Incompatible element

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

Hidden melting signatures recorded in the Troodos ophiolite plutonic suite: evidence for widespread generation of depleted melts and intra-crustal melt aggregation

Abstract: The main plutonic complex of the Troodos ophiolite, north of the Arakapas Fault Zone, has been re-examined both from field and geochemical perspectives. Ion microprobe analyses of clinopyroxene crystal cores show that the range of melt compositions added to the lower crust far exceeds that of published lavas in the main Troodos massif. This suggests that the lower crust acted as a filter into which a large range of melt compositions were added and out of which a homogenised (and generally fractionated) derivative was extracted. This crustal-level aggregation homogenised diverse melt fractions from a broad range of degrees of melting. Depleted melts with U-shaped rare earth element (REE) patterns were a significant component of the melts added to the crust, but because of their low incompatible element abundances, mixing with less depleted melts prior to eruption masked their signature in the lavas. The discovery that highly depleted melts constituted a significant component of the melts added to the Troodos crust, but not of the lavas, demonstrates that the spatial distribution of lava-types is not necessarily a good indicator of where different parental melt compositions are generated within the mantle. Compared with normal mid-ocean ridge basalts, the Troodos parental melts were (1) generally depleted in immobile incompatible trace elements, (2) less depleted in light REE (LREE) than would be expected for the concomitant depletion in middle and heavy REE, (3) enriched in Sr with respect to the LREE and (4) more oxidised. Modelling of these characteristics suggests a mantle source that had previously lost a significant melt fraction under relatively reducing conditions. This was followed by remelting under more oxidising conditions in an environment in which Sr and LREE were added to the source consistent with previous models of a supra-subduction zone setting.

Identification of a subduction zone component in the Higganum dike, Central Atlantic Magmatic Province: A LA‐ICPMS study of clinopyroxene with implications for flood basalt petrogenesis

Abstract: Determining the primary compositions of continental flood basalts has proven allusive because of the masking effects of crustal contamination. The Higganum dike (Connecticut, USA) that fed the Talcott basalt of the Central Atlantic Magmatic Province hosts strongly zoned clinopyroxenes. The cores are Mg- and Cr-rich, containing up to 1 wt % Cr2O3. The grains show optical and major, minor, and trace element continua between cores and their mantles and rims, indicating that the cores are early crystallizing phenocrysts, not xenocrysts. Laser ablation inductively coupled plasma mass spectrometry analyses of the cores were used to calculate liquid compositions of the most primitive component in the Higganum system and to make inferences about composition of continental flood basalt parental magmas. The Cr-rich cores yield calculated liquids with Mg numbers of 63. The high-Cr contents of these calculated liquids indicate the absence of significant clinoproxene or chromite fractionation. Olivine or orthopyroxene fractionation may have lowered the liquid Mg number but would not have significantly influenced the calculated incompatible trace element patterns. Extended rare earth element diagrams for the most Cr-rich liquids show enrichment in incompatible elements, similar to arc basalts. Prominent negative Nb anomalies are present as are distinct positive Pb anomalies indicative of the influence of subduction zone fluids in the mantle source. Calculated liquid Ba/La values range from 3 to 30, also suggestive of fluid input. These data indicate that the mantle source experienced subduction zone fluid metasomatism. Subsequent crustal contamination is evident in bulk-rock Nd and Sr isotopic compositions.