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

Wednesday, 17 December 2014
Takele Chekol1,2 and Eizo Nakamura2, (1)T.Y.K. Mineral and Water Exploration, Development and Consultancy PLC, Addis Ababa, Ethiopia, (2)Okayama University, Institute for Study of the Earth's Interior, Misasa, 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.