Composition of the Nuvvuagittuq "faux-amphibolite": implications for the formation of Earth's early crust
Until recently, the only Hadean samples available were detrital zircons from the Jack Hills conglomerate (∼4.4 Ga). The chemistry of these zircons provided evidence for their derivation from a granitic melt indicating the presence of a felsic crust early in the Earth's history. Recent 142Nd work on the Nuvvuagittuq greenstone belt suggests that it formed at ∼4.28 Ga, making it the only know remnant of Hadean crust preserved on Earth. The dominant lithology of the belt, know as the ''faux-amphibolite", has a mafic composition and consist of gneisses ranging from cummingtonite amphibolite to garnet-biotite schist composed of variable proportions of cummingtonite + plagioclase + biotite + quartz ± garnet. The composition of the faux-amphibolite ranges from basalt to basaltic andesite (42-66 wt.% - modal value: 52 wt.%) and is characterized by low Ti and high Al contents with LREE-enriched profiles and flat HREE. These geochemical characteristics most resemble those of modern day calc-alkaline volcanic suites. The faux-amphibolite is also relatively depleted in Ca, Na and Sr with many samples having CaO contents < 1 wt.%. Such low Ca contents are unlikely to represent the original composition of the igneous precursor and are interpreted to reflect intense weathering, perhaps in a warmer and more acidic Hadean surificial environment. These compositional characteristics suggest that the protolith of the faux-amphibolite was an altered basalt, or perhaps immature sediments derived primarily from a mafic source. The high Al and low Ti of the andesitic compositions suggest the early crystallization of Fe-Ti oxides and late appearance of plagioclase consistent with fractionation at elevated water pressures. Regardless of the tectonic setting responsible for its emplacement, the Nuvvuagittuq faux-amphibolite appears to represent a ∼4.3 Ga mafic volcanic suite ranging from basalt to basaltic andesite that has significantly lost Ca and Sr due to intense weathering.
An Early Shelter for Life on Earth? S and O Isotope Evidence From the Nuvvuagittuq Greenstone Belt, Northeastern Superior Province, Canada
The Nuvvuagittuq Greenstone Belt (NGB,) is one of Earth's oldest Eoarchean volcano-sedimentary suites, and was emplaced prior to 3.75 Ga (Cates and Mojzsis, S.J. 2007), and likely as early as 4.28 Ga (O'Neil et al. 2008). As revealed by recent detailed mapping, the NGB geology is dominated by cummingtonite-bearing amphibolites (formerly called Faux-amphibolite, (O'Neil et al. 2008)) and a series of conformable gabbroic and ultra mafic sills. Minor horizons in the belt include banded iron formations (BIF) with cm-scale quartz-rich and magnetite-rich laminations, and a pyrite-bearing quartzite in gradational contact with the BIF. These rocks and may represent the oldest remains of the sedimentary record on Earth. We performed multiple O-isotope measurements of individual minerals (quartz, garnet, amphibole and magnetite) from three NGB lithologies (BIF, faux amphibolites and quartzite). In BIF samples, δ18O values cover a narrow range (from 1.36 per mil magnetite to 4.98 per mil with one outlier at 9.99 per mil), whereas silicate minerals in the faux reveal a more scattered range that is more depleted in light isotopes (7.77 per mil ≤ δ18O ≤ 13.38 per mil). One quartzite sample has also been analyzed and reveals the most 16O-depleted composition yet measured from the belt (δ18O = 15.44 per mil). The δ17O and δ18O values from these samples define a fractionation line for multiple oxygen isotopes with a slope of 0.528 ± 0.004 (MSWD = 0.47), statistically indistinguishable from the slope (0.524 +± 0.002) of the Archean Terrestrial Fractional Line (TFL) determined from other Archean rocks and minerals. These results show no evidence for the drastic O-isotope heterogeneity that would likely accompany the late heavy bombardment of the Earth-Moon system. We also performed multiple S-isotope ratio measurements (δ34S, Δ33S, Δ36S) in samples covering the entire lithological suite of the NGB. Samples from the quartzite and BIF display a narrow range of δ34S values (0.8 per mil ≤ δ34S ≤ 3.3 per mil), in good agreement with ranges reported so far from early Archean sediments. The same samples exhibit non-zero Δ33S and Δ36S values (respectively ranging from 0.18 to 2.27 per mil and from -2.9 per mil to -0.6 per mil) that are negatively correlated (Δ36S ~ -0.9 Δ33S) and conform to the linear array that characterizes most of the Archean Eon. Finally, the NGB BIF and silica formation reveal a tight correlation between Δ33S and δ34S values (Δ33S ~ 0.9 δ34S) that matches previous observations from Neoarchean and Paleoarchean samples. In previous studies, the δ34S - Δ33S - Δ36S correlations observed here have been taken to reflect both a restricted chemistry of the atmosphere and a dynamic microbiologically-dominated sulfur cycle. Taken together, and considering the age of the NGB, the S- and O-isotope results suggest that conditions conducive to life on Earth were established very early in Earth's history. Either there was apparently no significant perturbation of the early Earth system by the late heavy bombardment, or the rocks of the NGC record a time interval that was not affected by this dramatic event. This talk will explore the implications of these two possibilities for the early establishment of a stable environment suitable for the emergence of life.
