Minik Rosing | University of Copenhagen (original) (raw)

Papers by Minik Rosing

Geological Survey of Denmark and Greenland Bulletin, Jul 17, 2019

Geochemical perspectives letters, Sep 1, 2018

Tracking the secular evolution of 142 Nd/ 144 Nd anomalies is important towards understanding the... more Tracking the secular evolution of 142 Nd/ 144 Nd anomalies is important towards understanding the crust-mantle dynamics in the early Earth. Excessive scatter in the published data, however, precludes identifying the fine structure of 142 Nd/ 144 Nd evolution as the expected variability is on the order of few parts per million. We report ultra-high precision 142 Nd/ 144 Nd data for Eoarchean and Palaeoarchean rocks from the Isua Supracrustal Belt (SW Greenland) that show a well-resolved 142 Nd/ 144 Nd temporal variability suggesting progressive convective homogenisation of the Hadean Isua depleted mantle. This temporally decreasing 142 Nd/ 144 Nd signal provides a direct measure of early mantle dynamics, defining a stirring timescale of <250 Myr consistent with vigorous convective stirring in the early mantle. The 142 Nd/ 144 Nd evolution suggests protracted crustal residence times of ~1000-2000 Myr, inconsistent with modern-style plate tectonics in the Archean. In contrast, a stagnant-lid regime punctuated by episodes of mantle overturns accounts for the long lifetime estimated here for the Hadean proto-crust.

AGU Fall Meeting Abstracts, Dec 1, 2013

AGUSM, May 1, 2002

ABSTRACT The age distribution of mass-independent, sulfur isotope fractionation anomalies reporte... more ABSTRACT The age distribution of mass-independent, sulfur isotope fractionation anomalies reported by Farquhar et al (2000) has attracted much interest. The distribution, as originally published, showed Archean sulfide and sulfate minerals with anomalies ranging from D33S = -1.3 to +2.0 permil (D33S = d33S-0.515*d34S; for mass dependent fractionation (MDF), D33S = 0.0; for mass independent fractionation (MIF), D33S not = 0.0). Younger samples, however, beginning at an age of 1.9 Ga showed -0.2 < D33S < 0.3. The sharpness of the transition from the large anomalies of the Archean and Paleoproterozoic to smaller values post-1.9 Ga and the coincidence of the transition with the postulated time of oxygenation of Earth's atmosphere suggested that a hitherto unknown record of Earth's atmospheric chemistry had been discovered. But, questions have been raised about the analytical data and their interpretation. It is evident that the discovery, if verified, potentially has significant implications for life's evolution. Thus, testing the features of the reported age distribution rates a very high priority. We have tested the reported stratigraphic distribution by analyzing sulfides from a wider range of geographic sites and from different geologic ages than were available in the original study. We have tested the analytical techniques by comparing bulk fluorination with laser fluorination methods. The new laser fluorination, sulfur isotope microprobe (supported by JPL Grand Challenge program, P. Conrad and K. Nealson co-PI's) uses a CO2 laser for analysis of mineral fragments and powders. A KrF excimer laser is used for in situ, spot analyses of mineral surfaces. We conducted 3 types of analyses: (1) in situ analyses of sulfides with a KrF excimer laser; (2) whole grain analyses of mineral separates of sulfides with a CO2 laser; and (3) whole grain analyses with a CO2 laser of Ag2S prepared by bulk chemical leaching of sulfide-bearing samples. In situ analyses of pyrite with a KrF excimer laser show variations in D33S of from -1.5 to +3.0 in a 30-m drill core of Archean black shale. Pyrites from the 2.3 Ga Timeball Hill and Rooihoogte formations, S. Africa, provide the oldest record for the change from MIF to MDF with -0.2 < D33S < +0.3. A small MIF signal, |D33S|

AGU Fall Meeting Abstracts, Dec 16, 2014

Rapport, Dec 31, 1983

A coherent stratigraphy is recognised in the highly deformed, amphibolite facies early Archaean I... more A coherent stratigraphy is recognised in the highly deformed, amphibolite facies early Archaean Isua supracrustal belt. The supracrustal belt consists of layered rocks (in which sedimentary structures are locally preserved), ultramafic rocks and units of garbenschiefer (a massive Mg-Al rich, leucoamphibolite). The layered supracrustal rocks form two sequences, which are separated from each other tectonically. When folding is taken into account, these sequences are now less than 200 m thick. Sequence A forms most of the belt. In it there is a transition upwards from predominantly layered amphibolites with banded iron formation horizons to calc-silicate rocks, carbonates and layered felsic metasediments. Sequence B i~restricted to the western edge of the eastern part of the supracrustal belt. It changes upwards from predominantly layered felsic metasediments to ferromagnesian mica schists. The supracrustal belt is regarded as a thin fragment from a thieker, more extensive volcanosedimentary pile. The early Archaean gneisses adjacent to the supracrustal belt consist of early multiphase tonalites which were first intruded by mafic dioritic dykes and then by granitic sheets. The granitic sheets were originaIly horizontal to gently inciined and form up to 40 per cent of the gneiss complex. Interdigitation of supracrustal rocks and gneisses in the Isukasia area is due to both the style of intrusion of the gneisses and to tectonic intercalation. Archaean basic dykes that cut the supracrustal belt and adjacent gneisses are of several generations. Within and south of the supracrustal belt they are generally strongly deformed and have been recrystallised under amphibolite facies conditions; but in the north of the area they are generally better preserved. The dykes cut across several generations of structures in the supracrustal belt and the adjacent gneisses. * 'Isukasia' refers to the same place as 'Isua' in previous literature on the Isua supracrustal belt. 'Isua' was changed to 'Isukasia' by the Greenlandie Place Names Commission in 1980.

