Os Isotopic Signatures Research Articles

METASOMATISM IN THE HYBLEAN UPPER MANTLE: THE OS ISOTOPE CONTRIBUTION

Abundant upper mantle (peridotite and pyroxenite) xenoliths occur in Miocene basaltic diatreme pipes and Quaternary lava flows on the Hyblean Plateau (Sicily, Southern Italy). Peridotites are spinel-facies protogranular-textured harzburgites and lherzolites (Fo90, En89, Cr-diopside, Cr-rich spinel), that equilibrated at temperatures between 950 and 1050°C. Even the freshest Hyblean peridotites exhibit some serpentine content. Major-element distribution and abundance reflect depletion (high MgO, low Al2O3) related to one or more melt extraction events, but evidence of modal and cryptic metasomatism does exist. For instance, phlogopite of metasomatic origin occurs rarely in these peridotites. Geochemical evidences of metasomatism includes Light Rare Earth Element (LREE) (e.g. LaN/YbN = 9-19). Sr-Nd and He isotopic signatures are also consistent with the refertilization of the lithospheric peridotite matrix (full descriptions in Sapienza and Scribano, 2000, Sapienza et al., 2005 and references therein). To better define the metasomatic reservoir, we performed in situ Os isotope analyses on sulphides from mantle peridotite collected from Valle Guffari Miocene diatreme. Eight peridotite xenoliths were cut in blocks, stuck on thin section glasses and polished on one side. Sulfides were imaged using a Cameca SX-100 electron microprobe. Then, Os isotope analyses were performed using a Merchantek LUV266 nm laser ablation microprobe (LAM) attached to a Nu Plasma multi-collector ICP (MC-ICP). The laser spot size was ~60 mm. Only 4 peridotite samples contain measurable sulfides, resulting in 11 analyses. Analytical details are in Pearson et al. (2002). Sulfides in the Hyblean peridotites are Ni-rich, often Ferich, and enclose a Cu-rich phase (Fig. 1). Their shape is variable, from spheroidal to irregularly shaped, usually displaying curvilinear margins (Fig. 1). The grainsize ranges from tens to ~250 mm. Sulfides are mainly related to the serpentine network. A few sulfides are olivine-enclosed but are too small to be analysed. 187O/188Os ratios range from 0.110408 to 0.124398, which correspond to sub-chondritic values. 187Re/188Os ratios range from 0.0296463 to 1.41346, i.e. supra- to subchondritic values (Fig. 2). Five out of 6 sulfides from one sample (GE12) show similar 187Os/188Os but different 187Re/188Os, while the remaining sulfide and the other samples all align along an oblique array showing a negative correlation between 187Os/188Os and 187Re/188Os. Assuming the 187Re decay constant after Smoliar et al. (1996), 187Re/188Os- CHUR and 187Os/188OsCHUR after Walker and Morgan. (1989), we calculate Re-depletion ages (TRD) and model age (TMA). TRD represents the minimum melt-depletion age of the lithospheric mantle, since the calculation minimizes the effect of metasomatism-driven Re addition, while TMA assumes that the considered mantle portion remains in closedsystem conditions through time (Walker et al., 1989). Low- 187Os/188Os sulfides from peridotite GE12 show Paleoprotezoic to Archean TRD ages, while the other sulfides are Neoand Mesoproterozoic. TMA yields meaningless age (future or even older than the Earth’s formation), reflecting recent disturbance of the Re/Os ratio. GE12 sulfides show the same unradiogenic 187Os/188Os composition (except for one sulfide) but 187Re/188Os ranging from 0.8-1.4 (Fig. 2). This indicates that Os isotope composition does not depend on time-integrated in situ 187Re, but rather suggests the metasomatic effects with Re addition probably occurring shortly prior the eruption. The negative correlation between 187Os/188Os and 187Re/188Os in the other samples also supports the metasomatic origin of Re, and rather suggests mixing between two reservoirs with different Re-Os signatures (Wang et al., 2003; Fig. 2). Thus, the Hyblean peridotites we have studied experienced melt extraction event(s) – which would remove Re from the system - and underwent later metasomatic Re-enriching event(s). Although these preliminary Re-Os data - and the large associated uncertainties - on peridotite sulfides do not allow the calculation of a very detailed ages, TRD ages are realistic and require a Proterozoic-Archean minimum age for these portions of the Hyblean lithospheric mantle. Most sulfides in peridotite GE12 testify to recent Re addition, while the other peridotite sulfides and one GE12 sulfide seem to be the results of mixing between two metasomatic end-members. The low-187Os/188Os sulfides in sample GE12 may be representative of the low-187Os/188Os endmember. The occurrence of sulfides with different Os isotope signatures in sample GE12 confirms the importance of using in situ technique in such petrological study.