Using Pyroxene and Amphibole Compositions to Determine Protolith of Banded Quartz- Amphibole-Pyroxene Rocks on Akilia, Southwest Greenland: a Lithology Suitable for Hosting Earth's Oldest Life?
At ∼ 3.8 Ga in age, Earth's oldest known supracrustal rocks are exposed in SW Greenland and are comprised dominantly of mafic igneous rocks with less common sedimentary units, included banded iron formation (BIF). The great antiquity of the supracrustal rocks and repeated claims for a fossil record makes Greenland one of the prime astrobiological destinations on Earth, however, many primary characteristics of these rocks have been overprinted during multiple high-grade metamorphic events, which results in complex field relationships (e.g., Myers and Crowley, 2000; Whitehouse and Fedo, 2003). One example of this concerns an ∼ 5 m thick lithology dominated by bands of quartz, amphibole, pyroxene interpreted by some as BIF (Mojzsis et al., 1996; Nutman et al., 1997; Dauphas et al., 2004) on Akilia, SW Greenland. Correct identification of these rocks is of the utmost importance because they are reported to contain grains of apatite with 13C- depleted graphite inclusions that have been claimed as evidence for the oldest (> 3800 Mya) life on Earth (Mojzsis et al., 1996; Nutman et al., 1997; McKeegan et al., 2007). We analyzed mafic mineral compositions by electron microprobe from samples collected from a detailed measured section and from sample 92-197, the rock originally claimed to host Earth's oldest chemofossil. Ultramafic rocks from outside the quartz-amphibole-pyroxene (QAP) lithology are dominated by enstatite, anthophyllite, and hornblende and possess bulk trace-element signatures indicative of an igneous origin. Sample AK 38, a band of mixed pyroxene and amphibole that occurs within the QAP unit also has a bulk trace- element composition consistent with an ultramafic protolith, but contains Fe-rich clinopyroxene (Mg# = ∼ 50). AK 38 amphiboles are dominated by actinolite, although a few analyses of anthophyllite point towards an original Mg-rich protolith. Other QAP samples contain Fe-rich clino- and orthopyroxenes, actinolite and hornblende. Magnetite is typically only found in trace amounts and only occurs in greater amounts (5-10%) in two QAP sub-units (AK33 and AK42). A protolith consisting of ultramafic rocks with disseminated carbonate combined with a Fe-rich carbonate band, brought to granulite facies metamorphism, can produce the observed pyroxene and amphibole compositions in the QAP lithology (Bonnichsen, 1975). Ca-rich pyroxenes could have formed during metamorphism from an original Mg-rich ultramafic protolith that contained disseminated secondary carbonate. Ca-poor pyroxenes and amphiboles could have been derived from a Fe-rich metasomatic carbonate band located within an ultramafic protolith. Examples of rocks similar to this occur in the nearby, lower metamorphic grade Isua Greenstone Belt. Even though these Isua samples have an undisputed ultramafic protolith, they exhibit mineral and whole-rock chemistries more similar to the Akilia QAP rock than to the Akilia ultramafic unit. This suggests that carbonate-bearing metasomatic fluids could have modified the original mineralogical and geochemical signatures of the ultramafic protolith, and makes it plausible that a similar modification occurred in the Akilia QAP lithology, rendering a BIF protolith unnecessary.
