R.W. Carlson

Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths - Implications for the chemical evolution of subcontinental mantle

Twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae and Premier kimberlites of southern Africa were analysed for Os, Sr, Nd and Pb isotopic composition. 187Os186Os for 9 of the 12 samples, including both low-and high-temperature peridotites, are below any published value for the present mantle. Initial 187Os186Os in the peridotites show some correlation with Mg/Fe suggesting that the Re-Os fractionation was caused by melt removal that occurred at least 2 Ga ago. Sr and Nd isotopic compositions (87Sr86Sr = 0.7059 to 0.7078, ϵNd = −7.4 to −21) for acid washed clinopyroxenes from samples equilibrated at low temperature (<1200°C) are outside the range observed for oceanic basalts. Clinopyroxenes from high-temperature samples have Sr and Nd isotopic composition (87Sr86Sr = 0.7028 to 0.7032; ϵNd = 3.5 to 4.3) plotting within the oceanic mantle array, but have distinct Pb isotopic compositions (e.g. 207Pb204Pb = 15.07 and 16.18). Initial Os, Sr and Nd isotopic compositions for the Letseng-la-terae (Group I) kimberlite are within the range observed for rocks from the oceanic mantle, whereas these isotopic compositions in a Group II kimberlite from the Bellsbank group suggest a source for Group II kimberlites that is isotopically similar to the low-temperature peridotites. The compositional and Os isotopic characteristics of the low-temperature samples indicate that they may be residues from high-degrees of partial melt extraction. The distinct Os and Pb isotopic characteristics of the samples analysed here suggest that both low- and high-temperature peridotites reside in an ancient stable lithospheric “keel” to the craton that has been isolated from chemical exchange with the sub-lithospheric mantle for time periods in excess of 2 Ga.

Stabilisation of Archaean lithospheric mantle: A ReOs isotope study of peridotite xenoliths from the Kaapvaal craton

Os isotopic compositions of lithospheric peridotite xenoliths erupted by kimberlites in the Kaapvaal craton are almost exclusively less radiogenic than estimates of Bulk Earth (187Os/188Os as low as 0.106) and require long-term evolution in low Re/Os environments. Using Re depletion model ages which assume complete Re removal during formation, the data indicate that cratonic lithosphere stabilisation occurred at, at least, 3.5 Ga, when the lithosphere was over 200 km thick. This thick lithosphere persisted into the Phanerozoic to be sampled by kimberlites. Younger, Proterozoic and Phanerozoic Re depletion ages are interpreted to be largely the result of open system behaviour involving Re addition by metasomatic processes. Some of the younger ages may represent the addition of new lithospheric material during periods of major igneous activity. A mid-Archaean age for the Kaapvaal cratonic mantle concurs with Archaean ReOs ages found in samples of Siberian and Wyoming cratonic mantle. Both shallow (spinel facies) and deep (diamond facies) Kaapvaal peridotites have similar ages (3.3–3.5 Ga) suggesting that 150 km of mantle lithosphere may have accumulated very rapidly. Os isotope estimates for the timing of separation and stabilisation of Kaapvaal cratonic mantle overlap the main period of cratonic crust building and stabilisation (3.5-2.7 Ga). A similar overlap between crust and mantle stabilisation is evident for the Siberian craton. Archaean lithospheric mantle is compositionally different to that formed post-Archaean. The Kaapvaal peridotites have very low FeO compared to post-Archaean peridotites and show a large spread in Mg/Si. Some samples are anomalously Si-enriched compared with post-Archaean mantle samples. This compositional distinction and the varied NdOs isotope systematics are difficult to explain in terms of accepted models involving ancient melt depletion and subsequent metasomatism. Crystal segregation/cumulate processes have been suggested as a mechanism for forming the compositional range observed in Kaapvaal peridotites. This type of process may have occurred during harzburgite crystallisation from high-degree (> 50%) mantle melts associated with Archaean plume activity. A role for hot mantle plumes in generating the thick lithospheric keels beneath the Kaapvaal and Siberian cratons is supported by the possibility of their rapid formation and their thermal stability with respect to post-Archaean lithosphere. The coincidence of mid-Archaean cratonic mantle differentiation with periods of major crust building and stabilisation on the Kaapvaal and Siberian cratons suggests a link between crust generation and stabilisation and lithospheric mantle formation in the Archaean. Thermal energy from the plume may have been the impetus for major crust building at the time of lithosphere stabilisation, possibly by underplating of basaltic magmas. Direct involvement of mantle plumes in episodes of major mantle and possibly crust differentiation would imply that modern style plate tectonics may not have been the primary mechanism of planetary differentiation in the early Earth. Archaean ages for peridotites originating up to 200 km deep suggest that the mechanical boundary layer beneath continents is at least this thick.

