Troels F. D. Nielsen

Importance of iron-rich tholeiitic magmas at divergent plate margins: A reappraisal

When a tholeiitic liquid differentiates, it may give rise to either iron-poor, silica-rich (rhyolitic) differentiates or to iron-rich, silica-poor differentiates, as found in the Skaergaard intrusion. Iron-rich differentiated liquids are rare among erupted rocks, but are found in small quantities from several localities at divergent plate margins. Among the Tertiary basalts and intrusions in East Greenland, normal erupted basalts may be mixtures of primitive liquid and differentiated iron-rich liquid, which exists at depth but normally does not reach the surface because of its high density. The evolved liquids of the Skaergaard intrusion were of this kind. Data from mid-oceanic ridges confirm this view. We believe that iron-rich differentiated liquids, despite their scarcity on the surface, are much more voluminous at depth, as picrites, at the other side of the density minimum attained during liquid evolution, are thought to be. The trend toward iron enrichment develops when the tholeiitic magma differentiates in a closed system at a relatively low oxidation state, whereas the trend toward silica enrichment and iron depletion occurs when the magma has interacted with the oxidized and hydrated surroundings in the crust.

Crustal contamination in Palaeogene East Greenland flood basalts: plumbing system evolution during continental rifting

Abstract Palaeogene central East Greenland flood basalts have Sr, Nd and Pb isotope characteristics consistent with crustal contamination of Icelandic-type basalts. In the Lower Basalts of central East Greenland, amphibolite facies gneiss is the dominant contaminant at low stratigraphic levels, whereas contamination with granulite facies gneiss prevails in the upper part of the lava succession. The contamination probably occurred by partial melting of the crust, and can be modelled by assimilation–fractionation–crystallisation processes. Similar contaminants have been detected in lavas from ODP Leg 152 off the SE Greenland coast, although in a reverse sequence. Contamination in Lower Basalts and ODP Leg 152 lavas, which erupted through an Archaean/Proterozoic substrate, differs from lavas in NE Greenland, which overly the Caledonian fold–belt and large Mesozoic sediment deposits. In the NE Greenland lavas, the dominant contaminant appears to be a dacitic component formed by partial melting of Mesozoic sediments. Changes in magma supply rates from the mantle may be the determining factor for the sequence of contaminants and may also control the depth of ponding in crustal chambers. The compositional variation of the least contaminated East Greenland basalts suggests that melts were produced from mantle sources similar to present-day Iceland and North Atlantic MORB at the time of continental breakup but distinction between mantle sources is very complicated. REE systematics indicate a maximum in initial depth of mantle melting at Kangerlussuaq during formation of the East Greenland flood basalts, consistent with a plume being centered in that region.

Re–Os isotope geochemistry of Tertiary picritic and basaltic magmatism of East Greenland: constraints on plume–lithosphere interactions and the genesis of the Platinova reef, Skaergaard intrusion

Abstract Re–Os abundance and isotopic studies on a small number of samples from the lowermost part of the East Greenland lava pile and the nearby Skaergaard intrusion show that picrite and ankaramite lavas tend to have high Os abundances and low Re/Os ratios, with most rocks having near-chondritic values for initial 187 Os / 188 Os . One olivine basaltic sample with very high 187 Os / 188 Os has most likely been affected by upper crustal contamination. There is no clear evidence for recycled crustal material in the ancestral Iceland plume source or for a significant degree of interaction of the picrites with ancient subcontinental lithospheric mantle of the North Atlantic craton. The Platinova Reef, the precious metal enriched zone in the Skaergaard intrusion, has very high Au and Pd contents but very low Pt and Os contents compared to platiniferous horizons in other layered intrusions. These characteristics are almost certainly due to the very late stage in the evolution of the Skaergaard magma chamber at which the Platinova Reef formed. The Platinova Reef yields a Re–Os TCHUR model age that is younger than the age of the intrusion, which was most likely produced by some form of post depositional disturbance to the Re–Os isotopic system. The high PGE contents of the East Greenland volcanic rocks indicate that they were formed from magmas that had the potential to form major accumulations of Ni–Cu–PGE sulphide deposits. Had similar magmas undergone significant interaction with continental crust elsewhere in East Greenland, they may have formed massive magmatic sulphide deposits. Thus, the potential for the discovery of large magmatic sulphide deposits of the Norilsk-Talnakh-type in East Greenland must be considered excellent.

Quartz content and the quartz-to-plagioclase ratio determined by X-ray diffraction: a proxy for ice rafting in the northern North Atlantic?

