Origin of basalts by hybridization in andesite-dominated arcs

Mafic magmas are common in subduction zone settings, yet their high density restricts their ascent to the surface. Once stalled in the crust, these magmas may differentiate, assimilate crust and other melts and mushes to produce hybridised intermediate magmas. The Soufriere Hills Volcano on Montserrat is a ‘type locality’ for these hybridisation processes and yet, just 3 km south of the crater, voluminous basalts have erupted from the South Soufriere Hills volcano within the same time period as the Soufriere Hills Volcano was erupting hybrid andesites (131 - 128 ka). Basaltic South Soufriere Hills magmas have 48 - 53 32 wt% SiO2 and 4 - 6 wt% MgO. They were hot (970 - 1160 °C), volatile-rich (melt inclusions contain up to 6.2 wt% H2O) and were stored at 8 – 13 km prior to eruption (based on olivine and pyroxene-hosted melt inclusion volatile geochemistry). Melt inclusions do not preserve basaltic liquids: they are andesitic to rhyolitic in composition, related to one another by a line of descent controlled by simple closed-system fractionation. Whole rock compositions, however, are best described by a hybridisation model involving “back”-mixing of andesitic to rhyolitic melts with mafic crystal phases such as magnetite, olivine, orthopyroxene and clinopyroxene. Phenocryst zoning illustrates repeated mixing events between evolved melts and mafic phenocrysts, which, when coupled with the heterogeneity of crystal compositions, strongly suggests that although the bulk composition is basalt (containing Fo80 olivine), they were assembled from disparate ingredients, likely derived from mafic crystal mushes and more evolved melt lenses of variable composition. The mixing events occur days to weeks prior to eruption. We propose that the South Soufriere Hills basaltic magmas, with their higher bulk density over andesites from neighbouring volcanoes, ultimately may have been eruptible owing to both the transtensional tectonics imposed by offshore grabens (related to the oblique subduction of the Lesser Antilles) and to surface unloading caused by large scale edifice collapse. Our observations support the idea that compositional changes in arcs might reflect not only changes in source compositions, but also effects caused by patterns in crustal strain and tectonics.MC and SFLW thank NERC for financial support via grant NE/K000403/1.This is the accepted manuscript. The final version is available from OUP at http://dx.doi.org/10.1093/petrology/egv00

The Role of Tephra in Enhancing Organic Carbon Preservation in Marine Sediments

Preservation of organic carbon (Corg) in marine sediments plays a major role in defining ocean-atmosphere CO2 levels, Earth climate, and the generation of hydrocarbons. Important controls over sedimentary Corg preservation include; biological productivity, Corg isolation from oxidants (mainly dissolved O2) in the overlying water column and sediments, and Corg – mineral association in sediments. Deposition of the products of explosive volcanism (tephra) in the oceans directly enhances Corg burial through all these mechanisms, and indirectly through enhanced formation of authigenic carbonate (Cauth) derived from sedimentary Corg. In the modern oceans, it is suggested that tephra deposition may account for 5–10% of the Corg burial flux and 10–40% of the Cauth burial flux. However, during certain periods in Earth's history, extensive explosive volcanism may have led to enhanced Cauth precipitation on a sufficiently large scale to influence the global ocean-atmosphere carbon cycle. Changes in tephra-related Corg preservation may also have played a role in increasing Corg preservation rates in local marine basins, at the oxic-anoxic boundary and enhanced the generation of hydrocarbon deposits in these settings

The origin of pelletal lapilli in explosive kimberlite eruptions

Kimberlites are volatile-rich magmas from mantle depths of ≥150 km and are the primary source of diamonds. Kimberlite volcanism involves the formation of diverging pipes or diatremes, which are the locus of high-intensity explosive eruptions. A conspicuous and previously enigmatic feature of diatreme fills are 'pelletal lapilli'—well-rounded clasts consisting of an inner 'seed' particle with a complex rim, thought to represent quenched juvenile melt. Here we show that these coincide with a transition from magmatic to pyroclastic behaviour, thus offering fundamental insights into eruption dynamics and constraints on vent conditions. We propose that pelletal lapilli are formed when fluid melts intrude into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which locally scavenged particles are simultaneously fluidised and coated by a spray of low-viscosity melt. A similar origin may apply to pelletal lapilli in other alkaline volcanic rocks, including carbonatites, kamafugites and melilitites

The role of tephra in enhancing organic carbon preservation in marine sediments

Preservation of organic carbon (Corg) in marine sediments plays a major role in defining ocean-atmosphere CO2 levels, Earth climate, and the generation of hydrocarbons. Important controls over sedimentary Corg preservation include; biological productivity, Corg isolation from oxidants (mainly dissolved O2) in the overlying water column and sediments, and Corg – mineral association in sediments. Deposition of the products of explosive volcanism (tephra) in the oceans directly enhances Corg burial through all these mechanisms, and indirectly through enhanced formation of authigenic carbonate (Cauth) derived from sedimentary Corg. In the modern oceans, it is suggested that tephra deposition may account for 5–10% of the Corg burial flux and 10–40% of the Cauth burial flux. However, during certain periods in Earth's history, extensive explosive volcanism may have led to enhanced Cauth precipitation on a sufficiently large scale to influence the global ocean-atmosphere carbon cycle. Changes in tephra-related Corg preservation may also have played a role in increasing Corg preservation rates in local marine basins, at the oxic-anoxic boundary and enhanced the generation of hydrocarbon deposits in these settings