A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems

¹⁴³Nd/¹⁴⁴Nd ratios, and Sm and Nd abundances, are reported for particulates from major and minor rivers of the Earth, continental sediments, and aeolian dusts collected over the Atlantic, Pacific, and Indian Oceans. Overall, Sm/Nd ratios and Nd isotopic compositions in contemporary continental erosion products vary within the small ranges of ¹⁴⁷Sm/¹⁴⁴Nd= 0.115 ± 0.01 and¹⁴³Nd/¹⁴⁴Nd= 0.51204 ± 0.0002 (ε_Nd = −11.4 ± 4). The average period of residence in the continental crust is estimated to be1.70 ± 0.35Ga.

These results combined with data from the literature have implications for the age, history, and composition of the sedimentary mass and the continental crust: (1) The average “crustal residence age” of the whole sedimentary mass is about 1.9 Ga. (2) The range of Nd isotope compositions in the continent derived particulate input to the oceans is the same as Atlantic sediments and seawater, but lower than those of the Pacific, demonstrating the importance of Pacific volcanism to Pacific Nd chemistry. (3) The average ratio of Sm/Nd is about 0.19 in the upper continental crust, and has remained so since the early Archean. This precludes the likelihood of major mafic to felsic or felsic to mafic trends in the overall composition of the upper continental crust through Earth history. (4) Sediments appear to be formed primarily by erosion of continental crust having similar Sm/Nd ratios, rather than by mixing of mafic and felsic compositions. (5) The average ratio of ¹⁴³Nd/¹⁴⁴Nd≈ 0.5117 (ε_Nd ≈ −17) in the upper continental crust, assuming its mean age is about 2 Ga. (6) The uniformity of the SmNd isotopic systematics in river and aeolian particulates primarily reflects efficient recycling of old sediment by sedimentary processes on a short time scale compared to the amount of time the material has resided in the crust.     

Oxygen isotope evidence for the origin of enriched mantle beneath the mid-Atlantic ridge

Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ¹⁸O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ¹⁸O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ¹⁸O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea.     

The role of lower continental crust and lithospheric mantle in the genesis of Plio–Pleistocene volcanic rocks from Sardinia (Italy)

The first comprehensive chemical and Sr–Nd–Pb isotopic data set of Plio–Pleistocene tholeiitic and alkaline volcanic rocks cropping out in Sardinia (Italy) is presented here. These rocks are alkali basalts, hawaiites, basanites, tholeiitic basalts and basaltic andesites, and were divided into two groups with distinct isotopic compositions. The vast majority of lavas have relatively high ⁸⁷Sr/⁸⁶Sr (0.7043–0.7051), low ¹⁴³Nd/¹⁴⁴Nd (0.5124–0.5126), and are characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and European) Neogene-to-Recent mafic volcanic rocks (²⁰⁶Pb/²⁰⁴Pb=17.55–18.01) (unradiogenic Pb volcanic rocks, UPV); these rocks crop out in central and northern Sardinia. Lavas of more limited areal extent have chemical and Sr–Nd–Pb isotopic ratios indicative of a markedly different source (⁸⁷Sr/⁸⁶Sr=0.7031–0.7040; ¹⁴³Nd/¹⁴⁴Nd=0.5127–0.5129; ²⁰⁶Pb/²⁰⁴Pb=18.8–19.4) (radiogenic Pb volcanic rocks, RPV), and crop out only in the southern part of the island. The isotopic ratios of these latter rocks match the values found in the roughly coeval anorogenic (i.e. not related to recent subduction events in space and time) mafic volcanic rocks of Italy (i.e. Mt. Etna, Hyblean Mts., Pantelleria, Linosa), and Cenozoic European volcanic rocks. The mafic rocks of the two Sardinian rock groups also show distinct trace element contents and ratios (e.g. Ba/Nb>14, Ce/Pb=8–25 and Nb/U=29–38 for the UPV; Ba/Nb<9, Ce/Pb=24–28 and Nb/U=46–54 for the RPV). The sources of the UPV could have been stabilised in the Precambrian after low amounts of lower crustal input (about 3%), or later, during the Hercynian Orogeny, after input of Precambrian lower crust in the source region, whereas the sources of the RPV could be related to processes that occurred in the late Palaeozoic–early Mesozoic, possibly via recycling of proto-Tethys oceanic lithosphere by subduction.     

Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity

Basalts from young seamounts situated within 6.8 m.y. of the East Pacific Rise, between 9° and 14°N latitude, display significant variations in ¹⁴³Nd/¹⁴⁴Nd (0.51295–0.51321), ⁸⁷Sr/⁸⁶Sr (0.7025–0.7031), and(La/Sm)_N (0.415–3.270). Nd and Sr isotope ratios are anti-correlated and form a trend roughly parallel to the “mantle array” on a¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr variation diagram. Nd and Sr isotope ratios display negative and positive correlations, respectively, with(La/Sm)_N. The geochemical variations observed at the seamounts are nearly as great or greater than those observed over several hundred kilometers of the Reykjanes Ridge, or at the islands of Iceland or Hawaii.

Samples from one particular seamount, Seamount 6, display nearly the entire observed range of chemical variations, offering an ideal opportunity to constrain the nature of heterogeneities in the source mantle. Systematics indicative of magma mixing are recognized when major elements, trace elements, trace element ratios, and isotope ratios are compared with each other in all possible permutations. The source materials required to produce the end-member magmas are: (1) a typical MORB-source-depleted peridotite; and (2) a relatively enriched material which may represent ancient mantle segregations of basaltic melt, incompletely mixed remnants of subducted ocean crust, or metasomatized peridotite such as that found at St. Paul's Rocks or Zabargad Island. Due to the proximity of the seamounts to the East Pacific Rise (EPR), the source materials are thought to comprise an intimate mixture in the mantle immediately underlying the seamounts and the adjacent EPR. Lavas erupted at the ridge axis display a small range of isotopic and incompatible trace element compositions because the large degrees of melting and presence of magma chambers tend to average the chemical characteristics of large volumes of mantle.

If the postulated mantle materials, with large magnitude, small-scale heterogeneities, are ubiquitous in the upper mantle, chemical variations in basalts ranging from MOR tholeiites to island alkali basalts may reflect sampling differences rather than changes in bulk mantle chemistry.     

Martian mantle mineralogy investigated by the Lu–Hf and Sm–Nd systematics of shergottites

Chemical heterogeneities in the Martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history. Shergottite meteorites from Mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths. The coupled ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd isotope systematics in 9 shergottites are used here to investigate these issues. Three compositional groups in the shergottites display distinct isotope systematics. One group, commonly termed as depleted, is characterized by positive ¹⁷⁶Hf_i from + 46.2 to + 50.4 and ¹⁴³Nd_i from + 36.2 to + 39.1. Another, termed as enriched, has negative ¹⁷⁶Hf_i = − 16.5 to − 13.2 and ¹⁴³Nd_i = − 7.0 to − 6.5. The third group is intermediate between the depleted and enriched groups with positive ¹⁷⁶Hf_i = + 30.0 to + 33.4 and ¹⁴³Nd_i = + 16.9. Together, they describe mixing curves between ¹⁷⁶Hf/¹⁷⁷Hf, ¹⁴³Nd/¹⁴⁴Nd, Lu/Hf, and Sm/Nd, implying that they sample two distinct sources in the Martian mantle. All shergottites are characterized by (Sm/Nd)_source < (Sm/Nd)_sample, but (Lu/Hf)_source > (Lu/Hf)_sample. This decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the Martian upper mantle. The genesis of shergottites can be modeled using non-modal equilibrium partial melting in a source initially composed of 60% olivine, 21% clinopyroxene, 9% orthopyroxene, and 10% garnet, with degrees of partial melting of 8.8% and 3.9%, respectively, for the two successive events. The enriched end-member of the shergottite mixing curve is best modeled by late-stage quenched residual melt resulting from the crystallization of a magma ocean. The depleted shergottite source may be modeled as a mixture of cumulates and residual melt, as convection in the Martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers. Consequently, equilibrium crystallization is preferred to model the crystallization of the Martian magma ocean. The models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in Mars.     

