Julian R. Goldsmith

Oxygen isotopic fractionation in the system quartz-albite-anorthite-water

Abstract Oxygen isotopic fractionations have been determined between quartz and water, albite and water, and anorthite and water at temperatures from 300 to 825°C, and pressures from 1.5. to 25 kbar. The equilibrium quartz-feldspar fractionation curves can be approximated by the following equations: 1000ln αQ−PI = (0.46 + 0.55β)106T−2 + (0.02 + 0.85β) between 500 and 800°C 1000ln αQ−PI = (0.79 + 0.90β)106T−2 — (0.43 − 0.30β) between 400 and 500°C where β is the mole-fraction of anorthite in plagioclase. Application of these isotopic thermometer calibrations to literature data on quartz and feldspar gives temperatures for some metamorphic rocks which are concordant with quartz-magnetite temperatures. Plutonic igneous rocks typically have quartz-feldspar fractionations which are substantially larger than the equilibrium values at solidus temperatures, indicating substantial retrograde exchange effects.

Oxygen isotope fractionations involving diopside, forsterite, magnetite, and calcite: Application to geothermometry

Oxygen isotope fractionations between diopside, forsterite, magnetite, and calcite have been studied experimentally at high pressures (P = 15–16 kbar) and temperatures (T = 600–1300°C) with the carbonate-exchange technique of Claytonet al. (1989). The fractionations determined for these minerals can be combined with the data of Claytonet al. (1989) to give an internally consistent set of mineralmineral fractionations of the form 1000 ln α = A × 106T−2, where the coefficient A is given in the following table: 7. Cc Ab An Di Fo Mt Qtz 0.38 0.94 1.99 2.75 3.67 6.29 Cc 0.56 1.61 2.37 3.29 5.91 Ab 1.05 1.81 2.73 5.35 An 0.76 1.68 4.30 Di 0.92 3.54 Fo 2.62 Full-size table Table options View in workspace Download as CSV The diopside-calcite and forsterite-calcite fractionations of the present study are in excellent agreement with the theoretically-derived fractionations of Kieffer (1982). Mineral-mineral fractionations obtained by the carbonate-exchange technique are also in fair agreement with those derived from hydrothermal experiments except where the fractionations involve quartz or calcite. In those cases, the results of the present study indicate that the experimentally-determined quartz-water and calcite-water fractionations are systematically too small. Application of the present calibrations to natural samples yields reasonable crystallization temperatures for volcanic rocks. In plutonic igneous rocks and granulites, however, thermometers involving magnetite indicate extensive retrograde re-equilibration. Using the quartz-pyroxene thermometer, it may be possible in favorable cases to recover high temperature data from granulites.

Mechanisms of hydrothermal crystallization of quartz at 250°C and 15 kbar

Oxygen isotopic exchange between quartz and water, using a novel technique in which both 18O/16O and 17O/16O fractionations were measured, yielded an equilibrium fractionation Δ18 = 9.0 at 250°C and 15 kbar. The reaction proceeds predominantly by solution of fine grains and growth of larger grains. Exchange by solid-state diffusion is immeasurably slow at this temperature. Under the same experimental conditions, cristobalite behaves quite differently, becoming transformed to sub-micron quartz crystals in a few minutes. The phase transformation is accompanied by a kinetic isotope effect yielding quartz in isotopic disequilibrium with water. It is possible that such disequilibrium products are also formed in other experiments involving phase transitions or mineral syntheses.

Oxygen isotope fractionation in quartz, albite, anorthite and calcite

Laboratory measurements of equilibrium oxygen isotope fractionation in quartz, albite, anorthite, and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600°C and above. Exchange in these systems is as rapid as exchange between silicates and water. Fractionation factors can be summarized in the following equations: °qc = 0.38·106T−2ΔAbc = −0.57·106T−2ΔAnc = −1.59·106T−2 from which the silicate-pair fractionations are readily obtained. These results are compared with published theoretical estimates as well as data derived from hydrothermal experiments. Some significant differences are found. In particular, it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data. The new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems.

Subsolidus Phase Relations in the System CaCO3-MgCO3

The solid solubility, disorder, and decomposition relationships in the system CaCO3-MgCO3 were investigated in the pressure range of from 10 to 10,000 bars and at temperatures of from 700° to 1,200° C. A sealed-tube technique was used for the most part in an internally heated pressure vessel designed for rapid operation. The top of the solvus outlining the two-phase field of magnesian calcites and dolomite is at a temperature of 1,075° C. at the composition Ca57Mg43, and above the solvus a single-phase region extends from CaCO3 to CaMg(CO3)2. The solubility gap between CaMg(CO3)2 and MgCO3 is considerably larger, and solubility of Ca++ in MgCO3 and Mg++ in CaMg(CO3)2 is quite limited at 1,100° C. Cation disorder in stoichiometric dolomite becomes observable at approximately 1,000° C. and is complete at approximately 1,200° C., at which temperature CaMg(CO3)2 has the calcite-type structure. There is no evidence for anything but a continuous (higher-order) transformation over this 200° C. interval. Disorder is enhanced by substitution of Ca++ for Mg++ beyond the ideal 1:1 ratio in dolomite; the most Ca-rich dolomite showing any order exists at the top of the solvus. Thus a region exists in which, at temperatures above the solvus, disorder takes place at constant temperature along with the substitution of excess CaCO3 into dolomite. Both simple substitutional disorder and mixed-layer effects are observed in quenched samples near dolomite in composition. The relation of these high-temperature equilibrium states to the naturally occurring metastable low-temperature protodolomite-like materials is discussed. Consideration is also given to the relation between calcite I and calcite II in the system. Data on the

THE MICROCLINE-SANIDINE STABILITY RELATIONS,

P_{CO_{2}}

Some Hydrothermal Syntheses of Dolomite and Protodolomite

-T relations of CaCO3, MgCO3, and CaMg(CO3)2 are presented in an appendix.

Limits on the effect of pressure on isotopic fractionation

Dolomite concentrates from sedimentary rocks ranging in age from Eocene to Ordovician have been examined by powder and single-crystal X-ray methods. They not infrequently contain about 5 mol per cent excess