Apatite-Graphite in Quartz-Pyroxene Rock From Akilia Island; Primary Biogenic Origin Questioned by Oxygen Isotope Evidence.

Recent field and geochemical research (1) has questioned the claim for the earliest traces of life on Earth (2) in a banded quartz-pyroxene dominated rock from Akilia Island, southern West Greenland. This rock, once considered a banded iron formation (BIF), has now been interpreted as a highly deformed ultramafic igneous rock that has undergone pervasive silica addition. We present oxygen isotope analysis by laser fluorination/mass spectrometry on quartz in a suite of samples from the banded rock on Akilia, as well as similar rocks on Ingnerssuartut Island (ca. 10 km south of Akilia). All studied rock samples show δ¹⁸O values consistently around +12 ‰ relative to SMOW, which is significantly lower than δ¹⁸O values of quartz from BIFs of the related amphibolite-facies Isua Supracrustal Belt, ca. 150 km northeast of Akilia. Instead, the data fall in the δ¹⁸O range of granites and pegmatites. Typical low-grade metamorphic BIFs have quartz δ¹⁸O of ca. +20 ‰ and magnetite δ¹⁸O of ca. +3 ‰ . If one assumes closed-system conditions, isotope mass balance shows that the observed low values for quartz can only be obtained by isotope exchange between quartz and magnetite during metamorphism if an original BIF consisted of at least 50-80 wt % magnetite. This is not consistent with the observed mineralogy; magnetite is a minor mineral phase in the Akilia quartz-pyroxene rock, and some of the layers in the succession are nearly pure quartzites. It is possible, however, that open-system isotope exchange occurred between quartz of an original chert/BIF and a low δ¹⁸O metamorphic fluid that equilibrated with surrounding igneous rocks. In order to explain the observed low δ¹⁸O values, a pervasive fluid infiltration and high fluid/rock ratio is required, for a sufficiently long time to allow complete recrystallization of the original quartz grains. Quartz from our Akilia samples does not show any variation in δ¹⁸O, suggesting that if it was ever part of a chert/BIF, then pervasive fluid infiltration would have been required for the entire outcrop. Such fluids would dissolve graphite during recrystallization if not carbon-saturated, and would exchange δ13C if carbon saturated. Either way the preservation of carbonaceous inclusions in apatite crystals, that were previously interpreted as biogenic in origin, appears highly unlikely. It could be argued that we have sampled only those parts of the outcrop that were most intensely infiltrated with silica, and our δ¹⁸O values would simply reflect metasomatic quartz, not the original chert/BIF. However, we obtained δ¹⁸O values from the exact same location in which the isotopically light carbon was discovered. We conclude that the observed mineralogy and quartz δ¹⁸O do not favor a BIF protolith. The apatite-graphite associations in the Akilia quartz-pyroxene rock must have formed during or after the last main metamorphic event, and thus are at least younger than ca. 2800 Ma. The graphite inclusions may be metasomatic in origin or may be derived from the introduction of organic carbon during metamorphism. (1) Fedo, C. M., Whitehouse, M. (2002) Science, 296, 1448-1452. (2) Mojzsis, S. J., et al. (1996) Nature, 384, 55-59.