Incompatible element

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

Igneous geochemistry of mafic rocks in the Betts Cove ophiolite, Newfoundland

Abstract: The Betts Cove ophiolite, Newfoundland, consists of cumulate ultramafics, gabbro/clinopyroxenites, sheeted dikes and pillow lavas. The pillow lavas are divisible into three compositional groups: lower lavas ( 0.7 wt.% TiO2). The lower and intermediate lavas are very depleted in Ti, Zr, Y, P, and REE and have high Al2O3/TiO2 ratios relative to ‘normal’ oceanic tholeiite. The extreme depletion of these lavas and their dike equivalents (diabase and picrites) suggests they were derived by melting a severely depleted lherzolite. Conversely, the upper lavas, a volumetrically small part of the ophiolite, are compositionally similar to fractionated oceanic tholeiite and thus, their source material may be like that postulated for modern ocean floor basalts. Whereas the majority of basalts in the Betts Cove ophiolite are depleted in ‘incompatible’ elements, most dikes and lavas from the Blow-MeDown ophiolite, western Newfoundland, are not and have incompatible element concentrations similar to modern oceanic tholeiite. The chemical differences between the two ophiolite massifs are related to melting of ultramafic source materials which are in different states of depletion brought about by previous melting episodes.

Chemical constraints on lithosphere composition and evolution beneath the Colorado Plateau

Abstract: We have measured Sr and Nd isotopic composition and rare earth element (REE) abundances in a variety of inclusions and host minettes from the Navajo Volcanic Field of the Colorado Plateau in order to develop geochemical constraints on the composition and evolution of the mantle beneath the relatively undisturbed Proterozoic crust of the Colorado Plateau. Spinel and garnet peridotites and eclogites derived from less than 80 km in depth have geochemical signatures (e.g., light REE (LREE) depletion) similar to oceanic lithosphere; we interpret these rocks to be fragments of lithosphere, possibly part of an island arc which was accreted to the continent in the Proterozoic. Isotopic evidence suggests that some of the peridotites were hydrated by an incompatible element-poor, H2O-rich fluid, whereas the eclogites were metasomatized by a Na2O-rich phase with high 87Sr/86Sr. Some of the peridotites have highly radiogenic Nd isotopic ratios which reflect long-term isolation from mantle convection and show that they probably are not samples of recently subducted lithosphere. Garnet peridotites derived from at or near the base of the lithosphere are isotopically distinct from the shallow xenoliths and similar to ocean island basalts. Many of these same peridotites are enriched in the LREE. An inverse correlation between temperature of equilibration and 87Sr/86Sr in these garnet peridotites may reflect infiltration of relatively low 87Sr/86Sr melts into refractory, relatively high 87Sr/86Sr wall rock (lithosphere?). These high-temperature, low 87Sr/86Sr melts were possibly parental to the megacrystalline inclusions associated with the garnet peridotites. Overall, our data for the Colorado Plateau suggest that highly depleted, shallow lithosphere is underlain by material enriched in incompatible elements. This latter material was located near the inferred Oligocene base of the lithosphere and its composition may reflect mixing between asthenosphere and lithosphere. Melting of this material as a consequence of warming of the Colorado Plateau lithosphere during the late Tertiary may explain the origin of the Hopi Buttes alkaline magmas. The host minettes are isotopically similar to the bulk earth and their major element chemical variation can be explained by fractional crystallization. However, isotopic heterogeneity in the same eruptive center and decreasing Nb and Ta abundances with magmatic evolution indicate a complex petrogenesis involving exotic phases. The variation of isotopic composition with trace element ratios is inconsistent with bulk assimilation of crust, and we suggest that it reflects melting of a veined source containing phlogopite, apatite, and possibly a Nb-Ta-rich phase in addition to normal peridotite minerals.

Along-ridge petrological segmentation of the mantle in the Oman ophiolite

Abstract: Oman ophiolite mantle has been sampled over a distance of about 400 km, all along the paleo-ridge axis Primary phases have been analyzed in 174 peridotites (mainly harzburgites) and major and trace element contents measured in 90 and 156 samples, respectively Most samples display depleted characteristics with very low incompatible element bulk rocks and very low HREE contents On the basis of the spinel Cr# and in agreement with Yb concentrations in the bulk rocks, an average of 165% (Fmax) of melt extraction is estimated These rocks show light REE enrichments marked by high LREE/MREE ratios that well correlate with the extent of melting The light REE were possibly gained during the latest stage of melting in an open-system melting model, or through interaction with influxed fluid after melting Chemical data have been processed Fourier Transforms to study the along-ridge variations, which gives results similar to those obtained using the seven point running average (Le Mee et al, 2004) When plotted along ridge, spinel Cr# display variations with two types of wavelengths, defining four 50–100 km long segments (70 km in average) and numerous 10–20 km shorter ones making undulations within the longer ones All segments have a center marked by high values of spinel Cr# (≈ Fmax) and edges with the lowest values The large, 50–100 km segments (70 km in average) may correspond to large asthenospheric mantle upwellings between major deep mantle discontinuities, while the smaller ones possibly relate more superficial mantle instabilities similar to the structural diapirs of Nicolas et al (1988a) We consider that the variation in degree of melting in the short-scale instabilities relates fluid/melt flux melting variations By comparison with mid-oceanic ridge models, the long Oman segments can correspond to second-order segments and the smallest to third- to fourth-order ones Our data on the geometry of the melting zones will constrain models of the dynamics of the mantle beneath ridges They provide a new perspective for further characterization of the segments in the Oman ophiolite