Multiple Dehydration Events Beneath Island Arc, Evidence from U-Th-Ra Isotopic Disequilibria in Mt. Fuji Lavas, Japan.

Trace element, Sr-Nd isotopes compositions and U-Th-Ra isotopic disequilibria data are presented for Fuji basalts of the last 10,000 yr. Both Sr and Nd isotopes signatures of Fuji basalts show very narrow range (0.703276-0.703363 and 0.51304-0.51306, respectively), indicating that a homogenous mantle source beneath the volcano. All of the Fuji lavas have ²³⁸U-²³⁰Th disequilibrium with excesses of ²³⁸U. These signatures are typical for arc lavas and not MORB and OIB lavas. Samples from individual stages show linear trends having a trajectory cross the equiline at almost the same value at (²³⁰Th/²³²Th) = 0.84. This common line can thus be regarded as isochron age of ~90 kyr, representing the time intervals since the onset of the disequilibrium by metasomatism of the mantle wage by fluids released from the dehydrated subducting slab. However, (²²⁶Ra/²³⁰Th) activity ratios in Fuji lavas range from ~1 up to 3.2 indicating that less than 8000 yr elapsed since the generation of the disequilibria by metasomatism of the mantle wedge by fluids released from the subducting slab. Ba/Th ratio shows clear positive correlation with (²³⁸U/²³⁰Th) disequilibrium, however, it shows no correlation with (²²⁶Ra/²³⁰Th), instead remains constant for the whole range of (²²⁶Ra/²³⁰Th) disequilibria, indicating that the fluid that carried Ra was not carried Ba. Multiple dehydration events can be the preferred explanation in order to reconcile the U-Th and Ra-Th disequilibria age information. Therefore, dehydration reactions in the slab, some 90 kyr ago, were released fluids and transporting fluid-mobile elements Ra and U into the overlaying mantle wedge. Consequently, Ra and U depleted in the slab. But, ²²⁶Ra with time would have been continually formed in the slab by in-growth from the fluid-immobile ²³⁰Th that left after the first dehydration episode. As a result, any later fluid addition into the mantle wedge is capable of adding considerable amount of ²²⁶Ra but not extra U, so that the U-Th disequilibria recorded the first fluid addition event around 90 kyr.