ReOs, SmNd, and RbSr isotope evidence for thick Archaean lithospheric mantle beneath the Siberian craton modified by multistage metasomatism

A suite of peridotite xenoliths from kimberlites intruding the Siberian craton indicate the presence of lithospheric mantle over 150 km thick at 350 Ma. We report Sr-Nd isotope data for minerals from the peridotite xenoliths together with osmium isotopic compositions for whole-rocks and two olivine separates. Additionally, the osmium isotopic composition of a carbonatite from Fort Portal, Uganda, has been measured in order to evaluate the effect of carbonatite metasomatism on mantle ReOs systematics. Osmium isotope compositions of peridotite xenoliths from the Mir and Udachnaya kimberlites vary from those characteristic of the oceanic mantle, to considerably less radiogenic values (187Os1880s, 0.16469 to 0.10812), comparable to those previously found in other cratonic peridotites. In contrast, two eclogite xenoliths from Udachnaya have extremely radiogenic Os, 187Os188Os, up to 9.67. The lowest peridotite osmium isotopic compositions require Re depletion in the mid-Archaean (3.2 Ga) and this age is interpreted as the time of differentiation of the Siberian cratonic lithospheric mantle. Archaean depletion ages for spinel peridotites of relatively shallow origin and garnet peridotites and dunites containing diamond indicate that the depleted lithosphere reached from the Moho to 150 Km depth at this time and has been stable for 3 Ga. ReOs and SmNd model ages for two eclogite xenoliths are also in the range of 2.7 to 3.1 Ga and support an ancient origin for the Siberian lithosphere. The oldest peridotite depletion ages and the eclogite model ages correspond to the oldest crustal ages obtained from the Anabar Shield of the Siberian craton, and suggest that the initiation of major crust formation and stabilisation of a thick cratonic keel were coeval. In general, the Siberian low-temperature peridotites are not as enriched in incompatible elements as those from the Kaapvaal craton yet their diopsides possess similar, low SmNd. The low incompatible element concentrations but LREE/MREE enrichment seen in some Siberian lherzolites suggest they may be the products of disequilibrium melting. Neodymium and strontium isotopic compositions of minerals from the peridotites are extremely heterogeneous (ϵNd(350), −55.1 to 491; 87Sr86Sr, 0.70253 to 0.72235). Subcalcic garnets of diamond inclusion-like composition within megacrystalline peridotites have ϵNd(350 values varying from −55.1 to −12.1. Depleted mantle model Nd ages are as old as 3.2 Ga permitting an ancient, enriched origin similar to that suggested for diamond inclusions (Richardson et al., 1984). Alternatively, consideration of the complex garnet SmNd isotope systematics and the presence of unsupported radiogenic Sr together with marked trace element zonation (Shimizu et al., 1994) suggest that these subcalcic garnets crystallised recently (close to the time of kimberlite eruption) from ancient, LREE-enriched, high RbSr precursors. We propose that the isotope systematics of subcalcic garnet diamond inclusions can also be interpreted in terms of a recent origin.

Depletion and enrichment history of subcontinental lithospheric mantle: An Os, Sr, Nd and Pb isotopic study of ultramafic xenoliths from the northwestern Wyoming Craton

Abstract Elemental and isotopic compositions of spinel peridotite, pyroxenite and glimmerite xenoliths in Eocene minette dikes from the Highwood Mountains and Eagle Buttes, Montana, U.S.A., reveal a prolonged, yet episodic, history of melt removal and addition within the shallow lithospheric mantle of the Archean Wyoming Craton or its modified margin. Ancient, but highly variable, enrichment in incompatible elements is indicated by extreme Sr, Nd and Pb isotopic compositions ( 87 Sr 86 Sr = 0.705 to 1.02 ; ϵ Nd = −9 to −43; 206 Pb 204 Pb = 15.8 to 23.2 ). Very low 187 Os 188 Os (0.110 or less), corresponding to Re depletion model ages ( T RD ) of 2.7 to 2.9 Ga in some of the peridotites reflects melt removal during the Archean. At least one product of Archean melt migrating through this mantle section is preserved as a websterite xenolith that gives 2.7 Ga model ages for both the SmNd and ReOs isotopic systems. The majority of xenoliths, however, define PbPb and SmNd isochrons of mid-Proterozoic age and have ReOs T RD ages of 2 Ga or less. The mid-Proterozoic age could reflect either the time of formation of these peridotites in the shallow mantle or a time of severe overprinting of the incompatible element budget of pre-existing material by interaction with migrating fluids and/or melts. Glimmerite veins within one harzburgite sample yield 1.8 Ga monazite UPb ages and probably represent the products of crystallization of the fluid/melt responsible for the incompatible element enrichment. The material introduced in the Proterozoic was derived from much older, presumably Archean, crustal materials as shown by marked negative Eu anomalies in many samples and highly evolved initial Sr and Nd isotopic compositions. The data highlight the complex chemical evolution experienced by mantle lithosphere and suggest a coupling between the timing of processes affecting the lithospheric mantle and those recorded in the overlying crustal section.

