Imprint of meteoric water on the stable isotope compositions of igneous and secondary minerals, Kap Edvard Holm Complex, East Greenland

Abstract

Hydrogen and oxygen isotope analyses have been made of hydrous minerals in gabbros and basaltic xenoliths from the Eocene Kap Edvard Holm intrusive complex of East Greenland. The analyzed samples are of three types: (1) primary igneous hornblendes and phlogopites that crystallized from partial melts of hydrothermally altered basaltic xenoliths, (2) primary igneous hornblendes that formed during late-magmatic recrystallization of layered gabbroic cumulates, and (3) secondary actinolite, epidote and chlorite that formed during subsolidus alteration of both xenoliths and gabbros. Secondary actinolite has a δ¹⁸O value of — 5.8‰ and a δD value of — 158‰. These low values reflect subsolidus alteration by low-δ¹⁸O, low-δD hydrothermal fluids of meteoric origin. The δD value is lower than the —146 to – 112‰ values previously reported for amphiboles from other early Tertiary meteoric-hy-drothermal systems in East Greenland and Scotland, indicating that the meteoric waters at Kap Edvard Holm were isotopically lighter than typical early Tertiary meteoric waters in the North Atlantic region. This probably reflects local climatic variations caused by formation of a major topographic dome at about the time of plutonism and hydrothermal activity. The calculated isotopic composition of the meteoric water is δD = - 110 ± 10‰, δ¹⁸O ≈ - 15‰. Igneous hornblendes and phlogopites from pegmatitic pods in horn-felsed basaltic xenoliths have δ¹⁸O values between- 6.0 and - 3.8‰ and δD values between - 155 and- 140‰. These are both much lower than typical values of fresh basalts. The oxygen isotope fractiona-tions between pegmatitic hornblendes and surrounding hornfelsic minerals are close to equilibrium fractiona-tions for magmatic temperatures, indicating that the pegmatites crystallized from low—δ¹⁸O partial melts of xenoliths that had been hydrothermally altered and depleted in ¹⁸O prior to stoping. The pegmatitic minerals may have crystallized with low primary δD values inherited from the altered country rocks, but these values were probably overprinted extensively by subsolidus isotopic exchange with low-δD meteoric-hydrothermal fluids. This exchange was facilitated by rapid self-diffusion of hydrogen through the crystal structures. Primary igneous hornblendes from the plutonic rocks have δ¹⁸O values between + 2.0 and + 3.2‰ and δD values between - 166 and - 146‰. The ¹⁸O fractionations between hornblendes and coexisting augites are close to equilibrium fractionations for magmatic temperatures, indicating that the hornblendes crystallized directly from the magma and subsequently underwent little or no oxygen exchange. The hornblendes may have crystallized with low primary δD values, due to contamination of the magma with altered xenolithic material, but the final δD values were probably controlled largely by subsolidus isotopic exchange. This inference is based partly on the observation that coexisting plagioclase has been extensively depleted in ¹⁸O via a mineral-fluid exchange reaction that is much slower than the hydrogen exchange reaction in hornblende. It is concluded that all hydrous minerals in the study area, whether igneous or secondary, have δD values that reflect extensive subsolidus isotopic equilibration with meteoric-hydrothermal fluids.