Reid R. Keays

A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction

Abstract Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5–8 Myr earlier at the end of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring of huge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci. Lett. 166 (1999) 177–195] and Wignall [Earth Sci. Rev. 53 (2001) 1–33] suggested that the earlier Late Permian mass extinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China. However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution Ion Microprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the main phase of EFB volcanism, to be 259±3 Ma. Hence, the formation of the EFB is coincident with a proposed extinction event at 256–259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.

Sulfide melt-silicate melt distribution coefficients for noble metals and other chalcophile elements as deduced from MORB: Implications for partial melting

Abstract Analyses of chalcophile elements in coexisting sulfide and glass of basalts from the FAMOUS area of the mid-Atlantic ridge have been obtained by combined instrumental and radiochemical neutron activation analysis and directly coupled plasma spectrometry of hand-picked separates. In one sulfiderich sample, 526-1, both the glass and sulfides were analyzed for the noble metals. The sulfide meltsilicate melt Nernst partition coefficients determined for Au and Ir are 1.5–1.9 × 10 4 and 1.2–1.6 × 10 4 , respectively. That for Pd is estimated to be 3.5 × 10 4 . Partition coefficients for Ni are within the range 500–900, those for Co are between 26–51 and a single determination for Cu yields a value of 1383. The distribution coefficients for Ni, and possibly Cu and Co, are shown to be compositionally dependent. The data have been used to model the behavior of chalcophile elements during generation of MORB in its source region. The model assumes that all the platinum group elements, Se, Cu and S in the mantle reside in sulfides and that sulfide-saturated melt in the mantle contains 800 ppm S. Starting with a bulk upper mantle composition based on data from mantle xenoliths, 20–23% batch partial melting yields a silicate melt and a residual mantle sulfide that, with the exception of Ir, have compositions similar to sample 526-1 glass and sulfide, respectively. This implies that sulfide similar to that in 526-1 is left as a residue of partial melting to produce MORB and that primitive MORB compositions are determined by their equilibration with that sulfide. In contrast to MORB glass, the relative abundances of the platinum group elements, Se, Cu, and S in boninites closely resemble those of the sulfide that separated from MORB. This supports a model which holds that while the relative PGE abundances of boninite are controlled by mantle sulfide, no sulfide residue remains after the generation of boninite magma. That the modelling fails to account for Ir abundances supports the possibility that the distribution of Ir is controlled by a phase(s) in addition to sulfide.

The role of komatiitic and picritic magmatism and S-saturation in the formation of ore deposits

Abstract The introduction of high temperature, high magnesium komatiitic and picritic magmas into the Earths upper crust has given rise either directly or indirectly to many of the worlds major ore deposits. There is a direct association between komatiitic magmatism and Kambalda-style NiCuPGE deposits. The link between magma type and mineralization is not as clear cut in the case of the Norilsk CuNiPGE deposits, but nevertheless there is an association between picritic magmatism and ore deposit development. Komatiitic and/or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits. Recent Nd isotope data generated by Kent et al. (1994) provide strong evidence for the hypothesis proposed by Keays in 1983 that the Au in Archaean Au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids. The importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic/ultramafic magma types that are S-undersaturated at the time of magma formation. S-saturation of magmas leads to marked depletion in the chalcophile metals. For example, Czamanske and Moore (1977) estimated that a third of the original Cu and Ni contents of MORB were lost from the magmas due to extraction into magmatic sulphides. The depletion in Au and the PGE is much greater due to their much larger partition coefficients (Keays and Scott, 1976). Komatiitic and picritic magmas are S-undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in S through earlier partial melting events. Most mafic magmas, produced by less than 25% partial melting, are S-saturated and hence depleted in the ore forming chalcophile elements. In contrast, komatiitic and picritic magmas have their full complement of chalcophile metals. When they do become S-saturated, they form sulphides that are strongly enriched in Ni, Cu, Au, PGE and other strongly, chalcophile metals; these may accumulate to form ore deposits directly (e.g. magmatic NiCuPGE sulphides) or be dispersed in komatiitic/picritic rocks or their fractionation products making these source rocks for hydrothermal Au or VMS deposits.