Re-Os isotope studies of Mindanao adakites: Implications for sources of metals and melts

Whether the downgoing slab in modern subduction zones is able to melt is an important unresolved aspect of plate tectonics. Rare arc lavas with high Sr/Y ratios and low Y and heavy rare earth element concentrations, called adakites, are often attributed to melting of young subducted crust in subduction zones. Osmium isotopic data obtained for 13 adakite samples from Mindanao, Philippines, conflict with the slab-melting model. Ten of the samples have unradiogenic 187Os/188Os ratios inconsistent with partial melting of the ca. 50 Ma Philippine Sea lithosphere subducting beneath Mindanao. The Os isotope signatures are similar to those of young mid-oceanic-ridge basalts and normal arc rocks, consistent with a mantle source. In addition to implications for mass transfer in subduction zones, the apparent association of adakites with Cu-Au deposits makes understanding their origin, and that of the associated metals, economically interesting. This study suggests that a reevaluation is required of the slab-melting hypothesis in general, and that the metals associated with the Mindanao adakites come from the mantle.

Sources and genesis of the Chinkuashih Au–Cu deposits in northern Taiwan: constraints from Os and Sr isotopic compositions of sulfides

The Chinkuashih district at northern Taiwan hosts one of the largest Au deposits in the western Pacific gold province. Gold were precipitated from hydrothermal solutions as native gold or incorporated into sulfides at a temperature range of 200–350 °C. The sulfides in ore mines have 187Os/ 188Os ratios varying from 0.139 to 0.249. The positive 187Os/ 188Os–1/Os correlation is consistent with derivation from the hybrid fluids containing various proportions of mantle and crustal components. The crustal component was the meteoric water that had acquired its Sr and Os isotopic signatures from the local sedimentary formations and dacitic intrusions. The mantle component was the magmatic fluid segregated from the dacitic magma by fractional crystallization. Based on the 187Os/ 188Os–1/Os correlation, the hybrid fluids forming the Chinkuashih sulfides contained less than 30% magmatic fluid, except for one sulfide sample from Hsumei, which required >40% magmatic fluid. Compared to meteoric water, the magmatic fluid contained a higher Os content (130 times higher) and was enriched in Os relative to Sr with an Os/Sr ratio two orders higher than that of the crustal fluid. Consequently, the Os budget in the hybrid fluids was controlled by the magmatic fluid, although the meteoric water was volumetrically dominated. If gold and Os behave similarly in chemistry, the Chinkuashih gold deposits are of mantle origin and the area where sulfides with the greatest mantle Os signature may host undiscovered gold deposits. Finally, the 187Os/ 188Os ratios of sulfides show no relationship with the mineral assemblages of sulfides, implying that the sulfide mineral assemblages reflect local surfacial redox conditions rather than the chemical characteristics of parental fluids.

The Hadean upper mantle conundrum: evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Pb isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland

