Search

The formerly continuous Vitiaz Arc broke into its Vanuatu and Fijian portions during a reversal of subduction polarity in the Miocene. Basaltic volcanism in Fiji that accompanied the breakup ranged from shoshonitic to low-K and boninitic with increasing distance from the broken edge of the arc that, presumably, marks the broken edge of the slab. The Sr-Pb-Nd isotope ratios of the slab-derived component in the breakup basalts most closely match those of the isotopically most depleted part of the Samoan seamount chain on the Pacific Plate that was adjacent to the site of breakup at 4–8 Ma, and differ from those of subsequent basalts in spreading segments of the surrounding backarc North Fiji and Lau Basins. Subduction of the Samoan Chain along the Vitiaz Trench Lineament may have controlled the limit of polarity reversal and, hence, where the double saloon doors (Martin, 2013) opened. Prior to breakup, Fijian volcanics were more similar isotopically to the Louisville Seamount Chain. Key Points - The breakup between Fiji and Vanuatu may have been triggered by subduction of Samoan seamounts - Shoshonitic to low-K and boninitic volcanism accompanied breakup with increasing distance from the break - The mantle source of later basalts in surrounding backarc basins and islands came from beneath the Pacific Plate north of the breakup site

Viti Levu, Fiji, provides one of the best exposed Phanerozoic analogues for the formation of juvenile continental crust in an intra-oceanic setting. Tonalites and trondhjemites are present in several large (75-150 km(2)) adjacent, mid-Cenozoic plutons. We report major and trace element data including rare earth element (REE) and high-precision high field strength element (HFSE) compositions, new Hf-Nd-Sr-Pb isotope data, and zircon U/Pb-ages, O-Hf isotopes, and trace elements, from five different plutons. The Eocene Yavuna pluton and the Miocene Colo plutons are mainly composed of tonalites and trondhjemites and represent the exposed middle crust of the former Vitiaz island arc. The plutons can be divided into three suites. One suite is light REE (LREE) depleted with some trace element ratios lower than average normal mid-ocean ridge basalts (N-MORB). A second suite has flat REE patterns similar to local island arc basalts. Both suites occur near the coast of Viti Levu, include a wide compositional spectrum from gabbro to tonalite, and can be produced mostly by fractional crystallization of mafic precursor melts. The third suite is characterized by LREE enrichments with higher La-N/Yb-N (2.3-4.9), higher Zr/Y (4.3-7.1), and lower Nb/Ta (9.6-12.4). They occur closer to the center of the island and are bimodal trondhjemite-gabbro intrusions. These characteristics are consistent with formation mostly by partial melting of mafic crust. Trace element modeling shows that the trace element ratios of the third suite can be produced by 10-20 % melting of the mafic crust in the presence of residual amphibole, resulting in the retention of the medium REE (MREE) and diagnostic trace element ratios including low Nb/Ta and high Zr/Y. Geochemical similarities of the LREE enriched suite to typical low -pressure Archean tonalites-trondhjemites-granodiorites (TTGs) imply a common petrogenetic origin and similar mechanisms for the generation of juvenile Archean and modern differentiat

Magmas from SW Pacific back-arc basins have geochemical and isotopic signatures indicating variable mantle and subduction-derived components. Basalts from South Fiji Basin (SFB) are little influenced by subduction, but come from variably enriched mantle, resulting from mixing between enriched mantle, like FOZO, and depleted mantle, like DMM. The same components are present in the Havre Trough mantle, but Havre Trough basalts come from a mantle wedge to which a greater proportion of subduction-derived components are added. Their slab-derived components are isotopically similar to locally subducting sediment, with variable Sr and Pb from altered oceanic crust. Their compositional diversity correlates with morphology, previously described as contrasting Arc-type and Rift-type back-arc regimes. Geochemical modeling indicates that material is added as both supercritical fluids and slab melts below the back-arc, is locally distinct, and correlate with differences in the predicted slab-surface pressure and temperature conditions. Deeper slab surfaces correspond to higher-temperatures at a given distance from the volcanic front, but not necessarily with an increase in the amount of slab-derived material. Slab fluxes for rift-type basalts are consistent with predicted slab-surface temperatures at or below the water-saturated solidus. However, some are consistent with melting in equilibrium with residual rutile, zircon, and monazite, so melting may have occurred by fluid fluxing of the slab surface, requiring external fluids from within the slab. Arc-type basalts are explained by thermal anomalies in the mantle wedge, which may correspond to locally hotter slab-surface temperatures and more fractionated slab-derived component signatures in their source.

Little is known about the independent usefulness of MossTalk Words, a computerized training program for word retrieval, nor how training effects generalize to other untrained words. In a single-participant design, we investigated effects of independent training with MossTalk multi-mode matching exercises in four individuals with stroke-induced aphasia. Three of four participants improved in picture naming for trained words. One showed limited generalization to words in untrained semantic categories. Three improved in standardized testing, particularly for auditory comprehension, which is addressed in the matching exercises. Improvements are possible with independent training programs such as MossTalk Words.