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11.
Subduction factory processes beneath the Guguan cross-chain, Mariana Arc: no role for sediments, are serpentinites important? 总被引:1,自引:0,他引:1
Robert J. Stern Ed Kohut Sherman H. Bloomer Matthew Leybourne Matthew Fouch Jeff Vervoort 《Contributions to Mineralogy and Petrology》2006,151(2):202-221
We need to understand chemical recycling at convergent margins and how chemical interactions between subducted slab and the
overlying mantle wedge affect mantle evolution and magmagenesis. This requires distinguishing contributions from recycled
individual subducted components as well as those contributed by the mantle. We do this by examining magmatic products generated
at different depths above a subduction zone, in an intra-oceanic arc setting. The Guguan cross-chain in the intra-oceanic
Mariana arc overlies subducted Jurassic Pacific plate lithosphere at depths of ~125--230 km and erupts mostly basalt. Basalts
from rear-arc volcanoes are more primitive than those from the magmatic front, in spite of being derived by lower degrees
of melting of less-depleted mantle. Rear-arc magmas also show higher temperatures and pressures of equilibration. Coexisting
mineral compositions become more MORB- or OIB-like with increasing height above the subduction zone. Trace element and isotopic
variations indicate that the subduction component in cross-chain lavas diminishes with increasing depth to the subduction
zone, except for water contents. There is little support for the idea that melting beneath the Mariana Trough back-arc basin
depleted the source region of arc magmas, but melting to form rear-arc volcanoes may have depleted the source of magmatic
front volcanoes. Enrichments in rear-arc lavas were not caused by sediment melting; the data instead favor an OIB-type mantle
that has been modestly affected by subduction zone fluids. Our most important conclusion is that sediment fluids or melts
are not responsible for the K--h relationship and other cross-chain chemical and isotopic variations. We speculate that an
increasing role for supercritical fluids released from serpentinites interacting with modestly enriched mantle might be responsible
for cross-chain geochemical and isotopic variations.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
12.
White steenbras Lithognathus lithognathus (Teleostei: Sparidae) is an overexploited marine fish species endemic to South Africa. Overexploitation in recreational, subsistence and commercial fisheries has resulted in stock collapse and the need for improved management of the species. Adults are thought to undertake large-scale annual spawning migrations, yet studies of their movement indicate low levels of connectivity among coastal regions. To address this, mitochondrial DNA sequencing and genotyping of microsatellite loci in the nuclear genome were conducted to determine the genetic stock structure and level of gene flow in this species. Genetic diversity was high throughout the species’ core distribution, with no evidence of isolation by way of distance or localised spawning. Low, non-significant pairwise fixation indices (FST, RST and Jost’s Dest) indicated low genetic differentiation and high levels of gene flow. The observed results, and agreement between mitochondrial and microsatellite DNA, confirm that white steenbras exists as a single genetic stock with high levels of gene flow throughout its distribution. 相似文献
13.
Robert J. Stern Yoshihiko Tamura Robert W. Embley Osamu Ishizuka Susan G. Merle Neil K. Basu Hiroshi Kawabata Sherman H. Bloomer 《Island Arc》2008,17(1):70-89
Abstract West Rota Volcano (WRV) is a recently discovered extinct submarine volcano in the southern Mariana Arc. It is large (25 km diameter base), shallow (up to 300 m below sealevel), and contains a large caldera (6 × 10 km, with up to 1 km relief). The WRV lies near the northern termination of a major NNE‐trending normal fault. This and a second, parallel fault just west of the volcano separate uplifted, thick frontal arc crust to the east from subsiding, thin back‐arc basin crust to the west. The WRV is distinct from other Mariana Arc volcanoes: (i) it consists of a lower, predominantly andesite section overlain by a bimodal rhyolite‐basalt layered sequence; (ii) andesitic rocks are locally intensely altered and mineralized; (iii) it has a large caldera; and (iv) WRV is built on a major fault. Submarine felsic calderas are common in the Izu and Kermadec Arcs but are otherwise unknown from the Marianas and other primitive, intraoceanic arcs. 40Ar–39Ar dating indicates that andesitic volcanism comprising the lower volcanic section occurred 0.33–0.55 my ago, whereas eruption of the upper rhyolites and basalts occurred 37–51 thousand years ago. Four sequences of rhyolite pyroclastics each are 20–75 m thick, unwelded and show reverse grading, indicating submarine eruption. The youngest unit consists of 1–2 m diameter spheroids of rhyolite pumice, interpreted as magmatic balloons, formed by relatively quiet effusion and inflation of rhyolite into the overlying seawater. Geochemical studies indicate that felsic magmas were generated by anatexis of amphibolite‐facies meta‐andesites, perhaps in the middle arc crust. The presence of a large felsic volcano and caldera in the southern Marianas might indicate interaction of large normal faults with a mid‐crustal magma body at depth, providing a way for viscous felsic melts to reach the surface. 相似文献