Hydrogen and oxygen isotope fractionation in hydrous minerals as indicators of fluid source in modern and fossil metasomatic environments [electronic resource]
Keywords: Geological and Environmental Sciences.

Oxygen and hydrogen isotope properties of hydrous silicate minerals formed by weathering, hydrothermal, metamorphic and igneous processes provide a record of fluid-rock interaction. We utilize this isotopic record to 1) determine the source of geothermal fluids in two active geothermal systems in Iceland, and to evaluate the consequences of fluid-rock interaction on host rock, fluid and magma chemistry, and 2) to better characterize Earth's surface environments during the early Archaean. Geothermal systems within the active volcanic zone of Iceland provide a unique natural laboratory for studying fluid-rock interaction in magma-hydrothermal systems where the Mid-Atlantic ridge emerges onto land. The fluids of the Reykjanes geothermal system in southwest Iceland are derived from hydrothermally modified seawater. The anomalously low hydrogen isotope composition of these fluids is not due to mixing with local meteoric fluids, as previously supposed, but to diffusional exchange with relict hydrous alteration minerals, such as epidote, which retain an isotopic signature of glacially derived Ice Age fluids that existed early in the evolution of the geothermal system. In contrast, the meteoric-water dominated Krafla geothermal system, in northeast Iceland, displays wide isotopic heterogeneities in modern geothermal fluids and hydrothermal epidote that reflects a complex fluid evolution involving boiling, condensation and contamination by magmatic volatiles. A silicic melt that intruded the Iceland Deep Drilling Project drillhole IDDP-1 within the Krafla geothermal system appears to be largely derived from partial melting of hydrothermal alteration minerals, given the almost identical hydrogen isotope composition of glass sampled from drill cuttings and hydrothermal epidote. The oxygen isotope values of the rhyolite glass show the characteristically low-[lowercase Delta]18O values typical of Icelandic lavas, and result from mixing of a dominant mantle-derived basalt source and a lesser contribution of lighter oxygen from the incongruent melting of hydrothermally altered basalts within the Krafla caldera. The oxygen and hydrogen isotope characteristics of metamorphic fluids recorded in alteration minerals have applications to fossil metasomatic systems as well as modern ones. Serpentinites from the [greater than or equal to] 3.8 Ga Isua Supracrustal Belt (ISB) of West Greenland locally preserve isotope characteristics of their original formation by seawater alteration of ocean crust and suggest that the early Archaean oceans had oxygen isotopes comparable to modern day seawater, but a hydrogen isotope composition that is lower than modern seawater by 25 ± 5%. The hydrogen isotopes of Archaean oceans places mass balance constraints on the extent of hydrogen escape before the rise of atmospheric oxygen ~2.5 Ga, and by extension the maximum atmospheric methane levels during the early Archaean. The oxygen isotope composition predicted by these serpentinites suggests that the ocean was isotopically buffered by hydrothermal interaction with ocean crust by 3.8 Ga. Finally, chromian muscovite-quartz-carbonate veins in the ISB have oxygen and hydrogen stable isotope, elemental and mineralogical characteristics that are genetically similar to orogenic gold deposits in the fore-arc regions of Phanerozoic accretionary margins. We show that in both modern orogens and in the supracrustal sequence at Isua, these veins are the result of seawater-derived fluids liberated from subducting lithosphere interacting with ultramafic rocks in the mantle wedge and lower crust, before migrating up crustal-scale vertical fracture zones. The presence of these veins in the ISB and other Archaean-age deposits indicates that plate tectonic processes comparable to modern-day subduction existed as early as 3.8 Ga.