Abstract

Transmission and analytical electron microscope (TEM/AEM) study of complex calcic amphibole assemblages in metagabbros from the Cheyenne belt, southeast Wyoming, has revealed the amphiboles to be pervasively exsolved on a very fine scale.

Electron microprobe analyses indicate that the bulk composition of the amphiboles ranges from actinolite (Altot: 0.559) to magnesio-hornblende (Altot = 2.587). AEM analyses of the exsolved phases show them to be actinolite with an average composition of K0.03Na0.12 (Na0.18Ca1.71Fe0.08Mn0.03)(Fe1.55Mg3.05Al0.38Ti0.02)[Al0.38Si7.62]O22(OH)2 and hornblende with an average composition of K0.14Na0.36(Ca1.88Fe0.16Mn0.02)(Fe2.4Mg1.8Al0.76Ti0.05)[Al1.48Si6.52]O22(OH)2. The tschermakite exchange, [6]Al,[4]Al ↔ (Mg,Fe),Si, predominates during the exsolution process, accompanied by minor edenite exchange, NaA,[4]Al ↔ ☐,Si. The hornblendes are always significantly richer in Fe and have a higher A-site occupancy than the actinolites.

The exsolution microstructure occurs as a pervasive fine-scale tweed texture with lamellae typically on the order of 5–15 nm thick. The coarsest lamellae occur in compositional zones representing the middle of the actinolite-hornblende solvus, whereas increasingly finer textures are observed on both limbs. High-resolution TEM images indicate that the microstructure is coherent with diffuse lamellar interfaces. The lamellae occur in two symmetrical orientations, nearly parallel to (132) and (132). Since the unit-cell dimensions of the exsolved phases are so similar, these orientations are somewhat variable. The orientation of this microstructure differs significantly from more common lamellar orienlations (“100” and “101”) in exsolved monoclinic amphiboles. Optimal phase boundary calculations using EPLAG successfully predict the observed orientation for selected combinations of unstrained actinolite-hornblende lattice parameters. More importantly, they show that optimal phase boundaries may occur over a wide range of orientations depending upon actual differences in the two lattices, which are sensitive functions of composition and perhaps pressure and temperature.

Selected-area electron diffraction (SAED) patterns ofcoarse tweed areas have revealed small satellite spots around the main Bragg reflections, indicating that the microstructure is relatively periodic, with measured periodicities ranging from 25 to 35 nm. The small satellite spots around single reciprocal lattice points, pervasive tweed texture, diffuse lamellar interfaces, and small size of the exsolution lamellae are consistent with a spinodal decomposition mechanism.

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