Radiogenic Isotope Signatures Research Articles

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

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].

Lead isotope compositions as guides to early gold mineralizations; the North Amethyst vein system, Creede District, Colorado

The North Amethyst vein system, which is hosted by approximately 27 Ma Carpenter Ridge Tuff and approximately 26 Ma Nelson Mountain Tuff, has two mineral associations separated by brecciation and sedimentation in the veins. The early association consists of quartz, rhodonite, hematite, magnetite, electrum (Au (sub 0.3-0.5) Ag (sub 0.7-0.5)) , and Mn carbonate, Au-Ag sulfide, Ag sulfosalt, and base metal sulfide minerals. The later mineral association cuts the Mn- and Au-bearing assemblages and consists of quartz, calcite, sericite, chlorite, hematite, adularia, fluorite, base metal sulfides, and Ag-bearing tetrahedrite.Our detailed studies show that the Pb isotope compositions of paragenetically early galenas associated with Au-rich mineralization in the North Amethyst vein system are relatively unradiogenic ( 206 Pb/ 204 Pb: 18.826-18.881, 207 Pb/ 204 Pb: 15.588-15.602, and 208 Pb/ 204 Pb: 37.790-37.926) compared to Pb isotope compositions of galenas formed later at about 25 Ma during Ag and base metal mineralization ( 206 Pb/ 204 Pb: 19.041-19.115, 207 Pb/ 204 Pb: 15.627- 15.672, and 208 Pb/ 204 Pb: 37.829-38.057). New Pb isotope data for the central and southern parts of the Creede district, which are located 5 to 7 km south of the North Amethyst area, agree with the results of a regional study by Doe et al. (1979) that included five samples from the main part of the Creede district. Galenas and adularia from the central and southern Creede district and galenas from the Bondholder district, 7 km north of the North Amethyst area, are similar to the Pb isotope compositions of galenas formed later in the North Amethyst area during Ag and base metal mineralization. Galenas from the Alpha-Corsair vein, which was mined for Ag prior to 1910, are isotopically similar to galenas associated with North Amethyst Au-stage mineralization. This isotopic similarity suggests that unexplored segments of the Alpha-Corsair structure may have the same mineralogy as the North Amethyst Au stage; thus, the Alpha-Corsair structure has the potential for high gold contents.Pb isotope compositions from the late stage of the North Amethyst vein system and from the Bondholder and central and southern Creede mining districts are more radiogenic than the host volcanic rocks of the central cluster of the San Juan volcanic field. Our Pb isotope results indicate that early Au mineralization of the North Amethyst area may represent the product of an older and relatively local hydrothermal system distinct from that of the younger base metal and Ag mineralization found throughout the region. Fluids that deposited Au minerals may have derived their Pb isotope composition by a greater degree of interaction with shallow, relatively less radiogenic volcanic wall rocks. The younger, base metal and Ag-rich mineralization that overprints the Au mineralization in the North Amethyst area clearly has a more radiogenic isotopic signature, which implies that the later mineralization derived a greater component of its Pb from Proterozoic source rocks, or sediments derived from them.Paragenetically early sulfide-rich vein assemblages have the least radiogenic galenas and generally also have the highest Au contents. Thus, identification of paragenetically early vein assemblages with relatively unradiogenic Pb isotope compositions similar to those of the North Amethyst area provides an additional exploration tool for Au in the central San Juan Mountains area.

Tertiary and quaternary magmatism in Central Sulawesi: Chronological and petrological constraints

40K- 40Ar ages (27 new ones) and geochemical data (major and trace elements) allow the recognition of four main magmatic events in Central Sulawesi (South of Palu and in the Toraja area). The oldes magmatic activity took place during Late Eocene and Oligocene, and led to the emplacement of island arc tholeiites and calc-alkaline magmas as intrusions cutting the Tinombo Formation near Donggala (34.5 My) and the Kambuno granitic body in the Toraja area (29.9 My). The Lamasi Volcanics followed, but their KAr ages, which range from 33 to 15 My, cannot be safely interpreted as magmatic ages. They have a MORB-type affinity, flat to slightly depleted incompatible trace-element patterns and Nb and Ta negative anomalies indicative of their BABB affinity. Their origin and mode of emplacement are still enigmatic. Then an important magmatic event, with a strong shoshonitic affinity, occurred in a short time interval (10.1−11.9 My), leading to the emplacement of intrusions cross-cutting the Mamasa and Kambuno granitic bodies and to the eruption of lava flows (Sekata, Talaya and Sesean Volcanics). This shoshonitic episode is probably equivalent to the post-subduction magmatic associations emplaced in South Sulawesi during the late Miocene and might derive from the melting of mantle material after the mantle was metasomatised during a former episode of subduction. The last magmatic event occurred between 6.5 and 0.6 Ma. The corresponding products are granitic rocks (Kulawi, Kamarora, Palopo), rhyolites (Gimpu, 1.9 My) and widely distributed rhyolitic pyroclastics known as the Barupu tuff (0.6 Ma). All these magmatic rocks are characterised by their K-rich calc-alkaline composition, by the lack of basaltic and intermediate magmas (SiO 2 > 60%) and by high enrichments in incompatible elements a radiogenic Sr isotopic signature. This is consistent with the hypothesis of a strong crustal imprint by melting of an underthrusted continental crust in a collisional context.

