Mathematics – Logic
Scientific paper
Apr 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979gecoa..43..603w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 43, Issue 4, pp.603-614
Mathematics
Logic
4
Scientific paper
D / H , 18 O / 16 O and 13 C / 12 C 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 D 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 18 O (+9.3 to +14.4) and ( 13 C (-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 18 O 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 18 O = +4.1 to +11.5 and D = -50 to -86; one sample containing minor amounts of lizardite-chrysotile serpentinite has D = -92, outside this range. However, most rodingites of basaltic or gabbroic parentage are more restricted in 18 O (+4.1 to +8.6). Such a wide range in 18 O 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 18 O / 16 O and 13 C / 12 C 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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