Stuart Black

U-series isotopes and destructive plate margin magma genesis in the Lesser Antilles.

High precision TIMS230Th232Th isotope ratios are presented from a suite of lavas which span the length of the Lesser Antilles island arc. Compositions vary from low K, tholeiitic lavas with low87Sr86Sr in the north, to high K, calc-alkaline to alkalic lavas with higher87Sr86Sr in the south. The northern lavas are characterised by(230Th238U) 1. A three-component model is required for magma genesis in the Lesser Antilles. Sediments from the subducting slab melt and variably enrich the mantle wedge, while the subducting oceanic crust dehydrates, releasing fluids enriched in U, Ba, Sr and K, but with negligible Th and low87Sr86Sr, which promote partial melting in the wedge.Ta/Zr increases with87Sr86Sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment. Mass balance calculations are consistent with a relatively constant fluid flux (except for U) and little variation in the degree of partial melting (10–15%) along the arc, and they suggest that the sediment contribution to the arc lavas increases from 2% in the north to 15% in the south. In the northern parts of the arc, the addition of U by these fluids overprints any increases inTh/U that might have been caused by partial melting, and results in lavas with(230Th238U) 1 m yr−1, which imply channelled flow through the mantle wedge.

The role of magma mixing in triggering the current eruption at the Soufriere Hills Volcano, Montserrat, West Indies

The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non-uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.

Liquid immiscibility between trachyte and carbonate in ash flow tuffs from Kenya

Three thin, syn-caldera ash flow tuffs of the Suswa volcano, Kenya, contain pumiceous clasts and globules of trachytic glass, and clasts rich in carbonate globules, in a carbonate ash matrix. Petrographic and textural evidence indicates that the carbonate was magmatic. The trachyte is metaluminous to mildly peralkaline and varies from nepheline- to quartz-normative. The carbonate is calcium-rich, with high REE and F contents. The silicate and carbonate fractions have similar 143Nd/144Nd values, suggesting a common parental magma. Chondrite-normalized REE patterns are consistent with a carbonate liquid being exsolved from a silicate liquid after alkali feldspar fractionation. Sr isotopic and REE data show that the carbonate matrix of even the freshest tuffs interacted to some degree with hydrothermal and/or meteoric water. A liquid immiscibility relationship between the trachyte and carbonate is indicated by the presence of sharp, curved menisci between them, the presence of carbonate globules in silicate glass and of fiamme rich in carbonate globules separated by silicate glass, and by the fact that similar phenocryst phases occur in both melts. It is inferred that the carbonate liquid separated from a carbonated trachyte magma prior to, or during, caldera collapse. Viscosity differences segregated the magma into a fraction comprising silicate magma with scattered carbonate globules, and a fraction comprising carbonate globules in a silicate magmatic host.Explosive disruption of the magma generated silicate-and carbonate-rich clasts in a carbonate matrix. The silicate liquid was disaggregated by explosive disruption and texturally appears to have been budding-off into the carbonate matrix. After emplacement, the basal parts of the flows welded slightly and flattened. The Suswa rocks represent a rare and clear example of a liquid immiscibility relationship between trachyte and carbonate melts.

Identification and dating of tephra layers from Quaternary sedimentary sequences of Inner Anatolia, Turkey

Abstract Results presented here are part of a research programme to investigate the environmental evolution in central Anatolia during the Pleistocene. In this study, tephra are used as chronostratigraphical markers between lacustrine sequences studied in cores or in sections surrounding eruptive centres of the Central Anatolian Province. At different locations in the Konya Plain and Cappadocia, tephra have been identified, characterized and linked to eruptions of the nearby volcanoes. Seven different tephra have been clearly identified in lacustrine sequences of the Konya Plain. 14 C dates on shells or organic matter, as well as 230 Th / 234 U ages on gypsum, give a chronological framework to the tephra layers. In addition, the study of the sediment fill of the Eski Acigol maar near Nevsehir has allowed the recognition of several eruptions during Late Glacial and Holocene in the vicinity of Acigol. The results show the first tephra to be identified in lacustrine sequences of Anatolia; they also show the Cappadocian volcanism to be very young (≤35 ka). This young volcanism is often of a basaltic type and does not include thick differenciated pyroclastic deposits.

U-series disequilibria in young (A.D. 1944) Vesuvius rocks : Preliminary implications for magma residence times and volatile addition

Abstract The results of a preliminary U-series study of the timescale of magmatic processes at Vesuvius are presented. Phonotephrites of the 1944 eruption of Vesuvius show 0–15% 230 Th – 238 U and 350–1150% 226 Ra – 230 Th disequilibria. Apparent U–Th internal isochrons for a lava and a cumulate nodule suggest crystal residence times of 12 and 39 ka, respectively. A tephra sample shows isotopic heterogeneity, possibly related to mixing of younger crystal-laden melt and older crystals giving apparent U–Th ages of 0.4 and 18 ka, respectively. Mineral 226 Ra – 230 Th disequilibria on Ba-normalised internal isochron diagrams suggest Ra–Th ages of 1730–3300 years for the same rocks and phenocrysts. Minor 226 Ra / 230 Th heterogeneity between minerals and groundmass (or whole rock) is evidence of open-system Ra–Th behaviour. This heterogeneity suggests that there have been recent, post-crystallisation changes in melt composition that affected 226 Ra more than 230 Th . Continued crystallisation in a Ra-enriched magma has subsequently resulted in Ra–Th disequilibria probably as a result of addition via a fluid-rich phase. Magma differentiation, residence time, transport, and pervasive gas addition at Vesuvius apparently occur over geologically short periods.