Peridotit xenoliths: a key to understanding exceptional depletion and re-enrichment of the lithospheric mantle beneath Greenland
Final Report Abstract
Our study revealed that the kimberlites beneath SW GNAC sampled two main peridotite types with respect to mineralogies and metasomatic styles, which occur at two depth intervals: (1) Lherzolites, harzburgites and dunites, some phlogopite-bearing, from ~100-170 km depth form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursory to ca. 150 Ma kimberlite eruption, in the course of rifting, decompression and lithosphere thinning. This was accompanied by progressive garnet-breakdown, texturally evident by pyroxene-spinel assemblages occupying former coarse grains, and compositionally by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, HREE) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletions in Cu, Ni and Co. (2) Phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, show evidence for recent pyroxene-breakdown. These lithologies occur at ~90-110 km depth and are inferred to have highly depleted protoliths. They are compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. While low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidising mantle at lower pressure and oxidative metasomatism, by residual, more silica-undersaturated melts that percolated upwards after interacting with the deeper lithosphere. A comparison with published data for mantle sections in the northwestern part of the craton, which at the time of 590-550 Ma magmatism extended to >210 km depth and where the shallow portion was strongly depleted, reveals a strong contrast with the intense metasomatism recorded in the shallow cratonic mantle lithosphere beneath SW GNAC, and in addition suggests loss of ~40 km of lithospheric mantle. This is also recorded in the progressive shallowing of magma sources during breakup of the North Atlantic craton. Highly siderophile element (HSE) abundances and Re-Os isotope compositions were obtained by isotope dilution inductively coupled plasma mass spectrometry of olivine separates from representative samples. Refertilised lherzolites are dominated by rare to frequent small (10s of m) sulphide inclusions in olivine whereas modally metasomatised phlogopite-bearing lherzolite and wehrlites tend to have higher modes of more Ni-rich sulphides, including abundant large interstitial grains (100s of µm). The olivine separates display depleted platinum-group element (PGE) systematics with Primitive Upper Mantle (PUM)-normalised Pd/Ir of 0.014 to 0.62, but have both depleted and enriched 187Os/188Os (0.1139 to 0.2724) that are uncorrelated with 187Re/188Os. Despite recent disturbance, four of ten olivine separates retain similarly depleted Os corresponding to Re-depletion model ages of 1.8 to 2.1 Ga. They may reflect Palaeoproterozoic refertilisation (lherzolitisation) during Laurentia plate assembly, with reintroduction of clinopyroxene and Os-rich sulphides into the originally refractory mantle lithosphere by basaltic asthenosphere-derived melts, followed by reccystallisation and occlusion in olivine. Unradiogenic Os is observed regardless of mineralogy, including in samples containing abundant interstitial sulphides, which are compositionally indistinguishable from those included in olivine. This provides indirect evidence that sulphide added during recent wehrlitisation and phlogopite-introduction can be Os-poor, and that the Os isotopic signature in these samples is controlled by Os-rich sulphides that precipitated from the basaltic melt. Depletions in compatible PGE (<0.5 x PUM for Ru, Ir, Os) in all but one olivine separate reflect nugget effects (amount of depleted vs. metasomatic sulphide inclusions) combined with loss due to scavenging by oxidising small-volume melts that intruded the lithosphere during recurrent rifting. In contrast, recent garnet-breakdown leading to Re liberation, and precipitation of sulphide from metasomatic melts originating in aged lithospheric metasomes, may explain disturbed Re-Os isotope systematics, including three samples with radiogenic 187Os/188Os. Combined, these multiple metasomatic events destroyed all vestiges of Neoarchaean inheritance in the samples investigated here. The concentration of phlogopite-rich lithologies in a narrow depth interval (~90-110 km) overlaps with a negative seismic velocity gradient that is interpreted as a midlithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of smallvolume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement, and may herald further future decratonisation in the Greenland part of the North Atlantic Craton.
Publications
- (2017) Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities. Journal of Petrology 58/12, 2311-2338
Aulbach S, Sun J, Tappe S, Höfer HE, Gerdes A
(See online at https://doi.org/10.1093/petrology/egy009)
