Peter W.U. Appel

Contrasting geochemical patterns in the 3.7-3.8 Ga pillow basalt cores and rims, Isua greenstone belt, Southwest Greenland : implications for postmagmatic alteration processes

Abstract Pillow basalts from the early Archean (3.7 to 3.8 Ga) Isua greenstone belt, West Greenland, are characterized by well-preserved rims and concentric core structures. The pillow rims and cores have different mineral assemblages, and chemical and isotopic compositions. The rims have systematically higher contents of Fe2O3, MgO, MnO, K2O, Rb, Ba, Ga, Y, and transition metals than the cores. In contrast, the cores possess higher concentrations of SiO2, Na2O, P2O5, Sr, Pb, U, Nb, and the light rare earth elements (REEs than the rims). These compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration. Variations of many major and trace element concentrations between the rims and cores of the Isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration. Al2O3, TiO2, Th, Zr, and the heavy REEs display similar values in both rims and cores, suggesting that these elements were relatively immobile during postemplacement alteration. In addition, the rims and cores have distinctive Sm-Nd and Rb-Sr isotopic compositions in that the rims are characterized by higher 143Nd/144Nd and 87Sr/86Sr ratios than the cores. The pillow basalts yield 2569 ± 170 Ma and 1604 ± 170 Ma errorchron ages on 143Nd/144Nd vs. 147Sm/144Nd and 87Sr/86Sr vs. 87Rb/86Sr diagrams, respectively. The Sm-Nd errorchron age may correspond, within errors, to a late Archean tectonothermal metamorphic event recorded in the region. The Sm-Nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of Nd, relative to Sm, during late Archean metamorphism. Although the Rb-Sr errorchron age overlaps with the timing of an early to mid-Proterozoic tectonothermal metamorphic event recorded in the region, because of a considerably large mean square of weighted deviates value and scatter in 86Sr/87Sr and 87Rb/86Sr ratios, this age may not have a precise geological significance. The 1.6 Ga Rb-Sr errorchron is likely to have resulted from the loss of radiogenic 87Sr. Collectively, the Sm-Nd and Rb-Sr data obtained from the 3.7–3.8 Ga Isua pillow basalt rims and cores are consistent with disturbances of the Sm-Nd and Rb-Sr systems by tectonothermal metamorphic events long after their eruption. In contrast to the Sm-Nd and Rb-Sr systems, the Lu-Hf system appears to be largely undisturbed by metamorphism. Five core samples and three rim samples yield a 3935 ± 350 Ma age, within error of the approximate age of eruption (3.7 to 3.8 Ga). Two rim samples that have gained Lu give an age of 1707 ± 140 Ma, within error of the Rb-Sr errorchron age. Initial 176Hf/177Hf ratios of the undisturbed samples at 3.75 Ga lie within ±1 e-unit of the chondritic value, suggesting no long-term depletion in the mantle source of the basalts.

Depositional setting and paleogeographic implications of earth's oldest supracrustal rocks, the >3.7 Ga Isua Greenstone belt, West Greenland

Abstract New structural and stratigraphic mapping in the Isua greenstone belt, West Greenland has revealed that the exposed lithologic succession is quite different to that depicted in earlier studies. The key to a better understanding of the stratigraphy has been the recognition of intense strain and metasomatic effects combined with ductile fault structures that segment the belt into a number of tectonic slices. In some of these slices, deformation and metasomatism are somewhat lower than in surrounding slices (though still significant), which permit the recognition of primary depositional features that may be used to compare with lithologies from elsewhere in the belt. The belt is dominated by amphibolite that in a number of places show well-defined pillows. Pillow breccias and basaltic debris flows also occur within this package. Strongly recrystallized ultramafic bodies that occur in the belt are interpreted as intrusions or komatiitic flows. The most common sedimentary rock type is chert/banded iron-formation. These lithologies have been strongly affected by brittle and ductile deformation in combination with coarse recrystallization. Siliciclastic detrital rocks such as conglomerate and sandstone are much less common in the belt, and where present, have intrabasinally derived sources. Highly deformed quartzo-feldspathic schist crops out in a number of places in the belt and most likely has multiple origins. Previously these schists have been considered to be felsic volcaniclastic rocks, though recent mapping, and geochemical, and isotopic studies support the hypothesis that at least some of these schists are highy deformed and carbonated tonalitic gneiss sheets or replaced pillow-lava successions. Carbonate rocks in the belt are now considered to be mostly, or entirely, replacement in origin, where metasomatizing fluids have particularly utilized amphibolite–chert contacts; additionally, some carbonate may represent the products of early sea-floor alteration. We envision a temporarily emergent, below wave base, sea-floor origin for the succession based on the assemblage of lithologies.