Many paleoceanographic reconstructions of the glacial North Atlantic include estimates of iceberg discharge, which are based on the variable abundance of ice-rafted detritus (IRD) in deep-sea sediments. IRD abundance is most often determined by the mechanical separation and painstaking counting of terrigenous particles larger than a specified threshold grain size, typically 150 mum. Here we present a new proxy for IRD based on X-ray diffraction (XRD) analysis of bulk sediments. This approach complements results obtained from standard techniques while offering several distinct advantages. In addition to the rapid production of objective data, XRD measurements on bulk sediments are sensitive to a broader and more characteristic grain size range than counts of individual coarse lithic fragments. The technique is demonstrated in a study of 12 sediment cores from the North Atlantic. Bulk quartz content and the quartz-to-plagioclase ratio exhibit peak-to-peak correspondence to manual counting results, which verifies the identification of large IRD influxes. The XRD data also reveal variations between the manually identified peaks, suggesting increased sensitivity to low-level, distal, or sea-ice sources of IRD. A saw-tooth pattern emerges in many IRD events, which supports a link between ice rafting and atmospheric temperature changes over Greenland, and providing further evidence of the influence of climate on iceberg discharges

Skaergaardite, PdCu, a new platinum-group intermetallic mineral from the Skaergaard intrusion, Greenland

Abstract Skaergaardite, PdCu, is a new mineral discovered in the Skaergaard intrusion, Kangerdlugssuaq area, East Greenland. It occurs in a tholeitiic gabbro associated with plagioclase, clinopyroxene, orthopyroxene, ilmenite, titanian magnetite, fayalite and accessory chlorite-group minerals, ferrosapo- nite, a member of the annite-phlogopite series, hornblende, actinolite, epidote, calcite, ankerite, apatite and baddeleyite. The mineral is found in composite microglobules composed of bomite, chalcocite, digenite, chalcopyrite, with rare cobalt pentlandite, cobaltoan pentlandite, sphalerite, keithconnite, vasilite, zvyagintsevite, (Cu5Pd5Au) and Pt-Fe-Cu-Pd alloys, unnamed PdCu3, (Pd,Cu,Sn), Au3Cu and PdAuCu. Skaergaardite occurs as droplets, equant grains with rounded outlines, subhedral to euhedral crystals and as irregular grains that vary in size from 2 to 75 pm, averaging 22 pm. It is steel grey with a bronze tint, has a black streak, a metallic lustre and is sectile. Neither cleavage nor fracture was observed. The mineral has a micro-indentation hardness of VHN25 = 257. It is isotropic, non-pleochroic and exhibits neither discernible internal reflections nor evidence of twinning. Skaergaardite varies from bright creamy white (associated with bomite and chalcopyrite) to bright white (associated with digenite and chalcocite). Reflectance values in air (and in oil) are: 58.65 (47.4) at 470 nm, 62.6 (51.1) at 546 nm, 64.1 (52.8) at 589 nm and 65.25 (53.95) at 650 nm. The average of 311 electron-microprobe analyses gives: Pd 58.94, Pt 1.12, Au 2.23, Cu 29.84, Fe 3.85, Zn 1.46, Sn 1.08, Te 0.28 and Pb 0.39, total 99.19 wt.%, corresponding to (Pd0.967Au0.020Pt0.010)∑0.997(Cu0.820Fe0.120Zn0.039Sn0.016Te0.004Pb0.003)∑1.002. The mineral is cubic, space group Pm3m, a = 3.0014(2) Å, V = 27.0378 Å3, Z = 1. Dcalc is 10.64 g/cm3. The six strongest lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 2.122(100)(110), 1.5000(20)(200), 1.2254(50)(211), 0.9491(20)(310), 0.8666(10)(222), 0.8021(70)(321). The mineral has the CsCl-type structure. It is believed to be isostructural with wairauite (CoFe), synthetic CuZn (β-brass) and is structurally related to hongshiite (PtCu). Skaergaardite developed from a disordered Pd-Cu-rich metal alloy melt that had exsolved from an earlier Cu-(Fe) sulphide melt. Ordering of Pd and Cu (beginning at T ≈ 600°C) results in development of the CsCl structure from a disordered face-centred cubic structure.

Was a short-lived Baffin Bay plume active prior to initiation of the present Icelandic plume? Clues from the high-Mg picrites of West Greenland

Abstract The Tertiary volcanism of the North Atlantic region provides one of the most extensive and best described magmatic records of interaction between a starting plume and continental lithosphere, and the role of plumes in continental break-up. Many authors consider the entire volcanic province, from Baffin Island in the north-west to Scotland in the south-east, to be the product of a single “proto-Icelandic” starting-plume head initially centred beneath East Greenland, whose initial eruptions were strongly influenced by the pre-break-up structure of the continental lithosphere. We demonstrate that the presence of Mesozoic-Palaeocene extensional sedimentary basins that developed within older N-S trending mobile belts influenced the location of the earliest on-shore Tertiary basalts. Even taking this into account, however, the prevalence of high-magnesium picrites among volcanics bordering the Davis Strait in the extreme north-west of the province, and their much lower abundance in East Greenland directly above the supposed plume axis, contradict the single-plume model. Best estimates of the most refractory liquid compositions erupted in the Disko area imply potential temperatures beneath West Greenland of 1540–1600°C. Such high temperatures are more consistent with the axial region of a separate, hot plume beneath West Greenland than with the periphery of the Icelandic plume head. West Greenland picrites cannot be attributed to an expanding “hot doughnut” as they are not present in other peripheral areas likely to be swept by such a doughnut. Uniformly high FeO contents and comparison with McKenzie-Bickle melting parameterisations suggest the West Greenland picrites are the product of ∼ 25% melting constrained to depths of 60–90 by a lithospheric lid. Geographical zonation in incompatible element and radiogenic isotope enrichment (Holm et al., 1993) provides support for a separate, somewhat earlier plume centred beneath Baffin Bay. Picrites from Svartenhuk Halvo (north) are more enriched than equivalent picrites on Disko and Baffin Island (south). This pattern, reminiscent of trends along the Reykjanes Ridge, would be radial to a plume axis beneath Baffin Bay, but not to one centred beneath East Greenland.

