Rocks In Greenland Research Articles

An Eoarchean continental nucleus for the Fennoscandian Shield and a link to the North Atlantic craton

Abstract Enabling the build-up of continental crust is a vital step in the stabilization of cratonic lithosphere. However, these initial crustal nuclei are commonly either destroyed by recycling or buried by younger rocks. In the Fennoscandian Shield, the oldest rocks are ca. 3.5 Ga, but ca. 3.7 Ga inherited and detrital zircons suggest the presence of an older, unexposed crustal substrate. We present U-Pb, O, and Hf isotope data from detrital zircons of three major Finnish rivers as well as zircon O and Hf isotope data from previously dated rocks of the Archean Suomujärvi and Pudasjärvi complexes, central Finland. Combined, these data indicate a previously unidentified ca. 3.75 Ga crustal nucleus in the Fennoscandian Shield. This adds to the growing number of Eoarchean nuclei recognized in Archean terranes around the globe, highlighting the importance of such nuclei in enabling the growth of continental crust. The isotope signatures of the Fennoscandian nucleus correlate with equivalent-aged rocks in Greenland, consistent with a common Eoarchean evolution for Fennoscandia and the North Atlantic craton.

Metasomatic Modification of the Mesoarchaean Ulamertoq Ultramafic Body, Southern West Greenland

AbstractNumerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in southern West Greenland. As some of the oldest exposed ultramafic bodies, determining their origin, as mantle segments or magmatic cumulates, is an important yet controversial issue. The origin of these Archaean ultramafic rocks remains unclear, in-part because these rocks have undergone metasomatic modification since their formation, yet the effects of this metasomatism have so far not been assessed in detail, despite being crucial for understanding their geochemical evolution. Here, we examined the petrology, mineral and whole-rock chemistry of the largest ultramafic body located within the Mesoarchaean Akia terrane, known as the Ulamertoq ultramafic body, to elucidate the poly-metamorphic and metasomatic events that overprinted the protolith. Pronounced lithologic zoning from hydrous mineral-rich layers to orthopyroxene-rich ultramafic rocks at the boundaries between ultramafic rocks and the granitoid country rocks was formed locally by metasomatic reactions related to the granitoids. The main body of ultramafic rocks, far from the contacts, can be classified into four types based on mineral assemblage and chemistry. The fine-grained orthopyroxene aggregates and large poikilitic orthopyroxenes have low Cr2O3 and CaO contents, suggesting a secondary origin. Trace element compositions of orthopyroxene and/or amphibole in the main ultramafic rocks indicate that at least three types of metasomatic agents were required to form these minerals and the associated whole-rock chemical variations within the ultramafic body. Variations represent differences in the proportions of metasomatic orthopyroxene and/or amphibole and phlogopite added to a dunitic protolith. The main body of Ulamertoq ultramafics experienced metasomatism under granulite-facies. Retrograde cooling occurred, to 650°C–850°C and <1.8 GPa prior to local metasomatism via country-rock reaction. The presence of titanian clinohumite and its associated mineral assemblage in the least-metasomatised dunites suggest the possibility that the main ultramafic rocks went through a hydration/dehydration process at ~800°C–900°C and <2 GPa prior to metasomatic modification. This study demonstrates that it is important to consider the effects of multi-stage metasomatism and metamorphism in order to elucidate the origin of the Archaean ultramafic rocks in Greenland and elsewhere.

Author Correction: Reassessing evidence of life in 3,700-million-year-old rocks of Greenland.

In Extended Data Fig. 1 of this Letter, the map showed the field-work location incorrectly; this figure has been corrected online.

Reassessing evidence of life in 3,700-million-year-old rocks of Greenland.

The Palaeoarchean supracrustal belts in Greenland contain Earth's oldest rocks and are a prime target in the search for the earliest evidence of life on Earth. However, metamorphism has largely obliterated original rock textures and compositions, posing a challenge to the preservation of biological signatures. A recent study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland described a rare zone in which low deformation and a closed metamorphic system allowed preservation of primary sedimentary features, including putative conical and domical stromatolites1 (laminated accretionary structures formed by microbially mediated sedimentation). The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth's rock record. Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the 'stromatolites' are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial. The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales.

Claims of Earth's oldest fossils tantalize researchers

Rocks in Greenland contain structures that could have been laid down by living organisms 3.7 billion years ago.

Geology: Evidence of life in Earth's oldest rocks.