Here we present Sm-Nd, Re-Os, and Pb isotopic data of carefully screened, least altered samples of boninite-like metabasalts from the Isua Supracrustal Belt (ISB, W Greenland)that characterize their mantle source at the time of their formation. The principal observations of this study are that by 3.7–3.8 Ga melt source regions existed in the upper mantle with complicated enrichment/depletion histories. Sm-Nd isotopic data define a correlation line with a slope corresponding to an age of 3.69 ± 0.18 Gy and an initial εNd value of +2.0 ± 4.7. This Sm-Nd age is consistent with indirect (but more precise) U-Pb geochronological estimates for their formation between 3.69–3.71 Ga. Relying on the maximum formation age of 3.71 Gy defined by the external age constraints, we calculate an average εNd [T = 3.71 Ga] value of +2.2 ± 0.9 (n = 18, 1σ) for these samples, which is indicative of a strongly depleted mantle source. This is consistent with the high Os concentrations, falling in the range between 1.9–3.4 ppb, which is similar to the estimated Os concentration for the primitive upper mantle. Re-Os isotopic data (excluding three outliers) yield an isochron defining an age of 3.76 ± 0.09 Gy (with an initial γOs value of 3.9 ± 1.2), within error consistent with the Sm-Nd age and the indirect U-Pb age estimates. An average initial γOs [T = 3.71 Ga] value of + 4.4 ± 1.2 (n = 8; 2σ) is indicative of enrichment of their source region during, or prior to, its melting. Thus, this study provides the first observation of an early Archean upper mantle domain with a distinctly radiogenic Os isotopic signature. This requires a mixing component characterized by time-integrated suprachondritic Re/Os evolution and a Os concentration high enough to strongly affect the Os budget of the mantle source; modern sediments, recycled basaltic crust, or the outer core do not constitute suitable candidates. At this point, the nature of the mantle or crustal component responsible for the radiogenic Os isotopic signature is not known.Compared with the Sm-Nd and Re-Os isotope systems, the Pb isotope systematics show evidence for substantial perturbation by postformational hydrothermal-metasomatic alteration processes accompanying an early Archean metamorphic event at 3510 ± 65 Ma and indicate that the U-Th-Pb system was partially opened to Pb-loss on a whole rock scale. Single stage mantle evolution models fail to provide a solution to the Pb isotopic data, which requires that a high-μ component was mixed with the depleted mantle component before or during the extrusion of the basalts. Relatively high 207Pb/204Pb ratios (compared to contemporaneous mantle), support the hypothesis that erosion products of the ancient terrestrial protocrust existed for several hundred My before recycling into the mantle before ∼3.7 Ga.Our results are broadly consistent with models favoring a time-integrated Hadean history of mantle depletion and with the existence of an early Hadean protocrust, the complement to the Hadean depleted mantle, which after establishment of subduction-like processes was, at least locally, recycled into the upper mantle before 3.7 Ga. Thus, already in the Hadean, the upper mantle seems to be characterized by geochemical heterogeneity on a range of length scales; one property that is shared with the modern upper mantle. However, a simple two component mixing scenario between depleted mantle and an enriched-, crustal component with a modern analogue can not account for the complicated and contradictory geochemical properties of this particular Hadean upper mantle source.

Current limitations to the understanding of Re–Os behaviour in subduction systems, with an example from New Britain

Existing data for subduction-related magmas reveal a considerable diversity in both Re and Os concentrations and osmium isotope ratios. Unfortunately, with current knowledge, deciphering the relative roles of mantle, subducting slab and crust in generating these signatures remains a difficult undertaking. Fractional crystallisation imparts a significant influence on both Re and Os abundances which, in the case of Re, can be corrected for, although the extent to which Re volatility influences measured concentrations remains to be firmly established. With these provisos in mind, Re does appear to be more mobile in ‘fluid-dominated’ low-K tholeiite-type arcs. A compilation of existing isotope data strongly suggests that Os isotope signatures in continental arcs are compromised by interactions within the crust rendering them largely unsuitable for the estimation of subduction fluxes. Within intra-oceanic arcs, taking lavas from New Britain as an otherwise well-constrained example, estimation of the local upper mantle isotopic composition remains a major impediment to the quantification of slab-derived Os fluxes.

Re-Os and Lu-Hf Isotope Constraints on the Origin and Age of Pyroxenites from the Beni Bousera Peridotite Massif: Implications for Mixed Peridotite-Pyroxenite Mantle Sources

