28 research outputs found
The Thickness of the Mushy Layer on the Floor of the Skaergaard Magma Chamber at Apatite Saturation
We present a novel way of constraining the thickness of the crystal mush in fractionated layered intrusions using detailed microstructural analysis. The results are combined with geochemical data to create a snapshot of the crystal mush on the floor of the Skaergaard magma chamber in the period immediately before and after the saturation of the bulk liquid in apatite (the UZaâb boundary). The step-change in the fractional latent heat (that part of the total enthalpy budget associated with crystallization) accompanying the arrival of a new liquidus phase is recorded by a step-change in the median clinopyroxeneâplagioclaseâplagioclase dihedral angle, Îcpp, in fully solidified cumulates. Dihedral angles are formed during the last stages of solidification and hence the change of Îcpp associated with apatite-in marks a point close to the base of the mushy layer at the moment the bulk liquid became saturated in apatite, whereas the first appearance of abundant, homogeneously scattered, cumulus apatite crystals in the stratigraphy marks the top of the mushy layer at this moment. Comparison of the offset between these two markers in five widely spaced drill cores through the Skaergaard Layered Series suggests that the mushy layer was only a few metres thick at the UZaâb boundary in the centre and east of the floor, whereas it was âŒ100âm thick on the floor near the western margin. There is no correlation between the efficiency of liquid expulsion (as recorded by bulk-rock P2O5 concentrations and the stratigraphic distribution of reactive symplectites) and the recorded mush thickness at the moment of apatite saturation, suggesting that existing models of adcumulate formation that depend on mush thickness need to be reconsidered.M.B.H. and C.T. acknowledge support from a Royal Society Joint International Grant. M.B.H. was supported by the Natural Environment Research Council (grant numbers NE/F020325/1 and NE/J021520/1). C.T. was supported by the Carlsberg Foundation, the Danish Council of Independent Research, and the Danish National Research Foundation
Elemental distributions and mineral parageneses of the Skaergaard PGE-Au mineralisation: consequences of accumulation, redistribution, and equilibration in an upward-migrating mush zone
This is the author accepted manuscript. The final version is available from Oxford University Press via the DOI in this recordThe Skaergaard PGE-Au mineralisation, aka the Platinova Reef, is a syn-magmatic Platinum Group Element (PGE) and gold (Au) mineralisation that formed after crystallisation of âŒ74% of the bulk melt of the intrusion. It is hosted in a more than 600 m deep and bowl-shaped succession of gabbroic macro-rhythmic layers in the upper 100 m of the Middle Zone. The precious metal mineralisation comprises a series of concordant, but compositionally zoned, mineralisation levels identified by distinct PGE, Au and Cu peaks. They formed due to local sulphide saturation in stratiform concentrations of interstitial and evolved mush melts in six MLs over > 2000 years. The PGE-Au mineralisation is compared to a stack of gold-rimmed saucers of PGE-rich gabbro of upward decreasing size. Fundamentally different crystallisation and mineralisation scenarios have been proposed for the mineralisation, including offset reef type models based on sulphide saturation in the melt from which the silicate host crystallised, and the here argued model which restricts the same processes to the melt of the inward migrating mush zone of the magma chamber. The latter is supported by: i) a 3D summary of the parageneses of precious metal minerals and phases (> 4000 grains) from 32 samples across the mineralisation; ii) a 3D compilation of all bulk rock assay data; and iii) a principal component analysis (PCA) of PGE, Au, Cu, and selected major and trace elements. In the main PGE-mineralisation level (Pd5 alias Pd-Zone) the precious metal mineral paragenesis varies across the intrusion with precious metal sulphides and Au-alloys at the W-margin to Precambrian basement, precious metal plumbide and Au- and Ag alloys at the E-margin to flood basalts, and skaergaardite (PdCu) and intermetallic compounds and alloys of PGE-Au and Cu in the central parts of the mineralisation. Precious metal parageneses are distinct for a given sector of the intrusion, i.e., drill core (local control), rather than for a given stratigraphic or temporal interval in the accumulated gabbros. The precious metal âgrade times widthâ number (average g/t x metres) for the mineralisation at an upper and a lower cut off of 100âppb PGE or Au increases from âŒ20 to âŒ45âg toward the centre of the mineralisation due to ponding of precious metal bearing melt. A strong increase in (Pd+Pt+Au)/Cu and