Morphological instabilities during rapid growth of metamorphic garnets

Abstract

Abstract. Hydrothermal grossular-andradite garnets from contact aureoles in the Oslo region show morphological transitions from planar via cellular to hopper-like structures. Dodecahedral surfaces {110} dominate during the planar growth stage, whereas the stable crystal faces, developed during the cellular and hopper stages also includes the ikositetrahedron {211} and possibly the hexoctahedron {321}. Faceted cells develope when initially 'wavy' perturbations on the dodecahedral surfaces become tangential to lower-index planar surfaces.Inclusion patterns and morphologies of almandinerich garnets from Mageroy (northernmost Norway) that formed during a period of rapid heating, suggest an early stage of cellular growth followed by planar growth. The morphological transitions suggest that the hydrothermal garnets experienced an increase in the overstepping of the garnet precipitation reaction at some stage during their growth whereas the opposite was the case during growth of the Mageroy garnets. The present observations put constraints on the garnet growth rates and emphasize the importance of growth kinetics during metamorphic processes.

Constraints on Transport and Kinetics in Hydrothermal Systems from Zoned Garnet Crystals

“…A decrease in growth rate during growth of the outermost rim has been inferred from the morphological evolution of these skarn garnets (5). Alternatively, it may simply reflect depletion of L E E in the hydrothermal fluid.…”

“…In fact, the rim REE patterns are qualitatively similar to FEE patterns from acidic, high-temperature hydrothermal fluids (1 5-1 7). Therefore, we speculate that the rim FEE composition may to a large extent have been controlled by irreversible incorporation of REE initially adsorbed on the garnet surface during periods of rapid crystal growth (5). In such a case, the rim REE pattern would reflect surface-fluid REE partitioning rather than bulk lattice control.…”

“…1). The composition of the grossularrich layers was largely determined by the local mineral assemblage during periods of slow fluid influx and low crystal growth rates, whereas the composition of more rapidly grown (5) strength elements (HFEs) such as Zr, Y, and Ti. The lack of evidence that local sources of HFEs (notably zircon and sphene) were removed from the system indicates that the fluids did not reach equilibrium concentrations with respect to these components during growth of the andradite-rich rims.…”

“…The fact that many shifts temperature correlation established from can be traced among different cores makes modern precipitation (2) implies that the S1'0 shifts, in some cases, reflect tempera- suggest the involvement of other climatic processes. In a series of model experiments that effectively tested the sensitivity of climate to reduced North Atlantic heat transport, this mechanism could explain less than one-third the magnitude of air temoerature change over Greenland inferred from the ice core isotopic shifts (5). Recent estimation of the rapidity of the changes in the ice cores (6) suggests that the variations occur more rapidly than do variations in European climate records that are undeniably the product of North Atlantic surface ocean changes (7).…”

Diamonds from Wellington, NSW: insights into the origin of eastern Australian diamonds

“…Non-planar facets observed at the intermediate zones of some Group A diamonds, and the majority of Group B1 diamonds suggest rapid growth rates and conditions less stable than those that favour planar growth faces (O'Hara et al, 1968, Jamtveit andAndersen, 1994). …”