Abundance and distribution of gold, palladium and iridium in some spinel and garnet lherzolites: implications for the nature and origin of precious metal-rich intergranular components in the upper mantle

Abstract The abundance and distribution of Au, Pd, Ir, Cu, Co and Cr has been determined in mantle-derived spinel lherzolite xenoliths in basanites from Mt Porndon (Victoria, Australia) and Kilbourne Hole (New Mexico, U.S.A.) and in garnet lherzolites from the Matsoku and Thaba Putsoa kimberlites (Lesotho). Minerals in the lherzolites concentrate Au, Pd and Ir in the following sequence of increasing platinum group element (PGE) content; garnet, olivine, orthopyroxene, clinopyroxene, spinel. and demonstrate that there exists a real crystallochemical control on the distribution of PGE. Whole rock PGE abundances calculated from the modal mineralogy are less than actually determined and indicate that the bulk of the PGE (60–80%) occur in a sulphide-rich intergranular component. A metasomatic origin for this component is considered to be unlikely and it is proposed that it represents an immiscible sulphide melt which has been retained in the mantle after extraction of a sulphur saturated basic partial melt. This component may in the case of garnet lherzolites have been modified by metasomatic events in the mantle leading to Au depletion and rare earth element addition. Spinel lherzolites are relatively homogeneous at a given locality but differ in their PGE content regionally. The weighted average abundances of PGE in a spinel lherzolite upper mantle are 0.6 ppb. Au, 4.0 ppb Pd. 3.6 ppb Ir. Garnet lherzolites are very heterogeneous and insufficient data is available to allow calculation of geochemically meaningful averages. Spinel lherzolite-basalt based pyrolite contains 0.9 ppb Au, 4.3 ppb Pd, 3.0 ppb Ir, and indicates that the mantle contains an apparent excess of Au over a calculated abundance based upon the siderophilic equilibrium distribution of Au between core and mantle. This excess is considered to be due to failure to consider the chalcophilic nature of Au in the mantle and not to the addition of a meteoritic component to a mantle equilibrated with the core.

Precious metals in magnesian low-Ti lavas: Implications for metallogenesis and sulfur saturation in primary magmas

Boninites and related magnesian low-Ti magmas are generally regarded as partial melts of a moderately to severely depleted peridotite source. Incompatible lithophile element abundances indicate that this source was variably enriched in LREE, Zr, Sr, Ba and alkalis by some mantle metasomatic process. Low-Ti lavas from the Bonin-Mariana arc system, Cape Vogel, New Caledonia, Cyprus, Newfoundland and SE Australia have been analysed for Pd, Ir, Au, Cu, S and Se. Comparison of fresh glassy material with variably altered samples suggests sporadic loss of Au and Cu and essentially inert behaviour for Pd, Ir and Se during seawater and subsequent alteration. They are uniformly enriched in Pd (mean 15 ppb) and depleted in Cu (mean 20 ppm), S (mean < 54 ppm) and Se (mean 53 ppb) compared to average MORB (<0.8 ppb Pd, 72 ppm Cu, 800 ppm S and 196 ppb Se) and exhibit incompatible-like behaviour for these elements and Au. The data are compatible with fractionation of the chalcophile elements during multi-stage mantle melting. Primary MORB liquids are S-saturated in their mantle source and an immiscible sulfide component is retained in the mantle residue. This results in the preferential removal of metals having low DS/L- values (base metals) and concentration of those metals with high DS/L values (precious metals) in the residual mantle sulfide fraction. Subsequent remelting of this refractory source produces S-deficient precious metal-enriched magmas, as exemplified by boninites. The absence of correlation between incompatible lithophile element enrichment and chalcophile element abundances suggests that the latter were not added to the source during mantle metasomatism. The constraints imposed by the nature of the source region result in two fundamentally contrasting patterns of behaviour for exclusively chalcophile elements. Magmas generated in mildly depleted to undepleted source regions by low to moderate degrees of partial melting (e.g. MORB) are S-saturated and become rapidly impoverished in precious metals during the early stages of silicate fractionation, owing to the co-precipitation of an immiscible sulfide component. Magmas generated from a strongly depleted source are initially S-undersaturated and concentrate chalcophile metals in their liquid residua. The contrasting behaviour of chalcophile metals during the early crystallisation stage of MORB and low-Ti magmas lead to divergent predictions concerning the primary distribution of these metals in oceanic crust generated by these magmas. The similarity in composition of early Bushveld magmas and boninites suggests that these S-deficient, PGE-enriched magmas may be essential to the formation of platiniferous horizons in layered intrusions.