A suite of pyroxenites from the Beni Bousera peridotite massif, northern Morocco, have been analysed for Re–Os and Lu–Hf isotopic compositions. Measured sections of the massif indicate that pyroxenite layers make up between 1 and 9% by volume of the total outcrop. Clinopyroxenes from two Cr-diopside pyroxenites have unradiogenic Hf isotope compositions (Hfi -7·7 to -8·5) whereas those of the Al-augite suite are more radiogenic (Hfi 9·4 to 25·6). In general, the Nd–Hf isotope compositions of the pyroxenites lie close to the mantle array. One garnet pyroxenite lies significantly below the mantle Hf–Nd isotope array such that it requires an ancient history characterized by high Lu/Hf and Sm/Nd but low Lu/Hf relative to Sm/Nd. As with the Sm–Nd and Rb–Sr systems, parent–daughter and isotopic ratios for the Lu–Hf system have been recently decoupled by a partial melting event associated with transfer of the massif from mantle to crust. This created highly fractionated Sm/Nd and Lu/Hf ratios in many rocks and the pyroxenites can be referred to as ‘residual’. The near-solidus extraction of a siliceous melt from the pyroxenites is also a possible explanation for the orthopyroxene-rich margins to numerous pyroxenite layers, via reaction with peridotite. Pyroxenite Os isotope compositions are much more radiogenic than their host peridotites. In contrast to the non-systematic Nd and Hf model ages, a large portion of the pyroxenite Re–Os model ages cluster between 1·2 and 1·4 Ga, within error of the model ages defined by many Ronda pyroxenites and close to the precise 1·43 ± 0·07 Ga Lu–Hf isochron defined by clinopyroxenes from the peridotites. The Re–Os system thus seems to have been more robust to late-stage melting events that decoupled Sm/Nd and Lu/Hf isotope systematics in the pyroxenites. In contrast to pyroxenites measured from Ronda, some Beni Bousera pyroxenites have relatively radiogenic Os isotope compositions at high Os concentrations (0·18 to >2 ppb), comparable with values reported for some cratonic pyroxene-rich xenoliths. In contrast to cratonic eclogites, most pyroxenites analysed here and those reported in the literature lie close to the mantle Nd–Hf isotope array. The Nd–Sr–Pb–Hf isotopic compositions and stable isotope characteristics of these pyroxenites reflect signatures from recycled oceanic crust and sediment. Hence, mixing of such material, if present within the convecting mantle, with peridotite, could account for some of the heterogeneity seen in oceanic basalts. Small amounts of pyroxenite incorporated into peridotite can also produce the radiogenic Os isotope signatures evident in the source of oceanic basalts. However, these observations alone do not require pyroxenite to be an integral part of the convecting upper-mantle magma source region. The spectrum of Nd, Hf and Os isotope compositions also makes them a suitable component to explain some of the isotopic characteristics of the source regions of ultrapotassic magmas.

Dehydration and partial melting in subduction zones: Constraints from U‐series disequilibria

We present a critical reappraisal of U‐series data from arc volcanoes to constrain slab dehydration and melting processes using a global subduction zone data set. There is no clear evidence for significant mobilization of Th or Pa in dehydration fluids while the source region of arc rocks is relatively oxidized and mobility of U is strongly enhanced. It is argued that along‐arc U/Th and U/Pa isotope data reflect time‐integrated addition of U from the slab to the mantle wedge. The presence of large Ra‐Th disequilibrium correlated with Ba/Th ratios provides evidence for some very recent fluid addition and fast magma ascent. This is consistent with radiogenic Os isotope signatures in arc lavas that can only be preserved if there is no melt‐peridotite equilibration during melt transport. The 231Pa excesses found in most arc lavas provide clear evidence for a melting signature and require that the timescale of melt production cannot be as short as the timescale of melt migration (<100 annum). We propose a dynamic model in which melting is initiated by fluid fluxing and where melt is rapidly extracted out of the mantle via high‐velocity channels while 231Pa excess is produced at melting rates consistent with literature estimates of melt productivity. An alternative model with continuous flux melting would require a long melting timescale (1–5 Ma), a large melting region and the mobility of Th and Pa in fluids. A model with a time lag between melting and initial dehydration is more consistent with a thermal structure where the wet solidus is not reached before an intermediate depth.

The nature of metasomatism in the sub-arc mantle wedge: evidence from Re–Os isotopes in Kamchatka peridotite xenoliths

