Fluids in Subduction Zone Magmatism: Implications of Boron Geochemistry

Abstract

The roles of fluids, sediments, and slab melts in convergent margin magmatism are difficult to quantify and likely vary from one arc to another. Decoupling of trace elements with differential fluidand melt-mobility can illuminate specific subdnction processes; the fluid-mobile element (FME) boron is particularly useful in this regard. Variable B-enrichment (up to > 100 x ) in arc compared to within-plate lavas can be attributed to direct subduction contributions (as implied by correlations with l~ in arc suites). The fact that B enrichment is decoupled from that of fluid-immobile elements (REE, Zr, Ti, Nb, etc.) but parallels that of fluid-mobile elements (Pb, As, Sb) implies dominant transfer of the latter elements into arc magma sources via aqueous fluids rather than melts. B commonly is decoupled from or poorly correlated with Ba, Sr, and alkalies - suggesting that these elements are subject to more complex controls. In general, element mobility must be considered in light of element partitioning between all stable mineral, fluid, or melt phases. Enrichments of B (e.g. relative to Zr) vary widely from one arc to another, and generally correlate with such factors as 'slab length' or 'age of subducting crust', both of which are manifestations of slab thermal state. Locally, in cross-arc transects, Benrichment is strongest at the volcanic front (VF) and diminishes toward back-arc (BA) regions. Coupled with data for metamorphic suites that indicate progressive B-depletion with increasing grade, such trends suggest that B is mobilized from subducting slabs due to progressive slab dehydration. Thus, Benrichment in arc basalts appears to reflect the thermal evolution of subducted slabs. B-enrichment also appears to be correlated with degree of Uisotope disequilibria. Available B isotopic data for arc lavas tend to show both regional and local variations consistent with the above. Different ranges in boron isotopic composition of siliciclastic sediments (5lIB: -5--}-5%o), altered oceanic crust (811B: 4.3+_5.4%o), and peridotite (sllB: 8 to 13%o) an covariations between degree of B enrichment, 811B, and 878r/86Sr isotopic data may be used to discriminate the relative contributions of hydrous fluids vs. sediments to arc magma sources. In western Pacific arcs (old, cold slab), elevated 811B (> +3%0) in VF lavas is consistent with derivation of excess B from subducted seawateraltered oceanic crust rather than sediments, whereas low 8lIB and elevated 878r/86Sr in some arc lava suites (Lesser Antilles, Aeolian) point to sediment or crustal involvement as well. Moreover, inter-arc variations in 811B could in part reflect progressive decrease in 811B of the subduction component during devolatization of subducted slabs, with less intense metamorphism and lower liB-loss in cold subduction zones and vice versa. To highlight the relative importance of these processes in arc-magma genesis we have analysed volcanic rocks from arcs with different geological settings and subduction zone thermal structures. Tonarini & Leeman (this volume) discuss the behaviour of B in specific arcs. Overall, 8 lIB varies widely (between -10%o and +21%.) in arc lavas; the highest values apparently occur in lavas derived from highly depleted sources that are easily modified by the subduction component (Fig. 1). The B/Nb-B systematics similar to those of the Izu and Kuriles arcs (Ishikawa and Nakamura, 1994; Ishikawa and Tera, 1997) are sometimes observed in subsets of our data, but overall relations are more complex. Also, 87Sr/S6Sr is correlated with 811B inside each arc but not in general due to differing slab and sediment contributions worldwide. From plots of 'fluid-immobile element'/B ratios vs ~IlB, extrapolated to zero, ~llB values for the subduction component(s) can be estimated for each arc studied. Typical values are: +16%o for the South Sandwich arc (and perhaps locally in Tonga [Hunga Ha'apai]); +7%0 for Tonga, Mariana, Izu (Ishikawa and Nakamura, 1994), and Kurile (Ishikawa and Tera, 1997) arcs; +3.7%0 for the Aleutian arc; and