The origin of ocean island basalt end-member compositions: trace element and isotopic constraints

Ocean island basalt (OIB) suites display a wide diversity of major element, trace element, and isotopic compositions. The incompatible trace element and isotopic ratios of OIB reflect considerable heterogeneity in the mantle source regions. In addition to the distinctive Sr, Nd and Pb isotopic signatures of the HIMU, EMI and EMII OIB end-members, differences in incompatible trace element ratios among these end-members are of great help in identifying the nature and origin of their sources. Examination of trace element and isotopic constraints for the three OIB end-members suggests a relatively simple model for their origin. The dominant component in all OIB is ancient recycled basaltic oceanic crust which has been processed through a subduction zone, and which carries the trace element and isotopic signature of a dehydration residue (enrichment in HFSE relative to LILE and LREE, low Rb/Sr, but high U/Pb and Th/Pb ratios leading to the development of radiogenic Pb isotope compositions). HIMU OIB are derived from such a source, with little contamination from other components. Both the EMI and EMII OIB end-members are also dominantly derived from this source, but they contain significant proportions (up to 5–10%) of sedimentary components derived from the continental crust. In the case of EMI OIB, ancient pelagic sediment with high LILE/HFSE, LREE/HFSE, Ba/Th and Ba/La ratios, and low U/Pb ratios, is the contaminant. EMII OIB are also contaminated by a sedimentary component, in the form of ancient terrigenous sediment with high LILE/HFSE and LREE/HFSE ratios, but lacking relative Ba enrichment, and with higher U/Pb and Rb/Sr ratios. A model whereby the source for all OIB is ancient (1–2 Ga old) subducted oceanic crust ± entrained sediment (pelagic and/or terrigenous) is therefore consistent with the trace element and isotopic data. Although subducted oceanic lithosphere will accumulate and be dominantly concentrated within the mesosphere boundary layer, forming the source for hot-spots, such material may also become convectively dispersed within the depleted upper mantle as blobs or streaks, giving rise to small-scale chemical heterogeneities in the upper mantle.     

Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere

Peridotite xenoliths from the Eifel can be divided into incompatible element-depleted and -enriched members. The depleted group is restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotites. Isotopically the depleted group is very diverse with¹⁴³Nd/¹⁴⁴Nd ranging from 0.51302 to 0.51355 and⁸⁷Sr/⁸⁶Sr from 0.7041 to 0.7019, thus occupying a field larger than expected for oceanic-type subcontinental mantle. These xenoliths are derived from a mantle which appears to have diverged from a bulk-earth Nd and Sr isotopic evolution path 2 Ga ago as a consequence of partial melting. The combination of high¹⁴³Nd/¹⁴⁴Nd with high⁸⁷Sr/⁸⁶Sr in some members of the depleted-xenoliths suite is likely to be the result of incipient reaction with incompatible element-enriched fluids in the mantle. In the enriched group such reactions have proceeded further and erased any pre-enrichment isotope memory resulting in a smaller isotopic diversity (¹⁴³Nd/¹⁴⁴Nd 0.51256–0.51273,⁸⁷Sr/⁸⁶Sr 0.7044–0.7032). An evaluation of SmHf and YbHf relationships suggests that the amphibole-bearing lherzolites and harzburgites acquired their high enrichment of light rare earth elements by fluid infiltration into previously depleted peridotite rather than by silicate melt-induced metasomatism. Upper mantle composed of such metasomatized peridotites does not represent a potential source for the basanites and nephelinites from the Eifel. The isotopic and chemical diversity of the subcontinental lithospheric part of the mantle may result from it having remained isolated from the convecting mantle for times > 1 Ga.     

Osmium-strontium-neodymium-lead isotopic covariations in mid-ocean ridge basalt glasses and the heterogeneity of the upper mantle

Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (¹⁸⁷Os/¹⁸⁶Os and ¹⁸⁷Os/¹⁸⁸Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and SrPb isotopes. In this case, OsSrNdPb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.     

