HIMU melt and mafic granulites. In both models crystal fractionation, involving olivine and clinopyroxene, played an important role. Major element chemistry, textural evidence and isotopic data from clinopyroxene phenocrysts from the olivine-bearing nephelinites, suggest that the pyroxenes did not crystallize from their host liquids. The isotopic data from the clinopyroxene phenocrysts support an interpretation of crystal fractionation in an open magma system that was undergoing continuous isotopic change. This study emphasises the importance of using combined isotopic data from both whole rock and mineral phases to interpret the evolutionary history of a single eruptive centre.