Meteoritic plagioclase

The winonaites are compositionally similar to silicate inclusions in some IAB irons (described below). They have chondritic compositions, and relict chondrules have been found in some meteorites. They consist of olivine, pyroxenes, plagioclase, metal, troilite, and other minor minerals (Benedix et al., 1998), and most have been recrystallized. Like the acapulcoites, they have experienced only small degrees of melting. 

Slater-Reynolds, V. and McSween, H. Y. (2005) Peak metamorphic temperatures in type 6 ordinary chondrites an evaluation of pyroxene and plagioclase geothermometry. Meteoritics and Planetary Science, 40, 745—754. 

Feldspar Unlike enstatite and forsterite, feldspar is present in most meteorites. The meteoritic feldspars are dominated by plagioclase which are those feldspar compositions in the continuous solid solution series ranging from albite (NaAlSi308) to anorthite (CaAl2Si2C>8) compositions near the ends of this series are most common. The earliest CL studies of feldspar were prompted by the lunar program about 1970 and a series of papers compared the CL of lunar, meteoritic and doped synthetic plagioclase. Later observations were made for anorthite in the carbonaceous chondrites. 

Figure 4. Photomosaics of the cathodoluminescence of three low petrologic type meteorites (a) Semarkona (type 3.0, top), (b) Bishunpur (type 3.1, middle), and (c) Krymka (type 3.1, bottom). The scale bar shown refers to Semarkona, the scale bar for the others is the same as in Figure 5. In general, red luminescence is produced by Fe-free olivine and pyroxene, and blue and yellow CL is produced by chondrule mesostases of plagioclase composition.

The stony-iron meteorites are intermediate between chondrites and irons. These very rare meteorites are equal mixtures of iron/nickel alloys and silicate minerals. Pallasites are striking examples of this type of meteorites, consisting of green olivine crystals in a matrix of metallic iron. Another type of stony-iron meteorite, called mesosiderites, contain pyroxene and plagioclase feldspars, minerals that are common on Earth. 

The recently described metal-rich Tafassasset meteorite has an oxygen isotopic composition similar to CR chondrites and an equigranular texture (Bourot-Denise et al., 2002). It is, however, characterized by uniformly ferrous ohvine (Fa3o) and orthopyroxene (FS25), and the presence of albitic plagioclase (An24 4s), chromite, and merrilite it may be the first metamorphosed CR chondrite. 

Figure 31 Lunar meteorite North West Africa (NWA) 773 consists of two distinct lithologies cumulate olivine norite and regolith breccia. The cumulate portion is composed of olivine, pigeonite, augite, feldspar, and opaques (troilite, chromite, Fe-metal). The breccia portion contains fragments of cumulate portion as well as silica glass, hedenbergitic pyroxene, volcanic rocks, and unusual lithic clasts with fayalite + Ba-rich K-feldspar + silica + plagioclase (photograph courtesy of M. Killgore).

Takeda H., Yugami K., BogardD. D., and Miyamoto M. (1997) Plagioclase-augite-rich gabbro in the Caddo County lAB iron and the missing basalts associated with iron meteorites. In Lunar Planet Sci. XXVIII. The Lunar and Planetary Institute, Houston, pp. 1409-1410. 

Hashimoto A. and Grossman L. (1987) Alteration of Al-rich inclusions inside amoeboid olivine aggregates in the AUende meteorite. Geochim. Cosmochim. Acta 51, 1685-1704. Hinton R. W. and Bischoff A. (1984) Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites. Nature 308, 169-172. 

Wark D. A. (1987) Plagioclase-rich inclusions in carbonaceous chondrite meteorites liquid condensates Geochim. Cosmochim. Acta 51, 221-242. 

Shallowater and Mt. Egerton are unique meteorites that may be closely related to the aubrites. Shallowater is unbrecciated, contains higher abundances of both metal and troilite than other aubrites (Keil et al., 1989 Watters and Prinz, 1979), and the pyroxene is ordered orthopyroxene, rather than disordered pyroxene common to the group (Reid and Cohen, 1967). Shallowater also contains xenoliths composed of twinned clinoen-statite, forsterite, plagioclase, EeNi metal, and troilite that comprise 20 vol.% of the rock (Keil et al., 1989). Mt. Egerton is also unbrecciated and composed of centimeter-sized enstatite crystals with 21 wt.% interstitial EeNi metal. 

IjOvering J. F. (1975) The Moama eucrites—a pyroxene-plagioclase adcumulate. Meteoritics 10, 101 — 114. 

Pb-Pb age of LEW and ADOR (Nyquist et al., 2003). The same authors found Mg in plagioclase and constructed a two-point Al-Mg isochron that implies A1/ A1 = (2.3 0.8) X 10 at crystallization. This low value is reasonably close to that expected based on A1 decay from the canonical ratio given an absolute age 4,561 Ma. If these preliminary results hold, they will represent a new point of concordance between the Al-Mg and Mn-Cr systems in igneously differentiated meteorites. 

Figure 2 An FcKq, map of a thin section of the Shergotty meteorite. Gray phases are zoned pyroxenes, black phases are plagioclase (transformed to maskely-nite by shock), and bright phases are Fe-Ti oxides.

Renne P. R. (2000) Ar-40/Ar-39 age of plagioclase from Acapulco meteorite and the problem of systematic errors in cosmochronology. Earth Planet. Sci. Lett. 175(1-2), 13-26. 

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