ALBERT

Description: The orogenic Anita Peridotite in Fiordland, SW New Zealand, provides an opportunity to examine the composition of a large block of upper mantle exhumed from beneath a Cretaceous arc. This little-studied 1 km x 20 km massif is dominated by spinel-facies harzburgite and dunite. Olivine Mg# of 92–93, spinel Cr# of ~70, orthopyroxene with low Al 2 O 3 , and extremely depleted whole-rock geochemical characteristics indicate that the peridotite body experienced 〉30% melt extraction, probably within the spinel facies. Mineral compositions show some similarity to those of cratonic peridotitic mantle. Rare Cr-rich amphibole suggests that the peridotite has been subsequently re-enriched. Distinctive, coupled Eu and Sr anomalies in the amphiboles, which can be subdivided into three groups, are interpreted to show that they formed by hydration of metasomatic clinopyroxene–plagioclase aggregates. Measured amphibole 87 Sr/ 86 Sr (~0·705–0·706), Nd (~ +6·3 to + 11·1), 208 Pb/ 204 Pb (~37·8–38·9) and Hf (~ +5·6 to 36·9) indicate that the metasomatic agent, which caused crystallization of clinopyroxene and plagioclase, had an isotopic composition similar to ocean island basalt. On the basis of isotopic data and mineral chemistry, the enriched nature of the peridotite is interpreted to have been caused by percolation of small volumes of a mafic silicate melt. Additional evidence for the passage of such melts is the rare occurrence of hornblendite veins and orthopyroxene hornblendite dykes. This peridotite body therefore preserves evidence of extreme melt depletion and the passage of silicate melts and hydrous fluids within the sub-arc mantle.

Description: New geochemical and isotopic data are presented for volumetrically minor, depleted low-Ti basalts that occur in the Plateau Basalt succession of central East Greenland (CEG), formed during the initial stages of opening of the North Atlantic at 55 Ma. The basalts have mid-ocean ridge basalt (MORB)-like geochemistry (e.g. depleted light rare earth elements) and are distinct from the high-Ti lavas that dominate the sequence. Rare earth element geochemistry implies derivation from a source more depleted than the typical MORB source, and suggests polybaric melting and contributions from both spinel- and garnet-facies mantle. The low-Ti basalts have Sr–Nd–Pb–Hf isotopic characteristics that are similar to those of depleted magmas from Iceland (e.g. Theistareykir) and adjacent ridges (Kolbeinsey and Reykjanes) and distinct from global MORB (e.g. negative 207 Pb, and Hf and Nd isotope compositions that plot above the mantle reference line). Isotope and trace element data indicate the involvement of two depleted source components. One component has isotopic compositions similar to other depleted components identified in the North Atlantic and has high Rb/Zr and Ba/Nb. The second is isotopically less depleted with lower Rb/Zr and Ba/Nb. Small degrees of crustal contamination (〈 1%) by both amphibolitic and granulitic crust result in relatively large changes in isotopic composition ( c . 1% lower for 206 Pb/ 204 Pb and 0·1% higher for 87 Sr/ 86 Sr depending on the contaminant). Negative Nb suggests a MORB affinity for the low-Ti magmas; however, they are distinguished from global normal (N)-MORB on the basis of vertical deviations from the Northern Hemisphere Reference Line (negative 207 Pb and positive 208 Pb), and relative enrichments in Ba, Sr and Pb. The isotopic compositions of the low-Ti CEG basalts suggest correlation with modern depleted components beneath Iceland and adjacent ridges, considered to be derived from upper mantle sources polluted by the Iceland plume. However, small positive Pb peaks when normalized to MORB, and lower Nb distinguish the CEG low-Ti basalts from depleted Icelandic compositions. The lower Nb (〈 0) and 87 Sr/ 86 Sr, and suggestion of higher 206 Pb/ 204 Pb in crustally uncontaminated parental melts imply a closer affinity to compositions from the oceanic ridges surrounding Iceland (especially the Reykjanes Ridge), yet they are subtly distinct on the basis of available trace element data. We suggest that this depleted component was an integral part of the plume that melted primarily during the rapid lithospheric uplift and extension associated with continental break-up.

Description: We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.

Description: High-precision electron microprobe data for major and trace elements (nickel, calcium and manganese) in the margins and rims of groundmass olivine grains from the Majuagaa kimberlite ( sensu stricto ) in southern West Greenland are presented. Despite a range of olivine core compositions defining several zoning types most of the olivine grains have similar margins. The olivine cores are considered to be xenocrysts in the kimberlite magma whereas the margins represent cognate olivine crystallized from the kimberlite melt itself. We evaluate models of olivine margin formation by fractional crystallization, fractional crystallization with simultaneous digestion of xenoliths, and diffusion. Only fractional crystallization coupled with digestion of xenocrysts (primarily orthopyroxene), with subsequent minor diffusion, can account for the observed compositional profiles in the olivine margins. We propose that the digestion crystallization process, whereby the entrained xenolithic material reacts with the kimberlite melt, is important in the evolution of kimberlite magmas. We suggest that this type of reaction gives a distinct local flavour to the parental kimberlite melt and can account for the similarities and differences between the various kimberlitic ( sensu lato ) magma types present in southern West Greenland and worldwide.