When did life first arise on Earth? Analysis of ancient rocks in Greenland that contain structures interpreted as bacterial in origin suggest that Earth might have been an abode for life much earlier than previously thought. See Letter p.535 Stromatolites are sedimentary formations created by the layered growth of microorganisms in shallow marine settings. Fossil stromatolites constitute some of the earliest evidence for life on Earth. Allen Nutman et al. describe metamorphosed stromatolites deposited around 3,700 million years ago in what is now Greenland. This is more than 200 million years older than the previous record-holders for earliest-known fossils, so these stromatolites rank as the Earth's earliest fossils by some margin. Although there is indirect evidence from isotope geochemistry that the pedigree of life on Earth is even older, this report is likely to be controversial.

Hints of oldest fossil life found in Greenland rocks

Hints of oldest fossil life found in Greenland rocks

Clumped-isotope thermometry of magnesium carbonates in ultramafic rocks

Magnesium carbonate minerals produced by reaction of H2O–CO2 with ultramafic rocks occur in a wide range of paragenetic and tectonic settings and can thus provide insights into a variety of geologic processes, including (1) deposition of ore-grade, massive-vein cryptocrystalline magnesite; (2) formation of hydrous magnesium carbonates in weathering environments; and (3) metamorphic carbonate alteration of ultramafic rocks. However, the application of traditional geochemical and isotopic methods to infer temperatures of mineralization, the nature of mineralizing fluids, and the mechanisms controlling the transformation of dissolved CO2 into magnesium carbonates in these settings is difficult because the fluids are usually not preserved. Clumped-isotope compositions of magnesium carbonates provide a means to determine primary mineralization or (re)equilibration temperature, which permits the reconstruction of geologic processes that govern magnesium carbonate formation. We first provide an evaluation of the acid fractionation correction for magnesium carbonates using synthetic magnesite and hydromagnesite, along with natural metamorphic magnesite and low-temperature hydromagnesite precipitated within a mine adit. We show that the acid fractionation correction for magnesium carbonates is virtually indistinguishable from other carbonate acid fractionation corrections given current mass spectrometer resolution and error. In addition, we employ carbonate clumped-isotope thermometry on natural magnesium carbonates from various geologic environments and tectonic settings. Cryptocrystalline magnesite vein deposits from California (Red Mountain magnesite mine), Austria (Kraubath locality), Turkey (Tutluca mine, Eskişehir district) and Iran (Derakht-Senjed deposit) exhibit broadly uniform Δ47 compositions that yield apparent clumped-isotope temperatures that average 23.7±5.0°C. Based on oxygen isotope thermometry, these clumped-isotope temperatures suggest mineralization at shallow crustal depths in the presence of meteoric water. Hydrous magnesium carbonates from a 400-km latitudinal transect along the serpentinized-peridotite bodies of the California Coast Ranges record clumped-isotope temperatures between 14.2 and 22.7±2.8°C, in agreement with historical maximum temperatures during the rainy season for California. Talc–carbonate alteration of ultramafic rocks in Greenland (Isua Supracrustal Belt) and Vermont (Ludlow) yields clumped-isotope alteration temperatures of magnesite and dolomite between 326 and 490°C, broadly consistent with paragenesis and thermodynamic analysis for CO2 metasomatism of ultramafic rocks. These metamorphic carbonates extend the applicability of clumped-isotope thermometry to high-temperature magnesium carbonate systems and indicate equilibrium blocking temperatures for magnesite of ∼490°C. Our study demonstrates the applicability of the clumped isotope approach to provide information on the formation of magnesium carbonates as ore resources, surface records of climate, and metamorphic assemblages.

Manganiferous minerals of the epidote group from the Archaean basement of West Greenland

The chemical compositions and crystal structures of Mn3+-containing minerals from the epidote group in Greenland rocks are investigated and described in detail. They occur in hydrothermally altered Archaean mafic sequences within the gneissic complex of the North Atlantic craton of West Greenland. The Mn-containing minerals have a characteristic red to pink colour. A detailed microchemical study shows a significant inter- and intra-sample variation in Mn content. The samples from different parageneses can be classified as Mn-bearing epidote and Mn-bearing clinozoisite. The intra-sample variation in the content of Al, Fe and Mn is on a very fine scale, but still allows for identification of a negative correlation between Mn and Fe. Textures indicate different stages of growth. Crystal chemical data are compared with literature data and illustrate the basic systematic differences between the influence of Fe and Mn on the crystal structure of the epidote group minerals.