We have performed Re–Os isotope measurements on a suite of 21 Kamchatka mantle xenoliths including 19 harzburgites and two lherzolites, from the northern arc front (Valovayam Volcano), the southern arc front (Avachinsky Volcano), and behind the arc front in the south (Bakening Volcano). Os and Re concentrations vary from 0.02 to 8.2 and 0.003 to 0.437 ppb, respectively, and 187Re/ 188Os varies from 0.004 to 3.811. 187Os/ 188Os ratios range from 0.1226 to 0.1566. Regional variations in Re–Os isotope signatures are apparent, with peridotites from Avachinsky exhibiting the least radiogenic Os isotope signatures and lowest Re/Os ratios, and those from Bakening the most radiogenic Os and highest Re/Os. Peridotites from Valovayam span the distinct compositional fields defined by the Avachinsky and Bakening peridotites. All of the Kamchatka peridotites are, however, characterized by radiogenic 187Os/ 188Os compared to non-arc continental peridotites with comparable Re abundances or Re/Os ratios. The relatively radiogenic Os isotope signatures in the Kamchatka peridotites cannot easily be explained by contamination of the xenoliths by their host lavas, as this process would result in Re/Os ratios higher than observed in the xenoliths. In situ radiogenic ingrowth of high Re/Os mantle followed by recent Re depletion also cannot explain the observed radiogenic Os signatures in the Kamchatka peridotites, as the time required for radiogenic ingrowth would be significantly greater than the age of the lithospheric terranes that make up the respective regions of Kamchatka. The radiogenic Os isotope signatures in the Kamchatka peridotites are instead attributed to metasomatism of the Kamchatka sub-arc mantle wedge by radiogenic slab-derived fluids and melts. The regional variations in Re–Os isotope signatures are consistent with previous petrographic and geochemical studies of the Kamchatka mantle xenoliths that reveal multistage metasomatic histories resulting from interaction of the mantle wedge with a variety of slab-derived fluids and melts, including silicic slab-melt metasomatism associated with subduction of relatively hot, young (∼15–25 Ma) oceanic crust in the northern arc front, hydrous slab-fluid metasomatism associated with subduction of colder, old (∼100 Ma) oceanic crust in the southern arc front, and carbonate-rich slab-melt metasomatism in the southern segment behind the arc front, where the slab is deeper. Positive correlations between 187Os/ 188Os, La/Sm, and Ru/Ir in Avachinsky harzburgites support a model in which high f O 2 , Cl-rich, hydrous slab fluids transport LREE, Ru, and radiogenic Os into the mantle wedge beneath the southern arc front. Re is either not transported, or is not retained in the mantle during fluid–mantle interaction. Relatively higher Re and more radiogenic Os (but low Os abundances) in the Valovayam and Bakening peridotites indicate that both scavenging of mantle Os as well as exchange with radiogenic slab-derived Os, and incorporation of Re, occurs during interaction of the mantle wedge with oxidized, adakitic, and carbonate-rich slab melts. Similar ranges of Re–Os isotope signatures in peridotites from Avachinsky, Japan and Lihir, and from Valovayam and the Cascades, respectively, suggest that the age (temperature) and depth of subducting oceanic crust influences the Re–Os composition of metasomatized sub-arc mantle.

New insights into the Re–Os systematics of sub-continental lithospheric mantle from in situ analysis of sulphides

The Os isotopic compositions of mantle rocks generally are considered to be established during melt-depletion events and to be robust to subsequent disturbances (e.g. metasomatism). Consequently, Os isotopes are used to date the main melting event that a mantle section has undergone, i.e. transformation of fertile asthenospheric material into a depleted, buoyant lithosphere. However, Os resides almost entirely in Fe–Ni–Cu sulphides, which can be very mobile under mantle conditions. In situ laser ablation multi-collector ICP-MS measurement of Re/Os isotopic ratios in sulphides from spinel peridotite xenoliths demonstrates that whole-rock Os-isotope signatures record the mixing of multiple sulphide populations. Sulphides residual after melting events have unradiogenic Os isotopic compositions reflecting ancient melt depletion; those introduced by later metasomatism events contain more radiogenic Os. Therefore, the whole-rock Os isotopic signature can be strongly altered by metasomatic processes, and studies of mantle-derived xenoliths show that such disturbance is quite common in the lithospheric mantle. Because melt-depletion ages estimated from individual sulphide inclusions are systematically older than those obtained from whole-rock analysis, caution is essential in the interpretation of the Os model ages derived from whole-rock analysis, and their use and abuse in geodynamic models. This work suggests that sulphide could become a key phase in unravelling the formation and evolution of the lithosphere.

Temporal‐compositional trends in intraplate basalt eruptions: Implications for mantle heterogeneity and melting processes

