ALH84001

ALH84001 Petrographic Description

Antarctic Meteorite Newsletter, Volume 16, Number 3
December 1993

Macroscopic Description: Roberta Score
Eighty percent of this rectangular shaped achondrite is covered with dull black fusion crust. Remnants of flow marks are visible on two exterior surfaces. Areas not covered by fusion crust have a greenish-gray color and a blocky texture. Cleavage planes are obvious on some large crystal faces and the stone has a shocked appearance.

Small areas of oxidation are present in the interior of ALH84001. Abundant small, black chromite grains are scattered throughout the stone. Small fractures are numerous.

Thin Section (,64) Description: David W. Mittlefehldt
The section consists of a coarse-grained, cataclastic orthopyroxenite. The orthopyroxene is generally anhedral and up to 3.5 mm across. Common minor minerals are euhedral to anhedral chromite grains up to 500 µ across and interstitial maskelynite typically 100-300 µ in size. Accessory phases are augite, calcium phosphate, iron disulfide, and probable carbonates (by petrography, EDS spectra) of Mg, Ca, Mn and/or Fe. The sample shows cataclastic texture, with common veins of crushed orthopyroxene plus chromite (grain size up to ~30 µ) transecting coarse orthopyroxene. The latter exhibits patchy extinction. The original texture shows common 120° triple junctures between coarse orthopyroxene grains. Many orthopyroxene and chromite grain boundaries exhibit offsets along fractures. Mineral compositions are: orthopyroxene Wo _3.3En₆₉Fs₂₇; augite Wo₄₂En₄₅Fs ₁₃; maskelynite An₃₁Ab ₆₃Or₆; chromite 100 Mg/(Mg+Fe ²⁺) ~19, 100 Cr/(Cr+Al+Fe³⁺ ) ~70, TiO₂ ~2.2 wt%, Fe₂ O₃ (from stoichiometry) ~7.7 wt%. Orthopyroxene and chromite are relatively homogeneous in composition.

Although originally classified as a diogenite, there are several petrologic features which indicate that ALH84001 is not a diogenite. Iron disulfide has not been reported from diogenites, or any HED meteorite; troilite is the sulfide phase. Plagioclase as sodic as Ab₆₃ has not been reported from any HED meteorite; the most sodic plagioclase from diogenites is about Ab₃₅, but most are Ab₂₀. Carbonate has not been found in diogenites or other HED meteorites, including as Antarctic weathering products. Diogenite spinels typically contain <1 wt% TiO ₂ (except Yamato 75032-type diogenites, ~1.8 wt%), and Fe₂O₃ from stoichiometry is <1 wt %. Diogenites are typically monomict or polymict breccias composed dominantly of orthopyroxene clasts in a matrix of comminuted orthopyroxene. The original grain size of diogenites was of the order of cm, not mm. The cataclastic texture of ALH84001 is uncommon in other diogenites. Tatahouine does not exhibit patchy extinction, but does not contain crushed zones. Only ALHA77256 has a texture resembling that of ALH84001. Maskelynite is rare in HED meteorites, although eucrites Padvarninkai and ALHA81313 contain it.

The mineralogic and petrographic features of ALH84001 are more in accord with those exhibited by the SNC meteorites. Iron disulfide is present in several SNC meteorites. The composition of maskelynite in ALH84001 falls withing the range of other SNCs, and SNC plagioclase is commonly maskelynite. Calcium carbonates have been identified as pre-terrestrial aqueous alteration in some SNCs. Spinels in SNC meteorites commonly contain substantial calculated Fe³⁺. SNC meteorites show a wide variety of shock textures, including patchy extinction of coarse-grained orthopyroxene in EETA79001, similar to that in ALH84001, although transecting crushed zones have not been described.

Based on its petrography, ALH84001 is a new, and unusual, SNC meteorite classified as a SNC orthopyroxenite.

Oxygen Isotope Analysis: Robert Clayton
The oxygen isotope composition of ALH84001 is: δ ¹⁸O=+4.60; δ ¹⁷O=+2.65, excess ¹⁷O = 0.25. These analyses are indistinguishable from those of Nakhla and Lafayette.