Gregory T. Roselle

Role of the tectonic accretion channel in collisional orogeny

High-pressure relics studied in many collisional mountain belts are overprinted by subsequent Barrovian metamorphism that may reach migmatite grade in the central parts of such orogens. We propose that this evolution is linked to the development of a narrow tectonic accretion channel (TAC) at the subducting plate boundary. Geologic evidence from the “Southern Steep Belt” of the Central Alps and the tectono-metamorphic record of this orogen guided us in constructing numerical models of a TAC. Simulations indicate that the accretion of crust enriched in radioactive elements to mantle depth provides a mechanism to obtain P - T - t (pressure-temperature-time) trajectories in reasonable agreement with observations in the Alps. Similarly, the generation of granitoid melts predicted by the model during late-orogenic exhumation of the TAC is in line with the Alpine record. This case study suggests that accretion of upper-crust fragments to mantle depth, by underplating along a subduction fault, and subsequent extrusion of parts of the TAC along that same fault, may be fundamental processes in the dynamic evolution of many collisional orogens.

Nucleation-dominated crystallization of forsterite in the Ubehebe Peak contact aureole, California

The morphology and number of forsterite crystals in the Ubehebe Peak, California, contact aureole vary systematically as a function of metamorphic grade. From their first appearance to as close as ∼150 m from the intrusive contact, forsterite crystals are large (5–20 mm) and have a tabular habit ( a ≈ c ≫ b ). In contrast, forsterite near the contact is equigranular and much smaller (<1 mm in diameter). The number of crystals per mole of forsterite increases from 3.5 × 104 at the forsterite-in isograd to more than 1.5 × 108 near the contact. This trend is interpreted to result from an increase in the ratio of nucleation rates with respect to growth rates with proximity to the intrusion. The change in morphology from tabular to equigranular is explained by kinetic surface roughening. The nucleation and growth information gained from this study highlights the important role of nucleation kinetics in the crystallization of forsterite at the Ubehebe Peak aureole.

Experimental determination of anorthite solubility and calcium speciation in supercritical chloride solutions at 2 kb from 400 to 600°C

Abstract The solubility of the assemblage anorthite + andalusite + quartz and Ca speciation were investigated in supercritical chloride solutions between 400 and 600°C at 2 kb over a total chloride range of 0.005–5.6 m. Species interpretation is based on systematic regression of multiple speciation schemes by a nonlinear, weighted least squares procedure. Best fits to the data were obtained by including the Ca species CaCl 3 − and Ca(OH) + , in addition to the species Ca 2+ , CaCl + , CaCl 2 0 , H + , OH − , Cl − , H 2 O, Al(OH) 3 0 , H 4 SiO 4 0 , and HCl 0 . The logarithm of the equilibrium constant for the reaction CaAl 2 Si 2 O 8 + 2HCl = CaCl 2 0 + Al 2 SiO 5 + SiO 2 + H 2 O is given by: log K 3 = −17.13 (±0.41) + 16483 (±301)/ T (K). For the reaction, Ca (OH) + = Ca 2+ + OH − : log K 18 = −35.39 (±0.41) + 22558 (±283)/ T (K) was obtained. The logarithm of the dissociation constant for the reaction CaCl 3 − = CaCl 2 + Cl − was determined at 600°C to be -1.317 (±0.15). The formation constants for Ca-chloride species determined in the present study are in close agreement with those retrieved by Baumgartner (1991) from the wollastonite + quartz solubility data of Popp and Frantz (1979), which were obtained over a smaller range of total chloride concentration. Prediction of the dissociation constant for the portlandite hydrolysis reaction was made based on the present anorthite solubility data and is in excellent agreement with the portlandite solubility data of Walther (1986). Results of calculations on the solubility of the assemblage plagioclase + muscovite + andalusite + quartz indicate that the dominant Ca species are either Ca(OH) + or CaCl 3 − depending on total chloride concentration. With the formation of CaCl 3 − , increased solubility of calcium in saline fluids is predicted, relative to Na and K.

Crystallization processes in migmatites

Abstract An important prerequisite for interpreting microstructures in plutonic and metamorphic rocks is understanding crystallization processes. This study uses crystal size distribution (CSD) and grain shape parameters to investigate mineral-reaction-controlled crystal growth and melt solidification in migmatites of the Bayerische Wald (Variscan Orogeny, Germany). Partially molten rocks were studied because they allow the simultaneous investigation of crystallization of a melt and the growth of solid products of a metamorphic reaction. CSD and shape factors were obtained for three different minerals: (1) cordierite in a diatexite, where the melt remained in the system; (2) K-feldspar in a segregated K-feldspar + quartz leucosome; and (3) plagioclase from a segregated plagioclase + quartz leucosome. Cordierite crystals exhibit a nearly linear CSD pattern and subeuhedral crystal shapes. K-feldspar grains display an approximately linear CSD except for a marked increase at large crystal size classes. K-feldspar grains are characterized by strongly lobate grain boundaries. The CSD of plagioclase crystals is nearly linear, with a slight decrease at small crystal size classes. Plagioclase shape factors also indicate lobate grain boundaries, however, with lower values than K-feldspar. Cordierite CSD and shape data are consistent with interface-controlled crystallization. The CSD indicates that nucleation and growth is the rate limiting step during the cordierite-producing reaction, in contrast to porphyroblast growth in solid rocks where mass transfer is typically rate limiting. CSD combined with information on the duration of crystallization yields growth rates of 10-17 to 10-18 m/s. The plagioclase data are consistent with magmatic crystallization and are well-described by the communicating neighbor theory. The K-feldspar data also indicate magmatic crystallization. The “overproduction” of large grains of K-feldspar reflects the interplay between nucleation and growth rates at the initial stage of crystallization. Because of different environments between solid and anatectic rocks, the CSD may help to decipher metamorphic from anatectic migmatites.

Porosity and Permeability of Carbonate Rocks During Contact Metamorphism

In recent years, penologists have increasingly used quantitative models of metamorphic fluid—rock interaction to interpret field-based observations of hydrothermal alteration. Many of these models have specifically focused on stable isotope transport and fluid-rock exchange as a key towards understanding how long-lived hydrothermal activity is recorded in rocks. With few exceptions (Norton and Taylor, 1979; Furlong, Hanson and Bowers, 1991; Jamtveit, Bucher-Nurminen and Stijfhoorn, 1992; Hanson et al. 1993; Bowman, Willett and Cook, 1994), models of large-scale hydrothermal systems have been geared towards understanding basic mechanisms of fluid—rock interaction by considering idealized, generic systems (Cathles, 1977; Norton and Cathles, 1979; Bickle and McKenzie, 1987; Baumgartner and Rumble, 1988; Lassey and Blattner, 1988; Hanson, 1992, 1995; Gerdes, Baumgartner and Person, 1995b).