RARE EARTH AND TRACE ELEMENTS IN AMPHIBOLES OF OCEANIC GABBROS (MARK AREA, MID-ATLANTIC RIDGE) AT MEDIUM- TO LOW-TEMPERATURE SEAFLOOR ALTERATION

Abstract

The gabbro pile cored at the ODP sites 921-924 (Mid-Atlantic Ridge) underwent subseafloor alteration under polyphase retrograde recrystallization from very high temperatures in a ductile deformation regime to amphibolite and greenschist facies under transitional to brittle regimes (Gaggero and Cortesogno, 1997). To complement the study on the highest grade conditions (Cortesogno et al., 2000), amphiboles developed in different textural sites during the amphibolite to greenschist facies alteration of an olivine gabbro and an Fe-Ti oxide gabbro were analysed for REE, HFSE, LILE, LLE (Li, Be, B) and volatile elements (H, F, Cl) using SIMS. Brown, amphibolite facies, Mg-hornblendes grading to actinolitic hornblendes (Ti 16795-8645 ppm), which had replaced pyroxenes or filled veins, were analysed. The normalised C1 patterns of the pseudomorphic grains were, on the whole, comparable to those of the higher-temperature titanian amphiboles (Cortesogno et al., 2000), whereas the hornblendes filling the veins showed abundances of normalised Nb, La, Ce, Nd, Sm, Gd, Dy, Y, Er, Yb about 10 times higher with moderately increased Eu. On the contrary, Sr, Zr, Ti, Cr were lower. The REE and trace elements were largely controlled by heritage from clinopyroxene, water–amphibole partitioning and water–rock interactions. Green hornblendes (Ti 3492-207 ppm) that had filled the veins within the amphibolite facies, at lower temperatures, under the brittle regime, had C1 normalised patterns parallel to those of the brown hornblendes filling the fractures; the element contents being controlled by microsite equilibria. On the whole, the late actinolite filling of the fractures had low trace and RE element contents, and a positive Eu anomaly; the trace and REE element level decreased in the pseudomorphic grains. The tremolite pseudomorphic on olivine had a C1 normalised pattern parallel to, but with lower values than, the veinfilling hornblendes, except for very low Nb. The pattern was also comparable to the vein-filling actinolites, except for higher Dy, Y, Er, Yb and a negative Eu anomaly. On the whole the H2O content of the amphiboles decreased and F increased with the metamorphic temperatures, whereas Cl increased in the greenschist facies actinolites.