ALBERT

Description: We present experimental data on the partitioning of Li, Be, B, K, Mg, Sr, Ga, Rb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U, Hf, Zr, Nb and Ta between lawsonite and fluid, and zoisite and fluid at 3·0–3·5 GPa and 650–850°C. The aim is to provide data bearing on the trace element contents of fluids released during dehydration of subducting oceanic crust. Experimental trace element partition coefficients for lawsonite indicate a preference for the light rare earth elements (LREE) over the heavy REE (HREE) and for Be. These characteristics are consistent with the chemical composition of lawsonite in natural rocks. Experimental trace element partition coefficients for zoisite indicate a preference for HREE relative to LREE. This observation, consistent with earlier experimental data, is the reverse of the observed trace element compositions of natural zoisites, indicating the influence of other factors on the trace element contents of this phase. Lattice strain theory explains well the experimentally derived partitioning of divalent cations in the Ca-site between lawsonite and fluid. However, the weak relative fractionation of REE between lawsonite and fluid cannot be explained by lattice strain theory, as previously observed for zoisite–fluid REE partitioning. We combine our experimental data with thermodynamic models of mineral stability to model the compositions of fluids released during subduction of altered normal mid-ocean ridge basalt. The low La/Sm ratio associated with very high Ba/Th in arc magmas can be explained only if allanite is stable in the subducting oceanic crust. This suggests that the crustal fluid component involved in arc magma petrogenesis results from processes occurring in the warm, top part of the subducting slab. Decreasing lawsonite modal proportion with depth is associated with a large release of fluid characterized by low B/Be ratios that could explain the decreasing B/Be ratios in arc magmas with increasing distance from the trench. This implies that an important Be input in arc magma originates from the fluid generated during oceanic crust dehydration.

Description: Fonualei is unusual amongst subaerial volcanoes in the Tonga arc because it has erupted dacitic vesicular lavas, tuffs and phreomagmatic deposits for the last 165 years. The total volume of dacite may approach 5 km 3 and overlies basal basaltic andesite and andesite lavas that are constrained to be less than a few millennia in age. All of the products are crystal-poor and formed from relatively low-viscosity magmas inferred to have had temperatures of 1100–1000°C, 2–4 wt % H 2 O and oxygen fugacities 1–2 log units above the quartz–fayalite–magnetite buffer. Major and trace element data, along with Sr–Nd–Pb and U–Th–Ra isotope data, are used to assess competing models for the origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution of the within-dacite compositional array by closed-system crystal fractionation of a single magma batch. An origin by partial melting of lower crustal amphibolites cannot reproduce these data trends or, arguably, any of the dacites either. Instead, we develop a model in which the dacites reflect mixing between two dacitic magmas, each the product of fractional crystallization of basaltic andesite magmas formed by different degrees of partial melting. Mixing was efficient because the two magmas had similar temperatures and viscosities. This is inferred to have occurred at shallow (2–6 km) depths beneath the volcano. U–Th–Ra disequilibria in the basaltic andesite and andesite indicate that the parental magmas had fluids added to their mantle source regions less than 8 kyr ago and that fractionation to the dacitic compositions took less than a few millennia. The 165 year eruption period for the dacites implies that mixing occurred on a similar timescale, possibly during ascent in conduits. The composition of the dacites renders them unsuitable candidates as contributors to average continental crust.

Description: Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands (46°34'S, 166°53'E), south of mainland New Zealand, are amphibole- and plagioclase-phyric trachyandesites–andesites. The Solander and Little Solander Island trachyandesites–andesites have a narrow range of SiO 2 (60·26–62·19 and 58·20 wt % respectively), and high incompatible element concentrations (e.g. Sr 931–1265 ppm, Ba 619–769 ppm and Sr 2200–2269 ppm, Ba 733–798 ppm respectively). They have geochemical affinities with modern adakites (e.g. high Sr/Y ~66–105 and ~132–146, and depleted Y ~11–16 ppm and 16–17 ppm, respectively). Isotopically similar porphyritic quench-textured enclaves reflect mixing with intermediate (basaltic andesite) magmas with high incompatible element concentrations. The presence of porphyritic and equigranular enclaves, disequilibrium phenocryst textures and compositions, and cumulate nodules is consistent with the Solander sample suite having evolved in an open crustal magma storage system through combined crystal fractionation and mafic magma recharge. Mixing with incoming batches of hotter and more mafic magma is marked by the appearance of oscillatory- and simple-zoned amphibole and sieve-textured plagioclase phenocrysts with An-rich rim overgrowths. High XMg clinopyroxene crystals in the Little Solander trachyandesites indicate the late influx of more mafic compositions. Although fractionation is clearly an important process in producing diversity in the Solander magmas, mafic magma recharge is suggested to be responsible for the enrichment in incompatible element concentrations observed in the more evolved Solander and Little Solander Island rocks. The concave-up rare earth element patterns of the Solander and Little Solander trachyandesites and andesites are consistent with significant amphibole fractionation of hydrous magmas at crustal depths outside the garnet stability field.

Description: How the Earth’s earliest crust was formed and when present-day plate tectonics (i.e., subduction) and life commenced remain fundamental questions in Earth sciences. Whereas the bulk composition of the crust is similar to that of rocks generated in subduction settings, it does not necessarily follow that melting and crust formation require subduction. Many workers suggest that subduction may have only commenced toward the end of the Archean or later. Here we observe that both the stratigraphy and geochemistry of rocks found in Quebec, Canada, that have been variously argued to be 4.4 or 3.8 Ga in age, closely match those from the modern-day Izu-Bonin-Mariana forearc. We suggest that this geochemical stratigraphy might provide a more robust test of ancient tectonic setting than individual chemical or isotopic signatures in rocks or detrital minerals. If correct, the match suggests that at least some form of subduction may have been operating as early as the Hadean or Eoarchean. This could have provided an ideal location for the development of first life.