Description: An important task in assessing the magma source of an intraplate volcanic province is establishing the composition of the underlying lithospheric mantle. Pyroxenes in peridotite xenoliths from the ~10,000 km 2 Dunedin Volcanic Group in New Zealand reveal that the underlying lithospheric mantle is chemically and isotopically heterogeneous and has a complex thermal history. Portions of this mantle have light rare earth element–depleted clinopyroxene trace element concentrations and distinctly radiogenic Nd compositions (Nd (20 Ma) ≥ + 15.5) with model depleted mantle ages that are ≥100 m.y. older than the overlying Jurassic crust (type 1). The Nd isotopic composition of these moderately fertile domains is distinct from any Dunedin Volcanic Group magma, but the domains are embedded within enriched peridotitic mantle (type 2) that has formed through reaction with a light rare earth element–rich fluid that imparted an isotopic composition in strongly metasomatized xenoliths of 87 Sr/ 86 Sr (20 Ma) = 0.7028–0.7029, Nd (20 Ma) = + 5.0 to + 5.1, and 206 Pb/ 204 Pb = 19.9, 207 Pb/ 204 Pb = 15.5, and 208 Pb/ 204 Pb = 39.6. These isotope ratios overlap with the isotopically homogeneous high time-integrated U/Pb–like source signature of the host Dunedin Volcanic Group. However, all metasomatic and nonmetasomatic pyroxenes are zoned in temperature-sensitive elements (Al, Cr, Mg ± Ca), with trends indicating element exchange during cooling and the results of diffusion calculations implying that the zoning formed over hundreds of thousands to millions of years prior to late Oligocene–Miocene xenolith entrainment. These data, along with calculated pyroxene rare earth element homogenization diffusion rates, indicate that mantle metasomatism predated entrainment in the host magmas by millions of years. Furthermore, the presence of the cooling trends in all but one sample indicates that this upper lithosphere mantle preserves little or no sign of a rise in the geotherm at the time of magmatism. Zoning patterns in peridotite pyroxenes can therefore provide useful insight into the role of portions of the lithospheric mantle in formation of intraplate alkaline basalts.

Description: Field, drillcore and geochemical data are used to create a three-dimensional implicit model to assess the controls on gold mineralization at the Nalunaq orogenic gold deposit in South Greenland. Gold occurs in narrow quartz veins with variable dips averaging 34° SE that cut meta-basic rocks. The bulk of the mineralization is contained within a single gold–quartz vein, named the Main Vein. Within this vein, gold is concentrated into three ore shoots plunging 20–25° NE, corresponding to the South, Target and Mountain blocks of the Nalunaq gold mine. Gold anomalies in drillcores are identified updip and downdip from the current mine workings. Modelling reveals that structural controls have the greatest influence on the location of gold. Flexures in the Main Vein correspond to changes in the host rock lithology and the gold grade is highest where the quartz vein is steepest. Where late-stage faults intercept the Main Vein, gold grades are lower. The comprehensive gold assay data from the mine, which are integrated with structural observations in the implicit model, refine the structural interpretation of the Nalunaq gold deposit, highlighting the ore shoot geometry and delineating the minimum extents of mineralization beyond the currently mined areas.

Description: The role of lithospheric mantle metasomatized by CO 2 -bearing melts in the genesis of HIMU-like alkaline intraplate basalts is investigated using a suite of peridotite xenoliths from New Zealand. The xenoliths have Sr–Nd–Pb–Hf isotope compositions ( 87 Sr/ 86 Sr = 0·7029, Nd = + 5 to + 6, 206 Pb/ 204 Pb = 20·4 and Hf = +5 to +8) indistinguishable from the host low-silica basalts and, except for 207 Pb/ 204 Pb, overlapping with the HIMU mantle reservoir. Laser line scans across grain boundaries in the xenoliths show, however, that the host magma contribution is restricted to minor degrees of melt infiltration along grain boundaries during ascent, with the distinctive peridotite isotopic compositions having been imparted earlier by mantle metasomatism. Two mantle metasomatic styles are distinguished from pyroxene trace element concentrations (in particular, rare earth elements, Ti, Zr and Hf) and are interpreted to be the result of reaction of peridotite with CO 2 - bearing magmas. The occurrence of two subtly chemically different but isotopically indistinguishable styles of metasomatism in rocks with the same equilibrium temperatures within the same mantle column may be due to separate volatile-rich melts formed by different degrees of melting of a deeper carbonated peridotitic ± pyroxenitic source, or due to metasomatism having been imparted to different degrees on a variably depleted protolith. In either case, the formation of the HIMU-like enriched lithospheric mantle was achieved by percolation of volatile-rich melts, which probably rose from the asthenosphere. Melt modelling of representative depleted and subsequently enriched samples shows that low-degree melting of a CO 2 -bearing melt-metasomatized peridotite could yield a melt with a trace element composition very similar to that of the Zealandia HIMU-like alkaline basalts, but only if small volumes (~5%) of amphibole participated in the melting process. Although not observed in the studied xenoliths, amphibole is associated with mantle metasomatism by carbonatitic or CO 2 -bearing melts elsewhere in the world and has been found as xenocrysts with HIMU-like isotope compositions in some Zealandia basalts. The melt modelling results also imply that amphibole could buffer the trace element budgets of a low-degree melt regardless of the source peridotite composition; therefore, provided that hydrous metasomatized lithospheric mantle can be perturbed to melt, the contribution of amphibole would explain the similarities of alkaline ocean island basalt-like magmas in continental and oceanic settings. HIMU-like reservoirs formed by percolation of young volatile-rich (CO 2 + H 2 O) melts are widespread within the Earth’s lithospheric mantle and are a potential source for intraplate alkaline basaltic magmatism.

Description: The layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.