Correlations between mean annual temperature (MAT) and the weighted average oxygen isotope composition of yearly precipitation (delta(18)O(pt)) are well-known, but the utility of modern relations to make reliable estimates of temperature change over geological time is uncertain. This question has been addressed by using seasonal subsets of the global data base of temperature and isotopic measurements to represent two different climate modes. A comparison of middle- to high-latitude delta(18)O(pt)/temperature relations for each climate mode reveals (1) a significant offset between them, and (2) a difference in the strength of their correlations. The offset in relations is due to differences in temperature and water vapor budget in the tropics, and can lead to serious underestimates of temperature change. Differences in the strength of correlations arise from the influence of climate mode-specific, non-temperature factors on delta(18)O(pt). The overall result is that no single relation can be used in all cases to make unambiguous temperature estimates using a temporal record of delta(18)O(pt) values. One way to overcome these problems is to reconstruct delta(18)O(pt)/temperature relations for the time periods being investigated. If an appropriate proxy for delta(18)O(pt) is available, it may also be possible to estimate temperature without relying on delta(18)O(pt)/temperature relations. A promising alternative to these options is to use records of delta(18)O(pt) to test predictions of global climate models, an approach that may allow a reliable and more complete reconstruction to be made of climate change over geologic time. (C) 1999 Elsevier Science B.V. All rights reserved.

Neodymium-142 data for rocks from the Nuvvuagittuq greenstone belt in northern Quebec, Canada, show that some rock types have lower Nd-142/Nd-144 ratios than the terrestrial standard (epsilon Nd-142 = - 0.07 to - 0.15). Within a mafic amphibolite unit, 142Nd/144Nd ratios correlate positively with Sm/Nd ratios and produce a Sm-146-Nd-142 isochron with an age of 4280(-81)(vertical bar) (53) million years. These rocks thus sample incompatible- element- enriched material formed shortly after Earth formation and may represent the oldest preserved crustal section on Earth.

Andreasen and Sharma raise concerns about the neodymium-142 data and age that we reported for rocks from the Nuvvuagittuq greenstone belt in Quebec, Canada. We agree that the issue of accurate mass fractionation correction is important, but stand by our discussion of this issue in our original report and our conclusion that the variation in Nd-142/Nd-144 ratios reflects the decay of Sm-146 caused by Sm-Nd fractionation within 300 million years of Earth's formation.

U-Pb geochronology for the Nuvvuagittuq greenstone belt put a minimum age constraint of similar to 3.8 Ga for the supracrustal lithologies. Recent Nd-142 work raised the possibility that the dominant lithology of the belt formed at similar to 4.28 Ga, which would make it the only known remnant of Hadean crust preserved on Earth. The dominant lithology of the belt has a mafic composition that consists of gneisses ranging from cummingtonite amphibolite to garnet-biotite schist composed of variable proportions of cummingtonite + biotite + quartz, +/- plagioclase +/- garnet +/- anthophyllite +/- cordierite. The composition of this unit ranges from basalt to andesite and it is divided into two distinct geochemical groups that are stratigraphically separated by a banded iron formation (BIF). At the base of the sequence, the mafic unit is mainly basaltic in composition and generally has relatively low Al2O3 and high TiO2 contents, whereas above the BIF, the unit is characterized by high Al2O3 and low TiO2 contents and exhibits a wider range of compositions from basaltic to andesitic. The low-Ti unit can be further subdivided into a trace element depleted and a trace element enriched subgroup. The high-Ti unit is characterized by relatively flat REE patterns as opposed to the low-Ti gneisses, which display light REE-enriched profiles with flat heavy REE slopes. The incompatible element depleted low-Ti rocks have U-shaped REE profiles. The geochemical groups have compositional analogues in three types of ultramafic sills that exhibit the same stratigraphic succession. Generally, the mafic gneisses have low Ca, Na and Sr contents, with many samples having CaO contents < 1wt %. Such low Ca contents are unlikely to represent the original composition of their igneous precursors and are interpreted to reflect intensive alteration of plagioclase. These compositional characteristics along with the presence of cordierite + anthophyllite suggest that the protoliths of the mafic gneisses were mafic volcanic rocks exhibiting variable degrees of hydrothermal alteration. The high-Ti compositional type shares geochemical characteristics with tholeiitic volcanic suites with low Al2O3 and high TiO2 contents and is consistent with crystal fractionation at low pressures under dry conditions. In contrast, the low-Ti compositional group is geochemically similar to boninitic and calc-alkaline volcanic suites. The high Al2O3 and low TiO2 contents in the andesitic compositions suggest the early crystallization of Fe-Ti oxides and late appearance of plagioclase, and are more consistent with fractionation at elevated water pressures. The succession from 'tholeitic' to 'calc-alkaline' magmatism seen in the Nuvvuagittuq greenstone belt is typical of the volcanic successions of many younger Archean greenstone belts. Regardless of the exact tectonic setting, this volcanic succession suggests that the geological processes responsible for the formation and evolution of Archean greenstone belts were active at 3.8 Ga and perhaps as early as 4.3 Ga.

