Anorthite Grains Research Articles

High‐Temperature Rheology of Calcium Aluminosilicate (Anorthite) Glass‐Ceramics under Uniaxial and Triaxial Loading

The high‐temperature creep behavior of two fine‐grained (∼3 μm) anorthite‐rich glass‐ceramics was characterized at ambient pressure and under a confining pressure of ∼300 MPa. Experiments were done at differential stresses of 15–200 MPa and temperatures of 1200°–1320°C. Of the two materials, one had a tabular (lathlike) grain structure with finely dispersed second phase of mullite, mostly in the form of ∼3–5 μm grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (∼400 nm) mullite precipitates concentrated at the anorthite grain boundaries. The results of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitude faster than the equiaxed material. Creep in the tabular‐grained material at ambient pressure was accompanied by a significant extent of intergranular cavitation: pore‐volume analysis before and after creep in this material suggested that >75% of the bulk strain was due to growth of these voids. The equiaxed material, in contrast, showed a smooth transition from Newtonian (n= 1) creep at low stresses to non‐Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both materials underwent significant changes. Grain‐boundary mullite precipitates in the undeformed, equiaxed‐grain material were replaced by fine (∼100 nm), intragranular precipitates of silliminate and corundum because of a pressure‐induced chemical reaction. This was accompanied by a significant reduction in grain size in both materials. The substantial microstructural changes at high confining pressure resulted in substantially lower viscosities for both materials. The absence of mullite precipitates at the grain boundaries changed the behavior of the equiaxed material to non‐Newtonian (n= 2) at a pressure of ∼300 MPa, possibly because of a grain‐boundary sliding mechanism; the tabular‐grained material showed Newtonian diffusional creep under similar conditions.

Micro-distribution of oxygen isotopes in a refractory inclusion from the Allende meteorite

Isotope ratios of oxygen have been measured on a fassaite crystal and the surrounding area of a coarse-grained CAI from the Allende meteorite using secondary ion mass spectrometry. Values of δ 18O SMOW for fassaite, spinel, anorthite grains and alteration products are −27 ± 5, −38 ± 2, +5 ± 6, and +4‰, respectively, which are consistent with the results obtained on mineral separates. The O isotopic distributions are homogeneous either within individual single crystals of fassaite and anorthite or among the spinel grains within the analytical error. No gradual variation in O isotopic composition was detected even at the boundary between different phases or at cracks inside a phase with a spatial resolution of 10 μm. These observations strongly suggest that the partial exchange of O isotopes in the minerals in the CAI with the surrounding gas through diffusion is an unlikely origin for the observed inter-mineral heterogeneity of O isotopes in the Allende CAIs.

Xenoliths in recent basaltic andesite flows from Arenal Volcano, Costa Rica: inference on the composition of the lower crust

Basaltic andesite flows erupted between 1973 and 1980 from Arenal Volcano contain abundant inclusions of anorthosite, olivine gabbro, and pyroxenites, and megacrysts of olivine and anorthite. The anorthosites with large (20 mm) anorthite grains (An96-92) exhibit deformation twinning and granulation between grain boundaries. Some olivine gabbros have angular clasts of anorthite with bent twins, pyroxene, and olivine in a finer-grained matrix which is distinctly foliated. These textural features suggest that these inclusions were deformed. An exotic (xenolithic) origin is supported in part by the mineral compositions and the estimated temperatures of equilibration: a temperature of about 975° C is obtained by two-pyroxene and Fe-Ti oxide geothermometers for the gabbros, but two-pyroxene temperatures are higher (1064 to 1120° C) for the basaltic andesite host. The olivine gabbro is thought to have crystallized at a pressure between 8.5 and 9.5 kb; whereas the lava phenocrysts crystallized at a much lower pressure of less than 5 kb. These xenoliths probably represent fragments of the lower crust below Arenal volcano. The lava flows show evidence for some contamination especially from fragments of anorthite broken apart from the larger megacrysts and xenoliths. A few phenocrysts of plagioclase in the lava samples have deformation twins. The unusually high Al2O3 content (19.4 to 23.2 wt%) of the lava samples can be attributed directly to the addition of anorthite; in fact the observed chemical variation in the lava flows (the increasing alumina and lime contents with decreasing silica) can be explained by this contamination.