In:
Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 87, No. B8 ( 1982-08-10), p. 6559-6565
Abstract:
Oxygen isotope ratios of hydrothermally altered basalts from depths of up to ∼3 km are reported from three localities in Iceland: International Research Drilling Project (IRDP) core at Reydarfjordur, eastern Iceland (Tertiary age); drill cuttings from Reykjavik (Plio‐Pleistocene age); and Holocene drill cuttings from the active Krafla central volcano. Whole rock samples from these three localities have δ 18 O values averaging +3.9±1.3, +2.4±1.1, and −7.7±2.4‰, respectively. The observed values in the deeper samples from Krafla are as low as the values for any rocks previously reported. There seems to be a slight negative gradient in δ 18 O with depth at the former two localities and a more pronounced one at Krafla. Oxygen isotope fractionations between epidote and quartz and those between calcite and fluid suggest that the basalts were altered at temperatures of 300°–400°C. Low δD and δ 18 O of epidote and low δ 34 S of anhydrite indicate that the altering fluids in all three areas originated as meteoric waters and have undergone varied ‘oxygen shifts.’ Differences in the 18 O shift of the fluids are attributed to differences in hydrothermal systems; low water/rock ratios (≤0.1) are found in the IRDP core and the Reykjavik hole, but high water/rock ratios ( 〉 5) at Krafla. The convective hydrothermal activity, which is probably driven by silicic magma beneath the central volcanoes, has caused strong subsolidus depletion of 18 O in the rocks. The primary‐magmatic δ 18 O value of the rocks in the Tertiary IRDP core was about +3.9‰, which is lower than that obtained for fresh basalt from other places. Such exceptionally low δ 18 O magmas are common in Iceland and may occur as the result of oxygen isotope exchange with or assimilation of altered rocks that form a thick sequence beneath the island due to isostatic subsidence.
Type of Medium:
Online Resource
ISSN:
0148-0227
DOI:
10.1029/JB087iB08p06559
Language:
English
Publisher:
American Geophysical Union (AGU)
Publication Date:
1982
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