ALBERT

Description: The 3 km long Bellevue Core was recovered from the Upper Zone (UZ) and Main Zone (MZ) of the Northern Limb of the Bushveld Complex—the largest known layered mafic intrusion. We present abundance data for 57 trace elements hosted in plagioclase, clinopyroxene, low-Ca pyroxene and olivine, obtained by laser ablation inductively coupled plasma mass spectrometry. These data permit the first comprehensive investigation of trace element distribution in the Bushveld Complex on a mineral-by-mineral basis, and show that the Cr content of pyroxene is the only proxy that records three cryptic magma injections, coincident with two pyroxenite horizons and the UZ–MZ boundary. On the basis of Cr abundance, we infer that an ‘anomalous’ troctolitic horizon at the base of the Bellevue Core was related to injection of relatively primitive magma, analogous to the Lower or Critical Zone magmas in the Eastern and Western Limbs of the Bushveld Complex; this horizon defines the greatest inconsistency of magmatic stratigraphy between the Northern Limb and the Eastern and Western Limbs of the complex, and is prospective for platinum-group element exploration. However, the most significant finding of this study is that plagioclase, pyroxene and olivine have undergone extensive diffusive modification. The major and trace element geochemistry of these minerals records sub-solidus equilibration during the prolonged cooling of the Bushveld Complex, controlled by diffusion rates and local partitioning between adjacent minerals. By analogy, texturally mature cumulates in any layered intrusion are also likely to be chemically re-equilibrated.

Description: Grenada is the southernmost island in the Lesser Antilles arc, a chain of subduction-related volcanoes distinguished by its diversity of magma composition and unusually abundant plutonic xenoliths, many with cumulate textures. We have determined the mineral compositions of a newly collected, extensive suite of plutonic xenoliths from Grenada and examined their relationship with the lavas in an attempt to explore the role of intra-crustal processes on magmatic evolution. The plutonic assemblages are dominated by mafic phases with abundant hornblende and clinopyroxene, and include the only known plagioclase-free examples in the Lesser Antilles. Bulk compositions are unlike those of natural silicate melts and are consistent with the majority of the xenoliths having a cumulate origin. Experimental and thermobarometric evidence shows that the entire cumulate suite can be generated in a narrow pressure range (0·2–0·5 GPa) with different assemblages resulting from small variations in melt chemistry and temperature. Temperature estimates are consistent with the observed crystallization sequence of olivine -〉 clinopyroxene -〉 hornblende -〉 plagioclase. A spinel phase is present throughout ranging from Cr- to Fe 3+ -rich. The crystallization sequence requires elevated magmatic H 2 O contents (~7 wt % H 2 O) sufficient both to suppress plagioclase crystallization and to render this phase extremely rich in anorthite upon appearance; this is a characteristic of many island arc settings. Studied lavas from the M- and C-series span picrites and ankaramites to hornblende- and orthopyroxene-bearing andesites. MELTS modelling confirms experimental hypotheses that the two lava series can be derived from a common picritic magma, with M-series differentiation occurring in the uppermost mantle (~1·4–1·8 GPa) and C-series in the shallow crust (~0·2 GPa). Plutonic xenoliths from Grenada are notably different from those of the neighbouring island of St Vincent, the respective assemblages and mineral chemistry demonstrating the effect of small-scale changes in melt composition and magma storage conditions between these two islands. We suggest that the unusual petrological and geochemical characteristics of Grenada magmas are a result of proximity to the South American continent and associated localized thickening of the oceanic lithosphere. This increases the depth of magma generation and is reflected in the elevated LREE/HREE of the Grenada lavas, indicating that last equilibration with a garnet lherzolite source occurred at a depth of ≥60 km.

Description: In New Caledonia, blueschist and eclogite preserve, as inclusions in porphyroblastic minerals, a record of sulfide present during prograde subduction processes. Sulfide inclusions in prograde garnet and lawsonite became chemically isolated from the matrix whereas sulfide minerals in the matrix continued to equilibrate with matrix fluids, or grew later, during retrogression. Cu–Fe sulfide mineral inclusions have been found across metamorphic grade within silicate-defined metamorphic mineral zones spanning a crustal profile of ~30 km. Bulk area scans of sulfide inclusions provide compositions that represent mixtures of the solid sulfide that were included as the host silicate minerals grew. In general, single sulfide inclusion compositions and aggregate sulfide assemblages are distinct from those of matrix phases. High Cu contents in sulfide inclusions are interpreted to be a consequence of Fe lost from sulfide to growing garnet, rather than the result of intrinsically high Cu in the bulk-rock. The distribution of sulfide inclusion compositions across metamorphic grade, considered together with the available thermodynamic data, suggests that covellite/nukundamite-bearing inclusions in lawsonite, high in both Cu and S, disappear at higher grades as these sulfide minerals are no longer stable. Similarly, clustering near the ratio Fe:Cu = 1:1 may cease with increasing grade owing to the replacement of chalcopyrite/intermediate solid solution (iss) by the denser assemblage pyrite + pyrrhotite + bornite/digenite.

Description: Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (〉15 wt % MgO, 48–54 wt % SiO 2 ) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (〉25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ~15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.

Description: Tephras recovered by deep-sea drilling from fore-arc to back-arc locations across the Mariana Volcanic Arc record the last 34 Myr of the system’s evolution. Major and trace element abundances and Sr–Nd–Pb–Hf isotope ratios have been determined for tephra with high temporal precision and an average inter-tephra layer interval of ~1 Myr. Temporal variations of source-sensitive radiogenic isotopes and large-ion lithophile elements (LILE) are mostly decoupled from unchanging modes observed in silica and, with the exception of K, other archetypical crust-forming major elements. Modeling confirms that the temporal isotopic and elemental abundance trends are controlled by subducted slab and mantle sources. The Pb and Sr fluxes can be linked to fluids from altered oceanic crust (AOC), and are influenced by contributions from the mantle wedge and slab partial melts, whereas Hf mostly derives from the sub-arc mantle. Most plausibly, a K 2 O increase and fluctuations at ~10 Ma can be linked to collision of the leading trace of the Cretaceous-aged Western Pacific Seamount Province with the arc. This is inferred to have arrived at the Mariana Trench at ~15–16 Ma, coincident with the termination of spreading in the Parece Vela back-arc basin. A short period of slab melting followed, possibly induced by slab rollback that peaked at ~8–9 Ma and ended with incipient rifting in the Mariana Trough at ~7 Ma. Each of the periods of arc formation (52–24, 22–11 and 10–0 Ma) is characterized by a distinctive mixture of source materials that is not repeated through time. Mariana Arc crustal growth has occurred through the addition of predominantly mafic and silicic melts formed during relatively short time intervals, traceable via their chemically distinctive subducted slab inputs.