National Museum of Nature and Science, Japan

IODP Expedition 324 [1] recovered samples from Shatsky Rise oceanic plateau ranging from picritic basalts (15.6 wt% MgO) to more differentiated tholeiitic basalts (4.9 wt% MgO). In order to determine crustal magma chamber and magma evolution processes lava samples from several evolutionary stages should be investigated. We present results of the mineralogical study of lower MgO pillow and massive flow basalts (Site U1347). Major element glass and mineral compositions were analyzed by electron microprobe (JAMSTEC and University of Hannover). Glass H (sub 2) O and CO (sub 2) determinations were performed by FTIR spectroscopy. In general, basaltic glasses from Site U1347 are evolved tholeiitic basalts (5.2-6.8 wt% MgO), resembling typical EPR MORBs, located in the high-FeO and low-Al (sub 2) O (sub 3) fields of EPR basalts. The CaO/Al (sub 2) O (sub 3) ratios observed in U1347 basalts are amongst the highest known for all MORBs, probably indicating very low pressures of partial crystallization. H (sub 2) O contents in the glasses show MORB values, ranging from 0.18 to 0.6 wt%. CO (sub 2) contents are usually below detection limit of the FTIR method (<50 ppm). The glass compositions and the H (sub 2) O concentrations were used to simulate conditions of multiply saturation [2]. Our calculations demonstrate that basaltic melts could have been last equilibrated with Ol-Pl-Cpx association at 1110 to 1170 degrees C in the range of pressures between 1 atm to 3 kbar. These estimates represent the maximum values, since Ol is scarcely observed in natural Pl-Cpx-phyric lavas. The compositions of coexisting Pl and Cpx ( approximately 30 samples studied) show two compositional trends. One trend can be reproduced as a result of ideal fractional crystallization at pressures between (0.5-3 kbar). The second trend is the object for future investigations. Application of Cpx-melt and Pl-melt geothermobarometers [3] gives inconsistent results for the same samples and generally shows high P-T values using Cpx-melt (3.7-7 kbar, 1140-1220 degrees C) and low and negative pressure values using Pl-melt equilibria. [1] Sager et al. (2010) Proc. IODP, 324: Tokyo. [2] Almeev et al. (2008) JPet, 49, 25-45. [3] Putirka (2008) Rev. Min. Geoch. 69: 61-120.