Rare earth elements in the early Archaean Isua iron-formation, West Greenland

Abstract The rare earth element (REE) patterns in the 3.8 Ga-old Isua iron-formation are generally flat, resembling those of some primitive basalts. Samples with positive, negative or no europium anomaly were found. It is shown that diagenesis and metamorphism did not significantly change the REE patterns. The presence or absence of europium anomalies in iron-formations cannot be used as an indicator of the presence or absence of oxygen in the atmosphere during the Archaean and Precambrian. The REE contents cannot be used to distinguish Algoma-type from Superior-type iron-formations. There appears to be a striking similarity between the Archaean submarine exhalations and modern submarine hydrothermal systems. It is considered likely that Archaean and early Precambrian seawater had a chondritic REE pattern with a slight enrichment of light REE.

U-Th-total Pb dating of monazite from orthogneisses and their ultra-high temperature metapelitic enclaves : implications for the multistage tectonic evolution of the Madurai Block, southern India

Electron microprobe dating of monazite was performed on enclaves of sapphirine-bearing granulites and their host granites from the southeastern part of the Madurai Block in India. The granulites are strongly migmatitic and comprise quartzofeldspathic leucosome veins and restitic domains of sapphirine, garnet, orthopyroxene, sillimanite and cordierite. Inclusions of sapphirine-quartz intergrowths in garnet testify to an UHT stage of metamorphism which was followed by a stage of decompression during which garnet was replaced by coronitic opx-sill intergrowths. Monazite inclusions in garnet, orthopyroxene and quartz often yield a bimodal age distribution of Mid- to early Neoproterozoic (1500-850 Ma) ages in the cores which are rimmed by late Neoproterozoic age domains (630-580 Ma). Monazites in the granite and the charnockite exclusively yield Pan-African values. The occurrence of sapphirine-quartz intergrowths and the early Neoproterozoic age population in the same petrographical position infers that the crystalline basement of the Madurai Block already experienced a stage of UHT metamorphism prior to the Pan-African orogenic event.

Boron isotopic composition of tourmalines from the 3.8-Ga-old Isua supracrustals, West Greenland: implications on the δ11B value of early Archean seawater

Abstract The boron isotope composition of different occurrences of tourmalines in the 3.8-Ga-old Isua (West Greenland) supracrustals has been analysed by ion microprobe. Tourmalines in the mafic volcaniclastics have δ 11 B values between −12.6 and +5.8% with a mean at −3.2±5.1%. These values are close to the values previously reported for tourmalines in the Isua acid volcanic tuffs (−6.7±1.4%) and indicate no difference in δ 11 B value of the mantle or the continental crust between the present day and early Archean times. In contrast, tourmalines in a marine conglomerate have low and homogeneous δ 11 B values of −20.2±2.2%. For these tourmalines, which are of authigenic origin having formed through metamorphism of pre-existing marine clays, a δ 11 B of ∼ −8±7% can be calculated for the unmetamorphosed marine sediments deposited before 3.8 Ga at Isua. A value of +27±11% is calculated for the δ 11 B value of seawater at 3.8 Ga. The large error arises because the δ 11 B value of marine sediments are controlled by: (1) the ratio of detrital to chemically precipitated boron; and (2) the boron isotope fractionation between precipitated boron and dissolved boron in seawater. A high δ 11 B similar to the present-day value of +40‰ seems rather unlikely, unless the fraction of detrital B in marine sediments was as high as today. A lower value of +27 ± 11‰ for early Archean seawater is consistent with mass balance of boron between seawater, mantle and continental crust.

Oxygen isotopes of an Early Archaean layered ultramafic body, southern West Greenland: implications for magma source and post-intrusion history

A large number of metamorphosed ultramafic bodies are found in the region SW of the Isua Greenstone Belt in southern West Greenland, enclosed in Archaean banded tonalitic gneisses. These enclaves of dunite and harzburgite range in size from a few to several hundred meters. The Ujarassuit Nunat layered body occurs as a large enclave (800 m×100 m) entrained within tonalitic gneisses and preserves primary igneous layering and textures. This enclave has been subjected to amphibolite-grade metamorphism, but for the most part the magmatic mineralogy remains intact. Separated minerals have been analysed for δ18O by CO2 laser fluorination. δ18O values range from +4.49 to +4.89‰ for olivine and from +5.70 to +5.86‰ for orthopyroxene. All chromite values are <+2.5‰. Closure temperatures recorded for olivine–orthopyroxene pairs vary from 700 to 900 °C, much lower than expected for an ultramafic magma. Widespread late-magmatic re-equilibration has resulted in significant decreases in δ18O for chromites and small changes of <0.3‰ in olivine, but orthopyroxene values show little variation from typical magmatic signatures. The lowering of 18O in chromite is associated with significant Fe-enrichment. Thermometry suggests that the re-equilibration took place at 680–750 °C, with equilibration between plagioclase and amphibole in residual gabbro anorthosite melt around 700 °C. Hydration, possibly associated with this late melt stage, resulted in further changes to chromite chemistry, formation of secondary amphibole, phlogopite and chlorite and may have further enhanced 18O-depletion in disseminated chromites. All these events are related to late-magmatic processes. Only the later metamorphic mineral, ruby corundum, present in amphibole-harzburgite adjacent to an intrusive gneiss vein, clearly indicates an external oxygen input to this system. Olivines from an unlayered dunite body, 3.5 km away from Ujarassuit Nunat, have higher δ18O values around 5.25‰, typical of more recent high temperature mantle peridotite nodules, and suggesting that mantle oxygen composition at 3.8 Ga was already similar to the present composition.

