Oceanic plateaus are formed by a large volume of basaltic rocks on top of the oceanic lithosphere. Alteration of these basalt lava piles leads to significant chemical element exchanges between mantle and oceans that can strongly influence chemical budget. Here we report boron (B), chlorine (Cl), and other element concentrations in basalt samples from the Shatsky Rise to define alteration processes and to estimate the significance of oceanic plateaus in storing these elements. Sampling includes 121 basaltic lavas and 92 fresh glasses collected at various depths from Holes U1346A, U1347A, U1349A, and U1350A during Integrated Ocean Drilling Program Expedition 324. Loss on ignition (LOI) results indicate that alteration affected basalts from the summit sites (U1346 and U1349) more deeply than those from the flank sites (U1347 and U1350). The positive correlations between B, K, and LOI observed in the basalts indicate that low-temperature seawater-derived alteration was the predominant process affecting Shatsky Rise basalts. This is confirmed by the elevated B/K and modest Cl/K ratios of these altered basalts relative to the fresh glasses. In addition, B concentrations in the summit basalts (~132 ppm) are significantly higher than those in normal altered oceanic crust and are likely related to the presence of illite. This suggests that the Shatsky Rise oceanic plateau may be an important sink for B in the Pacific oceanic crust.

Abstract We analysed major and trace elements and Sr–Nd–Pb isotope ratios of Himeshima, Futagoyama and Aso volcanoes, northern Kyushu, Japan. The purpose of the study was to investigate the petrogenesis of Quaternary arc magmas of the northern Kyushu arc, Japan, through along-strike variations. The northern Kyushu arc is underlain by the Philippine Sea Plate, which is divided by the Kyushu–Palau Ridge into the Palaeogene West Philippine Basin segment (60–40 Ma) and the Miocene Shikoku Basin segment (26–15 Ma). The Sr/Y ratios of the arc lavas decrease from north to south along the volcanic front. Mixing relations in Sr–Nd–Pb isotope space suggest recycling of the subducted slab materials from the Philippine Sea Plate to the arc. The 87 Sr/ 86 Sr ratios decrease with increasing Sr/Y and SiO 2 , which argues against a genetic link of fractional crystallization between adakites and basaltic magmas. The observations further suggest that partial melting of the hot and young Shikoku Basin slab produces the high Sr/Y component visible in the arc magmas in the north, whereas dehydration of the older West Philippine Basin slab produces the low Sr/Y arc magmas in the south.

Abstract Minerals of mafic rocks from the SW Japan arc have been studied to deduce P – T –X H2O conditions and their variations in mafic arc magmas. Two-pyroxene thermobarometry of magmas from several volcanoes yields constant temperatures and variable pressures. MELTS fractional crystallization modelling is employed to show that such ‘pseudo-decompression paths’ (PDPs) are artefacts that derive from uptake of pyroxene antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded mafic melts. It is shown that PDPs can be used to constrain oxygen fugacities and initial water contents of the intruded magmas. These constraints, and plagioclase hygrometry, indicate that initial melt H 2 O contents change systematically along the SW Japan arc. Direct determination of hydrogen in olivine by secondary ion mass spectrometry yields consistently low olivine H 2 O contents of 11±4 ppm (1σ), with little, if any, along-arc variations. MELTS modelling indicates that crystallization of calcic plagioclase and olivine dominantly occurs during upper crustal differentiation of mafic melts. The combined data indicate that aphyric melts are released from the mantle wedge, taking up most if not all crystals from previously intruded plutonic rocks during rapid magma ascent to the surface.