To investigate formation of the Earth's earliest continental crust, partial-melting experiments were conducted (at 900-1100 degrees C and 0.5-3.0 GPa) on two greenstones from the 4.3 Ga Nuvvuagittuq complex of Quebec, Canada. For comparison, experiments were also conducted on a compositionally similar but modern arc volcanic (a Tongan boninite). At 1.5-3.0 GPa and 950-1100 degrees C, the experimentally produced melts are compositionally similar to the tonalite-trondhjemite-granodiorite (TTG) granitoids that compose most of Earth's early continental crust, including a 3.66 Ga tonalite that encloses the Nuvvuagittuq Complex. Because the degree of melting needed to produce the TTG-like melts is comparatively high (> 30%), the relative concentrations of most incompatible elements in the melts are similar to those in their greenstone parent rocks. These greenstones have compositional affinities with modern subduction zone magmas and do not resemble mid-oceanic ridge basalts. That arc-like mafic rocks could have been selectively involved in TTG formation (in spite of their volumetrically subordinate status in most greenstone terrains) must reflect tectonic circumstances that were specific to their generation. These must have enabled accumulations sufficiently deep to melt at the 1.5-3.0 GPa needed to generate TTG magmas from eclogitic sources. They are also likely to have been related to some form of crustal recycling whereby mafic crust and water were returned to the mantle and arc-like mafic magmas generated as a consequence. To what degree these circumstances replicated modern plate tectonics is difficult to say, but it seems likely that, as in the modern Earth, the Hadean crust was organized into different tectonic environments and that one of these gave rise to the first continental crust.

The Nuvvuagittuq Greenstone Belt (NGB) in Northern Quebec, Canada, is dominated by mafic and ultramafic rocks metamorphosed to at least upper amphibolite facies. Rare felsic intrusive rocks provide zircon ages of up to similar to 3.8 Ga (David et al., 2009; Cates and Mojzsis, 2007) establishing the minimum formation age of the NGB as Eoarchean. Primary U-rich minerals that may provide reliable formation ages for the dominant mafic lithology, called the Ujaraaluk unit, have yet to be found. Metamorphic zircons, rutiles and monazites are present in the unit and give variably discordant results with (207)pb/Pb-206 ages ranging from 2.8 Ga to 2.5 Ga. The younger ages overlap 2686 +/- 4 Ma zircon ages for intruding pegmatites (David et al., 2009) and Sm-Nd ages for garnet formation in the Ujaraaluk rocks suggesting this era as the time of peak metamorphism and metasomatism in the NGB, coeval with regional metamorphism of the Superior craton. Sm-Nd data for Ujaraaluk whole rocks scatter about a Sm/Nd vs. Nd-143/Nd-144 correlation (MSWD = 134) whose slope would correspond to 3.6 +/- 0.2 Ga if interpreted as an isochron. This "isochron" is seen to consist of a series of younger similar to 3.2-2.5 Ga slopes for the different geochemical groups within the Ujaraaluk, emanating from a baseline distribution older than 4 Ga. The Sm-146-Nd-142 chronometer is less affected by metamorphism at 2.7 Ga because of Sm-146 extinction prior to similar to 4 Ga. Expansion of the Nd-142 dataset for the Ujaraaluk and associated ultramafic rocks continues to show a good correlation between Sm/Nd ratio and Nd-142/Nd-144 that corresponds to an age of 4388 (+15)(-17) Ma. The dataset now includes samples with superchondritic Sm/Nd ratios that extend the correlation to Nd-142 excesses of up to 8 ppm compared to the terrestrial standard with a total range in Nd-142/Nd-144 of 26 ppm. The upper Sm/Nd ratio end of the Ujaraaluk correlation is defined by rocks that are interpreted to be cumulates to compositionally related extrusive rocks indicating that this crystal fractionation had to occur while Sm-146 decay was active, i.e. well before 4 Ga. Intruding gabbros give Nd-143 and Nd-142 isochron ages of respectively 4115 +/- 100 Ma and 4313(-69)(+41) Ma, also supporting an Hadean age for the gabbros and providing a minimum age for the intruded Ujaraaluk unit. 3.6 Ga tonalites surrounding the NGB, 3.8 Ga trondhjemitic intrusive veins, and a 2.7 Ga pegmatite show a deficit in Nd-142 compared to the terrestrial standard. These felsic rocks plot to the low Sm/Nd ratio side of the Ujaraaluk isochron and do not show a correlation between their Sm/Nd and Nd-142/Nd-144 ratios, which can be explained if they are melts of ancient LREE-enriched mafic rocks, such as the Ujaraaluk, with the melting occurring after Sm-146 was extinct. A subset of least disturbed Ujaraaluk samples has coherent isotopic compositions for both short-lived and long-lived Nd isotopic systems giving Nd-143 and Nd-142 isochron ages overlapping within error of 4321 +/- 160 Ma (MSWD = 6.3) and 4406(-17)(+14) Ma (MSWD = 1.0), respectively. This age represents our best age estimate for the Ujaraaluk unit. The NGB thus preserves over 1.6 billion years of early Earth history including an expanse of mafic crust formed in the Hadean.

