Lithium isotope as a proxy for water/rock interaction between hydrothermal fluids and the oceanic crust: A case study at Milos Greece

Abstract

Hydrothermal activity at Milos in the Aegean island (Greece) is mainly located at rather shallow depth (about 5 m) It is interesting to compare these chemical compositions and the evolution processes of the hydrothermal fluids at deep sea hydrothermal vents in Mid-ocean Ridge (MOR) Lithium (Li) is a highly mobile element and its isotopic composition varies at different geological settings Therefore Li and its isotope could be used as an indicator for many geochemical processes Since 6Li is preferential retained in the mineral phase while 7Li is leached into fluid phase during low-temperature basalt alteration the Li isotopic fractionation between the rocks and the fluids reflects sensitively the degree of water-rock interaction In this study Bio-Rad AG-50W X8 cation exchange resin was used for purifying the hydrothermal fluids to separate Li from other matrix elements The Li isotopic composition (δ7Li) was determined by Multi-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) with an average precision of better than 0 2? (2σ n=24) The Li concentration in the hydrothermal fluids falls between 0 03 to 10 31 mM During phase separation lithium boron chlorine iodine bromine sodium and potassium were enriched in the brine phase On the other hand aluminum sulphur and iron were enriched in the vapor phase There is no significant isotopic fractionation has occurred between the two phases The δ7Li values vary from +1 4 to +31 3? indicating significant seawater contamination have occurred These hydrothermal fluids fit well with seawater and brine two end-member binary mixing model There are several main controls in the δ7Li including seawater contamination seawater-basalt interaction and other potential sources The calculated water/rock ratio (W/R) is rather high for the Milos fluids significant amount of seawater recharge into the oceanic crust Moreover the oceanic crust in the region becomes higher altered since the W/R is rather high Hydrothermal activity in Milos may be a non-negligible source of Li and the Li isotope of the hydrothermal fluids can be used as a sensitive tool for studying water-rock interaction

Date of Award	2014 Aug 26
Original language	English
Supervisor	Chen-Feng You (Supervisor)