Earth and Planetary Science Letters, May 1, 2016

+12 ppm). Excesses in 142 Nd/ 144 Nd compared to the Nd standard for all the ISB rocks range betw... more +12 ppm). Excesses in 142 Nd/ 144 Nd compared to the Nd standard for all the ISB rocks range between +8 and +20 ppm, with a near Gaussian distribution around +12 ppm. This distribution could simply reflect the analytical error (±5 ppm) around a single 142 Nd/ 144 Nd ratio indicating that the samples formed after the extinction of 146 Sm from a source with a nearly uniform 142 Nd/ 144 Nd ratio. In contrast, the NGB shows a range of 142 Nd/ 144 Nd ratios from +8 to −18 ppm relative to the modern Nd standard and displays a flat distribution of 142 Nd/ 144 Nd ratios. The ISB samples show no significant correlation between their 142 Nd/ 144 Nd and Sm/Nd ratios, consistent with their formation in the Eoarchean via melting of a Hadean depleted mantle. In contrast, all NGB samples display a 142 Nd/ 144 Nd vs. Sm/Nd correlation, consistent with their crystallization in the Hadean. The mantle sources for both the ISB and NGB mantlederived rocks have a similar 142 Nd/ 144 Nd ratio at the possible age of formation of the NGB (∼4.3 Ga) suggesting the derivation of ISB and NGB rocks from a common early-formed depleted mantle source formed between 4.47 and 4.42 Ga with a 147 Sm/ 144 Nd ratio ∼0.218. This mantle appears to have been an important source component involved in the formation of the primitive crust during most of the Hadean and Eoarchean eons.

Earth and Planetary Science Letters, May 1, 1984

... 1). The samples for this study come from two areas: (1) around the mouth of Ameralik and on t... more ... 1). The samples for this study come from two areas: (1) around the mouth of Ameralik and on the islands to the south, and (2) the Isukasia area. ... 6. Intrusion of the ca. 3050 MaNhk gneisses, ac-companied by metamorphism and deformation. ...

Bulletin of The Geological Society of Denmark, Dec 20, 1985

Field relations between Tertiary intrusions and the lithologic units near Miki Fjord provide a re... more Field relations between Tertiary intrusions and the lithologic units near Miki Fjord provide a record of physical and chemical processes associated with crustal extension and the formation of the East Greenland continental margin. At several localities the Precambrian basement has been remobilized and partly incorporated in the large macrodikes northeast of the Skaergaard intrusion. Blocks of vesicular basalts sank into the macro dike magma and were metamorphosed to equigranular aggregates of olivine and ortho-and clinopyrox enes with concentrations of plagioclase filling amygdale-like structures. We suggest that contact metamor phic dehydration of hydrous alteration minerals in the basaltic xenoliths and diffusion of this water into the macrodike magma were responsible for the abundant rhythmic layering that occurs near swarms of the xenoliths in a manner similar to that proposed by Taylor & Forester (1979) and Mc Birney & Noyes (1979) for the Skaergaard intrusion. Concentrations of xenoliths of leucogabbro and migmatitic basement gneis ses are found near the top of one macrodike and in six smaller mafic dikes and sills that intrude the lower lavas. In contrast to the basalt blocks, the leucocratic xenoliths appear to have floated to the top of these mafic intrusions, thus providing an effective mechanism for transport of basement material during the early stages of crustal extension. Basement rocks and Tertiary basalts of the area have undergone hydrothermal alteration that is con centrated in fracture systems near the intrusive bodies. Alteration mineralogy in the basalts consists of calc-silicate assemblages similar to those found in active geothermal systems in Iceland. Extensive calcium metasomatism is evident in a number of coast parallel dolerite dikes that are mineralized by iron-rich prehnite. Thermodynamic analysis of the dehydration of prehnite to form epidote and garnet indicates that the fluids responsible for the formation of prehnite mineralized dikes were at temperatures less than about 250°C. Mineralized shear zones and mafic dikes of presumed Proterozoic age define a pronounced structural trend in the basement of the area. This east-west trend is parallel with the coastal dike swarm north of 68°N and with topographical trends within the basalts. We therefore suggest that the pre-Tertiary struc tures may in part control the deflection of the coastline at 68°N and other aspects of the Tertiary structural and intrusive development.

Goldschmidt Abstracts, 2020

AGU Fall Meeting Abstracts, Dec 1, 2016