ALBERT

Description: Major- and trace-element as well as Pb-isotope data are presented for greenschist- to amphibolite-facies greenstones from two locations: (1) metabasalt-breccias from the Galfipagos Rift at 100°W; and (2) metabasalt-breccias and unbrecciated greenstones from the Chile Ridge at 38°S. Greenstone-breccias from both locations display stockwork-like sulfide mineralizations related to the upflow portion of a hydrothermal convection system. Whereas Galfipagos Rift stockwork sulfides belong to the "normal" type containing Cu-Fe-sulfides, Chile Ridge stockwork sulfides are galena-rich Pb-Zn _ Cu-sulfides and represent a previously unknown type of sulfide mineralisation in a MORB environment. Geochemically, the greenstones can be divided into two types: (1) The unbrecciated galena-free greenstones from both Chile Ridge and Galfipagos Rift show at least a 10-20-fold Pb enrichment compared to fresh MORB. With respect to similar Pb enrichment measured in greenstones from two other locations, i.e. DSDP Hole 504B and Galfipagos Rift near 86°W, we suggest that this may be a general feature of all stockwork-mineralised oceanic greenstones. (2) The galena- and quartz-rich metabasalt-breccias from the Chile Ridge are up to 3000-fold enriched in Pb (up to 1000 Ixg/g Pb in the whole-rock analyses) compared to MORB and indicate Pb mineralisation two orders of magnitude higher than that of the "normal" greenstone-type. A mass-balance calculation carried out using crustal column thickness of 3000 m with a 200-m-thick greenstone layer and 0.15 m of galena-beating breccias shows that at the Chile Ridge ~ 42% of the entire Pb is concentrated in the greenstones. This suggests that the rest of the crustal column is depleted in 58% of its primary Pb content. This degree of depletion matches well with previous calculations that a 56% depletion of Pb in oceanic crust subjected to mantle-recycling via subduction would be necessary to yield a HIMU mantle source within 2 Ga. Despite the need for future investigations into the extent and volume of the Pb enrichment in Chile Ridge greenstones, we believe that this process of major Pb redistribution is capable of creating huge volumes of oceanic crust that on average are extremely Pb-depleted and which when recycled would produce the HIMU source. Keywords: Chile Ridge; Galfipagos Ridge; Hydrothermal alteration; Greenstones; Geochemistry; Mineralogy