Minik T. Rosing

Early history of Earth's crust-mantle system inferred from hafnium isotopes in chondrites.

The 176Lu to 176Hf decay series has been widely used to understand the nature of Earths early crust–mantle system. The interpretation, however, of Lu–Hf isotope data requires accurate knowledge of the radioactive decay constant of 176Lu (λ176Lu), as well as bulk-Earth reference parameters. A recent calibration of the λ176Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9u2009Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3u2009Gyr ago, but does not provide evidence for a complementary depleted mantle reservoir. Here we report Lu–Hf isotope measurements of different Solar System objects including chondrites and basaltic eucrites. The chondrites define a Lu–Hf isochron with an initial 176Hf/177Hf ratio of 0.279628 ± 0.000047, corresponding to λ176Lu = 1.983 ± 0.033 × 10-11u2009yr-1 using an age of 4.56u2009Gyr for the chondrite-forming event. This λ176Lu value indicates that Earths oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment. The depletion event must have occurred no later than 320u2009Myr after planetary accretion, consistent with timing inferred from extinct radionuclides.

∼ 3710 and ⪖ 3790 Ma volcanic sequences in the Isua (Greenland) supracrustal belt; structural and Nd isotope implications

Abstract The Isua supracrustal belt of southern West Greenland is the largest body of early Archaean supracrustal rocks, and as such has been the focus of numerous studies to characterise early Earth processes. Based on new geochronological constraints we re-interpret the Isua supracrustal belt stratigraphy and re-evaluate Nd isotopic constraints for the early Earth from Isua supracrustal belt rocks. Part of the belt (∼ 50%) consists of a package of mafic chloritic schists (the garbenschiefer unit), layered amphibolites and felsic rocks, mica (±kyanite) schists and banded iron formation (BIF), and is interpreted as a sequence of mafic to felsic volcanic/volcanosedimentary rocks, turbidites and pelites with minor gabbro. Graded felsic volcanic, in this package have yielded a Uue5f8Pb zircon age of ∼ 3710 Ma. Along the southern side of the belt, a package of amphibolites and ultramafic schists with BIF layers, interpreted as a mafic to ultramafic volcanic sequence, is cut by tonalite sheets with U-Pb zircon ages of 3790–3800 Ma. Thus the belt contains at least two unrelated supracrustal packages of different ages (⪖ 3790 Ma and ∼ 3710 Ma), that were probably juxtaposed in the early Archaean. Felsic volcanic rocks from a single ∼ 3710 Ma unit have consistent initial ϵ Nd values of +0.8 to + 1.8, suggesting minimal fractionation of Nd from Sm during later metamorphism. They may have formed from a depleted (but not ultra-depleted) mantle source only a short time before 3710 Ma, with or without a contribution from LREE-enriched crust of 3750–3870 Ma with ϵ Nd (3800) of + 2 to +4. There is debate as to whether the 3806 ± 2 Ma ‘A6’ felsic unit with ϵ Nd(3800) of + 1.0 to +2.9 is derived from volcanic rocks or from granitoid sheets. Thus in the published Isua supracrustal belt Nd database there are no samples which are definitely > 3710 Ma supracrustal or clearly mantle-derived rocks. Given the problems of locally intense alteration, the presence of supracrustal rocks of two or more ages and of older inherited crustal components in some samples, the Isua supracrustal belt is not the ideal locality to find definitive answers on early Archaean mantle evolution.

Stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt, southern west greenland☆

The highly deformed c. 3800 Ma Isua supracrustal belt is a fragment of a more extensive Early Archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic Amftsoq gneisses in the period 3800-3600 Ma. The supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 Ma, in the Late Archaean and locally at c. 1800 Ma. Layered sequences of rock of sedimentary and probable volcanic origin form over 50% of the belt. Bodies of high Mgue5f8Al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of Amitsoq gneisses. The layered rocks which are < 1 km thick are divided into two sequences, that are in faulted contact with each other. The way-up of these sequences has been determined from facing-directions of locally-preserved graded layering in felsic metasediments at several localities. The overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments. Clastic sedimentation took place against a background of chemical sedimentation, shown by interlayers of banded iron formation, metachert and calc-silicate rocks throughout the sequences. The felsic rocks locally preserve graded bedding and possible conglomerate structures, indicating deposition from turbidite flows and possibly as debris flows. Nodules in the felsic rocks contain structures interpreted as fiamme. There is an irregular enrichment in K2O/Na2O in many of the felsic rocks at constant SiO2 and Al2O3 content, interpreted as owing to alteration of original andesitic to dacitic volcanic rocks. Banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking, possibly under shallow water conditions. Lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source.

