Incompatible element

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

Origins of water content variations in the suboceanic upper mantle: Insight from Southwest Indian Ridge abyssal peridotites

Abstract: To investigate the origin of heterogeneous water distribution in the suboceanic lithospheric mantle, we performed a detailed petrological and geochemical analysis of abyssal peridotites collected from two localities (53°E and 63.5°E) on the Southwest Indian Ridge. These serpentinized peridotites display primary olivine-orthopyroxene-clinopyroxene-spinel assemblage and record equilibrium temperature of 1150–1200°C and around 1000°C for the 53°E and 63.5°E locations, respectively. The rocks were thus equilibrated in the spinel stability field prior to their exhumation to the seafloor. Our FTIR analyses show variable water contents in orthopyroxene ranging from 24 to 262 wt ppm H2O. Orthopyroxene in the 63.5°E peridotites is characterized by homogeneous and high water content (>200 ppm), whereas orthopyroxene in the 53°E peridotites displays a wider range of water contents (24–246 ppm). We first demonstrate that differences in equilibrium conditions (i.e., pressure and temperature) and mineral chemistry and serpentinization cannot explain the water content variations. Melting modeling show that a fractional melting in both garnet and spinel stability fields is needed to explain the MREE and HREE concentrations in clinopyroxene. Enrichment in LREE, high water contents, and high H2O/Ce, however, require a postmelting rehydration event such as metasomatism. Based on petrographic evidence and investigation of chemical heterogeneities at segment/dredge scale, we suggest that this event involves a metasomatic agent enriched in water and incompatible elements. We infer that the small-scale heterogeneities in water and trace elements may result either from successive infiltration of various amounts of melt/fluids as described in the formation of oceanic core complex or from spatial heterogeneities of melt infiltration as observed in peridotite massifs.

Geochemistry: earth holds its breath.

Abstract: Some inert-gas isotopes in Earth's atmosphere can only have come from deep inside the planet. We thought we knew how much gas Earth gives up, and how it does it — but a challenge has emerged to the prevailing model. The noble gases are widely used as tracers of mantle degassing on Earth and the other 'terrestrial' planets, but new work raises doubts over the practice. Specifically it is thought that the abundance of argon-40 in the atmosphere represents the total loss of gases from the interior over time via partial melting in the mantle followed by melt ascent to the surface and gas exsolution. New data on argon solubility in the olivine and orthopyroxene minerals of the upper mantle reveal two major difficulties with this scenario. First, the tacit assumption that all noble gases behave as incompatible elements during mantle melting is undermined by the finding that argon seems to be more compatible with the major phases of the terrestrial planets than was thought. And second, argon diffusion in these phases is slow. This challenges the common belief that the upper mantle is almost completely degassed of argon-40 and questions its value as a monitor of planetary degassing.

Geochemistry of basalt from the Ayu Trough, equatorial western Pacific

Abstract: The Ayu Trough is an approximately 600-km-long divergent margin between the Philippine Sea and Caroline plates located in the western Pacific near the equator. Characterized by rugged seafloor topography and deep axial depths that average greater than 4000 m, the Ayu Trough has been considered as a slow spreading center. Here we report geochemical analyses of Ayu Trough basalts (ATB) dredged from seven sites along the axis from 0°30′ to 4°30′N. The ATB vary geochemically from N-MORB (La/Sm N N > 1.4). N-MORB were recovered from five sites of axis wall, while E-MORB were sampled from one site, an axial volcanic ridge inside the median valley at 2°45′N. At one site, transitional (T)-MORB were recovered in addition to N-MORB. Variations in incompatible element distribution patterns and Sr–Nd–Pb isotopic ratios between the N-MORB and E-MORB samples are too large to be explained solely by melting of a uniformly depleted mantle source. It appears that at least two distinct mantle components, depleted and enriched, were involved in the formation of the ATB. N-MORB isotope ratios are similar to those associated with Indian-Ocean-type mantle, whereas those of E-MORB lie on a trend towards those of EM2. Based on incompatible element and isotope ratios, we suggest that the enriched component, source of E-MORB, beneath the Ayu Trough was generated in a subduction zone by metasomatism of low-degree melt and addition of a small amount of sediment. Although N-MORB generally conform to global correlations between the axial depth and basalt chemistry, E-MORB retrieved from the axial volcanic ridge include two distinct lava suites with (Sm/Yb) N ratios of ∼ 1.8 and 1.3, suggesting that they were generated at different mean depths of melting, and possibly different mean degrees of partial melting. If the low-degree melt samples are younger than the higher degree of melt samples, then this finding is consistent with inferences based on geophysical observations such as at least 50–70-m-thick sediment on the axis, that spreading in the Ayu Trough may have recently slowed dramatically or even ceased, leading to a diminished melt production and construction of a localized axial volcano.

Composition and geodynamic conditions of formation of the intrusive rocks of the Gromadnen-Vurguveem peridotite-gabbro massif, western Chukotka

Abstract: The Gromadnen-Vurguveem peridotite-gabbro massif is confined to one of the largest ophiolite complex of western Chukotka and composed mainly of intrusive rocks. This paper reports the first comprehensive compositional data for its plutonic rocks (petrochemistry, geochemistry, and compositions of minerals). In terms of petrography, two groups of rocks can be distinguished in the Gromadnen-Vurguveem peridotite-gabbro massif. The first group includes leucocratic gabbroids (mostly gabbronorites), composing most of the massif. The second group includes olivine-bearing cumulate rocks: olivine gabbros, troctolites, plagioclase-bearing dunites, and amphibolized wehrlites. The major element variations in these rocks suggest their affiliation to low-titanium, low-potassium, and high-alumina plutonic derivatives of island-arc magmatism. According to geochemical characteristics (distribution of REEs and indicator incompatible elements), the gabbroids of the first group are akin to both island-arc tholeiites and boninites. The olivine-bearing rocks of the second groups show boninitic affinity. Based on these observations, it was concluded that the intrusive complex of the Gromadnen-Vurguveem massif was formed during an early stage of the development of an ensimatic island arc.