Incompatible element

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

A neodymium and strontium isotopic study of the Mesozoic calc‐alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California

Abstract: Plutonic igneous rocks of the Sierra Nevada batholith exhibit a range of Nd isotopic composition described by eNd = +6.5 to −7.6. Similar rock types from the Peninsular Ranges have eNd = +8.0 to −6.4. In both batholiths, eNd correlates strongly with initial 87Sr/86Sr. Decreasing eNd values are accompanied by increasing 87Sr/86Sr and increasing δ18O; the correlation with δ18O being more pronounced for the Peninsular Ranges. The eNd values show regular geographic variations, as was found previously for initial 87Sr/86Sr. Three metasedimentary country rock samples from the Sierra Nevada region have low eNd values (−11 to −16) and Precambrian model Sm-Nd ages (1.5 to 1.9 AE). The country rock eNd values, and those of primitive oceanic island arcs (eNd = +8), bracket the data for the batholith rocks. The Nd, Sr, and O isotopic data can be explained if the batholiths are mixtures of island arc and metasedimentary components, the latter being of both Paleozoic and early Proterozoic age. This model appears to be consistent with existing Pb isotopic data. Consideration of O-Sr isotopic relations and the variation of 147Sm/144Nd with eNd suggests that assimilation of crustal rocks by magmas rising from the mantle and undergoing fractional crystallization could have been the major process responsible for the mixing of crustal- and mantle-derived components. The isotopic data, when combined with assumptions about the structure of the crust beneath the batholiths, suggest that about 50% of the crustal material presently within the geographic boundaries of the batholiths and above the Moho represents juvenile crust derived from the mantle in the Mesozoic. The remaining material appears to be mostly derived from 1.8-AE crust, yielding an average crust formation age of nearly 1 AE for this section of the crust. This result, which may apply to large portions of the Cordillera, suggests that the average age of the North American continent may be greater than previously estimated. The concentration of Nd correlates well with eNd in the batholith rocks and supports the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements. A 1.5-AE Sm-Nd model age for sedimentary rocks of the Mesozoic(?) Calaveras Formation indicates that the Nd in this “oceanic” terrain is dominated by continental detritus and demonstrates the potential of Sm-Nd isotopic studies for aiding in construction of tectonic models.

Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for Mantle Melting and Melt-Mantle Interaction in a Supra-Subduction Zone Setting

Abstract: Ocean Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu–Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40–0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine–spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite–magnetite–quartz) – 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ – 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20–25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.