Maya B. Kamenetsky

Kimberlite melts rich in alkali chlorides and carbonates: A potent metasomatic agent in the mantle

Kimberlite magmas, as the deepest probe into Earths mantle (>150 km), can supply unique information about volatile components (hydrogen, carbon, chlorine, sulfur) in mantle-derived melts and fluids. All known kimberlite rocks are not suitable for studies of mantle volatiles because of their pervasive postmagmatic alteration; however, this study discusses an exceptionally fresh group I kimberlites (<0.5 wt% H2O) from the Udachnaya-East diamondiferous pipe in Siberia. Kimberlite groundmass, in addition to euhedral olivine and calcite, is extremely enriched (at least 8 wt%) in water-soluble alkali chlorides, alkali carbonates, and sulfates (ratio 5:3:1), and often shows immiscibility textures. A primary magmatic origin of alkali chlorides and alkali carbonates is confirmed by the study of strontium isotopes in the water- and dilute acid-leachates of the groundmass (Sr-87/Sr-86 = 0.7069 and 0.7050) that contrast with much more radiogenic isotope composition of the Cambrian platform sedimentary rocks and the Udachnaya-East mine-site brines. Melt inclusions in groundmass olivine, composed of halite, sylvite, alkali-Ca carbonates, phlogopite, olivine, and CO2 fluid, were used to determine the composition and evolution of the kimberlite melt prior to emplacement. Melt inclusions show immiscibility between chloride and carbonate liquids at <600oC in heating stage experiments. The chloride and carbonate enrichment in the kimberlite parental magma suggests the presence of a powerful agent for chemical modifications (metasomatism) in the mantle and crust.

Sr, Nd, and Pb isotope evidence for a mantle origin of alkali chlorides and carbonates in the Udachnaya kimberlite, Siberia

The kimberlite rocks of the Udachnaya-East pipe (Siberia) are uniquely fresh and contain very high abundances of primary volatiles (Cl, CO2, S). Alkali elements and chlorine are extremely abundant in the reconstructed kimberlite melt compositions, and this enrichment is very important for our understanding of deep-mantle melting and melt transport. Here we present new isotopic data that confirm a mantle origin for these kimberlitic chlorides and carbonates, and constrain the kimberlite emplacement age as ca. 347 Ma. The initial Nd and Ph isotope ratios in a large salt aggregate, in a CI-S-enriched water leachate of the groundmass, and in the silicate fraction of the groundmass are very similar (epsilon(Nd) = +3 to +4, Pb-206/Pb-204 = 18.6, Pb-207/Pb-204 = 15.53), implying a comagmatic origin of the chlorides and carbonates and the silicates. Combined Sr, Nd, and Ph isotope data are used to rule out any significant contributions to the kimberlite chlorine budget from crustal sources, such as the Cambrian evaporite sequences of the Siberian platform. Our data support the interpretation that exsolved Na-K chloride and Na-K-Ca carbonate formed directly from original uncontaminated kimberlite magma. High Cl abundances in kimberlites suggest the presence of a Cl-rich reservoir in the deep sublithospheric mantle

Carbonate-chloride enrichment in fresh kimberlites of the Udachnaya-East pipe, Siberia: A clue to physical properties of kimberlite magmas?

Kimberlites, the deepest terrestrial magmas and the principal source of diamonds, must have low viscosity and high buoyancy, which govern their exceptionally fast transport from mantle depths to the surface. Appreciation of the rheological properties of kimberlite magmas relies on research into their temperatures and compositions. Understanding of the alkali and volatile element budget is central to these studies, but is hampered by contaminated and altered compositions of kimberlites worldwide. Kimberlites of the diamondiferous Udachnaya-East pipe (Siberia) are exceptionally fresh, with low H2O (<0.5 wt%), but high CO2 (up to 14 wt%), Cl (up to 6 wt%), and alkalies ( up to 6 wt% Na2O and 2.0 wt% K2O). After crystallization of olivine the kimberlite melt evolved towards essentially carbonate-chloride compositions. The groundmass assemblage and compositions of the Udachnaya-East kimberlite resemble modern halogen-rich natrocarbonatite lavas from the Oldoinyo Lengai volcano. Rheological measurements on the Oldoinyo Lengai lavas can be used to constrain properties of the kimberlite magma.

A Raman microprobe study of melt Inclusions in kimberlites from Siberia, Canada, SW Greenland and South Africa

Raman spectroscopy has been used for the identification of both common and uncommon minerals in melt inclusions in Group-I kimberlites from Siberia, Canada, SW Greenland and South Africa. The melt inclusions all contained high abundances of alkali-Ca carbonates, with varying proportions of cations, and Na-Ca-Ba sulphates. In accordance with its dry mineralogy, no hydrated carbonates or sulphates were detected in melt inclusions from the Udachnaya-East kimberlite. In contrast, the melt inclusions in kimberlites from Canada, South Africa and SW Greenland were found to contain bassanite, pirssonite, and hydromagnesite suggesting that greater amounts of water were present in their residual magmas. This suggests that enrichment in alkali carbonates and sulphates is widespread across a range of Group-I kimberlites and implies that they commonly have an alkali-, and sulphur-rich residual liquid.

