Light stable isotope systematics of granitic pegmatites from North America and Norway

Abstract

Oxygen, hydrogen and carbon isotope analysis of minerals, whole rocks and fluid inclusions were performed on pegmatites and associated wall rocks from ten pegmatite districts and individual occurrences of Precambrian—Late Cretaceous age. The pegmatites and pegmatite districts include the Evje—Iveland area, Norway; Harding pegmatite, New Mexico, U.S.A.; Tanco pegmatite, Manitoba, Canada; Kiawa pegmatite, New Mexico, U.S.A.; and the Crestmore pegmatite, California, U.S.A. The results of more than 350 isotopic analyses indicate no principal isotopic differences among pegmatites of widely different age. The δ 18O of quartz for analyzed pegmatites varies from δ 18O = +8.6 to +11.8‰- The δ 18O of quartz from a sanidine-bearing pegmatite in a sub-volcanic environment is +7.10‰. Late-stage lower-temperature quartz is typically the heaviest in any pegmatite. The whole-rock isotopic compositions of the pegmatites analyzed appear to largely reflect composition of the melt, although some isotopic exchange with the wall rocks may have occurred. Oxygen isotope fractionations can be used to indicate a magmatic temperature range for crystallization (540–750°C), compatible with experimental data on Li-bearing and simple granitic bulk compositions. Lower solidus temperatures for the Li-bearing pegmatites are reflected by larger isotopic fractionations. Continued, subsolidus crystallization (and replacement) of many pegmatite minerals is compatible with both isotopic, mineralogie and textural evidence. The hydrogen isotope composition of muscovite and biotite varies for most samples from −45 to −70‰, depending to a large degree on the temperature of crystallization. δ D of water from primary fluid inclusions from the same samples are enriched in D relative to the hydrous minerals. The hydrogen isotope fractionations between inclusion fluid and muscovite or biotite indicate in most cases a range in crystallization temperature compatible with that suggested on the basis of oxygen isotope fractionations and experimental data. Interaction with meteoric water is not typical of pegmatite emplacement and crystallization. The carbon isotope composition of C0 2 in most fluid inclusions is typically −6.1 to −7.1‰ Several δ 13C-values heavier than −6.1‰ (to −2.2‰) may record either contamination or isotopic effects due to separation of C02 from the melt.