Significance of apatite REE signatures in Early Archean Akilia and Isua supracrustal rocks, southern-west Greenland
Aivo Lepland1,2 and Gustaf Arrhenius1

1.Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0236, USA
2.Institute of Geology, Tallinn Technical University, Estonia Avenue 7,10143 Tallinn, Estonia

ABSTRACT

The ability of apatite to concentrate significant amounts of trace elements including rare earth elements (REE) that may reflect the geochemical state of the crystallization medium makes it valuable geological indicator mineral. In order to clarify the genesis of apatite and to evaluate its applicability as indirect biomarker in Early Archean supracrustal rocks of Isua belt and Akilia enclave (( 3.8 Ga), a detailed REE study using laser ablation ICPMS has been carried out on individual apatite grains extracted from metasedimentary units, mafic dykes/lavas and metasomatic veins. Because of the widespread metasomatic carbonation of Isua rocks and significant REE mobility in carbonate rich fluids, the heterochemical metamorphism is possible and recognition of primary REE signatures may be complicated.

The chondrite-normalized REE distribution patterns of apatite in most Isua cherts and banded iron formations (BIFs) are distinctive from non-sedimentary lithologies, and display relatively flat profile with variable positive Eu anomaly. Such REE distribution patterns are consistent with the REE data from several other Archean sedimentary formations, suggesting that apatite in Isua sediments has sampled the REE composition of Archean ocean and that the metamorphic overprinting was largely isochemical. However, in some metasedimentary samples the apatite REE profiles can be divided into two groups: one relatively flat and other slightly MREE enriched and LREE and HREE depleted. The latter group is interpreted as resulting from metamorphic re-equilibration of the original REE abundances in the rock, leading to preferential enrichment of MREE that have ionic radii similar to Ca2+ and fit best into apatite structure. Positive Eu anomalies are strongest in magnetite rich BIFs, while the cherts, that contain very little or no magnetite, display weak positive Eu anomalies. Such a correlation between magnetite abundance and Eu anomaly is consistent with BIF accumulation in immediate vicinity of hydrothermal venting sites that would have supplied both Fe and Eu. The weak positive Eu anomaly in Fe-poor cherts suggests their precipitation in areas outside of direct hydrothermal influence or during periods of low hydrothermal activity.

The ìhat-shapeî, strongly MREE enriched distribution patterns are typical for apatite in metasomatic, carbonate rich veins that cut across both sedimentary and non-sedimentary units. The similarity of apatite REE signatures in spatially separated veins cutting through different rock types suggests homogeneity of metasomatic fluids, unfractionated REE uptake and insignificant influence of the host rock upon the REE composition of the vein apatite. The REE abundances in apatite from mafic, chlorite rich rocks commonly display distinct negative Eu anomaly on the background of left-inclined, LREE depleted distribution profiles.

The REE distribution patterns of apatite in a quartzite unit of the Akilia enclave that has been interpreted as metachert/BIF, are characteristically Eu depleted, atypical of chemically formed Archean sediments. The observed negative Eu anomaly may reflect the Eu reduction and selective removal of relatively large Eu2+ during granulite facies metamorphism that Akilia rocks have experienced. However, such extensive alteration would require exceptionally high fluid/rock ratios to rework the protholith. If severe heterochemical alteration indeed has affected the Akilia quartzite then rock components other than REE were likely also to have been displaced. This complicates reliable identification of the sedimentary protholith on the basis of bulk rock chemical data. The amphibolite grade alteration of Isua metasediments has apparently not substantially altered their REE distribution, which retain a signature characteristic for Archean ocean with enhanced Eu concentration.

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