Additional estimates of continental surface Precambrian shield composition in Canada

Abstract New trace element analyses have been made on the composite Canadian Precambrian shield samples reported in 1967. The overall mean abundance of Cr has been revised to 35 ppm (from 99). New abundances similar to 1967 values are (in ppm): Ni, 19; Co, 12; Cu, 14; Zr, 300; Sr, 315; Ba, 1070; Rb, 110: individual 1967 Rb values were erroneous. Elements not previously determined have the following overall mean values (in ppm): Zn, 52; Sc, 7.0; Nb, 26; Hf, 6.9; La, 32; Ce, 65; Nd, 26; Sm, 4.5; Eu, 0.94; Gd, 2.8; Tb, 0.48; Ho, 0.62; Yb, 1.5; Lu, 0.23; Y, 21; Pb, 17; values in ppb are: Ir, 0.02; Au, 1.8; Tl, 520. Clear positive correlations among Mg-Cr-Ni-Ir-Au appear for all rock-types, marble and quartzite as well as mafic igneous. Regional differences are apparent for several elements: e.g. higher Au, Ir, Cr, Ni in Baffin Island and Northern Quebec composites, compared with Saskatchewan and Southwestern Quebec; high Ti, Zn, Nb, Zr, Hf, REE, Y, Sr, K/T1 abundances and negative Eu anomalies in Southwestern Quebec. The overall REE abundances (omitting Southwestern Quebec) differ from other surface continental crustal rock estimates.

Experimentally determined sulfide melt-silicate melt partition coefficients for iridium and palladium

Abstract Sulfide melt-silicate melt partition coefficients for Ir and Pd have been determined from experiments run in a piston-cylinder apparatus at 1450°C, 8 kbar and under ƒ O 2 ƒ S 2 conditions appropriate for mafic magmas. Preferred values of DIr and DPd [ D = ( wt % silicate melt ) ( wt % silicate melt ) ] are 3.5·104 and 3.4·104, respectively. This validates the common assumption that sulfide melt dominates the geochemistry of the platinum-group elements in igneous processes. It also indicates that in magmatic differentiation processes in which sulfide alone controls precious metal abundance, Pd and Ir should not significantly fractionate from each other. The Pd results are consistent with those deduced from analyses of coexisting MORB sulfide and glass and with other published experiments. No dependence of DPd on ƒ O 2 ƒ S 2 was observed. For Ir, disparate results were obtained depending on the nature of the starting material. Experiments using natural silicate glass as a starting material yielded consistent values of DIr in the range 2.5·104–5.4·104. The experiments included a compositional convergence, in which the Ir-bearing sulfide produced in one experiment was used as the starting material for the second experiment. The results using natural starting compositions are similar to the value of DIr deduced from analyses of MORB. In contrast, all experiments in which synthetic silicate starting compositions were used yielded erratic values of DIr from 13·104 to 152·104. There are several possibilities to account for these high and variable values: (1) an unidentified trace or minor element present in the natural compositions but absent in the synthetic ones might complex with Ir in the silicate melt and enhance its solubility, (2) this element might enhance reaction rate, or (3) an Ir-rich quench phase might have formed in the experiments using synthetic compositions but not in the natural ones and been removed during preparation of the charges for analysis. All of these possibilities suggest that the experiments using synthetic compositions are not appropriate for determining the behavior of Ir in nature. The preferred, experimentally-determined values of DIr and DPd do not explain the fractionation of these elements observed in natural systems. The observed relative and absolute abundances of Ir and Pd reflect either concentration of Ir or Pd in a phase other than sulfide or redistribution of the metals subsequent to their initial concentration.

