Jens C.Ø. Andersen

Included blocks (and blocks within blocks) in the Skaergaard intrusion: Geologic relations and the origins of rhythmic modally graded layers

The early Eocene Skaergaard intrusion of Greenland includes enormous numbers of rocks of both exotic and cognate origins. The lower parts of the Marginal Border Series contain abundant fragments of feldspathic peridotite that are possibly autoliths, intermixed with occasional xenoliths of Precambrian gneiss and metasomatized Cretaceous–Paleocene sediments derived from adjoining country rocks. The Upper Border Series includes one exceptionally large block of gneiss (several hundred meters across), and numerous smaller fragments, these originating from the intrusion9s footwalls, plus a few pieces of peridotite. The Layered Series contains countless autoliths of troctolite, gabbroic anorthosite, and oxide (magnetite-ilmenite) gabbro, broken from parts of the Upper Border Series that have otherwise been lost to erosion; at the upper midlevel of its western half, it contains a few xenoliths of basalt, derived probably from the now-eroded (Eocene) roof of the intrusion. A distinctive postintrusion composite basaltic dike at one place contains 40 or more xenoliths of troctolite, olivine gabbro, and gabbroic anorthosite that may represent parts of the Layered Series still hidden at depth. The Layered Series autoliths range from fragments a few centimeters on a side to blocks more than 400 m across, and they typically are coarser grained than their host cumulates, being in this respect more like Upper Border Series rocks. The autoliths are spread stratigraphically through the lower 70% of the exposed 2500 m thickness of the Layered Series and are generally concentrated in three broad stratigraphic zones. Their physical relationships to their host rocks—particularly the way they indent older layers beneath and are covered by younger layers above—provide abundant evidence that there was generally a sharp, well-defined interface between the top of the cumulate pile and the main body of magma in the intrusion while the Layered Series was forming. The distribution of the autoliths between and through the well-known, rhythmic, thin, modally graded layers shows that these layers were spread by magmatic currents; and their relations to the more extensive macrorhythmic layering suggest that it too was significantly shaped by currents. Many of the larger autoliths are crudely layered internally, and in places it is evident that their stratification existed before they broke loose; therefore, it must have formed in the Upper Border Series. One particularly large block of oxide gabbro exhibits extraordinarily well-developed modal and textural layering and includes small troctolitic autoliths of an earlier generation, and it provides evidence that currents also spread crystalline materials across the top of the magma body. Many of the very small autoliths in the Layered Series are highly anorthositic in composition, apparently because they were leached of mafic minerals, and some of the larger blocks show local patchy internal replacement by anorthosite. Most large blocks show little sign of postaccumulation modification, and some have thin, fine-grained augite-rich rims or rinds, demonstrating that even though they were out of thermal and chemical equilibrium with their host cumulates, they still were effectively armored against extensive chemical change. Also documented is a large block that was cut by several early basaltic dikes before it broke free from the top of the intrusion; these early dikes transgress small anorthositic replacement pipes in the block, showing that the replacement process also occurred in the upper border environment. Two mechanisms are described whereby graded cumulate layers can be sorted and deposited by magmatic crystal-liquid suspension currents. One, involving density surge currents, has been advocated previously; the other is a new concept based on boundary flow separation and reattachment vortex cells. The two mechanisms are used in complementary ways to illustrate the formation of (1) some of the principal Skaergaard structures involving blocks and layers; (2) modally graded layers in the Layered Series that rhythmically alternate with uniform layers; and (3) modally sorted layers in the Upper Border Series featuring “underside draping” beneath small included blocks. Explanations are provided for (1) why plagioclase did not float away from the tops of graded layers even though it was less dense than the liquid, and (2) how the liquid part of a current was fractionated away from the crystalline materials. Modal and grain-size data from Skaergaard intrusion graded layers are shown to be in excellent accord with characteristics predicted for layers sorted by currents; a synthesis diagram is presented illustrating how all the above processes may have functioned in concert in the intrusion.

Testing the validity of chrome spinel chemistry as a provenance and petrogenetic indicator

In situ and detrital chrome spinels from the Rum layered intrusion, Scotland, show a wide compositional range. In situ, within-seam spinels are relatively rich in Al and Mg, and poor in Ti, Cr, and total Fe. In contrast, detrital spinels have high Cr, total Fe, and Ti contents and low Al and Mg contents and were derived from spinels disseminated throughout the complex. Our data show that the widely used petrogenetic discrimination diagrams are not based on representative data and cannot, therefore, be applied in either provenance studies or the interpretation of altered ophiolitic bodies.