Selectively contaminated magmas of the Tertiary East Greenland macrodike complex

Chemical interaction between tholeiitic magmas of the East Greenland Tertiary macrodike complex and anatectic melts of the Precambrian basement produced a wide range of hybrid magmas. Field evidence indicates that, although coexisting magmas were stirred, mechanical mixing only occurred to a limited extent before segregation of magmas into a stratified system. The initial 87Sr/86Sr and 143Nd/144Nd isotope ratios for hybrid compositions fall between those of the mafic and felsic end-members. However, the covariation of these isotope ratios differs from that expected of bulk mixing. Major- and trace-element distributions in hybrid magmas are also inconsistent with simple mixing, as well as with fractional crystallization coupled with bulk assimilation (AFC) involving reasonable end-members of the macrodike-crust system. Rather, the chemical and isotopic modification of mafic and felsic magmas of the macrodike complex appears to have been controlled fundamentally by interdiffusion of silicate liquid species during mingling and buoyant roofward segregation of crust-derived granophyres. The relationships among juxtaposed hybrid magmas of the Miki Fjord macrodike are shown to be consistent with expectations of selective diffusional exchange based on available experimental interdiffusion data for silicate liquids. Comparison between these hybrid compositions and rocks from the felsic series of the Vandfaldsdalen macrodike suggest that the latter compositions were affected by a similar opensystem process operating presumably during the transient development of the felsic cap. Once hybrid magmas ponded at the roof of the intrusion they effectively were isolated from further exchange.

The theoretical effect of metasomatism on Sm-Nd isotopic systems

Abstract If Sm and Nd are mobilized during fluid-rock interaction, fractionation of the ratio between Sm and Nd in geological units may take place. The fractionation can be characterized by a fractionation factor F , which can attain any value between D Sm /D Nd and 1 /( D Sm /D Nd ) depending on the fluid/rock ratio during REE transfer. If such fractionations are not recognized this will induce errors in Sm-Nd isotopic interpretations of age and provenance of the affected geological units. The apparent ages and ϵ Nd values are functions of the true values, F , and the time lapse between petrogenesis and secondary disturbance. The functional relationships can be closely approximated by direct proportionalities.

SHRIMP U-Pb zircon geochronology of the late Archaean Ruinnæsset syenite, Skjoldungen alkaline province, southeast Greenland

Abstract A sample of the Ruinnaesset syenite of the southeast Greenland Skjoldungen alkaline province contains a homogeneous population of zircons up to 1 mm diameter that are devoid of new overgrowths and old inherited cores. U-Pb isotopic analyses of these zircons form a single population with weighted mean 207 Pb 206 Pb and 206 Pb 238 U ages of 2699 ± 4 Ma and 2690 ± 20 Ma (2σ), respectively. These dates are interpreted as giving the time of crystallisation of the syenite.

Assimilation and high-pressure fractional crystallization (AFC) recorded by Paleo-proterozoic mafic dykes, Southeast Greenland

Abstract The northern margin of the Nagssugtoqidian mobile belt in Southeast Greenland exposes a suite of moderately fractionated Fe-rich tholeiitic dykes of Paleo-proterozoic age. The dykes were intruded during extension of the crust prior to the development of the Nagssugtoqidian mobile belt. Although the dykes recrystallized under amphibolite facies conditions during Proterozoic orogenesis, they suffered little deformation. Excluding two very evolved samples, the compositions range between: 7.8–4.6 wt.% MgO, 44.5–52.9 wt.% SiO2 and 1.8–6.9 wt.% total alkalis. One group of mafic dykes shows distinct mantle-normalized trace element patterns with high abundance of low field strength elements and light rare earth elements and low abundances of high field strength elements. These characteristics are consistent with a process of fractional crystallization coupled with assimilation of the regional granulitic crust. Relatively high rates of assimilation to fractional crystallization (0.7) are required to generate the level of incompatible trace elements. This points to lower crustal conditions, and the assimilation is believed to have taken place at the base of the continental crust. Trace element variations indicate fractionation at high pressure involving clinopyroxene as the main extracted phase. We evaluate two fractionation models corresponding to a pressure of 0.9 and 1.5 GPa, respectively, and show that the trace element variations require polybaric fractionation at pressures from a maximum of 1.5 GPa to a minimum of 0.9 GPa.

Metasomatic Alteration of Ultramafic Rocks

Metasomatized ultramafic rocks from the 3800 Ma Isua supracrustal belt have been preferentially enriched in calcium, aluminium and silica, but show no or only little addition of potassium and rubidium. This unusual geochemical response to fluid infiltration can be explained in the light of a low buffered activity of aqueous silica during progressive silicification of the metasomatized units. Thermodynamic analysis shows that the selective element mobility was controlled by the stabilities of mineral assemblages in the ultramafic rocks during alteration.

The buffering capacity of open heterogeneous systems

Abstract Buffering of intensive thermodynamic variables in geological systems differs from the classical buffering in aqueous solutions. Whereas buffering in aqueous solutions is controlled by homogeneous equilibria, heterogeneous equilibria between several phases, which are often complex solutions themselves, control the intensive thermodynamic properties of geological systems. A necessary part of the definition of a buffer is the buffering capacity, which defines the rate of change in chemical potential of mobile components, as a consequence of their addition to or removal from the system. The rate of change of intensive parameters in open polyphase systems can be related to the abundances and compositions of reacting phases. A buffer can be characterized by a constant “β” which defines the proportionality between additions and losses of a given component and the change in its chemical potential.