Unique Angrite NWA 2999: The Case For Samples From Mercury

Angrite NWA 2999: A group of stones (totaling at least 2.4 kg) found in Northwest Africa in 2004 represent the tenth known example of the enigmatic angrite meteorites. Although most angrites have textures indicative of rapid cooling from melts, NWA 2999 has an overall plutonic, polygonal-granular texture (similar to Angra dos Reis and LEW 86010), but with distinctive large anorthite, spinel and recrystallized olivine porphyroclasts (up to 6 mm across) and 10-20 μm wide, discontinuous anorthite coronas around spinel grains (to our knowledge a feature unique among meteorites). The major minerals are calcic olivine (Fa39.8-41, CaO = 0.6-1.3 wt.%, FeO/MnO = 77-97) and Al-Ti-bearing diopside (Fs9.6-11.3Wo53-54, Al2O3 = 5 to 9 wt.%, TiO2 = 0.5 to 2.4 wt.%, FeO/MnO = 55-130), with subordinate Cr-pleonaste spinel (Cr2O3 = 4.7 to 8.7 wt.%, Al2O3 = 55 to 60 wt.%, Mg/(Mg+Fe) = 0.44-0.47) and pure anorthite (<0.02 wt.% Na2O), and accessory kamacite, troilite and S-bearing calcium silico-phosphate. Orthopyroxene, kirschsteinite and vesicles are absent. The anorthite coronas (and systematic compositional zoning in adjacent diopside and spinel) imply that this rock experienced solid state decompression from the spinel wehrlite to plagioclase wehrlite stability field, and last equilibrated at ~870°C and 6.7 kbar. Oxygen isotopic compositions of acid-washed samples by laser fluorination (mean of 3 analyses δ18O = 4.03, δ17O = 2.04, Δ17O = -0.086 +/- 0.01 per mil) are typical for angrites. Are Angrites From Mercury?: Angrites are mafic to ultramafic "igneous" or metamorphic rocks that probably derive from a large, differentiated planetary body; yet, in the absence of any "ground truth" the possibility that they are samples from Mercury rests only on circumstantial arguments: (1) the virtual lack of Na implies a highly refractory planet (near the Sun?) (2) oxygen isotopic compositions are close to and parallel to the TFL (like planetary rocks from Earth, Moon, Mars and Vesta) (3) preserved corona textures in NWA 2999 require a parent body capable of km-scale tectonic uplift of lithospheric material (by thrust faulting?) (4) each angrite specimen is texturally different with a unique CRE age (5) the wide range in CRE ages (55 to <6.1 Ma) suggests that the parent body (APB) is large enough to be struck repeatedly and may still be extant (6) very ancient formation ages (>4.555 Ga) imply very rapid core segregation and cooling following APB accretion (consistent with contraction?) (7) dynamical calculations predict that ~1% of material ejected from Mercury could reach Earth (8) the limited shock effects may mean that some angrites, including NWA 2999, were ejected by spallation; others may be impact melts (could vesicles in some quenched specimens be trapped impact rock vapor?).