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On B/Be Systematics of the Mexican Volcanic Belt

Boron and beryllium concentrations were measured in a diverse suite of well-characterized rocks from the Mexican Volcanic Belt (MVB). Low B and high Be result in relatively low B/Be ratios in the MVB, compared to other arcs. Nevertheless, B systematics resemble those of other arcs and provide insights into mantle processes. In the MVB, B enrichment depends first on magma type, and second on edifice type and location. B/Be values are highest (5–15) in andesites and dacites erupted from calcalkaline strato-volcanoes located along the volcanic front, such as Volcan Colima and V. San Juan. Rocks from strato-volcanoes located behind the volcanic front generally have lower values (1–5). B/Be values are also elevated in differentiated members of rock suites that show evidence for significant crustal assimilation. In the westernmost MVB, west of the Michoacán-Guanajuato Volcanic Field (MGVF), cindercone ejecta, including basalts, lamprophyres, and basanites, contain low B/Be values (\u3c5). The lamprophyres and basanites have very low B/Be, despite high Ba/Ce and other common measures of subduction signature. In the MGVF, where cinder cones occur exclusively, B/Be values in primitive calc-alkaline basalts are distinctly higher than those from alkaline basalts (3–8 vs. 1–3), indicating that high B/Be is a mantle-derived feature and not an artifact of crustal assimilation. Comparison among various elemental ratios indicates that Cs and U show enrichment patterns similar to B; all are enriched in calc-alkaline rocks, but not in lamprophyres or basanites. In contrast, Ba, K, and Sr, are enriched in both calc-alkaline rocks and the lamprophyres and basanites. Multi-stage processes and differing melting mechanisms are inferred to explain the variable characteristics of MVB volcanic rocks. First, slab-derived fluids, rich in fluid-mobile elements including B and Ba, infiltrate the mantle wedge. These fluids cause fluid-fluxed melting that produces calc-alkaline magmas enriched in all fluid-mobile elements. These lavas erupt from large, volcanic-front strato-volcanoes. The slab-derived fluids also metasomatize portions of the mantle wedge, producing phlogopite and/or amphibole. These phases have high partition coefficients for Ba-Sr-K, but may have low partition coefficients for B-Cs-U. Accordingly, subduction-zone metasomatism produces a mantle wedge enriched in Ba-Sr-K, but not necessarily in B-Cs-U. Decompression melting of this type of metasomatized mantle will consume the hydrous phases and produce magmas such as lamprophyres or basanites with high Ba/Ce, Sr/La, and K/La, but low B/Be, Cs/La, and U/La. This interpretation implies two types of subduction-zone signatures: one involving enriched Ba-Sr-K, elements that have longer residence times in the mantle wedge, and another involving enriched B-Cs-U, which all partition so strongly into fluids or melts that they have short residence times in the mantle, and are only enriched in magmas generated by fluid-fluxed melting or that have assimilated crustal material. Assimilation of granites and crustal rocks can also enrich differentiated lavas in B

Acoustic detectability of squid egg beds

Abstract only. Journal home page: http://scitation.aip.org/jasa

Geochemistry of Pliocene-Pleistocene volcanic rocks from the Izu Arc

The igneous geochemistry of lavas and breccias from the basement of Sites 790 and 791, and pumice clasts from the Pliocene-Pleistocene sedimentary section of Sites 788, 790, 791, and 793 were studied. Arc volcanism became silicic about 1.5 m.y. before the inception of rifting in the Sumisu Rift at 2 Ma, but eruption of these silicic magmas reflects changes in stress regime, especially during the last 130,000 yr, rather than crustal anatexis. Arc magmas have had a larger proportion of slab-derived components since the inception of rifting than before, but are otherwise similar. Rift basalts and rhyolites are derived from a different source than are arc andesites to rhyolites. The rift source has less slab-derived material and is an E-MORB-like source, in contrast to an N-MORB-type source overprinted with more slab-derived material beneath the arc. Rift magma types, in the form of rare pumice and lithic clasts, preceded the rift, and the earliest magmas that erupted in the rift already differed from those of the arc. The earliest large rift eruption produced an exotic explosion breccia ("mousse") despite eruption at >1800 mbsl. Although this rock type is attributed primarily to high magmatic water content, the clasts are more MORB-like in trace element and isotopic composition than are modern Mariana Trough basalts. After rifting began, arc volcanism continued to be predominantly silicic, with individual pumice deposits containing clasts that vary in composition by about 5 wt% SiO2, or about as much as in historical eruptions of submarine Izu Arc volcanoes. The overall variations in magma composition with time during the inception of arc rifting are broadly similar in the Sumisu Rift and Lau Basin, though newly tapped OIB-type mantle seems to be present earlier during basin formation in the Sumisu than Lau case