Tierra Blanca: An Unusual Achondrite From West Texas

Abstract

Tierra Blanca is an achondrite (sensu stricto) composed chiefly of orthopyroxene, olivine, plagioclase and clinopyroxene. The mineralogy, major element chemistry and texture are similar to Winona and also Mt. Morris, Acapulco and Antarctic meteorite ALHA 77081 to some degree. The designation of this group as “Winonaites,” as suggested by Prinz et al. (1980), seems justified and useful.

A petrologic and isotopic study of winonaites: evidence for early partial melting, brecciation, and metamorphism

“…Olivine, orthopyroxene, and clinopyroxene were not distinguished but were collectively determined as mafic silicates. However, mafic silicates have been distinguished in previous studies (Graham et al, 1977;Prinz et al, 1980;King et al, 1981;Kimura et al, 1992), and the total mafic silicates and overall modes are similar to ours. Mineral compositions were measured with a Cameca SX-50 electron microprobe operated at an accelerating voltage of 15 keV, a current of 20 nA, and a fully-focused beam.…”

“…Members of the winonaite group which satisfy both the oxygen isotopic composition and mineralogical criteria include Winona, Pontlyfni, Tierra Blanca, Mount Morris (Wisconsin), Yamato 74025, Yamato 75300, Yamato 75305, Yamato 8005, and QUE 94535 (Graham et al, 1977;Prinz et al, 1980;King et al, 1981;Kimura et al, 1992;Clayton and Mayeda, 1996;Mason, 1996;Yugami et al, 1996;R. N. Clayton, pers.…”

“…Previous investigators have studied only individual winonaites (e.g., Tierra Blanca; King et al, 1981) or a subset of the winonaites (e.g., Graham et al, 1977;Kimura et al, 1992). In contrast, we examined all members of the winonaite group.…”

Evolution of the winonaite parent body: Clues from silicate mineral trace element distributions

“…The CaO concentrations used for the phosphates (apatite and merrillite) were determined from EDX analyses of representative grains from each meteorite, whereas SiO 2 concentrations for the silicates are averages of values determined by electron microprobe. SiO 2 concentrations for silicates from Tierra Blanca are the average values listed in King et al (1981). Four measurements in a fine-grained lithology from Pontlyfni correspond to a mixture of minerals; for these analyses, we averaged the SiO 2 concentrations of all the minerals involved (plagioclase, orthopyroxene, clinopyroxene, and olivine) and used this value for determining REE concentrations.…”

“…2c). These grains can reach up to 9 mm in size (King et al 1981;Benedix et al 1998) and most likely formed through solid-state metamorphism (e.g., de Wit and Strong 1975;Kretz 1994). Also present in Tierra Blanca are coarse-grained clumps of olivine, similar to those observed in Winona.…”