One of the world's largest wollastonite deposits was formed at the
contact of the northern Hunter Mountain Batholith (California, USA) in
Paleozoic sediments. Wollastonite occurs as zones of variable thickness
surrounding layers or nodules of quartzite in limestones. A minimum
formation temperature of 650 degrees C is estimated from isolated
periclase-bearing lenses in that area. Contact metamorphism of siliceous
carbonates has produced mineral assemblages that are consistent with
heterogeneous, and partly limited infiltration of water-rich fluids,
compatible with O-18/O-16 and C-13/C-12 isotopic patterns recorded in
carbonates. Oxygen isotope compositions of wollastonites in the study
area may also not require infiltration of large quantities of
externally-derived fluids that were out of equilibrium with the rocks.
8180 values of wollastonite are high (14.8 parts per thousand to 25.0
parts per thousand; median: 19.7 parts per thousand) and close to those
of the host limestone (19.7 parts per thousand to 28 parts per thousand;
median: 24.9 parts per thousand) and quartz (18.0 parts per thousand. to
29.1 parts per thousand; median: 22.6 parts per thousand). Isotopic
disequilibrium exists at quartz/wollastonite and wollastonite/calcite
boundaries. Therefore, classical batch/Rayleigh fractionation models
based on reactant and product equilibrium are not applicable to the
wollastonite rims. An approach that relies on local instantaneous mass
balance for the reactants, based on the wollastonite-forming reaction is
suggested as an alternative way to model wollastonite reaction rims.
This model reproduces many of the measured delta O-18 values of
wollastonite reaction rims of the current study to within +/- 1 parts
per thousand, even though the wollastonite compositions vary by almost
10 parts per thousand. (C) 2011 Elsevier B.V. All rights reserved.