Across-arc variation in {}40Ar/{}36Ar ratios of olivines in volcanic rocks from the Izu-Ogasawara arc, Japan

The Izu-Ogasawara arc is located at an intra-oceanic convergent margin between the Pacific and Philippine Sea plates. This arc is suitable to investigate the recycling of volatile materials concurrent with subduction process, because contribution of continental crustal component in arc magma can be negligible. Here we discuss noble gas isotopic compositions of olivines in volcanic rocks from the volcanic front and back-arc regions of this arc to investigate the volatile behavior in slab-derived fluid during subduction processes. The samples from both regions have similar {}³He/{}⁴He ratio of about 8.0 {R}_A, which is in the range of the MORB value (8 ± 1 {R}_A), indicating that the contribution of helium in slab-derived fluid to the mantle wedge is negligible. However, {}⁴⁰Ar/{}³⁶Ar ratios of samples from the back-arc region range from 380 to 620, whereas those from the volcanic front region are 300 to 320. The {}⁴⁰Ar/{}³⁶Ar ratios of both regions are significantly lower than that of the MORB source (up to 40000) indicating that the contribution of slab-derived atmospheric argon is dominant in the mantle wedge. The difference in {}⁴⁰Ar/{}³⁶Ar ratios in both regions may reflect the different contribution of slab-derived component. Assuming that atmospheric argon in the highest {}⁴⁰Ar/{}³⁶Ar samples from each region are derived from subducting slab, but not due to the shallow level contamination, contributions of slab-derived {}⁴⁰Ar in each magma are about 90% in the volcanic front and 50% in the back-arc regions, respectively. If water content of source mantle which generates a primary melt is 0.14 wt%, and wedge mantle originally contains argon similar to the MORB source ({}⁴⁰Ar/{}³⁶Ar = 40000, {}⁴⁰Ar concentration = 7.4× {10}^-7 cc/g), slab-derived {}⁴⁰Ar/H{}₂O ratios in the source mantle are estimated to be 5.6× {10}^-3 cc/g in the volcanic front and 4.7× {10}^-4 cc/g in the back-arc regions, respectively. These {}⁴⁰Ar/H{}₂O ratios are comparable to those of air-saturated sea water, sediments and altered oceanic crust (2× {10}^-5 to 3× {10}^-4 cc/g). This suggests that subducting atmospheric {}⁴⁰Ar is assumed to be introduced into the mantle wedge associated with slab dehydration process beneath the arc.