The mineralogy of efflorescence on As calciner buildings in SW England

Abstract Arsenic is a very common by-product of the processing of Cu, Au and polymetallic ores worldwide, where the ore is roasted (calcined) to remove volatile elements. In southwest England, a diverse range of As-mineral species occur as efflorescent secondary mineral growths on historic calciner buildings. Gypsum occurs as abundant dendritic growths comprising either interlocking blades or tabular crystals. Ca-arsenate minerals are locally very abundant as white colloform masses. Positively identified Ca arsenates include pharmacolite, weilite and haidingerite. Other secondary minerals include arsenolite, scorodite, bukovskyite and an As-bearing potassium alum, together with a wide variety of unidentified minerals, including an Al-As-S phase and As-rich F-bearing phases. Gypsum contains As concentrations up to ~7 wt.%. Efflorescent growth at sites exposed to the prevailing weather systems is less abundant than at sheltered sites. This is interpreted as being due to ‘pressure washing’ of exposed sites by driving rain. Successive concentric growths of gypsum and Ca arsenate on masonry are interpreted as being the result of seasonal crystallization. Understanding both current and historical mining and mineral processing methods is critical in the evaluation of the potential impact on the modern environment. In particular, due to the abundance of As-bearing minerals in a wide range of ore types, many buildings worldwide are potentially significantly contaminated with As even though few are directly related to As production or handling. Characterizing the secondary As mineral species present at mine and mineral processing sites is critical in understanding the potential health risk these sites might pose.

Diversity of platinum-group element mineralization styles in the North Atlantic Igneous Province: new evidence from Rum, UK

Two generations of sulphide-hosted platinum-group element mineralization occur in the West Sgaorishal ultramafic plug, Rum. Disseminated Cu and Ni sulphides around the margin of the plug host a restricted platinum-group mineral assemblage that is dominated by Pd bismutho-tellurides and sperrylite (PtAs 2 ) with subordinate electrum (AuAg), froodite (PdBi 2 ) and unidentified Pt-Bi-Te phases. Later sulphide-rich dykes cross-cut the plug and host an assemblage dominated by Pd bismutho-tellurides, sperrylite and locally very abundant paolovite (Pd 2 Sn). Whole rock combined platinum-group element+Au concentrations are proportional to the sulphide abundance with slightly elevated values in disseminated sulphide lithologies ( > 400 pph) and very high values (> 2000 ppb) in the sulphide-rich dykes. Both generations have relatively flat chondrite-normalized plots indicating a primitive magmatic source. Negative δ 3 4 S values ( - 9.2 to - 18.3 ‰) indicate that the disseminated mineralization is due to contamination probably derived from Jurassic sedimentary rocks leading to sulphur saturation and collection of platinum-group elements. The sulphide-rich dykes must have entrained a platinum-group element-rich sulphide liquid collected from a much larger volume of magma. The presence of platinum-group elements and sulphide-rich dykes with δ 3 4 S values between - 10.8 and - 15.0 ‰ indicates that parts of the Rum Layered Suite became sulphur saturated through magmatic contamination. It appears likely that platinum-group element mineralization styles within the southern North Atlantic Igneous Province are diverse and may be present in a wider variety of mineralogical associations than previously recognized.

Platinum-group mineralization in the Tertiary Igneous Province: new data from Mull and Skye, Scottish Inner Hebrides, UK

In recent years, platinum-group mineral deposits have been reported from several of the Tertiary igneous complexes in East Greenland. These intrusions form part of the same igneous province as the Tertiary igneous centres in northwest Scotland, and recent work confirmed the presence of abundant platinum-group minerals in the Rum Central Complex. In this paper we report for the first time the presence of abundant and diverse platinum-group minerals from the Tertiary Ben Buie intrusion on Mull and the Cuillin Complex of Skye, Inner Hebrides, Scotland. The platinum-group minerals in the Ben Buie intrusion are associated with chromitite layers in ultramafic cumulates. The grains are small (1–2 μm), locally very abundant, and dominated by Pd–Bi–Te–Sb phases. In the Peridotite Series of the Cuillin Complex, the platinum-group minerals are dominated by laurite commonly enclosed within chromite or silicate. A combined orthomagmatic–hydromagmatic origin is proposed for the mineralization. However, the style and type of mineralization differs between each of the igneous centres. Platinum-group element mineralization is a ubiquitous feature of the Tertiary Igneous Province in northwest Scotland and in Greenland and is intimately linked to the regional tectonic setting.

Solubility of indium-tin oxide in simulated lung and gastric fluids: Pathways for human intake

From being a metal with very limited natural distribution, indium (In) has recently become disseminated throughout the human society. Little is known of how In compounds behave in the natural environment, but recent medical studies link exposure to In compounds to elevated risk of respiratory disorders. Animal tests suggest that exposure may lead to more widespread damage in the body, notably the liver, kidneys and spleen. In this paper, we investigate the solubility of the most widely used In compound, indium-tin oxide (ITO) in simulated lung and gastric fluids in order to better understand the potential pathways for metals to be introduced into the bloodstream. Our results show significant potential for release of In and tin (Sn) in the deep parts of the lungs (artificial lysosomal fluid) and digestive tract, while the solubility in the upper parts of the lungs (the respiratory tract or tracheobronchial tree) is very low. Our study confirms that ITO is likely to remain as solid particles in the upper parts of the lungs, but that particles are likely to slowly dissolve in the deep lungs. Considering the prolonged residence time of inhaled particles in the deep lung, this environment is likely to provide the major route for uptake of In and Sn from inhaled ITO nano- and microparticles. Although dissolution through digestion may also lead to some uptake, the much shorter residence time is likely to lead to much lower risk of uptake.

