Hibonite and coexisting zoisite and clinozoisite in a calc-silicate granulite from southern Tanzania

Abstract

SUMMARY. The third terrestrial occurrence of hibonite is reported from granulite-facies rocks in the Furua Granulite Complex in southern Tanzania. The mineral forms yellowish-brown lath-shaped crystals in a grossular-anorthite rock containing subordinate sphene (clino)zoisite, hercynite, apatite, ilmenite, and corundumilmenite intergrowths.Electron-microprobe analyses indicate a generalized formula (Cal_ xRE~) [(Al,Fe3+)12_ 2~+ x(Ti,Si) Three compositional types of (clino)zoisite are distinguished: 1.8-3.I wt 700 Fe203 (orthorhombic and monoclinic), 3.9-6.0 wt % FezO3 (monoclinic), and 5.8-7.9 wt 9/o Fe203 with an average of 6. 3 wt % RE203 (monoclinic).Thermometric and barometric data for coexisting pyroxenes and garnet from adjacent rocks indicate granulite-facies equilibration conditions of 75o to 85o ~ and 6 to I I kb. During retrogression with increasing partial H20 pressures, hibonite reacted with plagioclase and garnet to form spinel, sphene, and RE-bearing clinozoisite. Corundum-ilmenite intergrowths probably resulted from the breakdown of an Fe-h6gbomite.THE rare mineral hibonite, with the theoretical composition of CaO'6AI203, is well known from Ca-Al-rich inclusions in carbonaceous chondritic meteorites (Keil and Fuchs, I971; Blander and Fuchs, I975; Macdougall, i979; Smith, i979). In the Allende and Leoville meteorites, for instance, hibonite occurs in association with anorthite, perovskite, gehlenite, spinel, wollastonite, andradite, and fassaite. Textural, thermodynamic, and isotopic anomaly data suggest that hibonite in meteorites may be regarded, at least in several cases, as an early high-temperature condensate from a primitive solar nebula (Shimizu et al., I978; Smith, x979; Lorin, pets. comm.).The presence of hibonite in terrestrial rocks, @~

Crystal chemistry of meteoritic hibonites

“…Despite its rarity, hibonite (ideally CaAl12019 ) has attracted considerable interest in meteorite research because it is potentially the highest temperature condensate from solar nebular gas [Grossman, 1972;Blander and Fuchs, 1975;Fegley, 1982;Kornacki and Fegley, 1984]. Although reported at only three locations on earth [Maaskant et al, 1980], hibonite is a constituent of refractory inclusions in several carbonaceous chondrites (e.g., Murchison, Allende, Leoville, Vigarano) where it may display blue or orange pleochroism, in contrast to dark gray or brown colorations in terrestrial rocks. A surprising geochemical feature of many meteoritic hibonites is their enrichment of vanadium (odd atomic number) relative to Fe and Cr [Allen et al, 1978…”

“…Hibonite, metamorphosed limestone, Madagascar; brownish-grey; Fe203 calculated assuming 0 --19, • cations --13; analysis includes Ce203 --0.2%; Nd203 --0.1%; ThO 2 --1.5%[Maaskant et al, 1980;Curien et at., 1956]. (2) Hibonite, Furua Granulite Complex, southern Tanzania; yellowish-brown; Fe203 calculated assuming0 = 19, • cations = 13; average of 14 analyses; includes Ce203 --2.1%; La203 = 1.8%; Nd203 --0.2%;[Maaskant et aL, 1980]. (3) Hibonite, in Ca-AI-Ti-rich achondritic inclusions in Leoville, Kansas, carbonaceous chondrite; blue fluorescence; average of 10 analyses; includes MnO = 0.02%[Keil and Fuchs, 1971].…”

Inter- and Intracrystalline Isotopic Disequilibria: Techniques and Applications

“…Historically, the concept of removing growth zones or intracrystalline subsamples from larger samples for chemical analyses was governed by the scientific benefit of such analyses. All sub-disciplines in geology have gained some advantage from applying such techniques from evaluating sulfur isotopes in sulfur-bearing phases in breccias (Lambert et al 1982), to obtaining X-ray diffraction data from micromilled minerals in calc-silicate granulites (Maaskant et al 1980), to identifying fish provenance by micromilling otoliths for Sr isotopic analyses (Kennedy et al 2002). Applying micromilling in igneous petrology for the purposes of evaluating isotopes in magma-related materials was first accomplished by Davidson et al (1990) in which the cores of plagioclase feldspar crystals, and a small droplet of chilled mafic melt in the same hand sample, were milled using small bits and a basic drill press.…”

Zoned hibonites from Punalur, South India

“…Such low concentrations of R E E distinguish the Punalur hibonite rims from other terrestrial occurrences (e.g. Maaskant et al, 1980), which all contain significant R E E . In contrast, the concentration of R E E in the cores of the Punalur hibonites ( Z R E E xii -= 0.55-0.65) greatly exceeds previously analysed terrestrial hibonites, with the cores containing in excess of 50% of the R E E endmember (see below).…”

“…Burns and Burns, 1984). Terrestrial occurrences are restricted to four localities in metamorphosed calcareous rocks (Curien et al, 1956;Kuzmin, 1960;Yakovlevskaya, 1961;Maaskant et al, 1980). Reported analyses show that terrestrial, unlike meteoritic hibonites, contain appreciable rare earth element (REE) concentrations, with the REE replacing Ca on the 12-fold coordination site.…”