Incompatible element

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

Origin of late Cenozoic basalts at the Cima volcanic field, Mojave Desert, California

Abstract: Major element, trace element, and isotopic data from late Cenozoic alkali basalts comprising the Cima volcanic field, southeastern California, are used to characterize basalt sources beneath this portion of the Mojave Desert over the past 8 m.y. The basalts are dominantly trachybasalts with trace element compositions similar to modern ocean-island basalts (OIB), regardless of the presence or absence of mantle-derived xenoliths. In detail, the basalts can be divided into three groups based on their ages and on their trace element and isotopic characteristics. Those basalts <1 m.y. in age, and the majority of those 3–5 m.y. old, belong to Group 1 defined by high eNd values (7.6 to 9.3), low 87Sr/86Sr (0.7028 to 0.7040), low whole rock δ18O (5.8‰ to to 6.4‰), and a restricted range of Pb isotopic compositions that generally plot on the mid-ocean ridge basalt (MORB) portion of the northern hemisphere reference line. The 3 to 5-m.y.-old basalts have rare earth element (REE) and other incompatible element abundances that increase regularly with decreasing %MgO and apparently have undergone more extensive differentiation than the younger, <1 m.y.-old basalts. The Group 2 and 3 basalts are minor constituents of the preserved volcanic material, but are consistently older (5–7.6 m.y.) and have lower eNd (5.1 to 7.5) values than the Group 1 basalts. These basalts have distinctive trace element signatures, with the Group 2 basalts having higher Ni, lower Hf, and slightly lower middle REE abundances than the Group 1 basalts, while the Group 3 basalts are characterized by higher and more fractionated REE abundances, as well as higher Ca, P, Ti, Th, Ta, and Sc contents. The isotopic and trace element characteristics of all the basalts are interpreted to have been largely inherited from their mantle source regions. The isotopic compositions of the Group 1 basalts overlap the values for Pacific MORB and for late Cenozoic basalts in the California Coast Ranges interpreted to have been derived from upwelling MORB asthenosphere. We suggest that the Group 1 basalts were all derived from light REE (LREE)-enriched portions of the Pacific MORB source, which rose into the slab “gap” that developed beneath the southwestern United States during the late Cenozoic transition from a convergent to a transform plate margin. The Group 2 and 3 basalts either represent smaller degrees of melting of the MORB source, or melting of mafic portions of the subcontinental lithospheric mantle currently present beneath the region. Ancient, LREE-enriched mantle lithosphere has not been a primary source of basaltic magmatism in this region at any time over the past 8 m.y.

Young basalts of the central Washington Cascades, flux melting of the mantle, and trace element signatures of primary arc magmas

Abstract: Basaltic lavas from the Three Sisters and Dalles Lakes were erupted from two isolated vents in the central Washington Cascades at 370–400 ka and 2.2 Ma, respectively, and have distinct trace element compositions that exemplify an important and poorly understood feature of arc basalts. The Three Sisters lavas are calc-alkaline basalts (CAB) with trace element compositions typical of most arc magmas: high ratios of large-ion-lithophile to high-field-strength elements (LILE/HFSE), and strong negative Nb and Ta anomalies. In contrast, the Dalles Lakes lavas have relatively low LILE/HFSE and no Nb or Ta anomalies, similar to ocean-island basalts (OIB). Nearly all Washington Cascade basalts with high to moderate incompatible element concentrations show this CAB or OIB-like compositional distinction, and there is pronounced divergence between the two magma types with a large compositional gap between them. We show that this trace element distinction can be easily explained by a simple model of flux-melting of the mantle wedge by a fluid-rich subduction component (SC), in which the degree of melting (F) of the peridotite source is correlated with the amount of SC added to it. Distinctive CAB and OIB-like trace element compositions are best explained by a flux-melting model in which dF/dSC decreases with increasing F, consistent with isenthalpic (heat-balanced) melting. In the context of this model, CAB trace element signatures simply reflect large degrees of melting of strongly SC-fluxed peridotite along relatively low dF/dSC melting trends, consistent with derivation from relatively cold mantle. Under other conditions (i.e., small degrees of melting or large degrees of melting of weakly SC-fluxed peridotite [high dF/dSC]), either OIB- or MORB (mid-ocean ridge basalt)-like compositions are produced. Trace element and isotopic compositions of Washington Cascade basalts are easily modeled by a correlation between SC and F across a range of mantle temperatures. This implies that the dominant cause of arc magmatism in this region is flux melting of the mantle wedge.

Geochemistry and tectonic environment of Ordovician meta-igneous rocks in the Rudawy Janowickie Complex, SW Poland

Abstract: A bimodal suite of Ordovician metavolcanic rocks in the Rudawy Janowickie Mountains and the Lasocki Ridge (Sudetic region, SW Poland) is dominated by voluminous epidote amphibolites and greenschists derived from basalt, dolerite and gabbro precursors. Less abundant felsic components comprise both mylonitized granitic sheets that intrude the mafic rocks, and minor interlaminated volcanic rocks ranging from andesite to rhyolite in composition. These rocks are collectively referred to as the Rudawy Janowickie Complex. In chemical terms the mafic volcanic rocks define three north-south-trending provinces (western, central and eastern) which are separated tectonically by broad major mylonite zones. The western province contains all the alkali basalts in the study area and minor tholeiites, all exhibiting within-plate features: specifically high Zr/Y (>4) and enriched chondrite-normalized incompatible element patterns. The central province contains two fractionated tholeiitic suites (Leszczyniec and Okraj groups) with low Zr/Y ratios (2.0-3.5), depleted to flat REE patterns, and incompatible element compositions ranging from normal MORB to slightly enriched MORB. The eastern province has some features in common with the central province, notably metabasalts with normalized flat REE patterns, although it is also characterized by metagabbro bodies with enriched light REE patterns ((La/Yb) N 4-7), but depleted high field strength element contents. In tectonic terms the Rudawy Janowickie complex is interpreted as a former intracratonic rift floored by oceanic crust and bounded to the west by an attenuated continental margin. Existing data do not allow a definitive conclusion about the exact nature of the eastern margin of the basin: it might, in part reflect the presence of a distant subduction zone.