Gerhard Brügmann

Noble metal abundances in komatiite suites from Alexo, Ontario and Gorgona Island, Colombia

Abstract The distribution of the chalcophile and siderophile metals Cu, Ni, Au, Pd, Ir, Os and Ru in an Archaean komatiite flow from Alexo, Ontario and in a Phanerozoic komatiitic suite of Gorgona Island, Colombia, provides new information about the geochemical behaviour of these elements. Copper, Au and Pd behave as incompatible elements during the crystallization of these ultramafic magmas. In contrast, Ni, Ir, Os and Ru concentrations systematically decrease with decreasing MgO contents, a pattern characteristic of compatible elements. These trends are most probably controlled by olivine crystallization, which implies that Ir, Os and Ru are compatible in olivine. Calculated partition coefficients for Ir, Os and Ru between olivine and the melt are about 1.8. Compared to primitive mantle, parental komatiitic liquids are enriched in (incompatible) Cu, Au and Pd and depleted in (compatible) Ir, Os and Ru. Within both Archaean and Phanerozoic komatiites, noble metal ratios such as Au Pd , Ir Os , Os Ru and Ru Ir and ratios of lithophile and siderophile elements such as Ti/Pd, Ti/Au are constant and similar to primitive mantle values. This implies that Au and Pd are moderately incompatible elements and that there has been no significant fractionation of siderophile and lithophile elements since the Archaean. Platinum-group element abundances of normal MORB are highly variable and always much lower than in komatiites, because MORB magma is saturated with sulfur and a variable but minor amount of sulfide segregated during mantle melting or during the ascent of magma to the surface. Sulfide deposits associated with komatiites display similar chalcophile element patterns to those of komatiites. Noble metal ratios such as Pd Ir , Au Ir , Pd Os and Pd Ru can be used to determine the composition of the host komatiite at the time of sulfide segregation.

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth’s mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.

Melt percolation monitored by Os isotopes and HSE abundances: a case study from the mantle section of the Troodos Ophiolite

Combined siderophile and lithophile element systematics in mantle rocks can be used to monitor melt percolation processes in the Earth’s mantle. Here we present a coherent dataset from a single melt channel from the mantle section of the Troodos Ophiolite Complex on Cyprus. The melt channel is composed of a dunite vein that is surrounded by harzburgite. Dunite and harzburgite both have refractory Cr-spinel (Cr/(Cr+Al) of 0.58–0.60). Likewise, clinopyroxenes in both the dunites and harzburgites have strongly depleted REE patterns with (Gd/Yb)N values varying from 0.03 to 0.07. Such consistent lithophile element patterns suggest that the harzburgite and dunite interacted with the same melt during the melt percolation process. The distribution of the highly siderophile elements (HSEs) (Os, Ir, Ru, Pt, Pd and Re) in the melt channel cannot be explained by conventional partial melting models, but can be explained by melt-peridotite reaction. The harzburgites have slightly suprachondritic Os isotope ratios (187Os/188Ost=90 Ma=0.1288–0.1311) compared to the 187Os/188Ost=90 Ma of the carbonaceous chondrite reference (0.1264), and their HSE concentrations overlap with the range observed for lherzolites and harzburgites world-wide. In contrast, the dunites are significantly enriched in 187Os (187Os/188Os90 Ma=0.1335–0.1374), like volcanic rocks from island arcs world-wide. HSE patterns in the dunites are also typical for mantle melts, in that they are enriched in Pd, Pt and Re relative to Ir, Os and Ru, which are lower than in the primitive mantle. Hence, the harzburgites and dunites have complementary HSE concentrations and ratios. In addition, HSE ratios such as Ir/Os, Re/Os, systematically increase from the harzburgite towards the dunite ((Ir/Os)N: 0.36–1.8; (Re/Os)N: 0.14–9.5). This implies that Ir, Os and Ru behave incompatibly and become fractionated from each other during the melt percolation process. These features are interpreted to reflect the progressive reaction of a mantle melt with spinel–lherzolite to form harzburgite and eventually dunite. We suggest that an upper mantle peridotite was infiltrated by a radiogenic mantle melt typical for subduction-related volcanism. At low melt/rock ratios a harzburgite residue is left behind and its HSE distribution and the REE pattern of cpx can be explained by open-system melting if one assumes the HSEs to behave incompatibly. Continued melt percolation eventually produces dunites, and all mantle sulfides are removed from the peridotite. Thus, the sulfides and the HSE distribution in the dunites are not of residual origin but are dominated by sulfides that segregated from a sulfide-saturated melt with a radiogenic Os signature. The HSE variation in harzburgites and dunites from the melt channel can be interpreted as a mixing line that has HSE-bearing sulfides from the melt and from the residual mantle as end members. We conclude that HSEs become significantly mobilized and fractionated during melt percolation processes, thus providing useful proxies for melting and enrichment processes in the Earth’s mantle.

