Incompatible element

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

MIneralogic variation in a layered ultramafic cumulate sequence at the North Arm Mountain Massif, Bay of Islands ophiolite, Newfoundland

Abstract: In order to investigate magmatic processes operative within magma chambers underlying oceanic spreading centers, a suite of 59 samples was collected over an 8-m stratigraphic interval at the base of the cumulate ultramafic unit exposed on the North Arm Mountain Massif in the Bay of Islands ophiolite complex, Newfoundland. Interlayered dunites and wehrlites are present in the 8-m section. Low abundances of incompatible elements (Ti, Y, Zr) and other evidence suggest that these rocks originally formed as adcumulates. Olivine compositions are in the range Fo84.5–89.9 and clinopyroxene Mg # (= (Mg/Mg+Fe2+) × 100) range from 89.7 to 93.3. Spinel compositions are highly variable, and Mg # and Al2O3 contents of spinels have been modified by subsolidus reequlibration with olivine and clinopyroxene. Cryptic variation patterns defined by the Mg # of olivine and clinopyroxene are characterized by successive cryptic regressions that have a repeat interval averaging ∼1 m. The cryptic regressions are believed to record influxes of fresh, relatively unfractionated magma into a magma chamber or conduit. The gradual decrease in the Mg # of olivine and clinopyroxene and the decrease in olivine Ni contents between successive cryptic regressions indicate that ∼10% crystallization of each batch of magma occurred before a new batch of magma entered the crystallization site of these cumulate rocks. In some cases, the cryptic regressions in mineral chemistry correlate with abrupt lithologic changes (from wehrlite to dunite), suggesting rapid displacement of the fractionated magma by more primitive incoming magma or rapid mixing of fresh incoming magma and resident magma. In other cases, inverse fractionation trends and fine scale interlayering of the cumulate rocks appear to represent zones of less rapid mixing between fractionated and fresh magmas. Comparison of cryptic variation patterns at the base and the top of the cumulate ultramafic section on North Arm Mountain indicates that in some cases the liquids that crystallized to form the upper part of the unit were more primitive (higher MgO/FeO) than the liquids that crystallized to form the base of the unit. This feature is an indication of the complexity of processes that may operate in magma chambers underlying present-day oceanic spreading centers.

Lead contents of S-type granites and their petrogenetic significance

Abstract: An evaluation of Pb and Ba contents in S-type granites can provide important information on the processes of crustal partial melting. Primary low-T S-type granites, which form mainly by fluid-absent muscovite melting, may acquire a significant enrichment in Pb when compared to higher-T S-type granites for a given Ba content. We consider the following factors are responsible for this enrichment: Muscovite is a major carrier of Pb in amphibolite facies metapelites, and thus large quantities of Pb can be liberated upon its breakdown. The typical restite assemblage of Qz + Bt + Sil ± Pl ± Grt ± Kfsp that forms during low-T, fluid-absent muscovite melting can take up only minor amounts of this Pb. This is because the crystal/melt Pb distribution coefficients for these restite minerals are low to very low. Only K-feldspar is moderately compatible for Pb, with a crystal/melt distribution coefficient of ~3, but its modal content in restites is usually low. At the same time, the restite assemblage will retain much Ba owing to the very high Ba uptake in both biotite and K-feldspar, which is an order of magnitude higher than for Pb. Thus, during a low-T anatectic event involving a low degree of crustal melting, Pb (as an incompatible element) can become strongly enriched in the partial melt relative to Ba and also relative to source rock values. In the case of higher-T anatexis and larger partial melt amounts, the Pb becomes less enriched and the Ba less depleted or even enriched relative to source rock values. During fractional crystallization of a S-type granite magma, Ba behaves strongly compatibly and Pb weakly compatibly. The concentrations of both elements decrease along the liquid line of decent. Owing to this sympathetic fractionation behavior, the primary, source-related Pb–Ba fingerprint (with weak or strong Pb enrichment) remains in evolved S-type granites. This facilitates a distinction between primary low-T S-type granites, which are related to muscovite melting, and secondary low-T S-type granites that evolve through fractional crystallization from a higher-T parental magma. We show in this paper that a simple logarithmic Pb versus Ba diagram can be a valuable aid for interpreting the petrogenesis of S-type granite suites.