Oxygen and carbon isotope evidence for seawater-hydrothermal alteration of the Macquarie Island ophiolite

Abstract

Rocks of the Miocene Macquarie Island ophiolite, south of New Zealand, have oxygen and carbon isotopic compositions comparable to those of seafloor rocks. Basalt glass and weathered basalts have δ 18O values at 5.8–6.0‰ and 7.9–9.5‰, respectively, similar to drilled seafloor rocks including samples from the Leg 29 DSDP holes near Macquarie Island. Compared to the basalt glass, the greenschist to amphibolite facies metaintrusives are depleted in 18O (δ 18O=3.2–5.9‰) similar to dredged seafloor samples, whereas the metabasalts are enriched (δ 18O=7.1–9.7‰). Although the gabbros are only slightly altered in thin-section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4.5 km. There is an approximate balance between 18O depletion and enrichment in the exposed ophiolite section. The carbon isotopic composition of calcite in the weathered basalts (δ 13C=1.0–2.0‰) is similar to those of drilled basalts, but the metamorphosed rocks have low δ 13C values (−14.6 to 0.9‰). These data are compatible with two seawater circulation regimes. In the upper regime, basalts were weathered by cold seawater in a circulation system with high water/rock ratios (≫1.0). Based on calcite compositions weathering temperatures were less than 20°C and the carbon was derived from a predominantly inorganic marine source. As previously suggested for the Samail ophiolite, it is postulated that the lower hydrothermal regime consisted of two coupled parts. At the deeper levels, seawater circulating at low water/rock ratios (0.2–0.3) and high temperatures (300–600°C) gave rise to 18O-depleted gabbro and sheeted dikes via open system exchange reactions. During reaction the seawater underwent a shift in oxygen isotopic composition (δ 18O=1.0–5.0‰) and subsequently caused 18O enrichment of the overlying metabasalts. In the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water/rock ratios (1.0–10.0) and temperatures in the range 200–300°C. The relatively low water/rock ratios in the hydrothermal regime are supported by the low δ 13C values of calcite, interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts.