Incompatible element

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

Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies

Abstract: We have compiled extensive gravity and bathymetry data for the whole Mariana Trough, which were collected during several Japanese scientific cruises over the last few years. This study aims to clarify the lateral distribution of the local differences in geochemical signatures, which have been observed locally in the Mariana Trough. Shipboard free-air gravity anomaly data from eight Japan Agency for Marine-Earth Science and Technology (JAMSTEC) cruises were compiled with those crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA) were calculated by subtracting the predictable gravity signal due to the seawater/crust and crust/mantle density boundaries. The crustal thickness variation along the spreading axis was estimated from the MBA. Different features in crustal thickness, its variation, and segment length for each segment, allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the Mariana Trough. Segment in region A (to the north of 20°35′N) shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources. A variety of crustal thickness values and of along-axis crustal thickness variations in region B (between 15°38′N and 20°35′N) suggests two types of segments. One is similar to a slow spreading ridge segment that has a plume-like mantle upwelling under the spreading axis, and the other is a magma-starved segment. Region C (between 14°22′N and 15°38′N) is a less magmatic region (individual crustal thickness averages of 3.4–4.1 km). Region D (to the south of 14°22′N) has higher individual crustal thickness averages of 5.9–6.9 km, suggesting higher magmatic activity with a sheet-like mantle upwelling under the spreading axis. Different features in the MBA for off-axis areas suggest that these four regions have existed since the Mariana Trough started spreading. Moreover, comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust, which are locally distributed in both regions B and C, probably result from mantle source depleted of water and incompatible elements. This suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the Mariana Trough.

The pliocene volcanism of the voras Mts (Central Macedonia, Greece)

Abstract: Major element chemistry, K/Ar ages and trace element data are reported for volcanic rocks from the Voras volcanic complex of Central Macedonia (Greece). Petrological data show that the Voras volcanic rocks consist essentially of intermediate members of the high-K calc-alkaline and shoshonitic series, the most abundant rock types being high-K andesites and dacites, latites and trachytes. K/Ar ages determined on selected samples show values ranging between 5.0 and 1.8 m.y., indicating that the Voras volcanism was active from Pliocene up to the Lower Pleistocene. The high Th, Hf, Ba and LREE shown by some representative samples together with their strongly fractionated light and heavy REE patterns and the absence of significant negative Eu anomalies indicate a magma genesis by low degrees of partial melting of a source enriched in large ion lithophile elements, leaving a garnet-bearing and plagioclase-free residue. This source is believed to be represented by a mantle garnet peridotite enriched in incompatible elements.

Petrology and geochemistry of gabbroic and related rocks from site 894, Hess Deep

Abstract: The petrogenesis of gabbroic rocks and associated basaltic dikes recovered at Ocean Drilling Program Site 894 has been evaluated using petrography, mineral chemistry, and bulk-rock chemistry as well as Nd isotope analyses. The igneous lithologies recovered at Site 894 include gabbronorite, gabbro, olivine gabbro, olivine gabbronorite, and basalt. The gabbroic rocks and the crosscutting dikes show similar isotope and trace elements signatures, and they are therefore suggested to be cogenetic. The trace element data indicate that the rocks formed from a highly depleted parental magma. Isotopic compositions of Site 894 rocks are within the range of mid-ocean-ridge basalt, and similar to gabbroic rocks recovered from Site 895. The gabbroic rocks show no systematic variation in mineral compositions downhole, but exhibit large variations on a local scale. Plagioclase compositions, for example, vary by up to 25 mole percent An in a single thin section. The rare earth element contents vary by 1 order of magnitude, which is close to the total range seen in the crustal section of many ophiolite complexes. There is, however, no systematic, large-scale variation in bulk-rock compositions downhole. The downhole variations are characterized by abrupt (meter scale) variations in compatible and incompatible trace element contents. There appears to be a decoupling in the behavior of incompatible and compatible trace elements in the gabbroic rocks. Cr shows a well-defined trend when plotted against Ca# whereas TiO2 and Zr do not. The lack of any correlation between TiO2 and Zr with Ca numbers suggests that the abundance of oxides and incompatible element-bearing phases such as zircon and apatite (which is present in the rocks) is unrelated to the composition of the silicate matrix. The local enrichments of incompatible elements in rocks that are relatively rich in compatible elements indicate that the geochemistry of the Site 894 gabbros is not controlled by mineral crystallization and fractionation alone. We suggest that the Site 894 rocks formed along the roof of the magma reservoir where a sequence of mesocumulates and basaltic gabbros developed owing to limited buoyancy-driven migration of interstitial melts. Late magmatic liquids migrated locally into the almost-crystallized matrix. These late magmatic liquids enriched the rocks in incompatible elements but had little influence on the distribution of compatible trace elements and major elements in the rocks. The trace element geochemistry of the rocks may be understood in terms of fractional crystallization and various mixing processes.

Rare earth element and related chemistry of some drilled southern Indian Ocean basalts and volcanogenic sediments

Abstract: The rare earth elements (REE), selected incompatible elements, and overall compositions of some Deep-Sea Drilling Project (DSDP) leg 26 (southern Indian Ocean) basalts and volcanogenic sediments are reported. Variations in REE distributions are not always relatable to existing tectonic features that characterize the sampled sites. The olivine-rich basalt at site 250 is transitional between midocean ridge basalt (Morb) and alkali basalt and is tentatively considered to have been produced by hot spot activity. The age relationships of site 257 basalts are not clear, but the uppermost unit (flow?) has a distinct light REE and incompatible element enrichment and is apparently very much younger than the lower units, which show light REE depletion. Like the basalts, the volcanogenic sediments of Ninety East Ridge (sites 253 and 254) and Naturaliste Plateau (site 258) are enriched in light relative to heavy REE. Alteration seems to have little effect on the REE abundances except for the high La/Ce ratio. In the volcanogenic sediments it may result from an admixture of Ce-poor biogenic calcite. Site 256 and uppermost 257 basalts from the Wharton Basin may have been derived from the same single source as the rocks forming the Ninety East Ridge, Naturaliste Plateau, Broken Ridge, Kerguelen Plateau, and Amsterdam-Saint Paul islands.