Igneous Layering in Basaltic Magma Chambers

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.

Zirconosilicates in the kakortokites of the Ilímaussaq complex, South Greenland: Implications for fluid evolution and high-field-strength and rare-earth element mineralization in agpaitic systems

Lithium cannot be determined by electron microprobe, but it may be an essential component in tourmalinesupergroup minerals. Therefore, its estimation is important for structural formula calculation and nomenclature. In this paper, we present a method to estimate Li content in tourmaline frommicroprobe data based on a multiple linear-regression model, which is not reliant on a particular normalization scheme. The results derived from this model are reasonably accurate, particularly for low-Mg tourmalines (<2 wt.% MgO) with Li2O contents higher than ∼0.3 wt.%. Furthermore, it provides a better fitness compared with estimations of Li assuming that Li fills any cation deficiency at the Y site.

RARE EARTH CHEMISTRY OF PEROVSKITE GROUP MINERALS FROM THE GARDINER COMPLEX, EAST GREENLAND

Abstract Perovskite group minerals, general formula ABX3, from the intrusive ultramafic alkaline Gardiner Complex, East Greenland, range from almost pure CaTiO3 (perovskite, sensu stricto), to the rare earth element (REE) variety, loparite-(Ce). Chemical zonation in the perovskites (sensu lato), is described by the substitutions 2Ca2+ = (Na++ REE3+) on the A-site and 2Ti4+ = (Fe3+ + Nb5+) on the B-site. Other trace elements detected include Th, Sr, Al, Si, Zr, Ta and Sn. Excellent agreement was found between the determinations of the REE by electron microprobe and neutron activation analysis. Chondrite-normalized REE patterns display enrichment in the light rare earths for perovskite, loparite, apatite, melilite and diopside. Mean perovskite/apatite partition coefficients from four of the Gardiner rocks were calculated as La = 10.4, Ce = 13.8, Nd = 13.9, Sm= 9.9, Eu = 7.7, Gd = 5.2, Tb = 5.6, Tm = 5.5, Yb = 2.7 and Lu = 1.6, indicating that perovskite concentrates all REE to a much greater extent than apatite. Light-REE enrichment occurs in both perovskite and apatite.

Petrological and geochemical characteristics of Mesoproterozoic dyke swarms in the Gardar Province, South Greenland: Evidence for a major sub-continental lithospheric mantle component in the generation of the magmas

Abstract The Mesoproterozoic Gardar Province in South Greenland developed in a continental rift-related environment. Several alkaline intrusions and associated dyke swarms were emplaced in Archaean and Ketilidian basement rocks during two main magmatic periods at 1300-1250 Ma and 1180-1140 Ma. The present investigation focuses on mafic dykes from the early magmatic period (‘Older Gardar’) and the identification of their possible mantle sources. The rocks are typically fine- to coarse-grained dolerites, transitional between tholeiitic and alkaline compositions with a general predominance of Na over K. They crystallized from relatively evolved, mantle-derived melts and commonly show minor degrees of crustal contamination. Selective enrichment of the large ion lithophile elements Cs, Ba and K and the light rare-earth elements when compared to high field-strength elements indicate significant involvement of a sub-continental lithospheric mantle (SCLM) component in the generation of the magmas. This component was affected by fluid-dominated supra-subduction zone metasomatism, possibly related to the Ketilidian orogeny ~500 Ma years prior to the onset of Gardar magmatism. Melt generation in the SCLM is further documented by the inferential presence of amphibole in the source region, negative calculated εNd(i) values (-0.47 to -4.40) and slightly elevated 87Sr/86Sr(i) (0.702987 to 0.706472) ratios when compared to bulk silicate earth as well as relatively flat heavy rare-earth element (HREE) patterns ((Gd/Yb)N = 1.4-1.9) indicating melt generation above the garnet stability field. The dyke rocks investigated show strong geochemical and geochronological similarities to penecontemporaneous mafic dyke swarms in North America and Central Scandinavia and a petrogenetic link is hypothesized. Considering recent plate reconstructions, it is further suggested that magmatism was formed behind a long-lived orogenic belt in response to back-arc basin formation in the time interval between 1290-1235 Ma.