A new global compilation of high precision trace element and isotopic analyses from mafic island arc lavas is explored to highlight the key geochemical features of volcanic front lavas Two distinct components from the slab can be identified in island arc lavas One component dominates the budgets of most incompatible trace elements and another affects a smaller range of elements, most notably Ba, Pb and Sr These contrasting slab components require different ultimate sources and mechanisms of transport to the mantle wedge The first component is argued to be a melt of the down-going sediment, while the second is likely an aqueous fluid derived from the altered mafic oceanic crust U-series nuclides constrain the timing of slab component additions The sediment component is close to 238 U 230 Th equilibrium implying >350ky since the last major fractionation of U and Th, plausibly the time since sediment melting Lavas dominated by the 'fluid' component can have extreme 226 Ra- 230 Th disequilibrium together with elevated ratios of the stable Ba/Th analogue In its simplest interpretation, this observation implies only a few thousand years elapse between 'fluid' release from the slab, and eruption of this component in arc lavas In most cases the two subduction components appear to be added to a recently melt-depleted mantle This suggests the mantle wedge is fed with material processed through a back-arc melting regime These first order geochemical observations and inferences place important constraints on physical models of the subduction zone

Tracers of the Slab

Experimental phase equilibrium and trace element partitioning data are reported for H 2 O-saturated mid-ocean ridge basalt at 2·5 GPa, 750-900°C and oxygen fugacities at the nickel-nickel oxide buffer. Garnet, omphacite and rutile are present at all temperatures. Amphibole and epidote disappear as residual phases above 800°C; allanite appears above 750°C. The Na-Al-rich silicate glass present in all run products is likely to have quenched from a supercritical liquid. Trace element analyses of glasses demonstrate the important control exerted by residual minerals on liquid chemistry. In addition to garnet, which controls heavy rare earth elements (HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanite buffers the light REE (LREE; La-Sm) contents of liquids to relatively low levels and preferentially holds back Th relative to U. In agreement with previous experimental and metamorphic studies we propose that residual allanite plays a key role in selectively retaining trace elements in the slab during subduction. Experimental data and analyses of allanite-bearing volcanic rocks are used to derive a model for allanite solubility in liquids as a function of pressure, temperature, anhydrous liquid composition and LREE content. The large temperature dependence of allanite solubility is very similar to that previously determined for monazite. Our model, fitted to 48 datapoints, retrieves LREE solubility (in ppm) to within a factor of 1· 40 over a pressure range of 0-4 GPa, temperature range of 700-1200°C and for liquids with anhydrous SiO 2 contents of 50-84 wt %. This uncertainty in LREE content is equivalent to a temperature uncertainty of only ± 27°C at 1000 K, indicating the potential of allanite as a geothermometer. Silicic liquids from either basaltic or sedimentary protoliths will be saturated in allanite except for Ca-poor protoliths or at very high temperatures. For conventional subduction geotherms the low solubility of LREE (+ Th) in liquids raises questions about the mechanism of LREE + Th transport from slab to wedge. It is suggested either that, locally, temperatures experienced by the slab are high enough to eliminate allanite in the residue or that substantial volumes of H 2 O-rich fluids must pass through the mantle wedge prior to melting. The solubility of accessory phases in fluids derived from subducted rocks can provide important constraints on subduction zone thermal structure.

Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes

Partial melting of magnesite-bearing peridotites was studied at 6-10 GPa and 1300-1700°C. Experiments were performed in a multianvil apparatus using natural mineral mixes as starting material placed into olivine containers and sealed in Pt capsules. Partial melts originated within the peridotite layer, migrated outside the olivine container and formed pools of quenched melts along the wall of the Pt capsule. This allowed the analysis of even small melt fractions. Iron loss was not a problem, because the platinum near the olivine container became saturated in Fe as a result of the reaction Fe 2 SiO 4 Ol = Fe Fe-Pt alloy + FeSiO 3 Opx + O 2 . This reaction led to a gradual increase in oxygen fugacity within the capsules as expressed, for example, in high Fe 3+ in garnet. Carbonatitic to kimberlite-like melts were obtained that coexist with olivine + orthopyroxene + garnet ± clinopyroxene ± magnesite depending on P-T conditions. Kinetic experiments and a comparison of the chemistry of phases occasionally grown within the melt pools with those in the residual peridotite allowed us to conclude that the melts had approached equilibrium with peridotite. Melts in equilibrium with a magnesite-bearing garnet lherzolite are rich in CaO (20-25 wt %) at all pressures and show rather low MgO and SiO 2 contents (20 and 10 wt %, respectively). Melts in equilibrium with a magnesite-bearing garnet harzburgite are richer in SiO 2 and MgO. The contents of these oxides increase with temperature, whereas the CaO content becomes lower. Melts from magnesite-free experiments are richer in SiO 2 , but remain silicocarbonatitic. Partitioning of trace elements between melt and garnet was studied in several experiments at 6 and 10 GPa. The melts are very rich in incompatible elements, including large ion lithophile elements (LILE), Nb, Ta and light rare earth elements. Relative to the residual peridotite, the melts show no significant depletion in high field strength elements over LILE. We conclude from the major and trace element characteristics of our experimental melts that primitive kimberlites cannot be a direct product of single-stage melting of an asthenospheric mantle. They rather must be derived from a previously depleted and re-enriched mantle peridotite.

Experimental Melting of Carbonated Peridotite at 6-10 GPa

Basaltic lavas from Hainan Island near the northern edge of the South China Sea have an age range of between late Miocene (about 13 Ma) and Holocene, with a peak age of late Pliocene to middle Pleistocene. The basaltic province is dominated by tholeiites with subordinate alkali basalts. Most analysed samples display light rare earth element (LREE) enriched REE patterns and ocean island basalt (OIB)-like incompatible element distributions. The basalts contain abundant undeformed high-Mg olivine phenocrysts (up to Fo90•7) that are high in CaO and MnO, indicating high-magnesian parental magmas. Independent barometers indicate that clinopyroxenes in the basalts crystallized over a wide range of pressures of 2–25 kbar (dominantly at 10–15 kbar) and that the melt cooled from about 1350°C to 1100°C during their crystallization. The compositional characteristics of the basalts indicate that their generation most probably involved both low-silica and high-silica melts, as represented by the alkali basalts and tholeiites, respectively. Our results show that the source region for the Hainan basalts is highly heterogeneous. The source for the tholeiites is mainly composed of peridotite and recycled oceanic crust, whereas the source for the bulk of the low-Th alkali basalts consists predominantly of peridotite and low-silica eclogite (garnet pyroxenite). Some high-Th (≥  4 ppm) alkali basalts may have been produced by partial melting of low-silica garnet pyroxenite (eclogite). We estimated the primary melt compositions for the Hainan basalts using the most forsteritic olivine (Fo90•7) composition and the most primitive bulk-rock samples (MgO > 9•0 wt % and CaO >8•0 wt %), assuming a constant Fe–Mg exchange partition coefficient of KD = 0•31 and Fe3+/FeT = 0•1.

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Temperature, Pressure, and Composition of the Mantle Source Region of Late Cenozoic Basalts in Hainan Island, SE Asia: a Consequence of a Young Thermal Mantle Plume close to Subduction Zones?

https://www.geol.umd.edu/%7Emcdonoug/Lithium%20Group/Trace%20elements%20cpx&amph.pdf

Trace element variations in clinopyroxene and amphibole from alkaline to peralkaline syenites and granites: implications for mineral?melt trace-element partitioning

SL Bennett

Papers

The effect of sodium and titanium on crystal-melt partitioning of trace elements