P-T path and metamorphic ages of pelitic schists at Murchison Falls, NW Uganda: Evidence for a Pan-African tectonometamorphic event in the Congo Craton

Pelitic schists from Murchison Falls, an area within the Late-Archean Congo Craton in NW Uganda, were studied with the aim to deduce the P-T path and the age of metamorphism. These rocks possibly represent a northern extension of a Post-Archean SW-NE trending metasedimentary series (Igisi Group), which separates the sediments in the Albert Rift from the assumed Archean Granulite Group, where it is superimposed on. We here present for the first time constraints for the metamorphic evolution and the metamorphic age of these metasediments, which demonstrate the significance of Pan-African orogenic events in the cratonic domains of central Africa. The metapelites of Murchison Falls contain the peak metamorphic assemblage kyanite+garnet+biotite+muscovite+quartz+plagioclase K-feldspar +rnonazite. The metamorphic conditions were obtained from zoned garnet porphyroblasts, relic plagioclase inclusions in garnet which have higher anorthite contents in the inner parts of the garnet compared to those included in the outer zones and to matrix plagioclase. Conventional thermobarometry using the anorthite-quartz-grossular-kyanite barometer and Fe-Mg garnet-biotite exchange equilibria yield peak conditions within the kyanite stability field at about 0.8 GPa and 650-680 degrees C. These conditions are in accordance with pseudosection calculations for a particular sample, which suggests temperatures in excess of 650 degrees C during formation of the equilibrium assemblage ms+ky+grt+bt+qtz. To determine the age of metamorphism we used U-Th-total Pb chemical dating of metamorphic monazite with the electron microprobe. Monazites show a one-stage growth history. U-Th-total Pb dating of two samples yield Pan-African ages of 621 +/- 26 and 633 +/- 27 Ma. No indications for older monazite relics were found. These data imply that after the Archean/Early Proterozoic granulite-facies metamorphism of the basement rocks, sediments were deposited within epicontinental basins. During the Pan-African orogeny these sediments experienced a single stage metamorphism at amphibolite facies conditions induced by a crustal thickening event.

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 Earths surface at a time when those conditions were favorable for life to begin. Greenlands Isua greenstone belt (IGB) abounds in metamorphosed volcanic and sedimentary rocks from about 3.75–3.70 Gyr, a time in Earths 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 Earths 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.

High bromine contents and low ClBr ratios in hydrothermally altered Archaean komatiitic rocks, West Greenland

Abstract In Archaean supracrustal rocks in West Greenland komatiitic and tholeiitic rocks have undergone alteration by circulating hydrothermal solutions, whereby the rocks were partly transformed into calc-silicate assemblages. The alteration, which also formed extensive stratabound scheelite mineralizations, took place partly at the sea floor and partly below the sea floor prior to metamorphism and deformation of the volcano-sedimentary pile. The altered komatiitic and tholeiitic rocks contain up to 418 ppm bromine and Cl Br ratios down to 1.6 compared with 60–1300 ppb bromine and Cl Br ratios of 430 in mid-ocean ridge basalts. Cl and Br was not detected by electron microprobe in any of the minerals found. It is suggested that the volcano-sedimentary pile was altered by hot brines which had attained high halogen contents by reaction with mafic and ultramafic rocks. During brine-rock interaction the Cl Br ratio of the brines decreased and the metal contents increased. The metalliferous bromine-enriched brines ascended towards the sea floor. The bromine and some of the metals precipitated in the upper part of the volcano-sedimentary pile and in the volcanic rocks at the sea floor. High bromine contents and low Cl Br ratios may indicate presence of major orebodies.

Tourmalinites in the 3800 Ma old Isua supracrustal belt, West Greenland

Abstract Tourmalinites have been found in the early Archaean Isua supracrustal belt in mafic sediments representing metamorphosed mafic tuffs. The tourmalinites are spatially associated with sulphides and an iron-formation consisting of alternating magnetite and quartz bands. Tourmalines are also found as scattered grains in the mafic tuffs as well as in a conglomerate. The tourmalines are intermediate in composition between schorl and dravite, and have remarkably low calcium contents compared with younger stratabound tourmalines. It is suggested that the tourmaline in the tourmalinites is submarine exhalative in origin, derived from brines which were enriched in boron as well as iron and sulphur. The tourmaline was either precipitated on the seafloor or in altered seafloor sediments. The scattered tourmalines may have been formed by metamorphism of boron-rich clays, where the boron was absorbed from seawater.