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Hydrogen, oxygen and carbon isotopic evidence for the origin of rodingites in serpentinized ultramafic rocks

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/0016-7037(79)90168-6
  • Earth Science
  • Geography


Abstract D H , 18O 16O and 13C 12C analyses were made of 14 whole rock and 28 mineral samples of rodingites associated dominantly with lizardite-chrysotile serpentinites from the West Coast of the U.S.A., New Zealand, and the Northern Appalachian Mtns. The δD values of the rodingite minerals are in three groupings: 5 monomineralic veins of pectolite, −281 to −429; 8 monomineralic veins of xonotlite, −112 to −135; all other minerals, including hydrogarnet, idocrase, prehnite, actinolite, nephrite, and chlorite, −34 to −80. Most calcites in rodingites have δ 18O (+9.3 to +14.4) and (δ 13C (−6.7 to +0.9) values similar to calcites in other Franciscan rocks, but distinct from the very low temperature calcite veins in serpentinites. The D H data, combined with δ 18O values of xonotlite (+5.7 to +10.9) and pectolite (+8.9 to +12.4) suggest formation from meteoric-type waters at low temperatures; the D H depletion of pectolite, however, is anomalous. Rodingite whole rock values range from δ 18 O = +4.1 to +11.5 and δD = −50 to −86; one sample containing minor amounts of lizardite-chrysotile serpentinite has δD = −92, outside this range. However, most rodingites of basaltic or gabbroic parentage are more restricted in δ 18O (+4.1 to +8.6). Such a wide range in δ 18O is consistent with the idea that most rodingites form over a relatively broad range of hydrothermal temperatures. Hydrogen isotopic data for most rodingite minerals (except xonotlite and pectolite) and for whole rocks are suggestive of non-meteoric waters. These D H data overlap those observed for veins of hydrous minerals found in Franciscan igneous rocks studied by Margaritz and Taylor (1976, Geochim. Cosmochim. Acta 40, 215–234), possibly suggesting evolved D-enriched, connate type metamorphic waters generated during high P, low T Franciscan-type metamorphism at temperatures (250–500°C) comparable to estimates based on mineral stabilities. Such an interpretation is supported by the 18O 16O and 13C 12C data for calcite in rodingites. The isotope data appear to contradict some of the conclusions derived from geologic and petrologic studies that indicate concomitant metasomatism and serpentinization of their presently observed host rock. These data appear most consistent with the interpretation that most rodingite minerals, with the exception of late-stage veins of xonotlite and possibly pectolite, may involve metasomatism in association with antigorite serpentinization of ultramafic rock. Subsequent upward tectonic transport in many instances may result in incorporation of the rodingites into their presently observed lizarditechrysotile host rock during or subsequent to pervasive shallow level serpentinization by meteoric waters.

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