Archean subduction recorded by Re-Os isotopes in eclogitic sulfide inclusions in Kimberley diamonds

Eclogitic sulfide minerals encapsulated in diamonds originating from the deepest part of the continental mantle keel beneath the Kaapvaal craton, southern Africa, and brought to the surface by the Kimberley kimberlites, show low Ni and Os contents and high Re/Os ratios characteristic of a basaltic protolith. The sulfide inclusions with the lowest Os contents give late Archean single grain absolute ages while those with higher Os contents yield a well-constrained 2.9 Ga isochron age and radiogenic initial Os isotope composition (γOs=+45). This indicates a significant time gap between basaltic precursor generation and eclogitic diamond crystallization, consistent with extended residence in a near-surface environment prior to subduction associated with accretion of the Kimberley block to the rest of the craton and subsequent diamond formation. These results suggest that subduction-related crustal recycling was already a viable process during continent formation in the middle Archean and may have been implicated in eclogitic diamond formation ever since.

Sulphide inclusions in diamonds from the Koffiefontein kimberlite, S Africa: constraints on diamond ages and mantle Re–Os systematics

Re–Os isotope compositions of syngenetic sulphide inclusions in both eclogite suite (E-type) and peridotite suite (P-type) parageneses in diamonds from the Koffiefontein mine, South Africa have been analysed using a technique capable of analysing single inclusion grains, or, in some cases multiple inclusions from the same diamonds. Sulphide inclusion Ni contents broadly correlate with Os abundances in that low-Ni (6.8–8.7% Ni), E-type sulphides have 4.7 to 189 ppb Os whereas the two high-Ni (25%), P-type sulphides have 5986 and 6097 ppb Os. Two P-type sulphides from the same diamond define the first mineral isochron obtained for a single diamond which has an age of 69±30 Ma with chondritic initial 187Os/188Os. This indicates that the sulphides, and hence the host diamond, crystallised close to the time of kimberlite emplacement (90 Ma), in the Mesozoic. This is supported by Pb isotopic measurements of a fragment from one of the sulphides, together with the absence of significant Type IaB nitrogen aggregation in the host diamond lattice. E-type sulphide inclusions have radiogenic Os isotopic compositions, 187Os/188Os 0.346 to 2.28, and Re–Os model ages from 1.1 to 2.9 Ga. They define an array on a Re–Os isochron diagram that may be interpreted as defining a single period of E-type sulphide growth at 1.05±0.12 Ga, with an elevated initial 187Os/188Os. Alternatively, two episodes of sulphide crystallisation, from a chondritic reservoir, may be invoked in the Archaean and in the Proterozoic. The results for both P- and E-type diamonds point to a spectrum of diamond crystallisation ages. High contents of both Re and Os, and the similarity of Re/Os ratios of sulphide inclusions in diamonds to whole rock eclogite and peridotite xenoliths indicate that small amounts of sulphides can dominate the mantle budget of both these elements during melting. Recent addition to the lithospheric mantle of high-Os material similar to that from which the P-type sulphides crystallised may explain the variable, sometimes young Os model ages seen in whole rock xenolith Re–Os data.

High precision iron isotope measurements of meteoritic material by cold plasma ICP-MS

The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ56Fe and ± 0.25 ‰ for δ58Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ56Fechondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.

