Shearer, and 13 co-authors, reported in 2008 that the recently recovered Antarctic achondrites Graves Nunatak 06128 and 06129 are unique meteorites that represent high-temperature asteroidal processes in the early solar system never before identified in any other meteorite. They represent products of early planetesimal melting (4564.25 ± 0.21 Ma) and subsequent metamorphism of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. This melting event is unlike those predicted by previous experimental or geochemical studies, and indicates either disequilibrium melting of chondritic material or melting of chondritic material under volatile-rich conditions.
Introduction
Achondritic meteorites commonly represent remnants of magmatic systems from differentiated planets (e.g., Moon, Mars) or asteroids (e.g., 4 Vesta). They may be products of early whole planetary body melting such as the lunar ferroan anorthosites, residuum of low degrees of melting and melt extraction such as the acapulcoites and lodranites, or products of episodic mantle partial melting such as lunar and Martian basalts. Newly discovered paired achondritic meteorites Graves Nunatak (GRA) 06128 and 06129 are mineralogically distinct from all of the other known achondrites and do not fit within the planetary or petrologic context of any known parent body. Specifically, these meteorites have a high abundance of sodic plagioclase and Ferich silicates, implying they have experienced more elemental fractionation and are more “evolved” than other planetary lithologies such as basalts from the Moon, Mars, or 4 Vesta.
Superimposed on this distinct high-temperature mineral assemblage is a high-temperature metamorphism and low-temperature alteration. As discussed below, these unique characteristics are inferred to result from asteroidal processes in the early solar system unlike those recorded in any other meteorites of asteroidal or planetary origin.
Sample Description
Paired meteorites GRA 06129 and GRA 06128 (hereafter, GRA) exhibit a heterogeneous, granoblastic texture (Fig. 1) with a modal mineralogy dominated by sodic plagioclase (~81 vol%). The plagioclase exhibits a limited range in composition from An16Ab82Or2 to An13Ab85Or2. Two pyroxenes (orthopyroxene and Ca-rich clinopyroxene) and olivine are the most abundant silicates after plagioclase, making up approximately 9 and 8% of the rock, respectively. The composition of olivine is fairly homogenous at Fo41–40 and Cr2O3 values from 0.00–0.07 wt%.
One olivine grain, adjacent to a spinel, has slightly higher Fo (Fo45–42) and Cr2O3 (0.02–0.31 wt%) contents, suggesting this compositional variability is related to subsolidus re-equilibration with the adjacent spinel grain. Olivine contains inclusions of Fe-Ni metal, troilite, and pentlandite. Troilite and pentlandite are commonly intergrown with small blobs or blades of Fe-Ni metal (64–68 wt% Ni), spatially associated with the pentlandite. These inclusions appear to be trapped sulfide melt that has undergone subsolidus re-equilibration. The high-Ca pyroxene ranges in composition from En39Fs16Wo45 to En44Fs29Wo27. Most pyroxene grains exhibit exsolution of low-Ca pyroxene lamellae that are 2 to 8 µm in width. The average composition of the
orthopyroxene is En53Fs45Wo2. Based on the stoichiometry of ~100 electron microprobe analyses, the average ferric iron (Fe3+/ Fe3++Fe2+) in the high-Ca pyroxene is 7%. Apatite overgrowths on merrillite (dehydrogenated whitlockite) occur in individual phosphate masses up to 600 µm in size. Apatite is halogen-rich(Cl ~5.0 wt%, F ~0.7 wt%), whereas the merrillite is halogen-poor with substantial Na in the CaIIA site (~2.5%). The modal abundance of phosphates is ~1.0%. Ilmenite and “spinel” are commonly intergrown and are distributed throughout the section in trace abundances. A megascopic view of the GRA meteorites indicates they have undergone significant low-temperature alteration.
The microscopic view indicates this alteration is pervasive adjacent to mineral surfaces and in fractures within mineral grains. Initial observations indicate at least three types of low-temperature alteration in this meteorite:
1) alteration that primarily consists of Fe-bearing phases;
2) alteration that consists of Fe-rich phases, sulfates, and perhaps sulfides; and
3) alteration of olivine to “iddingsite.”
It is uncertain to what degree this alteration is parent body vs. terrestrial. As expected from this unusual mineralogy, the bulk meteorite is high in Al2O3 (15.5 wt%), CaO (6.22 wt%), and Na2O (7.51 wt%). The CaO/Al2O3 is 0.40, while the K content is rather low K2O/(Na2O + K2O) wt% = 0.033. The relatively high modal abundance of phosphate is evident in the overall P2O5 of the bulk rock (0.19 wt%). Although the high Na content and abundant
phosphates imply a formation involving extensive elemental fractionation, the overall trace element characteristics do not support this inference. The GRA whole rock REE pattern (Fig. 2a) is roughly flat (1 to 2× chondrite) with a positive Eu anomaly (5× chondrite). Nickel and Co abundances are relatively high (2215 and 297 ppm, respectively) indicating GRA did not experience extensive amounts of olivine fractionation. Na/Sc and Sm/Sc ratios are higher than CI chondrite (Fig. 2b) and imply this lithology represents the accumulation of plagioclase and phosphates from a silicate melt.
Complete analytical details are in the Appendix1 available as a deposited item.
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