Manganiferous minerals of the epidote group from the Archaean basement of West Greenland

The chemical compositions and crystal structures of Mn3+-containing minerals from the epidote group in Greenland rocks are investigated and described in detail. They occur in hydrothermally altered Archaean mafic sequences within the gneissic complex of the North Atlantic craton of West Greenland. The Mn-containing minerals have a characteristic red to pink colour. A detailed microchemical study shows a significant inter- and intra-sample variation in Mn content. The samples from different parageneses can be classified as Mn-bearing epidote and Mn-bearing clinozoisite. The intra-sample variation in the content of Al, Fe and Mn is on a very fine scale, but still allows for identification of a negative correlation between Mn and Fe. Textures indicate different stages of growth. Crystal chemical data are compared with literature data and illustrate the basic systematic differences between the influence of Fe and Mn on the crystal structure of the epidote group minerals.

Earth's patchy late veneer

A 'late veneer' of meteoritic material, added after Earth's core had formed, may be the source of our noble metals. Its absence from some parts of Earth's mantle will now force a rethink about this late accretion. See Letter p.195 It has long been speculated that a 'late heavy bombardment' of Earth by meteoritic material replenished the mantle's budget of siderophile (iron-loving) elements, such as tungsten, that were largely lost to the core during its segregation. However, evidence for this 'late veneer' remains indirect, and its influence has been much debated. Matthias Willbold and colleagues present high-precision tungsten isotope analyses of ancient Greenland rocks and show that they have significantly higher 182W/184W ratios than modern terrestrial samples. This finding is in good agreement with the expected influence of a meteoritic late heavy bombardment. They speculate that both the tungsten isotope data and the observed decrease in 142Nd/144Nd neodymium ratios can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.

Isotope-geochemical constraints on the formation of the early Earth’ crust

Analysis of isotope-geochemical data obtained for the early crustal complexes of the Earth provided constraints on the formation time, scales of development, and geochemical features of protocrust. Most informative were isotope-geochemical and geochemical data on the oldest zircons with ages up to 4.4 Ga, short-lived 146Sm/142Nd isotope system, and lead isotope composition of the oldest rocks of Greenland. The presence of positive 142Nd anomaly in the rocks of West Greenland and negative anomaly in the amphibolites of the oldest Nuvvuagittuq greenstone belt of the Superior province (O’Neil et al., 2008) indicates the early differentiation of the Earth material into depleted mantle and enriched (basaltic) crust (Caro et al., 2006; Benett et al., 2007a, b; O’Neil et al., 2008). Pb-Pb isotopic systematics of the oldest crustal rocks from West Greenland and Labrador testifies that high μ enriched crust (238U/204Pb = 10.9) of basaltic composition already existed 3.9 Ga ago (Kamber et al., 2003). Based on isotope-geochemical and geochemical features of the oldest zircons in the Late Archean greenstone belts of the Yilgarn block (Western Australia), the crust of intermediate-felsic composition and water on the Earth’s surface already existed 4.4 Ga ago (Wilde et al., 2001).

Seawater-like trace element signatures (REE + Y) of Eoarchaean chemical sedimentary rocks from southern West Greenland, and their corruption during high-grade metamorphism

Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive La/La*SN, Eu/Eu*SN, and YSN anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units in the Isua supracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island), display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of “earliest life” rocks (sample G91/26) from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous assertions (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised, mafic rock (Fedo and Whitehouse 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43–60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures. There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but are beginning to lose the distinct La, Eu and Y positive anomalies and are interpreted as metasomatised chemical sediments. Finally there are some siliceous samples with completely different trace element patterns that are interpreted as rocks of non-sedimentary origin, and include metasomatised mafic rocks. The positive La/La*SN, Eu/Eu*SN and YSN anomalies found in Isua BIFs and other Eoarchaean Greenland samples, such as G91/26 from Akilia, suggests that the processes of carbonate ion complexation controlling the REE − Y patterns were already established in the hydrosphere at the start of the sedimentary record 3,600–3,850 Ma ago. This is in accord with the presence of Eoarchaean siderite-bearing marbles of sedimentary origin, and suggests that CO2 may have been a significant greenhouse gas at that time.

