Water/rock interactions and the origin of H₂O in granitic batholiths: Thirtieth William Smith lecture

Abstract

D\H and 18O\16O data have now been obtained on a wide variety of granitic batholiths of various ages. The primary δD values of the biotites and hornblendes are remarkably constant at about –50 to –85, identical to the values in regional metamorphic rocks, marine sediments and greeenstones, and most weathering products in temperate climates. Therefore the primary H2O in these igneous rocks is probably not ‘juvenile’, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducated lithosphere. Most granitic rocks have δ18O = + 7.0 to +10.0, probably indicating significant involvement of high -18O metasedimentary or altered volcanic rocks in the melting process; such an origin is required for many other granodiorites and tonaloites that have δ18O = + 10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ18O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low-18O magmas probably formed by melting or assimilation of hydrothermally altered roofrocks. However, the water/rockratios were typically low enough that over wide areas the only evidence for meteoric H2O exchange in the batholiths is given by low D/H ratios (δD as low as –180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves from through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks haves δD values lower than –120. The lowering of δD values commonly correlates.