Heterogeneous Accretion and the Moderately Volatile Element Budget of Earth

Earths Silver Lining The age of the oldest rocks on Earths surface is controversial, but, even if they are at their oldest estimate, hundreds of millions of years in our planets earliest history are still missing. However, in some rocks that until relatively recently resided in the mantle, the isotopic signature from the time of Earths formation is still preserved. Schönbächler et al. (p. 884) exploited this preservation to constrain models that describe the early material that assembled together to form Earth. Because the isotopic profile of silver in these rocks is nearly identical to that measured in a class of primitive meteorites, the earliest material probably had high volatile content. However, the fractionation of other isotopes suggests that the volatile content probably decreased over time in subsequent accretion events. With these isotopic model constraints, it is possible that one of the last major collisions—the Moon-forming giant impact—added considerable amounts of water and other volatile elements to Earth. Silver isotopes from mantle rocks suggest that Earth assembled from materials with variable volatile contents. Several models exist to describe the growth and evolution of Earth; however, variables such as the type of precursor materials, extent of mixing, and material loss during accretion are poorly constrained. High-precision palladium-silver isotope data show that Earth’s mantle is similar in 107Ag/109Ag to primitive, volatile-rich chondrites, suggesting that Earth accreted a considerable amount of material with high contents of moderately volatile elements. Contradictory evidence from terrestrial chromium and strontium isotope data are reconciled by heterogeneous accretion, which includes a transition from dominantly volatile-depleted to volatile-rich materials with possibly high water contents. The Moon-forming giant impact probably involved the collision with a Mars-like protoplanet that had an oxidized mantle, enriched in moderately volatile elements.

Re-Os isotope measurements of single sulfide inclusions in a Siberian diamond and its nitrogen aggregation systematics

Abstract We have measured the Re-Os isotopic compositions of individual syngenetic sulfide inclusions from three different growth zones within a central cross section plate cut from a single Siberian diamond. Individual sulfides in their diamond host were isolated by laser cutting. The sulfides, and hence the different growth zones of the diamond have been suggested to differ in age by up to 2 Ga on the basis of their Pb isotope compositions. Re-Os model ages of the four inclusions range from 3.1 ± 0.3 to 3.5 ± 0.3 Ga and suggest a Middle Archaean age for the diamond. A sulfide inclusion in the rim of the diamond is very different in elemental composition from those of the core and intermediate zones. It is enriched in Os, Re, Pb, and Zn and has more radiogenic Os and Pb isotopes. The inclusion is connected to the surface of the diamond by a healed crack, revealed by cathodoluminescence. The compositional distinction may be caused either by postformational interaction between an ancient sulfide and a fluid, possibly at the time of kimberlite eruption, or later stage growth of new diamond plus sulfide. Such chemical complexities, and the presence of healed fractures within the host diamond, emphasize the desirability of analyzing individual inclusions from well-characterized diamonds if isotope data for inclusions are to be better understood. Nitrogen contents and aggregation state in the core and intermediate zone of the host diamond closely approximate theoretically calculated isotherms based on consideration of experimentally determined nitrogen aggregation kinetics. The nitrogen content of the rim diamond is too low to obtain spectra that allow accurate deconvolution of relative aggregation levels for use in residence time calculations. The aggregation state of nitrogen in the core and intermediate growth zones is compatible with a long, ca. 3 Ga mantle residence time at normal lithospheric temperatures. The similarity of the sulfide inclusion Re-Os model ages to the oldest Re-Os ages from Siberian peridotite xenoliths confirms an ancient age for the Siberian lithospheric mantle and indicates that some diamonds formed closely after lithosphere stabilization.

Archaean ultra-depleted komatiites formed by hydrous melting of cratonic mantle

Komatiites are ultramafic volcanic rocks containing more than 18 per cent MgO (ref. 1) that erupted mainly in the Archaean era (more than 2.5 gigayears ago). Although such compositions occur in later periods of Earth history (for example, the Cretaceous komatiites of Gorgona Island), the more recent examples tend to have lower MgO content than their Archaean equivalents. Komatiites are also characterized by their low incompatible-element content, which is most consistent with their generation by high degrees of partial melting (30–50 per cent). Current models for komatiite genesis include the melting of rock at great depth in plumes of hot, diapirically rising mantle or the melting of relatively shallow mantle rocks at less extreme, but still high, temperatures caused by fluxing with water. Here we report a suite of ultramafic lava flows from the Commondale greenstone belt, in the southern part of the Kaapvaal Craton, which represents a previously unrecognized type of komatiite with exceptionally high forsterite content of its igneous olivines, low TiO2/Al2O3 ratio, high silica content, extreme depletion in rare-earth elements and low Re/Os ratio. We suggest a model for their formation in which a garnet-enriched residue left by earlier cratonic volcanism was melted by hydration from a subducting slab.