Rare earth and trace elements in igneous and high-temperature metamorphic minerals of oceanic gabbros (MARK area, Mid-Atlantic Ridge)

SIMS analyses have been carried out on clinopyroxenes, plagioclases and amp hiboles of six gabbroic samples from Holes 921-924 of the Ocean Drilling Pr ogram Leg 153 sited in the MARK area of the Mid-Atlantic Ridge at the ridge -transform intersection, to investigate the rare earth, trace and volatile element distribution in the lower ocean crust during igneous crystallizatio n and higher grade metamorphic conditions. The metagabbros underwent granul ite to subgreenschist facies conditions through three main tectono-metamorp hic phases: (I) ductile regime (750 < T < 1000 degrees C and P approximate to 0.3 GPa); (2) transitional regime (600 < T < 700 degrees C and P approxi mate to 0.2 GPa); (3) brittle regime (350 less than or equal to T < 600 deg rees C and P < 0.2 GPa). Igneous clinopyroxenes show Cl-chondrite normalize d patterns depleted in LREE, and nearly flat for HREE. The rare earth and t race element distributions in igneous clinopyroxenes and plagioclases indic ate that these minerals act as REE reservoirs, and comprise the main contri bution to the overall rock content. The abundances in igneous minerals refl ect the degree of fractionation of the parent liquids. In metamorphic clino pyroxenes recrystallized in anhydrous assemblages, the REE and trace elemen ts patterns mimic those of the primary ones. Conversely, clinopyroxene re-e quilibrated in amphibolebearing assemblages shows a significant increase in REE, Ti, Zr, Y and V, a negative Eu anomaly, and slight decreases in Sr an d Ba. An overall increase of REE and some trace elements is evident in hydr ous assemblages, with preferential partitioning in the amphibole. It shows high Ti (18196-22844 ppm), LREE depleted patterns and La-N/Sm-N = 0.10-0.33 , La-N/Yb-N = 0.10-0.30 Amphiboles from granoblastic assemblages show homog eneous patterns with no or a positive anomaly for Ti-N and negative anomali es for Sr-N and Zr-N Volatiles in amphibole are low, with Cl/F < 1; H2O% is significantly lower than the stoichiometric ratio (1.33-1.53%). The compos ition of the clinopyroxene and amphibole recrystallized in low-strain domai ns records evidence of incomplete re-equilibration, and element diffusion a nd partitioning is in part controlled by the textural site. The possible or igins of the fluids involved in the motamorphic recrystallization are discu ssed: (1) remobilization from igneous amphibole; (2) exsolution from evolve d melts, (3) introduction of seawater-derived fluids modified in rock-domin ated systems; (4) injection of highly evolved hydrous melts during the meta morphic process.