Magmatic inclusions in the search for natural silicate-salt melt immiscibility: methodology and examples

Immiscible phase separation during the cooling and crystallisation of magmas is an inherently fugitive phenomenon and melt inclusions may provide the only remaining evidence of this process. We detail those features of such inclusions that can both prove the existence of immiscible phase separation, and constrain the compositional signature of the process. To do so requires the combination of traditional methods (petrographic examination, microthermometry, etc.) with state of the art microbeam analytical techniques (laser Raman spectroscopy and proton-induced X-ray emission). Examples of inclusions in phenocrysts from barren and mineralised rocks are provided to illustrate the approach and validate the interpretations.

Matrix effects in Pb/U measurements during LA-ICP-MS analysis of the mineral apatite

U–Pb ages of several apatite reference materials, acquired by LA-ICP-MS over a 3.5 year period using the Otter Lake apatite as a primary standard, show systematic offsets (up to 3%) from reference ages obtained by isotope dilution mass spectrometry. The offsets are well outside known analytical errors from counting statistics, uncertainty in common Pb corrections or excess 206Pb in high-Th/U materials. The age offsets also do not correlate strongly with the concentrations of F, Cl, REE, Th and U, with accumulated radiation dose or with the depth of ablation pits. We suggest that the observed age offsets are related to elemental fractionation at the ablation site, but their direction and magnitude cannot at present be predicted from compositional differences in different apatite samples. Our results further suggest that Otter Lake apatite is too heterogeneous for use as a calibration standard. Two other apatite samples, OD306 (1596.7 ± 7.1 Ma) and the 401 apatite (530.3 ± 1.5 Ma) are introduced as potential U–Pb reference materials.

A story of olivine from the McIvor Hill complex (Tasmania, Australia): Clues to the origin of the Avebury metasomatic Ni sulfide deposit

Abstract Magmatic differentiation and/or assimilation and related segregation of immiscible sulfide liquid are generally believed to be critical processes in the formation of the majority of orthomagmatic Ni sulfide deposits. In recent years, a new class of Ni sulfide deposits formed by metasomatic and/or hydrothermal modification of peridotites has been recognized. The serpentinite-hosted Avebury Ni sulfide deposit (Tasmania, Australia), the largest known non-magmatic sulfide deposit, provides an unprecedented opportunity to understand sources of metals and fluids responsible for this style of economic mineralization. Our study shows that serpentinization of the Ni-bearing olivine in the Cambrian peridotites of the McIvor Hill complex was followed by metasomatic transformation assisted by heat and fluids supplied by the nearby Late Devonian granite intrusion. The role of the above in the formation of an economic concentration of Ni sulfides is supported by (1) abundant Ni-Fe alloys and sulfides associated with serpentinization of peridotitic olivine, (2) metasomatic olivine containing inclusions of serpentine and metalliferous brines, and (3) the Late Devonian age of the Ni sulfide deposit. The Avebury meta-somatic olivine is Ni-depleted and enriched in Mn relative to olivine of similar Fo content in nearby unmineralized peridotites, and to olivine in subduction-related mafic magmas generally. The unusual minor element chemistry of olivine is matched by a unique set of olivine-hosted multiphase inclusions composed of fibrous Mg-silicates and various Na-, K-, Fe-, Ca-, Mn-, and Ba-bearing chlorides/ hydrochlorides, sulfides, arsenides magnetite, REE minerals, and Fe-Ni alloys. Peridotite whole-rock Sr-Nd-Pb isotope data and U-Pb dating of metasomatic titanite support earlier suggestions that Ni mineralization is temporally and genetically related with the intrusion of the nearby 360 Ma Heemskirk Granite. It appears that the multiphase inclusions in metasomatic olivine demonstrate chemical signatures of both in situ serpentinites (entrapped alloys, sulfides, arsenides, and magnetite) and distal fluids (enrichment in Pb, Bi, Sn, Sb, Sr, Ba, Rb, Cs, and Ce). We propose that magmatic olivine in large ultramafic bodies provides almost infinite Ni to replacive serpentinites and constitutes a major reservoir of disseminated Ni mineralization. In the case of Ave-bury Ni was locally redistributed from olivine in the Cambrian peridotites to mainly Fe-Ni alloys and sulfides during serpentinization in the early Paleozoic. In the Devonian reheating and interaction with a granitic fluid in the contact aureole of the Heemskirk Granite led to de-serpentinization and formation of metasomatic high-Mn, low-Ni olivine with inclusions of serpentine and entrapped alloys, sulfides, arsenides, and magnetite, and metalliferous brines rich in “granitic” elements. Nickel released from serpentinite in this process was re-deposited near the margins of the peridotite to form the Avebury Ni orebody. Our model of serpentinization-related release of Ni from magmatic olivine, in situ precipitation of metallic, sulfide, and arsenide Ni-minerals, and their redistribution and recrystallization in hydrothermal conditions represents an alternative to Ni remobilization from magmatic sulfides.

