HR: 1340h
AN: T53B-1298    [Abstracts]
TI: 'Hydromagmatic' epidote in in-situ deep oceanic diorites from IODP Hole U1309D, Atlantis Massif, Mid-Atlantic Ridge, 30°N
AU: * Yamasaki, T
EM: toru_gabbro@w5.dion.ne.jp
AU: Maeda, J
EM: jinmaeda@mail.sci.hokudai.ac.jp
AB: Epidote is one of the key minerals for controlling hydrothermal vent fluid composition at the 'reaction zone' of mid- ocean ridge hydrothermal system. In slow-spreading ridges (SSR), there has been very few geophysical evidences yet to indicate the presence of a magma chamber. Therefore, short-lived small melt lens and/or near- solidus hot gabbroic rocks have been assumed as heat sources of high-temperature hydrothermal circulation at SSR. Epidote-bearing plutonic rocks in SSR are typically oxide gabbros, diorites, and trondhjemitic rocks. Why occurrence of epidote is rather restricted in such differentiated-magma derived oceanic plutonic rocks is a fundamental question concerning petrogenetic relationship between magmatic and alteration processes. We report euhedral, discrete and zoned epidotes in hydrothermally altered leucocratic diorites, and discuss petrological features of transition from the epidote-bearing leucocratic diorites to adjacent disseminated oxide gabbros recovered from IODP Hole U1309D, Atlantis Massif at Mid-Atlantic Ridge 30°N. Constituent mineral compositions and texture suggest that the diorites crystallized from incompatible element- and fluid-rich late- stage melt segregated from oxide gabbros at low pressure (~0.6 kbar). Zoned, euhedral epidotes in leucocratic diorites are essentially interpreted as magmatic product and their pistacite contents (= octahedral (Fe3+/(Fe3+ + AlVI) × 100) <33) suggest crystallization from oxidized, fluid-rich melt or high-temperature fluid itself. Sr isotopic initial ratio of the epidote-bearing leucocratic diorites (87Sr/86SrI = 0.70312) and associated disseminated oxide gabbro (87Sr/86SrI = 0.70262) are relatively low. Calculated water/rock ratio of those rocks (0.5-3.0) suggests involvement of relatively rock dominant fluids in the alteration. Overall our assessment suggests that high-temperature alteration started at the latest stage of magmatic processes, and could attain the W/R ratio required to explain common hydrothermal fluid composition. It implies epidote-bearing leucocratic diorites and related lithologies are substantial materials in the 'reaction zone' of hydrothermal circulation system at SSR. The autometamorphism reasonably explain restricted occurrence of epidotes in leucocratic plutonic rocks at SSR.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3036 Ocean drilling
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting