Lead isotope geochemistry of plagioclase in the Skaergaard intrusion by LA-ICP-MS: Assessing the effects of crustal contamination and link with East Greenland flood basalts

Cho, June; Scoates, James S.; Weis, Dominique; Amini, Marghaleray

doi:10.1016/j.chemgeo.2022.120723

Cited by 6 publications

(7 citation statements)

References 148 publications

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“…They have the same phases on the liquidus (olivine and plagioclase), Mg/(Mg + Fe), TiO 2 /FeO* ratios, and geochronological age. These compositional constraints exclude other suggestions for parental melts [e.g., Milne Land Formation lavas (34,35)]. The evolved Giekie Plateau Formation lavas (21,32,33) are all notably aphanitic, suggesting that the melt entering the Skaergaard chamber had either a very low amount of or was totally free of phenocrysts.…”

Section: Skaergaard Layered Intrusionmentioning

confidence: 52%

Catastrophic growth of totally molten magma chambers in months to years

et al. 2022

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The vertical growth rate of basaltic magma chambers remains largely unknown with available estimates being highly uncertain. Here, we propose a novel approach to address this issue using the classical Skaergaard intrusion that started crystallizing from all margins inward only after it had been completely filled with magma. Our numerical simulations indicate that to keep the growing Skaergaard magma chamber completely molten, the vertical growth rate must have been on the order of several hundreds to a few thousands of meters per year, corresponding to volumetric flow rates of tens to hundreds of cubic kilometers per year. These rates are several orders of magnitude higher than current estimates and were likely achieved by rapid subsidence of the floor rocks along faults. We propose that the Skaergaard is a plutonic equivalent of supereruptions or intrusions that grow via catastrophically rapid magma emplacement into the crust, producing totally molten magma chambers in a matter of a few months to dozens of years.

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Section: Skaergaard Layered Intrusionmentioning

confidence: 52%

Catastrophic growth of totally molten magma chambers in months to years

et al. 2022

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“…All proposed compositions, however, point to a bulk composition comparable to tholeiitic lavas contemporaneous with the intrusion and present during the opening of the North Atlantic. A bulk composition comparable to that of evolved basalt (magnesium number (Mg#) c. 45-50) of the Geickie Plateau Formation is suggested by Andreasen et al (2004) and Nielsen (2004), whereas Jakobsen et al (2010) and Cho et al (2022), preferred a composition comparable to that of the underlying Milne Land Formation. Nielsen et al (2019b), however, supported a correlation to Geikie Plateau Formation lavas based on comparatively simple, but seemingly robust, major element characteristics.…”

Section: Wwwgeusbulletinorgmentioning

confidence: 98%

The PGE-Au Mineralisation of the Skaergaard intrusion: precious metal minerals, petrography and ore genesis