Oxygen and Hydrogen Isotopes of the Buck Reef Chert: Implications for an Archean Paleoenvironment
The geochemistry of Archean cherts provides key evidence for the environmental conditions of the early Earth. Stable oxygen isotope ratios of cherts have been interpreted as evidence for high (55-85°C) Archean ocean temperatures, however temperature estimates rely on an assumption of the oxygen isotope composition of the Archean ocean. Paired oxygen and hydrogen isotopes of modern and ancient cherts are shown to record information related to the temperature of formation without direct knowledge of the original isotopic composition of formational waters. We present oxygen and hydrogen isotope data from the 3.42 Ga Buck Reef Chert that raises new questions regarding Archean ocean temperature and isotope composition. Oxygen and hydrogen isotope data form an array consistent with formation at low diagenetic temperatures. The most 18O enriched cherts record the lowest diagenetic temperatures and are consistent with formation in equilibrium with 18O and D-depleted waters at temperatures below 40°C. Geochemical and sedimentary evidence for shallow to deep marine formation suggests that the Buck Reef Chert may have precipitated in a temperate Paleoarchean environment that was significantly more 18O and D-depleted than a modern 'ice-free' ocean.
Emerged Oceanic Plateaux and Their Role in Regulating Archean Ocean and Atmosphere Composition
A geologist associates the Earth's surface division into land and oceans instinctively with continents and oceanic plates. Here I propose that for the Archean eon, we need to break with this concept. As an alternative I propose a three-fold division. Continental land, relatively thin oceanic plates covered by water and also much thicker oceanic plateaux that were at least episodically emerged and contributed ca. 50% of the total land mass. The rationale for this proposal is a long-standing conundrum locked up in ancient hydrogenous sediments precipitated from seawater. They contain elemental and isotopic records with mutually exclusive conclusions regarding the supply of elements. Namely, isotopic data, particularly Sr, are interpreted to imply a preponderance of hydrothermal flux to the ocean. The elemental abundance of Eu, however, apparently requires a much greater flux from land. Yet a higher flux from continental land mass would be visible in the Sr- isotope record. I will present additional evidence from the origin of the marine rare earth element (REE) pattern that deepens the conundrum, which can be solved if the Archean landmass included emerged oceanic plateaux in addition to the continents. The appeal of the idea is that the marine REE inventory, including Eu, is only influenced by relative fluxes from hydrothermal vents and land, regardless of the nature of the land. Strontium isotopes, on the other hand, cannot discriminate between hydrothermal flux and riverine input draining juvenile oceanic plateaux. Using this concept, I will present a simple quantitative model that explains the evidence with a landmass at the end of the Archean that was comparable in area to that of today but made up to ca. 60% by oceanic plateaux. My proposal has implications far beyond the REE and Sr fluxes to the ocean. In particular, it requires the Archean upper mantle to have been relatively cool, potentially allowing for subduction of the thin oceanic lithosphere along destructive plate margins. The disappearance of emerged oceanic plateaux, at ca. 2.6 Ga, is related to the temperature of plumes, which apparently dropped at this time. The re-organization of the landmass at the A-P boundary also significantly changed the supply of essential nutrients to the ocean. This included Ni, a key nutrient for methanogens. Furthermore, the existence of emerged oceanic plateaux throughout the Archean provided weathering template to bind the early atmospheric greenhouse in time for the planet-wide glaciation. In summary, the temperature distribution in the mantle, in particular the potential temperature in plume sources, not only governed the type of melts produced (e.g. komatiite vs. basalt) but by creating horizontal volcanic piles (plateaux) of sufficient thickness to emerge from the ocean, it was also the single most important factor affecting atmospheric composition and climate and therefore the evolution of life.
Supercontinents, Supermoutains, and the Rise of Atmospheric O2