ABSTRACT
Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re-187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55-57Wo3-4Fs40-42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50-53; Fe/Mn = 1.1-2.1), chromite (Cr# ~ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1-102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite â olivine â merrillite â plagioclase. In spite of the inferred high supersaturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re-Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O-Ti isotope systematics (Δ17O = -1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = -2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.
ABSTRACT
STATEMENT OF PROBLEM: The mechanisms of water accumulation in the inner space of a closed hollow obturator are not understood, and it is not known whether the porosity of acrylic resin is involved in these mechanisms. PURPOSE: The purpose of this study was to evaluate water absorption by solid and closed hollow obturators and to determine the mechanisms of water accumulation in the inner space of the closed hollow obturator. MATERIAL AND METHODS: Twenty solid and 20 closed hollow spherical specimens (30 mm in diameter) were fabricated from heat-polymerized acrylic resin (Acron Clear). Closed hollow specimens consisted of 2 hemispheres (30 mm in diameter, 1.5 mm in thickness) joined with autopolymerizing resin (Unifast II). Ten solid and 10 closed hollow specimens were immersed in distilled water at 37 degrees C, while the other specimens were stored in an atmosphere of 100% relative humidity at 37 degrees C. Each specimen was weighed every 24 hours for 180 days, and weight changes between each group were compared by 2-way ANOVA and the Tukey HSD test. Split-plot design ANOVA was used to compare the course of the weight change over 180 days (alpha=.05). RESULTS: The weight of the closed hollow specimens increased immediately and became saturated by day 30, with an increase of 1.44% from the initial weight. Water accumulation was not observed inside the closed hollow specimen during the 180-day storage period. The weight of solid specimens increased more slowly than that of closed hollow specimens; by day 180 the weight of the latter had increased by 1.16%. However, no significant difference in changes in mean mass of the same type of specimen was observed between the 2 storage conditions. CONCLUSION: These results suggest that water accumulation inside a closed hollow obturator is unrelated to the water absorption properties of the acrylic resin.