Because primitive basaltic magmas travel from their mantle sources to the surface without significant cooling and crystallization en route, compositional variations within them provide direct information on the sources and processes of partial melting in the mantle. Most studies of basalt magmagenesis have focused on chemical variations in magmas erupted from multiple vents in regional complexes or on long timescale (105–106years) variations in magmas from single shield volcanoes; few studies have aimed to measure changes in melt chemistry with time in individual eruptions. Available observations of individual basaltic eruption sequences, however, show remarkably similar temporal‐compositional trends in the form of decreasing incompatible elements and MgO, with increasing SiO2, as the eruption (or sequence of related, closely‐spaced eruptions) continued. Both trace element and isotopic variations indicate that this does not reflect fractional crystallization nor can it be explained simply by varying extents of partial melting of a single source. Instead, these trends are consistent with systematically changing proportions of mixing between melts produced by different degrees of partial melting of two compositionally distinct but apparently common sources in the mantle. One potential explanation for this is that a relatively deep mantle source melts to large degrees, producing melts that ascend through and mix with small‐degree melts of the lithosphere. The small‐degree lithospheric melts are then progressively exhausted during melt migration and eruption, so that late‐erupting melts contain the smallest proportion of the shallow, small‐degree melt. This cannot explain Os isotopic correlations with melt composition and radiogenic Os isotope signatures of the late‐erupting melts in several eruption sequences, however. An alternative scenario is that systematic changes in erupted melt chemistry with time reflect sequential eruption of melt from increasing depths in mantle melting regions that are compositionally zoned because of different solidii and melt productivities of distinct peridotite and pyroxenite lithologic domains. In either case, compositional variation in many eruptions of primitive, intraplate basalts is dominated by mixing of melts from distinct lithologies, and temporal‐compositional changes in such eruptions reflect the distribution and composition of melt with depth in mantle melting and melt migration regions.

Abyssal peridotite osmium isotopic compositions from cr‐spinel

Abyssal peridotites, fragments of residual upper oceanic mantle, are believed to have less radiogenic Os compositions and higher Os concentrations than primitive upper mantle (PUM, 187Os/188Os = 0.129 [Meisel et al., 1996]). We have measured 187Os/188Os in 10 whole rock abyssal peridotites representing non‐plume‐influenced mid‐ocean ridge segments. The 187Os/188Os ratios range from 0.1183–0.1582. This large range in Os composition, to both less radiogenic and more radiogenic values than primitive upper mantle, can be attributed to ancient melting and subsequent sequestering of isotopic signatures, melt‐rock reaction, or secondary alteration, or a combination of any of the three. Petrographic, electron microprobe, and hand‐sample inspection show the peridotites to have experienced varying amounts of serpentinization and weathering, accompanied by heterogeneity in 187Os/188Os values. In addition, a majority of the peridotites studied here are spatially associated with gabbros having equally, if not more highly, heterogeneous Os isotopic compositions and N‐MORBs with homogeneous Sr, Nd, and Pb isotopic signatures. This suggests secondary seawater alteration as the dominant influence on present‐day Os signatures. We show that careful separation, leaching, and analysis of Cr‐spinel from abyssal peridotite largely removes the radiogenic seawater Os isotopic signature, allowing a more accurate assessment of the Os signature of depleted mid‐ocean ridge basalt mantle (DMM). Cr‐spinel is a highly refractory mantle mineral and commonly host to tiny sulfide inclusions, which are carrier phases for Os. These sulfides are well protected by the Cr‐spinel from high‐temperature serpentinization and low‐temperature seafloor weathering, thereby preserving nonradiogenic DMM‐like Os isotopic signatures. Our analyses of treated Cr‐spinel fractions finds 187Os/188Os ratios that are dominantly less radiogenic (0.1238–0.1482) than corresponding whole rock compositions. Nonmagnetic Cr‐spinels show more highly variable Os compositions and concentrations than magnetic Cr‐spinels, overlapping with the more radiogenic whole rock values. Magnetic Cr‐spinels are dominantly less radiogenic than PUM, with high but variable Os concentrations, possibly resulting from a sulfide “nugget” effect. The systematically lower 187Os/188Os in Cr‐spinels compared to whole rocks shows that whole rock abyssal peridotites are largely compromised by radiogenic seawater interaction, often obscuring the mantle signature. Measurements of treated abyssal peridotite Cr‐spinels in some cases can circumvent this seawater alteration problem allowing more straightforward interpretation of isotopic values and thus further constraining the Os isotopic signature for the depleted upper mantle. Our results suggest that the 187Os/188Os for the upper mantle lies in the range 0.120–0.125.

Re–Os isotopic composition and PGE contents of proterozoic carbonaceous argillites, Virginia Formation, Northeastern Minnesota

Carbonaceous, sulfidic argillites from the Proterozoic Virginia Formation in Minnesota have been investigated in terms of platinum group elements (PGE) distribution and Re–Os isotopic signatures. Kerogen from protoliths and slightly metamorphosed equivalents is characterized by Re and Os enrichment factors relative to whole rock concentrations that range from 4 to 272. Re–Os isotopic values of kerogen in samples that show no evidence of extensive recrystallization fall near a 1.85 Ga reference isochron. This age is consistent with that estimated for the Virginia Formation, and suggests that dating of organic-rich Precambrian sedimentary rocks and the retrieval of data pertinent to the Os isotopic evolution of seawater may be possible using the Re–Os isotopic system.

