Incompatible element

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

Convection and melting at mid‐ocean ridges

Abstract: We present a thermodynamically self-consistent model of plate and buoyancy driven flow and melt generation beneath mid-ocean ridges. Mantle flow is driven by a rigid lithosphere and by buoyancy forces resulting from melting, depletion, and melt extraction. Melt is generated using the solidus of Kinzler and Grove (1992a). Constant viscosity models without melt buoyancy forces show that no significant narrowing of the melting region or pressure gradient focusing of the melt is achieved. Temperature-dependent viscosity models show that pressure gradients in a high viscosity lithosphere are insufficient for focusing melt to the ridge axis. Constant viscosity models with melt buoyancy forces show that significant narrowing of the melting region is possible at slow spreading rates but not at fast spreading rates. Melt buoyancy forces cause the crustal thickness to decrease with spreading rate. Aggregate melts are similar to those required to form mid-ocean ridge basalt (MORB) but are too depleted in incompatible elements. They are also insensitive to spreading rate and melt region shape but are sensitive melting rate distribution. Model residuum trends are similar to those in abyssal peridotites and imply that abyssal peridotites result from partial melting and not refertilization. Massif peridotites appear to result from refertilization of harzburgite with MORB primary melt. Vertically integrated melts show very similar trends to data from the off-axis Lamont seamounts on the flanks of the East Pacific Rise. This implies the melting region at fast spreading ridges is at least 80–100 km wide.

Chemical compositions of martian basalts (shergottites): Some inferences on basalt formation, mantle metasomatism, and differentiation on Mars

Abstract: Bulk chemical compositions of the shergottite basalts provide important constraints on magma genesis and mantle processes in Mars. Abundances of many major and trace elements in the shergottites covary in 2 distinct groups: Group 1 (G1) includes mostly highly incompatible elements (e.g., La, Th), and Group 2 (G2) includes mostly moderately incompatible elements (e.g., Ti, Lu, Al, Hf). Covariations of G2 elements (not necessarily linear) are consistent with partitioning between basalt magma and orthopyroxene + olivine. This fractionation represents partial melting to form the shergottites and their crystallization; the restite minerals cannot include aluminous phase(s), phosphate, ilmenite, zircon, or sulfides. Overall, abundances of G1 elements are decoupled from those of G2. In graphing abundances of a G1 element against those of a G2 element, G1/G2 abundance ratios do not appear to be random but are restricted to 4 values. Shergottites with a given G1/G2 value need not have the same crystallization age and need not fall on a single fractionation trajectory involving compatible elements (e.g., Ti versus Fe*). These observations imply that the G1/G2 families were established before basalt formation and suggest metasomatic enrichment of their source region (major carrier of G2 elements) by a component rich in G1 elements. Group 1 elements were efficiently separated from G2 elements very early in Mars' history. Such efficient fractionation is not consistent with simple petrogenesis; it requires multiple fractionations, "complex" petrogenetic processes, or minerals with unusual geochemistry. The behavior of phosphorus in this early fractionation event is inexplicable by normal petrogenetic processes and minerals. Several explanations are possible, including significant compatibility of P in majoritic garnet and the presence of P-bearing iron metal (or a phosphide phase) in the residual solid assemblage (carrier of G2 elements). If the latter, Mars' mantle is more oxidized now than during the ancient fractionation event.

Melt/rock interaction in remains of refertilized Archean lithospheric mantle in Jiaodong Peninsula, North China Craton: Li isotopic evidence

Abstract: Li contents and its isotopes of minerals in mantle peridotite xenoliths from late Cretaceous mafic dikes, analyzed in situ by Cameca IMS-1280, reveal the existence of melt/rock interaction in remains of refertilized Archean lithospheric mantle in Qingdao, Jiaodong Peninsula, North China Craton. Two groups of peridotites exist, i.e., low-Mg# lherzolite and high-Mg# harzburgites. The low-Mg# lherzolite has a relatively homogeneous Li concentration (ol: 2.01–2.11 ppm; opx: 1.77–1.88 ppm; cpx: 1.75–1.93 ppm) and Li isotopic composition (δ7Li in ol: 4.2–7.6‰; in opx: 6.0–8.3‰; in cpx: 5.3–8.4‰). The similarity in δ7Li value to the fresh MORB provides further evidence for the argument that the low-Mg# lherzolite could be the fragment of the newly accreted lithospheric mantle. The high-Mg# harzburgites have heterogeneous Li abundances (ol: 0.83–2.09 ppm; opx: 0.92–1.94 ppm; cpx: 1.12–4.89 ppm) and Li isotopic compositions (δ7Li in ol: −0.5 to +11.5‰; in opx: −6.2 to +11.1‰; in cpx: −34.3 to +10.1‰), showing strong disequilibrium in Li partitioning and Li isotope fractionation between samples. The cores of most minerals in these high-Mg# harzburgites have relatively homogeneous δ7Li values, which are higher than those of fresh MORB, but similar to those previously reported for arc lavas. These harzburgites have enriched trace elemental and Sr–Nd isotopic compositions. These observations indicate that in the early Mesozoic the lithospheric mantle beneath the southeastern North China Craton was similar to that in arc settings, which is metasomatized by subducted crustal materials. Extremely low δ7Li preserved in cpxs requires diffusive fractionation of Li isotopes from later-stage melt into the minerals. Thus, the Li data provide further evidence that the Archean refractory lithospheric mantle represented by the high-Mg# harzburgites was refertilized through melt/rock interaction and transformed to the Mesozoic less refractory and incompatible element and Sr–Nd isotopes enriched lithospheric mantle.

Incompatible Element Partitioning between Clinopyroxene and Basalt Liquid Revealed by the Study of Melt Inclusions in Minerals from Troodos Lavas, Cyprus

Abstract: Melt inclusions in clinopyroxene from Troodos lavas were studied by the methods of high-temper- ature optical thermometry, secondary ion mass spectrometry, and microprobe analysis to determine the partition coefficients of Ba, Sr, Y, Ti, La, Ce, Nd, Sm, Dy, Er, and Yb between low-Al, high-Mg clinopyroxene and bon- inite-like melt for temperatures of 1100-1190 ° C and a hydrous fluid pressure of 1 kbar. Variations of the parti- tion coefficients were found as a function of the temperature and the Al 2 O 3 content in clinopyroxene. The results proved the study of melt inclusions in minerals to be an efficient tool for determining the coefficients of trace element distribution between the crystalline phase and the liquid.