Controls on platinum-group elemental distributions of podiform chromitites : A case study of high-Cr and high-Al chromitites from Chinese orogenic belts

Abstract A study of podiform chromite deposits from the Asiatic Orogenic Belt and the Qilian-Qiangling-Kunlun-Himalaya Tectonic Domain provides new insights into the geochemistry of the PGEs in podiform chromite deposits and the genesis of the deposits themselves. The bulk of deposits, which occur in mantle peridotites of ophiolites, have typical ophiolitic PGE patterns that are depleted in Pt and Pd relative to the average upper mantle and have negatively sloping distributions on mantle-normalized diagrams. Type I (high-Cr) chromitites have higher Os, Ir, Ru, and Rh contents than Type II (high-Al) chromitites, although both have similar Pd and Pt. Most of the Type I and II chromite deposits have lower Pd and Pt contents than the upper mantle peridotites in which they occur. Podiform chromitites are essentially products of melt/rock interaction in the upper mantle; their Cr and PGEs were contributed by not only the invading magmas but also by the upper mantle host; the chromite deposits are, in part, metasomatic replacement bodies. The Type I (high-Cr) chromitite PGE patterns were produced by interaction between S-undersaturated boninitic magmas and depleted harzburgites, whereas the Type II (high-Al) chromitite PGE patterns were formed by interaction between initially S-saturated tholeiitic magmas and depleted harzburgites. The low to very low Pd and Pt contents of both Type I and Type II chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides, and hence Pd and Pt, prior to formation of the chromite deposits; in addition, no or little Pd and Pt were deposited by the invading magma which either remained S-undersaturated (boninite) or became (MORB) S-undersaturated due to interaction with the S-depleted harzburgitic mantle. It is suggested that the very low Ir, Os, and Ru contents of boninites in general might be due to loss of Ir during the formation of podiform chromitites. It is suggested that podiform chromitites with Type I PGE patterns were formed in an island arc environment, whereas those with Type II PGE patterns were formed in a back-arc setting.

The association boninite low‐ti andesite‐tholeiite in the heathcote greenstone belt, Victoria; ensimatic setting for the early lachlan fold belt

The Heathcote Greenstone Belt is composed mainly of Lower Cambrian metavolcanic rocks and is one of three outcropping belts of the apparent basement to the Lachlan Fold Belt in SE Australia. The greenstones may be assigned to two broad magma series. A younger tholeiitic series with mid‐ocean ridge basalt (MORB) affinities has intruded through, and been erupted upon low‐Ti, intermediate SiO2 lavas. The latter were originally boninites (both clinoenstatite‐phyric and more fractionated orthopyroxene‐phyric varieties) and plagioclase‐phyric, low‐Ti andesites. They have partially re‐equilibrated to the lower greenschist facies and outcrop mainly in the central segment of the Heathcote Greenstone Belt, where deeper stratigraphic levels are exposed. Tholeiitic lavas and sills metamorphosed to the prehnite‐pumpellyite facies dominate the northern and southern segments. As the association boninite/low‐Ti lavas/MORB is known only from modern West Pacific‐type settings involving island arcs and backarc basins, the e...

Sulfur and selenium systematics of the subcontinental lithospheric mantle: Inferences from the Massif Central xenolith suite (France)

Abstract Selenium has been analyzed in addition to S in 58 spinel peridotite xenoliths collected in Cenozoic alkali basalts from the Massif Central (France). The S concentration range now available for this suite, calculated from 123 samples, is the largest ever reported for alkali basalt-hosted xenoliths (