Silicon isotopes in ∼3.8 Ga West Greenland rocks as clues to the Eoarchaean supracrustal Si cycle

We report MC-ICP-MS Si-isotopic measurements ( δ 29Si recalculated as δ 30Si) on micro-subsamples (at 500 μm scale) from several major rock types of the ∼3.8-Ga-old Isua Greenstone Belt (IGB, southern West Greenland) and surrounding Eoarchaean terrains. With a large overall range of variations (− 2.80‰ < δ 30Si < + 0.68‰), they demonstrate strong involvement of surface fluids enriched with dissolved Si. The resistance of Si-isotopes to metamorphic resettings and metasomatic overprints is also established. Metabasaltic pillows and metasediments display similar 30Si-enriched signatures, suggesting that emergent surfaces of the Eoarchaean protocrust were composed of slightly weathered, hydrothermally altered, mafic–ultramafic bodies. Isua magnetite–quartz Banded Iron Formation (BIF) is strongly depleted in 30Si relative to all coeval rocks. This depletion supports Rayleigh-controlled precipitation from seafloor-vented hydrothermal fluids. In contrast, banded quartz–pyroxene rocks (from Akilia Island, some 150 km southwest of the IGB), which some authors have identified as BIF-related, yield quartz with Si-isotopic composition ( δ 30Si = − 0.36‰) similar to metamorphic-derived quartz ( δ 30Si = − 0.50‰). This supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers.

Vestiges of life in the oldest Greenland rocks? A review of early Archean geology in the Godthåbsfjord region, and reappraisal of field evidence for >3850 Ma life on Akilia

The Godthåbsfjord region of West Greenland contains the most extensive, best exposed and most intensely studied early Archean rocks on Earth. A geological record has been described of numerous magmatic events between ∼3.9 and 3.6 Ga, and evidence of life at >3.85 Ga and ∼3.8–3.7 Ga has been proposed from two widely-separated localities. Some of these claims have recently been questioned, and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the Isua greenstone belt are being reinvestigated and substantially reinterpreted. The first part of this article reviews the evolution of geological research and interpretations, outlining the techniques by which the geological history has been determined and the ensuing controversies. The second part re-examines crucial field evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of Akilia.

Exploring Earth's oldest geological record in Greenland.

Parallel with ongoing exploration of the planets, particularly those that might harbor life, attention is again focusing on deciphering the origins of life on Earth. Multidisciplinary research on ancient rocks in Greenland has become an important part of that focus and may clarify the environmental conditions of the Earth's surface at a time when those conditions were favorable for life to begin. Greenland's Isua greenstone belt (IGB) abounds in metamorphosed volcanic and sedimentary rocks from about 3.75–3.70 Gyr, a time in Earth's history that perhaps stands at the very boundary between two contrasting tectonic and environmental regimes. Deposition of the IGB rocks may be close to when Earth's atmosphere, oceans, emerging land masses, and developing sedimentary basins provided the necessary conditions for life not only to start or recommence, but also to survive. These rocks may contain chemofossils— chemical remnants of very early life—a possibility suggested by carbon isotope ratios, though not proven.

U–Pb zircon ages of Kangâmiut dykes and detrital zircons in metasediments in the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland): Clues to the pre-collisional history of the orogen

In the southern marginal zone of the Palaeoproterozoic Nagssugtoqidian orogen and its foreland (West Greenland), Archaean gneisses were intruded by the syn-kinematic Kangâmiut dyke swarm. Leucodioritic cores of two dykes have yielded U–Pb zircon ages of 2036±5 Ma and 2046±8 Ma (2σ). Dyke emplacement and associated crustal movements thus occurred well before the main plutonic and tectonic activity within the orogen (∼1900–1800 Ma). In the central Nagssugtoqidian orogen, pelitic to semi-pelitic metasedimentary rocks were intruded by 1920–1900 Ma quartz diorites and tonalites of the Arfersiorfik association, which have chemical and isotopic compositions comparable with igneous suites formed in arc environments. The supracrustal and intrusive rocks are separated from Archaean gneisses by décollements of high-grade mylonite and annealed marble, and are allochthonous units. Detrital zircons in two samples of metasedimentary rocks from these allochthons are mostly large, poorly rounded, 2100–1950 Ma old with only a few Archaean grains. Deposition of the metasediments must have taken place between ∼1950 and ∼1920 Ma, before the intrusion of rocks of the Arfersiorfik association. The zircons may have been shed from presently unrecognised pre-Arfersiorfik association igneous complexes (arcs?), whose emplacement coincided with intrusion of the ∼2040 Ma Kangâmiut dykes into deforming Archaean crust in what later became the southern margin of the orogen. Quartz-rich psammitic metasedimentary rocks are also present in the central Nagssugtoqidian orogen. Detrital zircons from one sample yielded a few 2150–2250 Ma grains, but are predominantly Archaean. They include a high proportion of 3300–3400 Ma grains (the oldest material yet found in the orogen). Thrust-bounded panels of metasedimentary rocks in Archaean gneisses occur at the boundary between the southern and central parts of the orogen coinciding with the northern limit of the Kangâmiut dykes. Of the detrital zircons in one metasediment sample from these panels a few yielded ages around 2100–2000 Ma, some were Archaean, and >50% were ∼2400 Ma. Deposition of these two sediment suites took place after ∼2100 Ma. The detrital zircon population shows that the sources for these sediments were different from that of the above-mentioned allochthonous rocks. Neither the 3300–3400 Ma nor the ∼2400 Ma detrital grains can be matched with any rocks in Greenland, indicating a distal source for these sediments.

