Oxygen isotopic variability associated with multiple stages of serpentinization, Duke Island Complex, southeastern Alaska

摘要

Ultramafic rocks of the Duke Island Complex in southeastern Alaska crystallized in a supra-subduction zone setting, butthe serpentinization of olivine-bearing rocks involved the incursion of late-stage meteoric waters. Three textural types of ser-pentine (primarily lizardite) have been identified which in part reflect progress in reactions during multiple stages of fluid infil-tration. The overall mesh texture of serpentine has been subdivided into a massive-type, found in dunites and wehrlites, and adendritic-type found in wehrlites and olivine clinopyroxenites. Serpentine veins represent a late-stage in the hydrothermalalteration process. Both FeO contents and δ180 values of the three textural types of serpentine are variable at the centimeterscale. Magnetite abundance in association with serpentine is also variable with up to 5 vol% of magnetite found in sampleswith dendritic serpentine. Continued reaction of Fe0-bearing serpentine with fluid appears to control the formation of mostmagnetite. Oxygen isotope ratios of the three textural types of serpentine are distinct, with the massive variety characterizedby δ18O values between -3‰ and 3‰, the dendritic variety showing values between 2‰ and 6‰ and the veins having thehighest values between 4‰ and 10‰. Although the δ18o values may vary by as much as 5%0 on the centimeter scale, δD valuestend to show relatively less variation with over 90% of the measured values between -100‰ and -120‰. The 0 and H iso-topic values are consistent with the involvement of meteoric water that had undergone variable degrees of isotopic exchangewith country rocks prior to reacting with olivine in the Duke Island Complex. Small-scale variability in both serpentine FeOcontent and δ180 values suggests that chemical and isotopic equilibria may have not been attained at larger than centimeterscales. Oxygen isotopic variability in serpentine produced during relatively low-temperature hydrothermal alteration is inlarge part a function of exchange mediated via fluid flow through microfractures.