Anomalous strontium and lead isotope signatures in the off-rift Öræfajökull central volcano in south-east Iceland: Evidence for enriched endmember(s) of the Iceland mantle plume?

The currently active off-rift central volcano Öræfajökull in south-east Iceland sits unconformably on much older (10–12 Ma) and eroded crust. The composition of recent volcanics ranges from basalt to rhyolite, but the series is more sodic alkaline than the common rift zone tholeiitic suites. In this study we present Sr, Nd, Pb and O isotopic data for a suite of Öræfajökull samples. The complete suite shows typical mantle values for oxygen isotopes. The ⁸⁷Sr/⁸⁶Sr ratios (average of 15 SAMPLES=0.703702) of the modern Öræfajökull rocks (basalts as well as rhyolites) are much higher than observed so far for any other Icelandic rocks. The ¹⁴³Nd/¹⁴⁴Nd ratios (average=0.512947; n=15) are lower than for rift rocks, but similar to rocks of the off-rift Snæfellsnes volcanic zone. Furthermore, the Öræfajökull rocks are enriched in the ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotope ratios compared to Icelandic rift basalts. The enriched nature of the suite indicates that Öræfajökull samples a source component that has characteristics common with EM2 type mantle. Furthermore, it is concluded that the silicic rocks of Öræfajökull formed by fractional crystallization from mafic melts rather than by partial melting of older crust.     

Oxygen isotope geochemistry of lavas from an oceanic to continental arc transition, Kermadec–Hikurangi margin, SW Pacific

New oxygen isotope data are presented for submarine lavas erupted close to the transition between the oceanic Kermadec island arc and the continental Taupo Volcanic Zone, New Zealand. Volcanic glasses display δ¹⁸O values ranging from +5.65‰ to +5.83‰, clinopyroxenes range from +5.23‰ to +5.78‰ and olivines range from +4.83‰ to +5.47‰. Coexisting glass and phenocrysts in the lavas are in isotopic equilibrium, with one exception. Oxygen isotope ratios of back-arc lavas erupted through oceanic crust are indistinguishable from mid-ocean ridge basalts or lavas erupted in nearby back-arc settings. Although lavas from the arc front display elevated oxygen isotope ratios, the magnitude of ¹⁸O-enrichment is too great to result from recycling of subducted material alone. A single back-arc lava erupted through continental crust is also relatively ¹⁸O-rich suggesting that the most likely origin for the high δ¹⁸O signature is limited amounts of interaction between continental crust and melts derived from a mantle wedge that has been variably fluxed by recycled oxygen. The results of modelling open system behaviour in this volcanic system highlight the need for strong controls on the composition of local contaminants. Application of ‘average' crustal lithologies, as in other volcanic provinces, may lead to erroneous conclusions regarding the involvement of local basement.     

Anomalously nucleogenic neon in North Chile Ridge basalt glasses suggesting a previously degassed mantle source

Fresh basalt glasses from the North Chile Ridge (NCR) in the southeastern Pacific have Ne isotopic compositions distinctly different from typical mid-ocean ridge basalts (MORB). In a three-isotope plot of ²⁰Ne/²²Ne vs. ²¹Ne/²²Ne, the NCR data define a correlation line with a slope smaller than that of the MORB correlation line, i.e. their Ne composition is more nucleogenic than that of MORB. ³He/⁴He ratios are slightly lower than the MORB average, whereas in a few stepwise heating fractions very high ⁴⁰Ar/³⁶Ar ratios up to 28,000 are found. One model to explain the data assumes contamination of the NCR mantle source by material from the continental or oceanic crust, but in addition to difficulties with quantitatively reconciling the noble gas patterns with such a model it seems unable to account for some geochemical characteristics of NCR basalts reported earlier [Bach et al., Earth Planet. Sci. Lett. 142 (1996) 223–240], such as depletions in highly incompatible elements and unradiogenic Sr isotope compositions. Therefore we favor the scenario of a mantle source which was depleted and degassed previously, possibly as a residue from mantle melting beneath the southern East Pacific Rise that was transported to the NCR and melted again. The time during which such a depleted reservoir would have to be separated from the MORB mantle is estimated at 10–100 Ma based on U/Th–Ne systematics, in reasonable agreement with the time scale deduced from the formation history of the NCR and the temporal evolution of the southeast Pacific.     

Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure

The isotopic composition of helium emitted from geothermal springs in the southern Tibetan plateau, reported as R_c/R_A (R_c=air corrected sample ³He/⁴He, R_A=air ³He/⁴He), ranges from 0.013 to 0.38, and defines two principal domains. In southernmost central Tibet, helium isotope ratios are typical of radiogenic helium production in the crust (R_c/R_A<0.05, crustal helium domain). Further north, there is a resolvable ³He anomaly consistent with a mantle contribution (R/R_A>0.1, mantle helium domain). The highest values of 0.27–0.38 R_A occur at the southern end of the Karakoram fault. The boundary between the two domains lies 50–100 km north of the Indus-Zangpo suture zone. There seems to be no association between the ³He anomaly and zones of active normal faulting and litho-tectonic crustal units, such as the ultramafic rocks of the Indus-Zangpo suture zone and the Gangdese intrusive belt. Although scavenging of mantle-derived helium, stored in large ultrabasic and basic intrusions in the crust, cannot be ruled out entirely, we argue that the ³He anomaly most plausibly reflects degassing of volatiles from young (Quaternary) mantle-derived melts intruded into the crust. As such, it defines the southern limit of recent mantle melting and mantle melt extraction beneath the Tibetan plateau. The southern limit of the ³He anomaly coincides with the junction between the Indian and Asian plates, in the region where the Indian lithospheric slab steepens and is subducted beneath Tibet as suggested by seismic studies. Recent mantle melting and melt extraction is confined to the Asian mantle, but the southern limit of the melt zone may have migrated northwards during the last 10 Ma as the Indian lithosphere has progressively underthrust the Himalayas and Tibet.     

An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B

DSDP Hole 504B is the deepest basement hole in the oceanic crust, penetrating through a 571.5 m pillow section, a 209 m lithologic transition zone, and 295 m into a sheeted dike complex. An oxygen isotopic profile through the upper crust at Site 504 is similar to that in many ophiolite complexes, where the extrusive section is enriched in¹⁸O relative to unaltered basalts, and the dike section is variably depleted and enriched. Basalts in the pillow section at Site 504 haveδ¹⁸O values generally ranging from +6.1 to +8.5‰ SMOW(mean= +7.0‰), although minor zeolite-rich samples range up to 12.7‰. Rocks depleted in¹⁸O appear abruptly at 624 m sub-basement in the lithologic transition from 100% pillows to 100% dikes, coinciding with the appearance of greenschist facies minerals in the rocks. Whole-rock values range to as low as +3.6‰, but the mean values for the lithologic transition zone and dike section are +5.8 and +5.4‰, respectively.

Oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone. The pillow section reacted with seawater at low temperatures (near 0°C up to a maximum of around 150°C) and relatively high water/rock mass ratios (10–100); water/rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section. The transition zone and dikes were altered at much higher temperatures (up to about 350°C) and generally low water/rock mass ratios ( 1), and hydrothermal fluids probably contained mantle-derived CO₂. Mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient ( 2.5°C/m) across a 70 m interval at the top of the lithologic transition zone. Progressive reaction during axial hydrothermal metamorphism and later off-axis alteration led to the formation of albite- and Ca-zeolite-rich alteration halos around fractures. This enhanced the effects of cooling and¹⁸O enrichment of fluids, resulting in local increases inδ¹⁸O of rocks which had been previously depleted in¹⁸O during prior axial metamorphism.     

Helium isotope geochemistry of some volcanic rocks from Saint Helena

³He/⁴He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, includingin vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited³He/⁴He ratio at St. Helena is 4.3–5.9 R_A. Helium isotope disequilibrium is present within the phenocrysts, with lower³He/⁴He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to -particle implantation from the surrounding(U + Th)-rich lavas.

