An Approach To The Gabbros From The Upper Part Of ODP735B Hole At The Southwest Indian Ridge

The Southwest Indian Ridge （SWIR） is one famous ultraslow spreading center,with its oceanic crustal structure different with the classic models. The core of OceanDrilling Program （ODP） Hole735B from SWIR is characterized by being lack ofMORB and mainly exposing gabbroic cumulates of oceanic crust. According to thevariation of the texture and mineralogy, and the bulk Mg#,12major lithologic unitsand5geochemical cycles can be recognized, respectively; the lithologic unit IV thatis sandwiched between Cycle1and Cycle2is quite remarkable because it is tooabundant in Fe-Ti oxide-bearing gabbros, and its origin is highly concerned anddebated by many scientists.In this paper, based on detailed petrographical observations, mineralogical andpetrogeochemical features （including the bulk major and trace element contents, andSr-Nd-Pb isotopic compositions） of gabbros from the Cycle1and Cycle2, and oxidegabbros from the Unit IV are studied using electorn microprobe analysis, XRF,ICP-MS, and TIMS. Through the comparisons of mineralogical and geochemicalfeatures between gabbros from the Cycle1and Cycle2, and oxide gabbros from theUnit IV, the following results has been achieved.1. In both Cycle1and Cycle2, gabbros appear more depleted in light rare earthelements （LREEs） upwards, with （La/Sm）Ndecreasing from the bottom to the top; it issuggested that the LREE pattern corresponds to the magma fractionation, and can beused as the new indexes to discriminate the geochemical cycles and to evaluate themagma evolution.2. The gabbroic samples from both Cycle1and Cycle2have the Sr-Nd isotopiccompositions (⁸⁷Sr/⁸⁶Sr:0.702777-0.702986;¹⁴³Nd/¹⁴⁴Nd:0.513072-0.513126)resembling MORB from the Central Indian Ridge. Also they have very unradiogenicPb (²⁰⁶Pb/²⁰⁴Pb:15.856-17.181;²⁰⁷Pb/²⁰⁴Pb:15.081-15.287;²⁰⁸Pb/²⁰⁴Pb:35.772-37.022) that is unlike any MORB, indicating the mantle source beneath ODP735BCore should contain an end-member with a highly unradiogenic Pb that should haveaffinity to old depleted mantle. 3. Detailed petrographical observations and structure-controllable mineralcompositional data indicate that it should be the hydrous fluids or liquid mixtures ofhydrous fluids and basaltic melts, rather than highly differentiated Fe-Ti-rich melts assome scientists supposed, that are involved in the formation of oxide gabbros in UnitIV.4. Oxide gabbroic sample from Unit IV is quite depleted in LREEs, implying thatthe―protoliths‖of oxide gabbros should be crystallized during the magmaticevolution of Cycle2. Contrasted to the gabbros from Cycle1and Cycle2, oxidegabbroic sample from Unit IV has very high Pb isotopic composition (²⁰⁶Pb/²⁰⁴Pb:19.011;²⁰⁷Pb/²⁰⁴Pb:15.613;²⁰⁸Pb/²⁰⁴Pb:38.384), which indicates that the exoticmaterials involved in the origin of oxide gabbros are not derived from the mantlesource beneath the735B Core.