Comparing yttrium and rare earths in hydrothermal fluids from the Mid-Atlantic Ridge: implications for Y and REE behaviour during near-vent mixing and for the Y/Ho ratio of Proterozoic seawater

摘要

Mg-poor hydrothermal fluids from the high-temperature discrete flow at the Broken Spur site at the Mid-Atlantic Ridge show high Y concentrations between 1880 and 2639 pmol/kg, and almost chondritic Y/Ho molar ratios between 52 and 55. A sample contaminated with ambient seawater is lower in Y (661 pmol/kg), and yields an elevated Y/Ho ratio of 84. The diffuse flow at the TAG hydrothermal mound shows between 628 and 1785 pmol/kg of Y, and Y/Ho molar ratios between 57 and 65. Similar Y/Ho ratios in black-smoker fluids and MORBasalts argue against an important role of Rare Earths and yttrium (REY) fluoride complexes in the solutions, but are compatible with a REY speciation dominated by chloride complexes and 'free' REY~(3+) ions. Close to the vent orifice, Y behaves conservatively during mixing of high-temperature hydrothermal fluid with entrained seawater. This is in marked contrast to the behaviour of the rare earth elements (REE) which are partly scavenged by Fe oxyhydroxides within less than l m distance from the vent orifice, resulting in a strong increase of the Y/Ho ratio. Non-conservative mixing behaviour of the REE may result in underestimation of REE concentrations in the hydrothermal end-member when calculations are based on conservative elements, such as Mg, An approach combining Mg concentration and Y/Ho ratio may reduce this problem. Despite the lower particle-reactivity of Y compared to the REE, there is no hydrothermal Y input into present-day oxic seawater. And marine hydrothermal vent sites are sinks for dissolved Y rather than sources. However, REE elemental and Nd isotopic systematics of Precambrian banded iron-formations reveal the existence of a high-temperature hydrothermal REY flux into Early Precambrian an-or suboxic seawater. The results of our study indicate a chondritic Y/Ho ratio of this black-smoker-type hydrothermal REY input, and suggest that Paleoproterozoic surface seawater showed super-chondritic Y/Ho ratios similar to those of present-day seawater.