Water/rock interactions and the origin of H 2 O in granitic batholiths: Thirtieth William Smith lecture

Abstract

D\H and 18 O\ 16 O 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 H 2 O 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 δ 18 O = + 7.0 to +10.0, probably indicating significant involvement of high - 18 O metasedimentary or altered volcanic rocks in the melting process; such an origin is required for many other granodiorites and tonaloites that have δ 18 O = + 10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ 18 O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low- 18 O 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 H 2 O 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