Chemistry, petrology and origin of banded iron-formation lithologies from the 3800 MA isua supracrustal belt, West Greenland

Abstract

Data are presented on the phase petrology of 60 samples of banded iron-formation (BIF) and related rock types from 35 localities spanning the known stratigraphic units in the 3.8-Ga Isua Supracrustal Belt, West Greenland. We report representative microprobe analyses for all silicate and carbonate minerals found in these samples. In addition, complete chemical analyses for major, minor and trace elements are presented for a subset of 28 samples. In order to make our data base more representative of actual field occurrences, we have deliberately included in our sample set certain quartz-rich rocks that have iron contents lower than the nominal limit of 15 weight percent ordinarily ascribed to BIFs. On the basis of field setting, mineral assemblage and rock composition, we recognize six iron-formation lithologies: (I) quartz-magnetite IF; (II) magnesian IF; (III) aluminous IF; (IV) graphitic IF; (V) Mg-Fe carbonate IF; and (VI) Ca-Mg carbonate IF. Although rocks containing abundant oxide, carbonate and/or silicate minerals occur, we consider it inappropriate to assign a ‘facies classification’ onto the Isua occurrences, given the extreme state of deformation and high grade of metamorphism experienced by these rocks. The most common assemblage is: quartz+magnetite±cummingtonite/grunerite±actinolite/hornblende±ripidolite (ferroan chlorite)±calcite; almandine-rich garnet, ferroan talc, or minor sulfide (pyrite±pyrrhotite) occur in a few samples. Two unique assemblages are: quartz+magnetite+grunerite+stilpnomelane; and quartz+magnetite+actinolite+grunerite+ferrosalite. Carbonate-rich samples contain the assemblages: ±quartz±magnetite±cummingtonite/grunerite± actinolite±ripidolite±ferroan dolomite ± siderite/ferroan magnesite. Minor graphite may be present in some of the carbonate-rich samples, as well as in several of the oxide- and silicate-rich samples. In most samples, evidence of retrogression is rare, although the status of ripidolite as a prograde mineral is ambiguous. Overall, the iron-formation assemblages are compatible with amphibolite-facies P-T conditions based on studies of pelitic rocks. An interesting feature of the Isua BIFs is the very wide range of compositions revealed for coexisting Ca-poor and Ca-rich amphiboles, reflecting the variable nature of the samples studied. Nevertheless, the Isua assemblages are very similar to those found in younger Archaean and Proterozoic BIF suites that have been metamorphosed to amphibolite facies. The average major element composition for the 28 analysed samples is (in wt.% on an H 2O- and CO 2-free basis): SiO 2, 55.9; TiO 2, 0.06; Al 2O 3, 1.36; Fe 2O 3, 11.8; FeO, 19.3; MgO, 6.7; MnO, 0.62; CaO, 4.0; Na 2O, 0.01; K 2O, 0.20; P 2O 5, 0.16. These values are in the normal range of averages for younger Archaean and Proterozoic BIFs, when one allows for dilution by quartz in our data set. The average abundances of ferromagnesian trace elements are all low (in ppm: Sc, 2.5; V, 33; Cr, 32; Co, 11; Ni, 58; Cu, 28; Zn, 84), and similar to data on younger BIFs, although the Ni contents of the Isua samples appear to be slightly higher. The relative distributions of these elements qualitatively resemble those reported in ‘Algoma-type IF’ as opposed to values in ‘Superior-type IF’. These results support the contention that, as early as 3800 Ma, restricted basins of deposition were established that had the appropriate physical-chemical conditions for acumulation of Fe-rich chemical sediments from a probable volcanogenic source. Abundances of the rare-earth elements (REE) span two orders of magnitude concentration, ranging from ∼ 0.1 to 20 times chondrites. Most samples show negative Ce anomalies, and all but one have a positive Eu anomaly. Average values are (in ppm): La, 2.34; Ce, 3.92; Nd, 2.03; Sm, 0.51; Eu, 0.369; Tb, 0.118; Yb, 0.585; Lu, 0.102. Abundances of most other trace elements are quite low with the exception of Sr in Ca-Mg carbonate IF (up to ∼ 100 ppm), and Zr in aluminous IF (up to ∼ 100 ppm). A ‘crustal’ Zr/Hf ratio of ∼40 characterizes the sample suite as a whole, but an evaluation of La-Sc-Zr-Hf systematics reveals that most samples are free of ‘contamination’, based on comparisons with data for contemporaneous Isua clastic metasediments and amphibolites. Attention is drawn to the close similarity between the abundances of the REE and ferromagnesian trace elements in the Isua samples and those found in present day deep-sea hydrothermal deposits, such as the Galapagos Mounds. Mixing calculations using present-day seawater REE (with its negative Ce anomaly), and REE in high-temperature mid-ocean ridge hydrothermal fluid (with its pronounced positive Eu anomaly) faithfully reproduce the average REE pattern of the Isua BIFs using a mixture of 100 parts seawater and 1 part hydrothermal fluid. Accordingly, we interpret the Isua BIFs as the products of precipitation from very dilute hydrothermal solutions that had passed through (mafic to ultramafic?) volcanic rocks, stripping them of their iron, silica and other trace elements, particularly Eu. A further implication of our results is that the Archaean seawater component possessed a negative Ce anomaly, suggesting that ancient oceans may have been substantially more oxidized than was heretofore recognized.