Age determination of Precambrian rocks from Greenland: past and present

Age determination is an essential part of many geological investigations. The ages of Phanerozoic sedimentary rocks are typically determined by palaeontological studies (often with high precision), but for Precambrian rocks as well as for younger igneous intrusions, precise ages can only be obtained by isotopic analysis of minerals or rocks. Isotopic (radiometric) age determination of Greenland rocks began in the early 1960s and has continued since with gradually improving methods. In this contribution, the development of geochronological knowledge of the Precambrian of Greenland is described in historical perspective, and an outline of new results is given. The history of geochronology can be roughly divided into three periods: 1) a period of single-sample K-Ar and Rb-Sr mineral or whole-rock age determinations; 2) a time when most ages were determined with the help of Rb-Sr and Pb-Pb whole-rock isochrons and multi-grain zircon U-Pb isotope data; 3) the present, where ‘single’ zircon U-Pb data are the preferred method to obtain rock ages. These stages in the development of radiometric dating methods partly overlap in time, and each has yielded very significant contributions to the knowledge of Precambrian evolution in Greenland.

High-technology metals in alkaline and carbonatitic rocks in Greenland: recognition and exploration

High-technology metals such as Zr, Y, Nb, Ta, ree and Ga are generally associated with alkaline and carbonatitic rocks. The recognition of alkaline magmatic provinces or carbonatitic complexes is, therefore, a first step in the exploration for high-technology metals. Low-density exploration data over large parts of South and West Greenland including airborne gamma-ray spectrometry and stream sediment surveys clearly delineate the extension of a major Proterozoic alkaline igneous province and two Phanerozoic carbonatite complexes. On a reconnaissance scale, Nb in stream sediment is the best indicator and gave distinct anomaly patterns over all of the alkaline and carbonatitic intrusive complexes. The anomaly patterns for Y, Zr, Ga, Mn, Sr, Zn, U, and Th also reflect the alkaline/carbonatite intrusions but they are less regular and consistent. In the alkaline Gardar igneous province in South Greenland, two prospects in the Ilimaussaq intrusion relate to rock units rich in eudialyte (Zr, Y, ree). Some of the eudialyte-rich zones were located by geological mapping and visual recognition of this red mineral; elsewhere in the fine-grained eudialyte-bearing rocks, analyses of chip samples and drilling were used. In the Motzfeldt intrusion of the Gardar province, zones of intense hydrothermal alteration are enriched in Nb, Ta, Zr, ree, U and Th. The mineralised zones were located by helicopter-borne gamma spectrometry and subsequently grid sampled to assess Nb and Ta contents. In the Sarfartôq carbonatite complex, alteration zones with massive pyrochlore (Nb) were located by airborne and ground radiometry combined with detailed stream sediment sampling. In the Qaqarssuk carbonatite, minor pyrochlore concentrations and ree-bearing veins have been discovered during exploration for apatite. Based on this experience it is concluded that alkaline and carbonatite complexes are easily recognised in low-density geochemical data. Ore zones have been located using radiometric methods combined with geological mapping and detailed geochemical surveying.

Registration of kimberlites and other potentially diamond-bearing rocks in Greenland

Kimberlites are potentially diamondiferous rocks. They have been found at a number of localities in Greenland (Fig. 1), notably in the Holsteinsborg-Sarfartoq region and south of Frederikshåb (Andrews &amp; Emeleus, 1971, 1975, 1976; Larsen, 1980; Scott, 1977, 1979, 1981; Thy et al., 1987). Prospecting for diamonds and kimberlites was carried out by several mining companies in West Greenland in the years 1970-1988.