The composition of iron formations in the >= 3.75 Ga yr old Nuvvuagittuq Supracrustal Belt in northern Quebec provides a proxy for seawater composition of the Eoarchean, and perhaps Hadean oceans, as well as constraints on the types of nutrients available to Earth's earliest life forms. Integrated petrologic and geochemical relationships, mapped between mineral phases in thin section and whole-rock chemistry, provide a framework for interpreting bulk and micro-scale variations in these chemical sedimentary precipitates. Results show that there are two distinct chemical sedimentary units in the Nuvvuagittuq belt: i) a banded iron formation (BIF) consisting of alternating micro-bands of magnetite, Ca-Mg-Fe-silicates and quartz, and ii) a more silicate-rich (Fe-poor) unit, the banded silicate-formation (BSF), of alternating micro-bands of quartz and Ca-Mg-Fe silicates. Precursor BIF and BSF deposits were likely layered amorphous silica and ferric-oxyhydroxides, fine-grained carbonate oozes and/or Ca-Mg-Fe rich silicate gels deposited in a marine setting. Low Al2O3, TiO2 and HFSE concentrations show that they are relatively detritus-free, with distinctively seawater-like REE + Y profiles and consistently positive Eu anomalies. These features suggest that the rocks preserved their seawater-like compositions despite metamorphic overprinting. The most significant trace elements in the sediments are Ni and Zn. Experimentally-derived partitioning coefficients show that Ni was enriched in Eoarchean seawater as compared to today (up to 300 nM), while Zn was fairly similar (up to 20 nM). Compositional resemblances between the Nuvvuagittuq sediments and those documented in the ca. 3.8 Ga Isua supracrustals (West Greenland) provide a plausible case that global ocean processes - in terms of trace metal abundances - had reached steady-state by the Eoarchean. (C) 2011 Elsevier B.V. All rights reserved.

The Nuvvuagittuq greenstone belt is dominated by mafic rocks, called the Ujaraaluk unit, that are mostly composed of cummingtonite-plagioclase-biotite with variable amounts of garnet. While the oldest zircons contained in thin intrusive trondhjemitic bands are similar to 3.8 Ga, Sm-146-Nd-142 systematics suggest that the Ujaraaluk unit is as old as 4.4 Ga. The Nuvvuagittuq greenstone belt is surrounded by Eoarchean ITGs that have geochemical and isotopic compositions consistent with their derivation by partial melting of a source similar in composition and age to the Ujaraaluk unit. New zircon dates reported here show the Nuvvuagittuq TTGs to consist at least of four distinct age units of 3.76 Ga, 3.66 Ga, 3.5-3.4 Ga and 3.35 Ga. The Hf isotopic compositions of zircons from the TTG are consistent with derivation from Hadean mafic crust. The 3.66 Ga to 3.35 Ga ITGs appear to have been formed primarily from melting of a source compositionally similar to the 4.4 Ga Ujaraaluk unit, whereas the more radiogenic Hf of the zircons from the 3.76 Ga TTGs may suggest derivation from melting of a source compositionally similar to 4.1 Ga intrusive gabbros. Alternatively, the distinct rare earth element patterns of the 3.76 Ga and 3.66 Ga TTGs suggest their derivation from sources with variable amounts of residual garnet and hence formation at different depths. The composition of the older TTGs is indicative of a deeper source that may have involved a greater interaction between the melt and the mantle to explain the more radiogenic Hf isotopic compositions of their zircons. Sources compositionally similar to the Ujaraaluk unit and intrusive gabbros appear to be the most likely candidates for the Hadean precursor of the Nuvvuagittuq TTGs. (C) 2013 Elsevier B.V. All rights reserved.