Correlated Os–Pb–Nd–Sr isotopes in the Austral–Cook chain basalts: the nature of mantle components in plume sources

Osmium (Os), strontium (Sr), neodymium (Nd) and lead (Pb) isotopes have been measured on a suite of aphyric basalts from 12 islands of the Austral–Cook island archipelago, an area which exhibits a range in Pb isotope compositions that encompasses almost the entire range displayed by ocean island basalts (OIB). Although the samples have Os concentrations (1.69–34.80 ppt) at the lower end of the range measured for OIB, they display a range of initial 187Os/188Os ratios (between 0.1279 and 0.1594) similar to that defined by olivine–phyric, Os-rich OIB. Positive Os–Nd, Os–Pb and negative Os–Sr isotope correlations are documented, indicating that the isotopic arrays represent various proportions of mixing between a HIMU-type end-member represented by Mangaia and Tubuai islands and characterized by radiogenic Os and Pb isotopic compositions, and an end-member represented by Rarotonga island which is characterized by unradiogenic Os and intermediate Sr, Nd and Pb isotopic compositions. The HIMU signature of the mantle component involved in Tubuai–Mangaia mantle sources requires long-term enrichments of U and Th relative to Pb and Re relative to Os, without associated increase in Rb/Sr, that are consistent with recycled oceanic crust. The end-member represented by Rarotonga basalts shows Os, Sr, Nd, and Pb isotopic signatures similar to those presumed for the ‘bulk silicate earth’ (BSE), which cannot be obtained by mixing the four mantle components (DMM, HIMU and EM1 and 2) generally used to circumscribe the Sr–Nd–Pb isotopic data of OIB. The primitive-like isotopic characters of this end-member might be ascribed to the presence of undepleted material from a lower segment of the mantle in the source of the Austral–Cook island basalts (and more specifically Rarotonga basalts); however, such a hypothesis is challenged by both the absence of a primordial 3He signature and the non-primitive Ce/Pb and Nb/U values for the Austral–Cook island basalts. Alternatively, assuming that the primitive-like isotopic composition of the Rarotonga samples reflects mixing proportions between the HIMU component and a mantle component characterized by unradiogenic Os, Nd and Pb and radiogenic Sr isotopic composition relative to BSE, the involvement of recycled, old subcontinental lithosphere in the genesis of this component should then be considered.

187 Os-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.

Comment on: "Enriched mantle - Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica)" by Antonini P. et al. (Contributions to Mineralogy and Petrology 136, 1-19; 1999)

O values, the magmas retain initial 87Sr/86Sr Compositions of greater than 0.709 (e.g. Hoefs et al. 1980). Consistent with this is the lack of a radiogenic Os isotope signature in these rocks (e.g. Molzahn et al. 1996) as would be expected with assimilation of continental crust. Comments here are not concerned with the AFC calculations employed to describe the variations within the suite of samples from the Prince Albert mountains studied by Antonini et al., but rather, the extrapolation of these to more primitive compositions.

Platinum-group elements and Os isotopic characteristics of the lower oceanic crust

We present osmium isotopic compositions and Os, Re and platinum-group element (PGE) contents in the lower oceanic crust gabbros from ODP Site 735B on the Southwest Indian Ridge. The average gabbros are very depleted in PGEs, e.g. the Os concentrations are very low and range from 0.5 to 38 ppt. One troctolite sample is an exception, containing, e.g. 376 ppt Os, 396 ppt Ir and 8 ppb Pd. Re contents are extremely heterogeneous, ranging from 128 ppt to 28 ppb. The present day 187Os/ 188Os varies from 0.140 to 0.467, and correlates with Rb/Cs, suggesting that seawater alteration affected the Os isotopic system. The average Os content in the lower oceanic crust is about 9 ppt while median value for Re is about 487 ppt. Such very high Re/Os is relatively similar to the upper MORB-type oceanic crust, indicating that recycled ocean crust basalts and gabbros will rapidly develop very radiogenic Os isotopic signatures.

Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites

Picrites from Nuussuaq Peninsula and Qeqertarssuaq (Disko) Island, West Greenland, preserve trace element and isotopic signatures reflecting the composition of the Icelandic plume head. Os isotope ratios are low (187Os/188Os(i)=0.1272–0.1371) in terms of global plume related magmatism, and this is coupled with anomalously high Os abundances and radiogenic 143Nd/144Nd(i) isotopes. Crustally contaminated basalts within the West Greenland sequences possess Os and Nd isotopic signatures consistent with mixing between initial plume head compositions and two discrete types of West Greenland continental crust. One crustal component is of a local sedimentary origin, and the other is typical of ancient felsic crust. The low Os isotopic signatures of the picritic units are considered to be those of the initial mantle plume. The fact that such low Os isotope ratios occur in sequences with high 3He/4He, and the non-systematic variation in Os isotopes with indices of fractionation/accumulation and Pb isotopes, argue against mixing with depleted MORB mantle or ancient subcontinental lithospheric mantle. The high Os concentrations in the picrites are attributed to high degrees of partial melting (>25%) of mantle containing no residual sulphide. This is consistent with models for plume heads in which anomalous mantle temperatures initiate melting at high pressures generating large degrees of partial melting. Unradiogenic Os and radiogenic Nd components in plume-related CFB magmatism may preserve contributions from a reservoir which is sampled only occasionally in young oceanic basalts. Such a reservoir shares Os isotopic features with the primitive upper mantle (PUM), which may also be manifest in the ‘Kea’ component of Hawaiian magmatism. Therefore, portions of the West Greenland continental flood basalt province arguably represent the first direct sampling of unmodified plume head material derived from the lower mantle or lower portions of the upper mantle.

The accretion rate of extraterrestrial 3He based on oceanic 230Th flux and the relation to Os isotope variation over the past 200,000 years in an Indian Ocean core

In the eastern equatorial Indian Ocean, the flux of extraterrestrial 3He, a proxy of interplanetary dust particles (IDPs), has been relatively constant over the past 200 ka. The flux is equal to (1.1±0.4)×10 −12 cm 3 STP cm −2 ka −1, a value obtained using the xs 230Th profiling method. Variations in mass accumulation rates (MARs) derived assuming a constant extraterrestrial 3He flux have a 40-ka periodicity similar to that observed in the δ 18O-derived MARs. This frequency is similar to that of the Earth's obliquity. Measured 187Os/ 188Os ratios are less radiogenic than present-day seawater (0.49–0.98), reflecting the mixing of Os derived from extraterrestrial, terrigenous and hydrogenous sources. When coupled with He data measured on the same samples, Os isotope data yield important information about the terrigenous component supplied to the eastern equatorial Indian Ocean. The amount of Os in the sample derived from the extraterrestrial component can be deduced with the help of the helium systematics. Once corrected for the extraterrestrial component of Os, Os isotope signatures, in conjunction with the 4He concentrations, suggest a supply of terrigenous material from Indonesian ultramafic and Himalayan crustal sources that clearly varies through time.

Os Isotope Systematics in the Canary Islands and Madeira: Lithospheric Contamination and Mantle Plume Signatures

Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr–Nd–Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. 187Os/188Os ratios in the basalts range from 0.129 to 0.183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a ‘radiogenic’ group with high 187Os/188Os from 0.152 to 0.183; (2) an ‘unradiogenic’ group with low 187Os/188Os from 0.129 to 0.138; (3) an ‘intermediate’ group with 187Os/188Os between 0.139 and 0.151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of ≤ 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher 87Sr/86Sr and 208Pb/204Pb but lower 143Nd/144Nd than samples with similar 206Pb/204Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower 206Pb/204Pb. The radiogenic Os and moderate 206Pb/204Pb signature of the Canary plume is consistent with a plume which contains 25–35% of relatively young (∼1.2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in 206Pb/204Pb isotopic composition.

Os Isotope Systematics in the Canary Islands and Madeira: Lithospheric Contamination and Mantle Plume Signatures

Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr–Nd–Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. 187Os/188Os ratios in the basalts range from 0.129 to 0.183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a ‘radiogenic’ group with high 187Os/188Os from 0.152 to 0.183; (2) an ‘unradiogenic’ group with low 187Os/188Os from 0.129 to 0.138; (3) an ‘intermediate’ group with 187Os/188Os between 0.139 and 0.151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of ≤ 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher 87Sr/86Sr and 208Pb/204Pb but lower 143Nd/144Nd than samples with similar 206Pb/204Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower 206Pb/204Pb. The radiogenic Os and moderate 206Pb/204Pb signature of the Canary plume is consistent with a plume which contains 25–35% of relatively young (∼1.2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in 206Pb/204Pb isotopic composition.