Rudashevsky¹,

Nielsen²,

Rudashevsky³

2023

GEUS Bulletin

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The Skaergaard PGE-Au Mineralisation, alias the Platinova Reef, is hosted in a series of mineralisation levels within a suite of bowl-shaped macrorhythmic layers in the upper Middle Zone of the Skaergaard intrusion. The intrusion is exposed 68°N in East Greenland. The occurrence defines its own type due to its exceptional structure and mineralogy. A wealth of mineralogical data is available in laboratory reports for individual samples and in peer-reviewed publications, but none of these account for the lateral and stratigraphic distribution of PGE and Au parageneses in the gabbros of the intrusion. In this study, we collate and describe the mineralogical data for the first-formed PGE-rich and last-formed gold-rich mineralisation levels and integrate these with petrogenetic models. Recovery of >4000 grains of precious metal phases allow a detailed study of their distribution and compositions throughout the mineralisation, re-equilibration during cooling, inter-grain relationships and relationships to Cu-Fe sulphides and the gabbroic host rocks. The sulphides are dominated by bornite, chalcocite and minor chalcopyrite. All other sulphides, such as pentlandite, are very rare. Fifty-four different precious metal phases are identified in this study, and include the new IMA approved minerals skaergaardite (PdCu), nielsenite (Pd3Pb) and naldrettite (Pd2Sb). Precious metal phases include (1) intermetallic compounds and alloys of Cu and Pd; (2) intermetallic compounds and alloys of Au and Cu (Ag); (3) sulphides of Pd, Cu (Ag, Cd, Hg, Tl); (4) arsenides of Pd (Pt, Ni) and (5) intermetallic compounds of Pd, Cu with Sn, Pb, Te (Sb, Bi). Skaergaardite (PdCu) is the dominant PGE mineral in the lower and main PGE mineralisation level (Pd5). It is accompanied at the western margin of the intrusions by the sulphides vasilite (Pd16S7) and vysotskite (PdS) but is rare at the eastern margin, which is dominated by plumbide zvyagintsevite (Pd3Pb). Gold phases include a suite of intermetallic compounds and alloys from AuCu3 to native gold and are dominated by tetra-auricupride (AuCu). Gold is concentrated in the tops of individual mineralisation levels and in the uppermost precious metal–bearing mineralisation level, followed by stratiform Cu-rich mineralisation levels. Precious metal parageneses demonstrate formation and re-equilibration from liquidus to subsolidus temperatures and control by local geochemical environments. The mineralisation is syn-magmatic and the result of fractionation and evolution in the remaining bulk-silicate liquid and crystal mushes. Fractionation led to sulphide saturation and formation of immiscible sulphide melt droplets. This was followed by reaction with mush melts and re-equilibration to lower temperatures, first under the roof and subsequently after slumping to the floor in mushes of macrorhythmic layers. Droplets of sulphide melt formed between 1030–1050°C and trapped precious metals. The subsequent reaction between sulphide melt and interstitial Fe-rich immiscible melt at c. 1015°C, and redistribution to coexisting melt and fluid, led to the separation of PGE, Au and Cu and their up- and inward transport. Magmatic fluids as well as volatile-rich residual silicate melts were retained in gabbros at the margins and resulted in precious metal parageneses in equilibrium with hydrous low-temperature silicate parageneses.

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“…2020, Cho et al . 2022). The 87 Rb‐ 87 Sr system is particularly well‐suited to study the spectrum of magmatic processes as radiogenic Sr (expressed as 87 Sr/ 86 Sr) can be measured in a variety of Ca‐bearing, rock‐forming minerals including plagioclase, carbonates and clinopyroxene, accessory phases such as apatite, and groundmass material.…”

Section: Figurementioning

confidence: 99%

New Reference Materials, Analytical Procedures and Data Reduction Strategies for Sr Isotope Measurements in Geological Materials by LA‐MC‐ICP‐MS

Mulder

Hagen‐Peter

Ubide

et al. 2023

Geostandard Geoanalytic Res

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Laser ablation multi‐collector mass spectrometry (LA‐MC‐ICP‐MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA‐MC‐ICP‐MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr‐baseline subtraction, Rb‐peak‐stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE2+ correction for rock glasses, yields the most accurate and precise 87Sr/86Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve < 100 ppm 2SE repeatability ("internal") precision for 87Sr/86Sr measurements for materials with < 1000 μg g‐1 Sr.

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Lead isotope geochemistry of plagioclase in the Skaergaard intrusion by LA-ICP-MS: Assessing the effects of crustal contamination and link with East Greenland flood basalts

Cited by 6 publications

References 148 publications

Catastrophic growth of totally molten magma chambers in months to years

Catastrophic growth of totally molten magma chambers in months to years

The PGE-Au Mineralisation of the Skaergaard intrusion: precious metal minerals, petrography and ore genesis

New Reference Materials, Analytical Procedures and Data Reduction Strategies for Sr Isotope Measurements in Geological Materials by LA‐MC‐ICP‐MS

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