Melting processes and fluid and sediment transport rates along the Alaska‐Aleutian arc from an integrated U‐Th‐Ra‐Be isotope study

Abstract

[1] A comprehensive data set for young lavas erupted along the Alaska-Aleutian arc is used to examine how fluid and sediment transport rates and melting processes vary in response to systematic changes in subduction rate and dip along the arc and across the ocean-continent boundary. Positive correlations between convergence rate, volcano volume, and 238 U excesses suggest that magmatic output is closely linked to the size of the fluid flux which occurred <10 kyr prior to eruption. Sediment-sensitive tracers like Th/Nb, Ce/Ce*, and 10 Be/ 9Be also increase with convergence rate. However, the inferred 10 Be/ 9 Be ratio of this component is low relative to that in the incoming sediments, and this could reflect either sediment removal by accretion or storage in the mantle wedge for $0.5-1 Myr. Th/Nb and Th/Nd ratios exceed those expected from bulk sediment addition in the center of the arc suggesting that the enhanced sediment signal reflects transfer in a partial melt phase, whereas partial melts from the subducted oceanic crust appear to be restricted to the western tear in the Pacific plate. Therefore the thermal structure at the slabwedge interface in the center of the arc must lie close to, and possibly between, the sediment and basalt solidii and is thus constrained to be in the region of 670-740°C at 3 GPa. Ra excesses in many of the lavas are modeled as the products of dynamic melting effects superimposed upon a fluid-fluxed mantle wedge. The transition from oceanic to continental lithosphere seems to play an important role in determining the relative effects of fluid addition and partial melting upon U-Th disequilibria. We infer that the change from fast and steep plate subduction in the Aleutians to shallow and slow subduction, coupled with increasing lithospheric lid thickness, in Alaska together control slab and wedge temperatures and the rate of matrix flow through the melting region.INDEX TERMS: 1040 Geochemistry: Isotopic composition/chemistry; 8434 Volcanology: Magma migration; 8450 Volcanology: Planetary volcanism (5480); KEYWORDS: uranium series, beryllium isotopes, Aleutians, Alaska, arc petrogenesis Citation: George, R., S. Turner, C. Hawkesworth, J. Morris, C. Nye, J. Ryan, and S.-H. Zheng, Melting processes and fluid and sediment transport rates along the Alaska-Aleutian arc from an integrated U-Th-Ra-Be isotope study,

Determination of 9Be in geological standard samples, JA-2 and JB-2, and of 9Be and 10Be in a basaltic rock sample for evaluation of uncertainty involved in 10Be/9Be ratio measurements

“…Our uncertainty as 9% (1σ) is larger than that reported in George et al (2003). Our uncertainty, however, may include heterogeneous distribution of 10 Be and it is not clear if it can be compared with that reported in George et al (2003) directly.…”

“…They reported reproducibility of 10 Be/ 9 Be ratio as less than 22%. George et al (2003) reported that reproducibility on 10 Be measurements are typically 5% (1σ) at concentrations >1 × 10 6 atoms g -1 without detailed descriptions. Our uncertainty as 9% (1σ) is larger than that reported in George et al (2003).…”

“…George et al (2003) reported that reproducibility on 10 Be measurements are typically 5% (1σ) at concentrations >1 × 10 6 atoms g -1 without detailed descriptions. Our uncertainty as 9% (1σ) is larger than that reported in George et al (2003). Our uncertainty, however, may include heterogeneous distribution of 10 Be and it is not clear if it can be compared with that reported in George et al (2003) directly.…”

Metasomatized lithosphere–asthenosphere interaction during slab roll-back: Evidence from Late Carboniferous gabbros in the Luotuogou area, Central Tianshan

“…REE fractionation patterns can reflect the nature of the mantle source and the extent of melting (George et al, 2003). Despite the uncertainties related to the possible accumulation of pyroxene and the choice of clinopyroxene-melt partition coefficients, we conclude that the REE patterns of equilibrium melts (Figs.…”

Newer Dolerite dykes, Jharkhand, India: a case study of magma generation, differentiation and metasomatism in a subduction zone setting

“…Both the Sm/La and Nb/Ce of Group-I dykes fall either within or very close to the data range of basalt/ dolerites of active continental margins. High Th/Nb ratio of Group-I dykes compared to Group II dykes suggests the involvement of partial melt of subducted sediments (Class et al, 2000;George et al, 2003;Plank, 2005) in Group-I dykes. Although the range of Th/Nb ratios of Group-I dykes matches with that of the active continental margin basalts, it also overlaps with that of IABs.…”