A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave³He/⁴He= 7.1 R_A.³He/⁴He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to³He/⁴He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low²⁰⁶Pb/²⁰⁴Pb). Overall, the HeSrPb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere.³He/⁴He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 R_A). This suggests that the recycled materials were injected into the mantle within the last 10⁹ yrs.     

Age and isotope geochemistry of the Archaean Pongola and Usushwana suites in Swaziland, southern Africa: a case for crustal contamination of mantle-derived magma

The igneous rocks of the Pongola Supergroup (PS) and Usushwana Intrusive Suite (UIS) represent a case of late Archaean continental magmatism in the southeastern part of the Kaapvaal craton of South Africa and Swaziland.

U-Pb dating on zircons from felsic volcanic rocks of the PS yields a concordia intercept age of 2940 ± 22Ma that is consistent with a Sm-Nd whole rock age of 2934 ± 114Ma determined on the PS basalt-rhyolite suite. The initial ε_Nd of−2.6 ± 0.9 is the lowest value so far reported for Archaean mantle-derived rocks. Rb-Sr whole rock dating of the PS yields a younger isochron age of 2883 ± 69Ma, which is not significantly different form the accepted U-Pb zircon age.

An internal (cpx-opx-plag-whole rock) isochron for a pyroxenite from the younger UIS yields an age of 2871 ± 30 Ma and initial ¹⁴³Nd/¹⁴⁴Nd that lies off the CHUR growth curve by ε_Nd −2.9 ± 0.2. However, Sm-Nd whole-rock data for the UIS yield an excessively high age of 3.1 Ga that conflicts with firm geological evidence showing the UIS to be intrusive into the PS.

The negative deviations of initialε_Nd from the chondritic Nd evolution curve suggest significant contamination of the PS and UIS melts by older continental crust. A mixing process with continental crust after magma segregation is supported by a high initial ⁸⁷Sr/⁸⁶Sr ratio of0.703024 ± 24 for a clinopyroxene sample from a UIS pyroxenite, compared with an expected value of 0.701 for the 2.9 Ga mantle. We therefore interpret the linear array of data points for the UIS gabbros as a mixing line between 2.87 Ga old magma and older continental crust.

Parallel LREE-enriched REE patterns, negative Nb-Ti anomalies, a distinctive and uniform ratio of Ti/Zr 46 and a narrow span of initial Nd indicate a common source for both the PS and UIS suites which is different from primitive mantle.     

Coupled Os–Os enrichments in the Earth’s mantle – core–mantle interaction or recycling of ferromanganese crusts and nodules?

¹⁸⁶Os enrichments in volcanic rocks and peridotite-derived iridosmine grains have been attributed to contributions from Earth’s outer core to the mantle, and apparently constrain the scale of mantle convection and an early timing for inner–outer core segregation more than 3.5 Gyr ago. Here, we highlight that marine ferromanganese crusts and nodules are characterised by high Pt/Os ratios and Pt–Os contents that develop much larger ¹⁸⁶Os excesses over geological time (≥0.2%/Gyr) than those hypothesised for Earth’s outer core (<0.005–0.01%/Gyr). ¹⁸⁷Os/¹⁸⁸Os ratios in ferromanganese crusts are radiogenic due to sequestering of continental Os from seawater. Similarly, ancient ferromanganese materials may have had ¹⁸⁶Os excesses (>0.1%) as a result of high Pt/Os ratios in continental crust, even prior to in-growth of ¹⁸⁶Os after formation due to their high Pt/Os ratios. Past recycling of small amounts of these materials into the Earth’s mantle will produce coupled ¹⁸⁷Os–¹⁸⁶Os excesses and little change in Re and platinum-group-element concentrations, as observed in Hawaiian picrites, and in contrast to the predicted result of outer core addition to the mantle. ¹⁸⁷Os and ¹⁸⁶Os enrichments in the Hawaiian mantle source are potentially consistent with it comprising recycled oceanic lithosphere, pelagic sediments and ferromanganese materials, and questions the notion that Os isotopes can be used to uniquely identify core–mantle interactions and the depth at which mantle sources for volcanism originate.     