The Skaergaard trough layering: sedimentation in a convecting magma chamber

The upper parts of the floor cumulates of the Skaergaard Intrusion, East Greenland, contain abundant features known as troughs. The troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase-rich material. The origin of the troughs and layering is contentious, attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls, or to in situ development by localised recrystallisation during gravitationally-driven compaction. They are characterised by outcrop-scale features such as mineral lineations parallel to the trough axis, evidence of erosion and layer truncation associated with migration of the trough axis, and disruption of layering by syn-magmatic slumping. A detailed microstructural study of the modal trough layers, using electron backscatter diffraction together with geochemical mapping, demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution–reprecipitation. Instead, the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning. We argue that the lineations and foliations are, therefore, a consequence of grain alignment during magmatic flow. Post-accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid, with upwards migration of the Si-rich conjugate into the plagioclase-rich upper part of the layers, whereas the Fe-rich immiscible conjugate remained in the mafic base. Both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal-rich currents descending from the nearby chamber walls.

Petrogenesis and tectonic association of rift-related basic Panjal dykes from the northern Indian plate, North-Western Pakistan: evidence of high-Ti basalts analogous to dykes from Tibet

Rift related magmatism during Permian time in the northern margin of Indian plate is represented by basic dykes in several Himalayan terranes including north western Pakistan. The field relations, mineralogy and whole rock geochemistry of these basic dykes reveal significant textural, mineralogical and chemical variation between two major types (a) dolerite and (b) amphibolite. Intra-plate tectonic settings for both rock types have been interpreted on the basis of low Zr/Nb ratios (< 10), K/Ba ratios (20–40) and Hf-Ta-Th and FeO-MgO-Al2O3 discrimination diagrams. The compositional zoning in plagioclase and clinopyroxene, variation in olivine compositions and major elements oxide trends indicate a vital role of fractional crystallization in the evolution of dolerites, which also show depletion in rare earth elements (REEs) and other incompatible elements compared to the amphibolites. The equilibrium partial melting models from primitive mantle using Dy/Yb, La/Yb, Sm/Yb and La/Sm ratios show that amphibolite formed by smaller degrees (< 5%) of partial melting than the dolerites (< 10%). The trace elements ratios suggest the origination of dolerites from the subcontinental lithospheric mantle with some crustal contamination. This is consistent with a petrogenetic relationship with Panjal trap magmatism, reported from Kashmir and other parts of north western India. The amphibolites, in contrast, show affinity towards Ocean Island basalts (OIB) with a relatively deep asthenospheric mantle source and minimal crustal contribution and are geochemically similar to the High-Ti mafic dykes of southern Qiangtang, Tibet. These similarities combined with Permian tectonic restoration of Gondwana indicate the coeval origin for both dykes from distinct mantle source during continental rifting related to formation of the Neotethys Ocean.

Age and petrogenesis of the Lundy granite: Paleocene intraplate peraluminous magmatism in the Bristol Channel, UK

The Lundy granite forms part of the Lundy Igneous Complex, which is the southernmost substantive expression of magmatism within both the British Cenozoic Igneous Province and the wider North Atlantic Igneous Province. Its Qz + Pl + Kfs + Bt ± Grt ± Tpz mineralogy and peraluminous character contrast with other British Cenozoic Igneous Province granites farther north but are similar to the granites of the adjacent Early Permian Cornubian Batholith. We present the results of mapping, petrographical and mineral chemical analysis, and the first U–Pb zircon ages for the granite (59.8 ± 0.4 – 58.4 ± 0.4 Ma) and cross-cutting basic dykes (57.2 ± 0.5 Ma), which confirm a Paleocene age for magmatism. Zircon inheritance is limited but two cores imply the presence of Early Palaeozoic igneous rocks in the unexposed basement of SW England. The anomalous southerly location of the Lundy Igneous Complex is a consequence of mantle melting arising from the superimposition of localized lithospheric extension, related to intraplate strike-slip tectonics, with the distal ancestral Icelandic plume. Granite generation primarily reflects crustal partial melting during the emplacement of mantle-derived melts. The change in geochemical character between the Lundy granite (peraluminous) and other British Cenozoic Igneous Province granites (metaluminous or subalkaline) indicates a fundamental crustal source control between contrasting peri-Gondwanan and Laurentian basement provinces. Supplementary material: Sample locations, petrographic summaries, mineral chemistry data and secondary ionization mass spectrometry U–Pb analytical data are available at https://doi.org/10.6084/m9.figshare.c.3886783