Immiscible sulfide melts in primitive oceanic magmas: Evidence and implications from picrite lavas (Eastern Kamchatka, Russia)

Abstract Silicate-sulfide liquid immiscibility in mantle-derived magmas has important control on the budget of siderophile and chalcophile metals, and is considered to be instrumental in the origin orthomagmatic sulfide deposits. Data on primitive sulfide melts in natural samples, even those representing most voluminous magmatism in oceanic rifts, are very scarce due to the small size and poor preservation of incipient sulfide melt globules. Here we present the first detailed report of the crystallized sulfides melts in the oceanic picrites of the (presumably) Cretaceous age Kamchatsky Mys ophiolite complex in Eastern Kamchatka (Far East Russia). Sulfide melts are present in three forms; (1) as inclusions in olivine (87.1–89.6 mol% Fo), (2) interstitial to the groundmass minerals (clinopyroxene, plagioclase, and Ti-magnetite) of studied picrites, and (3) as daughter phases in silicate melt inclusions hosted by olivine and Cr-spinel phenocrysts. The sulfide melt inclusions in olivine and the groundmass of studied rocks are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss) and intermediate (Fe–Cu–Ni; Iss) solid solutions. Several <0.5 μm Pd–Sn, Pt–Ag, and Au–Ag phases are recorded within the matrix sulfides, commonly along phase boundaries and fractures. Major elements (S, Fe, Cu, Ni, Co), platinum group elements (PGE), and gold analyzed in the homogenized olivine-hosted sulfide melt inclusions, and phases identified in the matrix sulfides record the range of magmatic sulfide compositions. The most primitive sulfide liquids are notably enriched in Ni and Cu [(Ni+Cu)/Fe, at% > 0.5], continuously evolve with crystallization of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss. Although the compositional systematics found in this study are consistent with those previously recorded, the compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) “nuggets” that exsolve at subsolidus temperatures and form during serpentinization of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can be also used for modeling sulfide saturation at crustal pressures and understanding behavior of the noble metals in primitive oceanic magmas.

Tectonothermal events in the Olympic IOCG province constrained by apatite and REE-phosphate geochronology

ABSTRACT The Olympic iron oxide–copper–gold province in South Australia contains numerous deposits and prospects, including the Olympic Dam Cu–U–Au–Ag deposit and the Acropolis prospect. The Acropolis prospect comprises massive, coarse-grained magnetite–apatite veins partly replaced by a hematite-stable assemblage. The apatite grains in the veins contain zones with abundant inclusions of other minerals (including monazite and xenotime) and low trace-element concentrations relative to the inclusion-free zones. The inclusion-rich apatite zones are interpreted to be formed from the recrystallisation of the inclusion-free apatite and remobilisation of U, Th and rare earth element (REE) from apatite into monazite and xenotime. Apatite, monazite and xenotime are all established U–Th–Pb geochronometers and offer the potential to constrain the alteration history of the Acropolis prospect. The LA-ICPMS U–Pb age of inclusion-free apatite is within error of the age of the host volcanic units (ca 1.59 Ga). Inclusion-rich apatite yields both near-concordant analyses that are within error of the inclusion-free apatite as well as highly disturbed (discordant) analyses. The most concordant analyses of monazite (Th–Pb) inclusions and xenotime (U–Pb) inclusions and rim grains indicate an alteration event occurred at ca 1.37 Ga and possibly also at ca 500 Ma. The disparity in age of the inclusion-rich apatite and the REE-phosphate inclusions (and rim grains) is suggested to be owing to the apatite being initially recrystallised at ca 1.59 Ga and modified again by a later event that also formed (or coarsened) most of the inclusions. Partial resetting of the majority of the monazite inclusions as well as the presence of significant amounts of common Pb has complicated the interpretation of the monazite results. In contrast, xenotime is a more robust geochronometer in this setting. The ages of the two post-1.59 Ga events that appear to have affected the Acropolis prospect do not correspond to any events known to have occurred in the Gawler Craton. The earlier (ca 1.37 Ga) age instead corresponds best with metamorphic–magmatic–hydrothermal activity in Laurentia, consistent with the proximity of Laurentia and the Gawler Craton inferred from palaeogeographic reconstructions. The later (ca 500 Ma) event corresponds to the Delamerian Orogeny and has been shown by prior studies to have also affected the Olympic Dam deposit.

Data for:Catastrophic events in the Quaternary outflow history of Lake Baikal

File with original data for U-Pb dating of detrital zircons by laser ablation inductively coupled plasma mass spectrometry