Steven B. Shirey

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.

Start of the Wilson Cycle at 3 Ga Shown by Diamonds from Subcontinental Mantle

Mineral inclusions in diamonds reveal that the modern pattern of continent movements began around 3 billion years ago. Mineral inclusions encapsulated in diamonds are the oldest, deepest, and most pristine samples of Earth’s mantle. They provide age and chemical information over a period of 3.5 billion years—a span that includes continental crustal growth, atmospheric evolution, and the initiation of plate tectonics. We compiled isotopic and bulk chemical data of silicate and sulfide inclusions and found that a compositional change occurred 3.0 billion years ago (Ga). Before 3.2 Ga, only diamonds with peridotitic compositions formed, whereas after 3.0 Ga, eclogitic diamonds became prevalent. We suggest that this resulted from the capture of eclogite and diamond-forming fluids in subcontinental mantle via subduction and continental collision, marking the onset of the Wilson cycle of plate tectonics.

Osmium Recycling in Subduction Zones

Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.

Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts: Evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada

The ≧2685–2752 Ma old granite-greenstone crust in the Rainy Lake area, Ontario, consists of metaigneous and metasedimentary rocks that range in composition from tholeiite to monzogranite and include anorthosite, trachyandesite, monzodiorite and high-silica rhyodacite. Major element, rare earth and other trace element data are the basis for modelling the formation of the crust by melting of large-ionlithophile element enriched and unenriched mantle, by melting of basalt at mantle to crustal levels and by melting of monzodiorite and tonalite at crustal levels. All metaigneous rocks lie on a 143Nd/144Nd vs. 147Sm/144Nd isochron with an age of 2737 ±42Ma and an initial 143Nd/144Nd of 0.509178 ±33 (eNd = +1.9). This age is consistent with U-Pb zircon ages, which suggests the Nd isotopic system has been unaffected since the crust-forming events. The positive initial eNds are further evidence for time-averaged depletion in Sm/Nd relative to CHUR for the Archean mantle. The similarity of the initial Nd isotopic composition for both mantle-derived and crustally-derived rocks suggests rapid recycling of crustal components, which were previously derived from depleted mantle sources. Initial 143Nd/144Nd ratios on individual rocks range from eNd = +3.3 to eNd = −0.4. Younger granitoids have lower eNd values (+1.5 to −0.1) relative to tholeiites and monzodiorites crystallized from mantle-derived melts (+3.3 to +1.0). Thus, incorporation of slightly older crust (ca. 100–200 Ma) in some of the granitoid source areas is possible. Mantle-derived rocks form an isochron of 2764 ±58Ma that represents a minimum age for enrichment processes in the mantle sources for the Rainy Lake area. Consideration of data from the Abitibi belt suggests such enrichment processes in the mantle may have preceded crust-forming events in a wide area of the Superior Province, perhaps by as much as 50–70 Ma.

Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and their Mineral Inclusions

Tiny minerals trapped inside Brazilian diamonds show that Earth’s carbon cycle extends down to the lower mantle. A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

Os isotope systematics in the Azores: implications for mantle plume sources

Abstract Os isotopes were measured in 25 Holocene alkali basalts and trachybasalts from 6 islands in the Azores Archipelago. Extreme variations in 187 Os 188 Os signatures extending to very radiogenic values (up to 0.195) are found in samples with less than 20 pg/g Os. This trend is similar to that found in St. Helena, and is attributed to minor assimilation of marine sediment [1]. In contrast, a relatively limited range in 187 Os 188 Os (0.128-0.137) characterizes the basalts with greater than 20 pg/g Os. This range in 187 Os 188 Os is inferred to represent the Os isotopic signature of the Azores plume. High Os concentration samples from the island of Sao Miguel range in 187 Os 188 Os from 0.131 to 0.137, a surprisingly limited variation given the very large ranges on this island of Sr, Nd, Pb, He and Th isotopic signatures [2–5]. This is consistent with a model in which the Sao Miguel EMII signature is produced by high degree plume melts which mix with low degree melts of shallowly residing subcontinental lithospheric mantle [2], although an origin due to sediment recycling cannot be ruled out based on the Os isotope data. The Azores and many other plumes appear to be characterized by a relatively narrow range in Os isotopic composition despite variable Pb isotopic compositions. The Os isotopic compositions of these plumes are, in general, more radiogenic than depleted MORB mantle and than any chondrite groups, and indicate that plumes contain an additional source of radiogenic Os. Because Os is a highly compatible element, it is likely that Os only records large percentage crustal recycling, such as that inferred for the end-member HIMU islands [1,6,7]. We propose that the radiogenic Os isotope signatures in other plumes are due to the incorporation of radiogenic lower mantle, perhaps ultimately due to an outer core contribution [8,9].

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.