dominance of (PdCu) alloys in the lower and central parts of the mineralisation demonstrate the partial dissolution of droplets of Cu-rich sulphide melt and fractionation of precious metal ratios. The precious metal parageneses, the distribution of precious metals in the mineralisation, and the PCA support initial accumulation of precious metals in the melt of the mush in the floor, followed by equilibration, sulphide saturation, and reactions with residual and immiscible Fe-rich silicate melt in a series of macro-rhythmic layers in the stratified and upward migrating mush zone in the floor of the magma chamber. Syn-magmatic and upward redistribution of precious metals sets the Skaergaard PGE-Au Mineralisation apart from conventional reef type and offset-reef type precious metal mineralisations, and characterize âSkaergaard typeâ precious metal deposits.Geological Survey of Denmark and Greenlan
Mineral phosphorus drives glacier algal blooms on the Greenland Ice Sheet
Melting of the Greenland Ice Sheet is a leading cause of land-ice mass loss and cryosphere-attributed sea level rise. Blooms of pigmented glacier ice algae lower ice albedo and accelerate surface melting in the ice sheetâs southwest sector. Although glacier ice algae cause up to 13% of the surface melting in this region, the controls on bloom development remain poorly understood. Here we show a direct link between mineral phosphorus in surface ice and glacier ice algae biomass through the quantification of solid and fluid phase phosphorus reservoirs in surface habitats across the southwest ablation zone of the ice sheet. We demonstrate that nutrients from mineral dust likely drive glacier ice algal growth, and thereby identify mineral dust as a secondary control on ice sheet melting.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland
The Platinova Reef, in the Skaergaard Intrusion, east Greenland, is an example of a magmatic CuâPGEâAu sulfide deposit formed in the latter stages of magmatic differentiation. As is characteristic with such deposits, it contains a low volume of sulfide, displays peak metal offsets and is Cu rich but Ni poor. However, even for such deposits, the Platinova Reef contains extremely low volumes of sulfide and the highest Pd and Au tenor sulfides of any magmatic ore deposit. Here, we present the first LA-ICP-MS analyses of sulfide microdroplets from the Platinova Reef, which show that they have the highest Se concentrations (up to 1200 ppm) and lowest S/Se ratios (190â700) of any known magmatic sulfide deposit and have significant Te enrichment. In addition, where sulfide volume increases, there is a change from high Pd-tenor microdroplets trapped in situ to larger, low tenor sulfides. The transition between these two sulfide regimes is marked by sharp peaks in Au, and then Te concentration, followed by a wider peak in Se, which gradually decreases with height. Mineralogical evidence implies that there is no significant post-magmatic hydrothermal S loss and that the metal profiles are essentially a function of magmatic processes. We propose that to generate these extreme precious and semimetal contents, the sulfides must have formed from an anomalously metal-rich package of magma, possibly formed via the dissolution of a previously PGE-enriched sulfide. Other processes such as kinetic diffusion may have also occurred alongside this to produce the ultra-high tenors. The characteristic metal offset pattern observed is largely controlled by partitioning effects, producing offset peaks in the order Pt+Pd>Au>Te>Se>Cu that are entirely consistent with published D values. This study confirms that extreme enrichment in sulfide droplets can occur in closed-system layered intrusions in situ, but this will characteristically form ore deposits that are so low in sulfide that they do not conform to conventional deposit models for CuâNiâPGE sulfides which require very high R factors, and settling of sulfide liquids
First measurement of coherent Ï0 photoproduction in ultra-peripheral XeâXe collisions at âsNN = 5.44 TeV
The first measurement of the coherent photoproduction of Ï0 vector mesons in ultra-peripheral XeâXe collisions at sNN=5.44 TeV is presented. This result, together with previous HERA Îłp data and ÎłâPb measurements from ALICE, describes the atomic number (A) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe+XeâÏ0+Xe+Xe process, measured at midrapidity through the decay channel Ï0âÏ+Ïâ, is found to be dÏ/dy=131.5±5.6(stat.)â16.9+17.5(syst.) mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on A of cross section for the coherent Ï0 photoproduction at a centre-of-mass energy per nucleon of the ÎłA system of WÎłA,n=65 GeV is found to be consistent with a power-law behaviour Ï(ÎłAâÏ0A)âAα with a slope α=0.96±0.02(syst.). This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit.publishedVersio