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

Joyashish Thakurta, Edward M. Ripley, Chusi Li

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

Original languageEnglish (US)
Pages (from-to)6298-6312
Number of pages15
JournalGeochimica et Cosmochimica Acta
Issue number20
StatePublished - Oct 15 2009
Externally publishedYes

Bibliographical note

Funding Information:
This work was supported by NSF Grant EAR 0335131 to Edward Ripley and Chusi Li. We thank Tom Patton of Quaterra Resources Inc. and Curt Freeman and Chris VanTreeck of Avalon Development Corporation for access to the drill core samples and helicopter support during field work. The manuscript was highly benefitted by the feedback from the associate editor David Cole, and comments from Ted Labotka and two other anonymous reviewers. We acknowledge constructive reviews of an earlier draft of this manuscript by James Brophy and Erika Elswick. We also thank Craig Moore, Peter Sauer, David Bish and Curtis Williams for help with lab work.


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