Abstract

Chert nodules in limestone in the aureole of the Christmas Mountains gabbro are rimmed by wollastonite in the interval 102-25 m from the intrusive contact and by tilleyite or spurrite and wollastonite within 25 m of gabbro. Wollastonite rims thicken from 4.5 mm at 102 m to 7 mm at 80 m to 30 mm at 49 m and diminish in thickness from 79 to 53 mm over the interval 13-2 m from the gabbro. Wollastonite has two textural components. Polygonal matrix wollastonite grains coarsen from 0.011 mm at 102 m to 0.053 mm at 46 m to 0.2-0.3 mm within 20 m of the contact, whereas scattered wollastonite porphyroblasts appear at 30 m and coarsen to impingement with a diameter of 1.07 mm at 12 m.

Kinetic models for normal grain growth and for layer thickening set a function of the square of the grain diameter or layer thickness equal to the product of a material constant and a temperature-time integral that includes the Arrhenius function for the diffusion coefficient. The time index for non-isothermal diffusion-controlled mineral growth is the temperature-time integral numerically evaluated along T-t curves for the thermal history of the contact aureole. The thermal history of the Christmas Mountains aureole is obtained using a numerical model for a cylindrical intrusion, radius = 823 m, that matches maximum temperatures in the contact aureole of 600, 940, 1000, and 1030 °C at 115, 23, 13, and 0 m by convection for 700 yr followed by crystallization and cooling. The log of the T-t integral varies linearly with distance from the gabbro, with a slope proportional to activation energy and an intercept proportional to the material constant.

A plot of the log of the square of the grain diameter of matrix wollastonite against distance from the gabbro is linear, and the normal grain-growth model yields the following Arrhenius function for the diffusion of oxygen in wollastonite grain boundaries. DGBO2/δ = 9.333 x 10-4 [m/s] exp (-185/RT) [kJ/mole], where δ is grain-boundary width.

The fixed ratio of tilleyite to wollastonite rim thickness on nodules from the inner aureole yields a molar ratio of tilleyite to wollastonite in C|T|W|Q structures of 1T:24W and requires consumption of calcite and quartz in the ratio 29C:26Q. Truncation of fully coarsened impingement wollastonite by columnar tilleyite implies that the tilleyite layer grew at expense of wollastonite and that wollastonite coarsened at temperatures greater than 940 °C. Solution of a system of mass balance, conservation, and flux ratio equations describing diffusion-controlled growth of a C|T|W|Q nodule matches the net mass transfer and observed textural features for Onsager diffusion-coefficient ratios of LCaO/LSiO2 = 42 and LCaO/LCO2<1.

Grain diameter of impingement wollastonite varies linearly with the radius of spherical wollastonite nodules that ceased growth on consumption of the quartz core, implying that coarsening coincided in time with wollastonite rim growth. Grain-boundary cross section of wollastonite decreased by two orders of magnitude during layer growth. Coupling of the kinetics of coarsening of impingement wollastonite with diffusion-controlled growth of the wollastonite layer yields the following Arrhenius function for the Onsager diffusion coefficient for diffusion of CaO in wollastonite grain boundaries. δLGBCaO = 1.731 x 10-4/RT [mole2/J.s]exp (-220/RT) [kJ/mole]. The pre-exponential term for grain-boundary diffusion of SiO2 in wollastonite is obtained from the relation LCaO/LSiO2 = 42.

Values of the diffusion coefficients derived from analysis of grain coarsening and layer growth in the Christmas Mountains contact aureole satisfy the diffusion-compensation relations for grain-boundary diffusion of oxygen and cations in oxides based on laboratory experiment. The value of the coefficient for diffusion of CaO in tilleyite grain boundaries at 1000 °C based on a time-integrated aver-age temperature is essentially identical to that obtained for wollastonite using the full non-isothermal analysis, suggesting that the values of Onsager coefficients for grain-boundary diffusion of a given cation are not strongly dependent on identity of solid phase

Wollastonite rims grow to 82% to 95% of their ultimate thickness during the period of heating that coincides with magmatic convection. Because growth of the wollastonite and tilleyite layers does not begin until temperature rises to 600 and 940 °C, respectively, rim growth is not synchronous across the aureole.

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