Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

Heterogeneity of Rift Magmatism in the Equatorial Province of the Mid-Atlantic Ridge (15 o N to 3 o S)

Abstract: New data on the compositions of chill glasses, which were sampled in the central part of the Romanche Fracture Zone during the Russian-Italian cruise of the R/Y Gelendzhik in 1996, and the first analyses of the concentrations of 28 lithophile elements in glasses from the Equatorial Mid-Atlantic Ridge (EMAR) made it possible to identify distinctive features of magmatism in this zone, which was earlier recognized as a specific tectono-magmatic province of MAR. The ratios of lithophile elements and Sr, Nd, and Pb isotopes suggest that the enrichment character of tholeiites from two areas of the province (the 14°-15° N anomaly in the north and the 2° N-1° S anomaly in the south) is similar. The enriched melts are noted for high 206Pb/204Pb = 20.0, 207Pb/ 204Pb = 15.6, 208Pb/204Pb = 39.5, and 87Sr/86Sr = 0.7030 at a 143Nd/ 144Nd ratio as low as 0.5128. In this sense, the melts are similar to the alkaline magmas in islands of the Cameroon line. Conceivably, the development of the geochemical anomalies in the north and south of EMAR in the process of melting involved the subcontinental and suboceanic mantle, which had been metasomatized by the intrusion and capture of enriched melts early during the opening of the Atlantic Ocean. Similarities and differences were determined for the magma generation conditions in the southern and northern provinces. It was established that the conditions of magma derivation in the Mid-Atlantic Ridge (MAR) near the Romanche Fracture Zone had been unstable over a long period of time (at least 40 m.y.), which resulted in the origin of lavas of different compositions and geneses within a single area. The lavas ranged from normal tholeiites (of the TOR-2 type) to tholeiites whose parental melts were enriched in Na and Si (Na type), alkaline melts, and lavas of mixed types variously enriched in incompatible elements and radiogenic isotopes. Copyright © 2002 by MAIK " Nauka/Interperiodica" (Russia).

Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia

Abstract: Dunite–harzburgite–olivine-bearing orthopyroxenite successions in the subvolcanic Chapesvara-I and Chapesvara-II intrusions in the Serpentinite Belt, western Kola Peninsula, are notably magnesian. The mean Mg# value (whole-rock) is 86.6, and the olivine is Fo84−89. The upper contact facies (UCF) displays a lower Mg# (81.6). It consists of grains of Fo92 and abundant chromian spinel, implying rapid crystallization of an almost unfractionated melt. On average, the whole-rock Al2O3/TiO2 value is 22.45, close to 22.9 (UCF) and to the primitive mantle, ~22. The rise of primitive ultramafic magma presumably occurred in a special tectonic setting at the boundary of the Paleoproterozoic Lapland Granulite Terrane and the Belomorian Composite Terrane of Archean age. The Chapesvara suite resembles examples of the Al-undepleted komatiites in the Barberton Belt, South Africa, with magmas of up to 30–35% MgO. The UCF rock yields an anomalously low molar MgO/SiO2 value, close to that of dunitic rocks located at the center of the Chapesvara-II body. This rock is the most primitive, as indicated by the maximum Fo content of olivine, the lowest value of (Gd/Yb)N, 0.52, and the lowest abundances of middle to heavy rare-earth elements (REE) in the chondrite-normalized spectrum. The crystallization of the Chapesvara-II sill-like intrusion likely proceeded in two stages, which are evident from the olivine compositions varying from the maximum Fo92 (UCF) to Fo≤89.5 (the central dunite zone). At Stage 1, the UCF rock (Fo92) crystallized first, close to the upper contact. The area of crystallization then shifted to a central portion of the Chapesvara-II body, in which the dunitic zone (Fo89.5) formed in situ (Stage 2). The compositional variations in chromian spinel are consistent with this suggestion. Two crystallization trends were recognized. The type-1 trend displays a relative maximum or minimum close to the center, and then diverges into two linear subtrends directed upward and downward. This pattern is manifested in the variations of Mg# in olivine and chromian spinel, the whole-rock contents of Al and Ca, and in levels of incompatible elements: Ti, V, Zr, Y, and Hf. The type-2 trend decreases or increases uniformly from top to bottom. Variations in amount of Ni in olivine, the Fe3+# index in chromian spinel, and in values of Mg# (rocks), follow a type-2 trend. Variations in total amounts of REE, Nb, and Th, which gradually increase downward, are also related to a type-2 trend. Thus, a contrasting development and possible interference of the two types of evolutionary trends were observed in the crystallization history of the Chapesvara-II sill-like body. A double-front crystallization, hitherto unreported, involved two fronts moving upward and downward, respectively. The upward subtrend appeared to be of subordinate importance, whereas the extent of fractional crystallization of the downward front was much greater. Crystallization proceeded from the top to the bottom, presumably because of the preferential loss of heat at the roof. Variations in the Fe3+# index indicate that the level of fO2 also increased downward with progressive crystallization. Convection cells were presumably the key mechanism of accumulation of the crystallizing olivine grains to form the central dunite zone close to the center of the sill-like intrusion. The observed characteristics of the Chapesvara complex indicate the existence of a primitive-mantle source and imply a highly magnesian composition of intruding magma not only for Chapesvara, but also for the Pados-Tundra layered complex and associated suites of the Serpentinite Belt in the Kola Peninsula.

Silicate-carbonate liquid immiscibility and crystallization of carbonate and K-rich basaltic magma: insights from melt and fluid inclusions

Abstract: This paper reports an investigation of the crystallization products of K-rich silicate and carbonate melts trapped as melt inclusions in clinopyroxene phenocrysts from the Dunkeldyk alkaline igneous complex in the Tajik Republic. Heating experiments on the melt inclusions suggest that the carbonate melt was formed by liquid immiscibility at 1180°C and ~0.5 GPa. The carbonate-rich inclusions are dominated by Sr-bearing calcite, and rich in incompatible elements. Most of the silicate minerals are SiO 2 -poor and rich in K, Ba and Ti. Leucite, kalsilite and aegirine are the earliest magmatic minerals. High Ba and Ti contents in the melt resulted in the crystallization of Ba-rich K-feldspar, titanite, perovskite and Ti-bearing garnet, and the rare Ba-Ti silicates fresnoite and delindeite. The last minerals to crystallize from volatile-rich melts and fluids were aegirine, gotzenite, K-Ba- and Ca-Sr-bearing zeolites, fluorite and strontium-rich baryte. Interaction of the early minerals with residual melts and fluids produced Ba-rich phlogopite and Sr-rich apatite.