TRACE-ELEMENT MOBILITY AND LITHIUM ISOTOPE-EXCHANGE DURING HYDROTHERMAL ALTERATION OF SEA-FLOOR WEATHERED BASALT - AN EXPERIMENTAL-STUDY AT350-DEGREES-C, 500 BARS

The relative mobility of trace incompatible elements and exchange of l ithium isotopes during alteration of basalt previously altered by expo sure to seawater at low temperatures (seafloor weathered) were experim entally determined at 350 degrees C, 500 bars. These data are importan t as a means to test models calling for hydrothermal alteration of wea thered basalt as a source of alkali elements for hot spring Vent fluid s at mid-ocean ridges. Weathered basalt used for the experiment was ch aracterized by high Cs, Rb, B, K, and Li concentrations, high Cs/Rb, a nd low K/Rb and Ba/Rb ratios, respectively, relative to fresh basalt. The basalt is enriched in Li-7 consistent with a history involving Li uptake during seafloor weathering. The experiment involved a Na-Ca-K-C l fluid and flexible cell hydrothermal equipment, which permitted peri odic sampling of the fluid phase at experimental conditions. The extra ction efficiency of all trace incompatible elements was significant an d ranged from 40 to 80%. Cesium, rubidium, lithium, and boron were par ticularly mobile, whereas Ba, Sr, and to a lesser degree, K were taken up by alteration minerals. Early stage release of Li was dominated by Li-7, most probably as a consequence of the dissolution of low temper ature alteration phases in the weathered basalt that were rendered uns table at the elevated temperatures of the experiment. Subsequently, ho wever, an increase in Li-6 in solution was observed owing to a combina tion of mineral dissolution (from Li-bearing components in fresh basal t) and isotopic equilibration effects. Owing to the mobility of Li dur ing hydrothermal alteration of weathered basalt, it is unlikely that d issolution of weathered basalt components in subseafloor reaction zone s (350-400 degrees C) could occur without noticeably affecting the lit hium isotope composition of vent fluids, as inferred previously from f ield studies. Thus, the relatively high Cs/Rb ratios of some vent flui ds, which were thought to be the result of hydrothermal alteration of weathered basalt, may be caused by other effects, such as partitioning of Rb relative to Cs into hydrothermal minerals at temperatures in ex cess of 400 degrees C. Cs/Rb ratios in vent fluids from a wide range o f Pacific and Atlantic-ridge localities, however, reveal relatively la rge variations in space and time, which suggest that a single process is unlikely to account for all of the data. The relative mobility of t race alkali elements in weathered basalt not only constrains models of alkali element enrichment in hot spring vent fluids at mid-ocean ridg es, but also provides data bearing on potential sources of alkali elem ents for are magmas in subduction zones. Copyright (C) 1998 Elsevier S cience Ltd.