Trace element mobility and lithium isotope exchange during hydrothermal alteration of seafloor weathered basalt: an experimental study at 350°C, 500 bars

The relative mobility of trace incompatible elements and exchange of lithium isotopes during alteration of basalt previously altered by exposure to seawater at low temperatures (seafloor weathered) were experimentally determined at 350°C, 500 bars. These data are important as a means to test models calling for hydrothermal alteration of weathered basalt as a source of alkali elements for hot spring vent fluids at mid-ocena ridges. Weathered basalt used for the experiment was characterized by high Cs, Rb, B, K, and Li concentrations, high Cs/Rb, and low K/Rb and Ba/Rb ratios, respectively, relative to fresh basalt. The basalt is enriched in ⁷Li consistent with a history involving Li uptake during seafloor weathering. The experiment involved a Na-Ca-K-Cl fluid and flexible cell hydrothermal equipment, which permitted periodic sampling of the fluid phase at experimental conditions. The extraction efficiency of all trace incompatible elements was significant and ranged from 40 to 80%. Cesium, rubidium, lithium, and boron were particularly mobile, whereas Ba, Sr, and to a lesser degree, K were taken up by alteration minerals. Early stage release of Li was dominated by ⁷Li, most probably as a consequence of the dissolution of low temperature alteration phases in the weathered basalt that were rendered unstable at the elevated temperatures of the experiment. Subsequently, however, an increase in ⁶Li in solution was observed owing to a combination of mineral dissolution (from Li-bearing components in fresh basalt) and isotopic equilibration effects. Owing to the mobility of Li during hydrothermal alteration of weathered basalt, it is unlikely that dissolution of weathered basalt components in subseafloor reaction zones (350-400°C) could occur without noticeably affecting the lithium isotope composition of vent fluids, as inferred previously from field studies. Thus, the relatively high Cs/Rb ratios of some vent fluids, 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 excess of 400°C. Cs/Rb ratios in vent fluids from a wide range of Pacific and Atlantic-ridge localities, however, reveal relatively large variations in space and time, which suggest that a single process is unlikely to account for all of the data. The relative mobility of trace alkali elements in weathered basalt not only constrains models of alkali element enrichment in hot spring vent fluids at mid-ocean ridges, but also provides data bearing on potential sources of alkali elemenmts for arc magmas in subduction zones.