Lavas from transects across the Kurile Islands are showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab. Enrichment factors for boron, cesium, arsenic, and antimony, all elements with strong affinities for water, decreased across the are. This decrease is presumably related to losses of water-rich fluids during the dehydration df the subducting plate. Enrichments of potassium, barium, beryllium-10, and the light rare earth elements remained constant; these species may move in silica-rich fluids liberated from the slab at greater depths.

Pb-Pb mineral isochrons were determined for three cumulate eucrites (Moama, Moore County, and Serra de Mage) and three noncumulate eucrites (Nuevo Laredo, Bouvante, and Stannern). Two noncumulate eucrites (Bereba and Cachari) show disturbed Pb isotope patterns. The mineral isochron ages of cumulate eucrites range from 4.40 Ga to 4.48 Ga. The latter is the age of Moore County (4.484 +/- 0.019 Ga), which was also dated by the Sm-Nd method at 4.456 +/- 0.025 Ga. Nuevo Laredo and Bouvante give the same Pb-Pb age: 4.514 +/- 0.015 Ga and 4.510 +/- 0.004 Ga, respectively. The Pb isochron of Stannern corresponds to 4.128 +/- 0.016 Ga, which is a reequilibration age. Both Cachari and Bereba show evidence of isotope resetting at about 4 Ga. A Pb-Pb multicorrelation approach is introduced and applied to the data.

B and Nb concentrations and B isotope composition were determined for lavas covering the entire length of the Kurile are. B/Nb and beta(11)B of these lavas show clear across-are Variations in which both are highest at the trench side and continuously decrease as the slab depth increases. B/Nb in the volcanic front lavas (8-25) is significantly higher than those of MORB and OIB, indicating that the mantle wedge is extensively metasomatized by B-rich fluid derived from the subducting slab. The frontal delta(11)B values are also out of the range of MORB and OIB, and systematic difference exists between Northern Kurile (NK: + 5.4 +/- 0.5 parts per thousand) and Central and Southern Kurile (CSK: + 4.3 +/- 0.3 parts per thousand). However, these two parameters approach or coincide with mantle values at the most back-are side. The delta(11)B and Sr-87/Sr-86 linearly correlate with Nb/B, suggesting control by simple mixing of two isotopically homogeneous components; that is, slab-derived fluid and mantle wedge. The estimated delta(11)B and Sr-87/Sr-86 of the fluid are +7.0 +/- 0.6 parts per thousand and 0.70326 +/- 0.00005 for NK, and + 6.3 +/- 0.3 parts per thousand and > 0.7033 for CSK, respectively. These fluid compositions are consistent with a source constituted by > 95% altered oceanic crust and < 5% oceanic sediment, with the slight difference between NK and CSK resulting from the different lithologies of subducting sediments. The mantle wedge beneath the Kurile are has B, Sr and Nd isotope compositions identical with MORB but shows significantly higher B/Nb. The above trends bear a close resemblance to those observed in the Izu are, strongly suggesting their universality in the are system. The across-are decline of B/Nb observed both in Kurile and Izu implies that the fluid flux from the slab continuously decreases as the slab depth increases. However, this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore-are region. In contrast to B, K is uniformly enriched across the are. This requires control by a K-bearing mineral phase either in the slab (phengite) or the mantle wedge (phlogopite), although such a mineral is responsible for only a small part of the whole fluid transport. (C) 1997 Elsevier Science B.V.

Nb/B ratios and delta(11)B values of Mariana lavas reveal the involvement of two isotopically distinct slab-derived fluids in the magma genesis. The evidence of one type of fluid is observed in lavas of the islands of Anatahan, Alamagan, Agrigan, and Uracas, in which simple mixing between fluid with very homogeneous delta(11)B values (+5.40 parts per thousand +/- 0.05 parts per thousand) and the mantle wedge controls the magma compositions. In lavas of the islands of Guguan, Asuncion, and Pagan, however, the fluids that mixed with the mantle wedge are significantly heterogeneous in delta(11)B (+5.4 parts per thousand to +7.6 parts per thousand) even within a single island. The compositional difference between these two types of fluid can result from different ratios of sediment to altered oceanic crust (AOC) in the slabs, and this hypothesis is consistent,vith the Sr, Nd, and Pb isotope data. We propose that the variable isotope compositions of the latter fluid may be derived from local changes in sediment/AOC ratios of the slab that are caused by subduction of the Magellan seamounts beneath the Mariana are.