Enrichment of the continental lithosphere by OIB melts: Isotopic evidence from the volcanic province of northern Tanzania

Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr= 0.7035−0.7058,ε_Nd = −5to+3, and²⁰⁶Pb/²⁰⁴Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated²³⁸U/²⁰⁴Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.     

Conflicting Sm-Nd whole rock and U-Pb zircon ages for Archean lavas from Newton Township, Abitibi Belt, Ontario

Archean komatiitic and tholeiitic lavas from Newton Township, Ontario, have a sufficient range of Sm-Nd ratios to define a well-constrained line on the normal ¹⁴³Nd/¹⁴⁴Nd vs. ¹⁴⁷Sm/¹⁴⁴Nd isochron plot. The data give an isochron age of 2826 ± 64Ma, and an initial ε_Ndof+2.65 ± 0.26. However, U-Pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite-tholeiite suite give an age of 2697 ± 1.1Ma. There is strong evident that the zircon age is the eruption age, suggesting that the older Sm-Nd age is incorrect and probably results from mixing between isotopically distinct mantle sources. At the time of eruption, the sources had ε_Nd values from about +4.2 to +1.6, indicating that the Archean mantle in this area was markedly heterogeneous and not uniformly depleted.     

Strontium isotopic composition and trace element data bearing on the origin of Cenozoic volcanic rocks of the central and southern Andes

The Cenozoic volcanic rocks of the southern Andes are characterized by low ⁸⁷Sr/⁸⁶Sr ratios (0.7040–0.7045), which are consistent with an origin in the downgoing slab of oceanic lithosphere or the overlying mantle. These values are distinctly lower than those from corresponding rocks of the central Andes.The calc-alkaline rocks of the central Andes exhibit higher Sr isotopic values (0.705–0.713) and variable Rb/Sr ratios. Different explanations are possible for this behaviour as well as for the positive correlation between ⁸⁷Sr/⁸⁶Sr and Rb/Sr expressed in an apparent isochron of 380 ± 50 m.y. It is postulated that these magmas result from a mixing process between a primary magma with basaltic affinities and crustal material of relatively young age.A model is proposed for the generation of the “andesitic” magmas of the central Andes by which crustal rocks of the upper part of the crust are added to the base of the crust by an accretionary process at the margin of the continent. Melts from these upper crustal rocks act as contaminants in “andesitic” magmas.The role of crustal material is still more significant in the generation of the ignimbritic magmas; they are considered to result from a two-stage melting process by which igneous rocks, belonging to a former stage of development of the Andes, are engulfed in the subduction zone, where they melt.     

Atmospheric noble gases in volcanic glasses from the southern Lau Basin: origin from the subducting slab?

Noble gas concentrations and isotopic compositions have been determined for four submarine volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin. The samples are the least differentiated ones from this area, and they display enrichments in fluid-mobile elements similar to the nearby island arc. ³He/⁴He ratios are slightly below average MORB (6.8–7.8 times atmospheric), whereas Ne, Ar, Kr, and Xe have isotopic compositions very similar to air. Together with previously published data from the Valu Fa Ridge and other spreading segments in the Lau Basin, our data show a systematic latitudinal variation of increasing Ne, Ar, Kr, and Xe abundances from north to south as well as Ne and Ar isotopic compositions changing from MORB-like to atmosphere-like in the same direction. Moreover, isotopic compositions and noble gas abundances of the lavas correlate strongly with Ba/Nb ratios and H₂O concentrations. Based on these observations and mass balance arguments, we propose that the atmospheric noble gases come from the subducting oceanic crust and are not due to shallow contamination with air dissolved in seawater or assimilation of old crust. Our data suggest that the noble gases released from the subducting slab are atmospheric and thus contain little or no solar He and Ne. In addition to the fact that ratios of He to heavy noble gases are small in aged ocean crust, He has possibly fractionated from the other noble gases due to its higher diffusivity, and thus He transport from the subducting slab into the mantle wedge is probably insignificant. We propose that the ³He/⁴He ratios lower than MORB observed in the VFR lavas result from radiogenic ingrowth of He in a highly depleted, and hence degassed, mantle wedge after the enrichment of U and Th released from the downgoing slab.