Re–Os systematics of UB-N, a serpentinized peridotite reference material

The reference material (RM) UB-N is a typical representative of earths upper mantle. It is a serpentinized garnet and spinel-bearing peridotite (a metamorphosed lherzolite) from the Vosges mountains, France, that is well characterized for major and many trace elements. In order to test whether UB-N is a suitable Re–Os reference material, 32 digestions in three different laboratories (CRPG/CNRS, MPI (Mainz) and University of Leoben) with four different digestion techniques (low-temperature acid attack, Carius tube dissolution, high-pressure asher (HPA-S) acid attack and alkali fusion) were performed. The results show that the low-temperature acid attack is unsuitable for the study of the Re–Os systematics of UB-N. Surprisingly, the well-established Carius tube acid digestion technique also fails to completely digest all Os-bearing mineral phases. Only alkali fusion and HPA-S acid attack yield the highest Os concentrations. Though sample inhomogeneity has been recognized (approximately 6% RSD for 2-g sample aliquots), it is possible to determine a well-defined average Os concentration of 3.85±0.13 ng g−1 (95% confidence; 19 digestions, fusion and HPA-S only). Rhenium-bearing minerals are very homogeneously distributed and replicates within each laboratory yield highly reproducible results independent of the digestion technique. A value of 0.2095±0.0040 ng g−1 (95% confidence; n=24) is assigned to the Re concentration. The best estimate for the whole-rock 187Os/188Os is 0.1278±0.0002 (95% confidence; n=12). The UB-N reference material now has well-understood Re–Os systematics that are typical of fertile upper mantle rocks. Analysis of this standard is, thus, highly recommended for the validation of Re–Os analytical procedures.

Precise Re–Os mineral isochron and Pb–Nd–Os isotope systematics of a mafic–ultramafic sill in the 2.0 Ga Onega plateau (Baltic Shield)

Abstract We present Re–Os, Sm–Nd and Pb–Pb isotope and trace element data for the Konchozero sill, a layered mafic–ultramafic intrusion in the Early Proterozoic Onega plateau, one of the oldest continental flood basalt provinces on Earth. The Sm–Nd and Pb–Pb combined mineral and whole-rock isochron ages of 1988±34 and 1985±57 Ma for the sill coincide with the age of ferropicrites from Pechenga (the Kola Peninsula). The lithostratigraphic, chemical and isotope evidence suggest the derivation of Pechenga lavas and the Onega plateau volcanics from a single mantle plume. Peridotite and gabbro whole-rock samples, and primary ulvospinel and ilmenite mineral separates from the sill yield a Re–Os isochron with a slope corresponding to an age of 1969±18 Ma, γ Os(T) =−0.61±5.9. This age is consistent with the other isotope data, and indicates the closed-system behavior of Re and Os in the rocks. The peridotites and ulvospinel have high Os concentrations (2.5–14 ppb) and low 187 Re/ 188 Os ratios (0.35–1.1), thus allowing a more accurate determination of the weighted average initial 187 Os/ 188 Os of 0.1144±0.0019 (2 σ pop ), γ Os(T) =+0.77±1.7. This value is lower than that determined by Walker et al. (Geochim. Cosmochim. Acta 61 (1997) 3145–3160) for the Pechenga lavas (γ Os(T) =+6.1±0.7), and implies a substantial Os-isotope heterogeneity in this ancient plume. Compared to the Onega plateau primary basalt magma, Pechenga ferropicrites are relatively enriched in iron and Ni, have lower (Nb/Th) N ratios (2.1 vs 1.1) and less radiogenic Nd-isotope compositions (e Nd(T) = +3.1 and +1.4, respectively), but share similar low-radiogenic Pb-isotope characteristics ( μ 1 =8.57 and 8.60). Incorporation of small amounts (1.5%) of outer core material into the hotter central part of the plume and subsequent contamination of the Pechenga ferropicritic magmas with the 2.9 Ga Belomorian gneisses can explain the observed chemical and isotope variations in the two provinces provided that the core had

A quantitative link between recycling and osmium isotopes

Recycled subducted ocean crust has been traced by elevated 187Os/188Os in some studies and by high nickel and low manganese contents in others. Here, we show that these tracers are linked for Quaternary lavas of Iceland, strengthening the recycling model. An estimate of the osmium isotopic composition of both the recycled crust and the mantle peridotite implies that Icelandic Quaternary lavas are derived in part from an ancient crustal component with model ages between 1.1 × 109 and 1.8 × 109 years and from a peridotitic end-member close to present-day oceanic mantle.

187Os-enriched domain in an Archean mantle plume: Evidence from 2.8 Ga komatiites of the Kostomuksha greenstone belt, NW Baltic Shield

The Re^Os data on Archean komatiites from the Kostomuksha greenstone belt in the Baltic Shield are presented. This greenstone belt has been previously interpreted to represent a former oceanic plateau formed by the emplacement of an ancient plume head [Puchtel et al., Earth Planet. Sci. Lett. 155 (1998) 57^74]. Samples of flowtop breccia, spinifextextured and cumulate komatiites and a chromite separate, all collected from the core of a 300 m deep diamond drill hole, yielded a Re^Os isochron with an age of 2795 ˛ 40 Ma and an initial 187 Os/ 188 Os of 0.1117 ˛ 0.0011 (Q 187 Os = +3.6 ˛ 1.0). The high positive Q 187 Os(T) implies that the komatiites were derived from a mantle source with a time-integrated suprachondritic Re/Os ratio. Recycling of oceanic lithosphere to produce the enriched 187 Os isotope signature is considered unlikely, as 15^25% crustal component is required to be incorporated into the plume source as early as 3.5^4.3 Ga. Such a substantial proportion of mafic material in the source would likely destroy the major and trace element characteristics of the komatiites. Our tentative interpretation is that the 187 Os-enrichment in the Kostomuksha plume represents an outer core signature. If confirmed by the ongoing Pt^Os isotope studies, the results would provide evidence for the existence of whole-mantle convection in the late Archean, and might place constraints on the timing of core differentiation in the early Earth. fl 2001 Elsevier Science B.V. All rights reserved.