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Hydrogen and oxygen isotopic studies of serpentinization of ultramafic rocks

Citation

Wenner, David Bruce (1971) Hydrogen and oxygen isotopic studies of serpentinization of ultramafic rocks. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-11082007-141710

Abstract

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Lizardite-chrysotile serpentines in alpine ultramafic bodies from North America show latitudinal variations in [...] and [...] which indicate that meteoric ground waters were important in their formation. In per mil relative to SMOW they exhibit the following geographic variations in [...] and [...], respectively: British Columbia -205 to -150, 1.9 to 8.2; Washington-Oregon -142 to -107; 4.1 to 8.0; California -108 to -82, 6.6 to 8.8; Caribbean-Central America -97 to -59, 3.3 to 8.7. The [...]-latitude correlation is exceedingly good, but the [...]-values appear to be in large part dependent upon the [...] of the country rock. This suggests that the waters involved in serpentinization had previously exchanged [...] with these country rocks. For example, the Cassiar, British Columbia, serpentinite body has [...]-195 to -162 and [...] = 8.0 to 8.2, whereas the serpentines in zoned ultramafic bodies of SE Alaska have [...] = -127 to -140, [...] = -5.0 to -1.6; the former body lies in a terrane of abundant [...]-rich limestones, whereas the zoned bodies are surrounded by low-[...] igneous rocks. All antigorites, irrespective of geographic location, have very uniform isotopic compositions ([...] = -66 to -39, [...] = 4.7 to 8.8). This is practically identical to the range exhibited by metamorphic chlorites,.indicating, that a deep-seated or metamorphic type water was involved in.antigorite-type serpentinization. This isotopic uniformity further indicates that little or no isotopic exchange has occurred subsequent to formation of the antigorites, because many samples were collected from northerly regions where the ground waters are very low in [...] and [...]. Five lizardite-chrysotile type serpentinites-from California, Guatemala and the Dominican Republic show oxygen isotope fractionations of 15.1 to 12.9 per mil between coexisting serpentine and magnetite ([...] magnetite = -7.6 to -4.6 per mil relative to SMOW). Nine antigorites (mainly from Vermont and SE Pennsylvania) show distinctly smaller fractionations of 8.7 to 4.8 per mil ([...] magnetite = -2.6 to +1.7 per mil). Two lizardite and chrysotile serpentinites dredged from the Mid-Atlantic Ridge exhibit fractionations of 10.0 and 12.5 per mil ([...] magnetite - -6.8 and -7.9 per mil, respectively), whereas antigorite from this area shows a value of 8.2 per mil ([...] magnetite = -6.2). These data all clearly indicate that the antigorites formed at higher temperatures than the chrysotile-lizardites. Electron microprobe analyses of magnetites from the above samples show that they are chemically homogeneous and essentially pure [...]. A few magnetite samples from serpentinites in near proximity to later granitic intrusions showa a wide variation of Cr content, and also give erratic oxygen isotopic results, suggesting non-equilibrium. A rough serpentine-magnetite geothermometer curve was constructed by extrapolation of (1) observed [...]-values of chlorite and Fe-Ti oxides in low-grade pelitic schists whose isotopic temperatures are known from the quartz-muscovite geothermometer, and (2) estimates of the [...] fractionation between chlorite and serpentine (assumed to be zero). This serpentine-magnetite geothermometer suggests approximate equilibrium temperatures as follows: continental lizardite-chrysotile, [...]; oceanic lizardite and chrysotile, [...], respectively; continental antigorites, [...]; and oceanic antigorite, [...]. The isotopic compositions of the waters involved in serpentinization of various ultramafic rocks have been estimated, utilizing the above "temperatures". These calculations indicate that most lizardite-chrysotile serpentinization probably involved exchanged meteoric waters, and thus must have occurred at relatively shallow levels in the earth's crust, consistent with the proposed isotopic "temperatures". However, this type of serpentinization is clearly not a weathering phenomenon taking place at the earth's surface. Isotopic evidence for such low temperature serpentinization ([...] of +11.8 to +12.5) has been discovered only for several “deweylite" samples from Pennsylvania and Delaware. Serpentines from the ocean floor (Mid Atlantic Ridge, Puerto Rican trench, and Blanco Fracture zone in the Pacific ocean) form a unique isotopic grouping, with [...] = -68 to -31, [...] = +0.8 to +6.7. The waters that would have coexisted with these serpentines (at the temperatures inferred from serpentine-magnetite 018 fractionations) have calculated isotopic compositions that cluster around SMOW. This strongly suggests that ocean water is the major component of the waters responsible for sub-oceanic serpentinization. A few of the low-[...] oceanic serpentines are, however, consistent with formation from magmatic waterocean water mixtures. No serpentines presently found in continental areas give any isotopic evidence of ever having been serpentinized in oceanic environments. This statement also applies to the serpentines in a typical ophiolite sequence, the Vourinos Complex, Greece. Serpentine from this body is unique, however, in that it exhibits remarkably large [...] variation, from +2.1 to +12.7. All other bodies that were examined in detail were found to have very uniform isotopic compositions.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geochemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Taylor, Hugh P.
Thesis Committee:
  • Taylor, Hugh P. (chair)
  • Albee, Arden Leroy
  • Silver, Leon T.
  • Epstein, Samuel
Defense Date:30 December 1970
Record Number:CaltechETD:etd-11082007-141710
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-11082007-141710
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4466
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:05 Dec 2007
Last Modified:26 Dec 2012 03:08

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