Lead isotopes in iron and manganese oxide coatings and their use as an exploration guide for concealed mineralization

Lead isotopes from Fe and Mn oxides that coat stream pebbles from around the Mount Emmons porphyry molybdenum deposit in Colorado were studied to assess the feasibility of using Pb isotopes to detect concealed mineral deposits. The Fe/Mn oxide coatings were analyzed to determine their elemental concentrations using ICP-AES. The Pb isotope compositions of solutions from a selected suite of samples were measured, using both thermal ionization and ICP mass spectrometry, to compare results determined by the two analytical methods. Heavy mineral concentrates from the same sites were also analyzed to compare the Pb isotope compositions of the Fe/Mn coatings with those found in panned concentrates. The Fe/Mn and 206Pb/ 204Pb ratios of the oxide coatings are related to the lithology of the host rocks; Fe/Mn oxide coatings on pebbles of black shale have higher Fe/Mn values than do the coatings on either sandstone or igneous rocks. The shale host rocks have a more radiogenic signature (e.g. higher 206Pb/ 204Pb) than the sandstone or igneous host rocks. The Pb isotope data from sandstone and igneous hosts can detect concealed mineralized rock on both a regional and local scale, even though there are contributions from: (1) metals from the main-stage molybdenite ore deposit; (2) metals from the phyllic alteration zone which has a more radiogenic Pb isotope signature reflecting hydrothermal leaching of Pb from the Mancos Shale; (3) Pb-rich base metal veins with a highly variable Pb isotope signature; and (4) sedimentary country rocks which have a more radiogenic Pb isotope signature. An investigation of within-stream variation shows that the Pb isotope signature of the molybdenite ore zone is retained in the Fe/Mn oxide coatings and is not camouflaged by contributions from Pb-rich base-metal veins that crop out upstream. In another traverse, the Pb isotope data from Fe/Mn oxide coatings reflect a complex mixing of Pb from the molybdenite ore zone and its hornfels margin, Pb-rich base-metal veins, and sedimentary country rocks. Stream-sediment anomalies detected using oxalic acid leaches can be evaluated using Pb isotope analysesof selected geochemical anomalies. Such an evaluation procedure, given regional target Pb isotope signatures for concealed mineralization, can greatly reduce the cost of exploration for undiscovered ore deposits concealed beneath barren overburden. Lead isotope measurements on aliquots of the same solutions showed that ICP-MS determinations are of low precision and vary non-systematically when compared with the Pb isotope values of the higher precision thermal ionization method. These variations and lower precision of the ICP-MS measurements are attributed to matrix effects.

Evidence from Re–Os isotopes for plume–lithosphere mixing in Karoo flood basalt genesis

THERE is abundant evidence to link continental flood basalt provinces with mantle plume activity1,2; however, flood basalts often have radiogenic isotope signatures that are outside the range for plume sources as recorded in ocean-island basalts. These signatures are more readily attributed to source regions within the continental lithosphere, and there are strong indications that it is often the mantle lithosphere that contributes isotopically 'enriched' (low-143Nd/<144Nd) material3,4. There are objections, however, to models that propose cold, refractory mantle lithosphere as an important source of basaltic magma5. The rhenium–osmium system may provide new constraints on the relative importance of plume and sub-continental lithospheric mantle (SCLM) in basalt genesis: samples of SCLM brought to the surface as xenoliths commonly have low 187Os/<188Os ratios6,7, whereas ocean-island basalts tend to have higher than chondritic 187Os/<188Os, and old continental crust has even higher (more radiogenic) ratios. Here we report initial 187Os/<188Os ratios for 190-Myr-old picrite basalts from the Nuanetsi region of the Karoo flood basalt province, which are most readily explained as mixtures between enriched SCLM and sub-lithospheric material, most probably derived from a mantle plume.

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, including in vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited 3He/ 4He ratio at St. Helena is 4.3–5.9 R A. Helium isotope disequilibrium is present within the phenocrysts, with lower 3He/ 4He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to α-particle implantation from the surrounding (U + Th)-rich lavas. A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave 3He/ 4He= 7.1 R A . 3He/ 4He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to 3He/ 4He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low 206Pb/ 204Pb ). Overall, the He Sr Pb 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 10 9 yrs.

Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex

The Coldwell Complex represents the largest alkaline intrusion associated with the Midcontinent Rift System in North America. This complex contains a plethora of rock types that have previously been subdivided into three intrusive centers. A detailed U-Pb zircon/baddeleyite age study of five samples indicates that the majority of the complex was emplaced into “cold” Archean crust at 1108±1 Ma and likely experienced a rapid cooling history. These data, combined with published U-Pb zircon/baddeleyite results for other rift related igneous activity, document the contemporaneous production and emplacement of tholeiitic and alkaline magmas at the onset of rifting. The Sr-Nd-Pb isotopic compositions of selected minerals from different phases of the complex display considerable scatter that is best explained by the presence of magmas with different initial isotopic compositions. The initial Sr and Nd isotopic compositions for clinopyroxene and plagioclase from one of the earliest gabbro phases (eNd=+0.5 to +1.6; eSr=+2.4 to +3.1) are identical to published data for primitive olivine tholeiites from the rift and indicate that the majority of magmas, both tholeiitic and alkaline, have a uniform, nearly chondritic isotopic composition. This very reproducible isotopic composition for rift magmatism can be explained by the dominance of a well-mixed mantle plume signature in magma genesis. The shift in isotopic compositions observed for the more evolved granite and syenite samples (eNd=−4.6 to −6.4; eSr=+10.2 to +13.8) combined with a less radiogenic Pb isotopic signature is consistent with derivation of these magmas from or interaction with an older granulite facies lower crust. The chondritic isotopic signature typical of most MRS volcanic and plutonic rocks is quite distinct from published results on associated carbonatites (eNd=+2.1 to +4.5; eSr=−8.0 to 2212;11.5) indicating the presence of at least two distinct subcontinental mantle isotopic reservoirs in this region.

Regional differences in PB isotopic compositions of feldspars in plutonic rocks of the northern Appalachian Mountains, U.S.A., and Canada: A geochemical method of terrane correlation

Feldspar Pb isotopic compositions from Paleozoic tonalitic to leucogranitic Caledonian plutons from the northern Appalachian Mountains of the U.S.A. and Canada range from nonradiogenic to radiogenic relative to the evolution curve of Stacey and Kramers (1975). Pb isotopic values in the feldspars are generally independent of modal and major and trace element contents of the plutons. Plutons with the least radiogenic Pb isotopic signatures (Northern group) have intruded allochthonous rocks closest to the North American craton, and plutons with the most radiogenic Pb (Southern group) have intruded Avalonian basement. Feldspars from plutons in the Central group have intermediate Pb isotopic compositions between the Northern and Southern groups and do not define an isotopically unique group. The predominant source of the felsic magmas was the continental lithosphere as indicated by the general Pb isotopic similarity of the plutonic feldspars with either Grenville or Chain Lakes basement in the case of the Northern group and with Avalonian basement rocks in the case of the Southern group. The source region for plutons in the Central group may have contained a mixture of Grenvillian and Avalonian components. Plutons in the northern Appalachians are as a group more evolved in their Pb isotopic compositions relative to Caledonian plutons in the British Isles.

Trace element evidence for the origin of ocean-island basalts

Ocean-island basalt (OIB) suites display significant and consistent differences in highly incompatible trace element ratios between islands which have end-member radiogenic isotope signatures. OIB derived from sources with a high U/Pb ratio have trace element characteristics consistent with a source dominated by recycled ancient oceanic crust. OIB derived from the isotopically distinct enriched mantle sources have trace element characteristics best interpreted in terms of sources which are mixtures of high U/Pb OIB with minor amounts (a few percent) of recycled sedimentary (pelagic and terrigenous) components.

A petrologic comparison of Triassic plutonism in the San Gabriel and Mule Mountains, southern California

Triassic magmatism in the southwest U.S. Cordillera forms a semicontinuous magmatic arc extending from northwestern Nevada to southeastern California. Quartz monzodioritic and quartz monzonitic rocks and associated diorites and granites are widespread in southeastern California, and we suggest that these rocks represent exposure of a structurally deeper part of the Triassic arc, where it was emplaced into comparatively thick Proterozoic crust. Elemental and isotopic data suggest that Triassic quartz monzodiorites and quartz monzonites in the Mule and San Gabriel Mountains were derived from a relatively undepleted, nonradiogenic mafic lithospheric source, with virtually no upper crustal interaction. Very limited data for associated Triassic(?) diorites indicate a wide range in composition and a surprisingly radiogenic isotopic signature. Younger Triassic(?) granites record a strong geochemical signature of interaction with continental crust, including inherited zircon and high initial Sr ratios but comparatively less radiogenic Pb isotopic compositions. The major and trace element geochemistry of Late Triassic plutonic rocks in southeastern California is similar in many respects to alkalic components of the Triassic arc in the Mojave Desert. However, contemporaneous rocks farther north have a calc‐alkalic signature, perhaps reflecting the variation in age and composition of lithosphere across which the Triassic arc was constructed.

Origin and evolution of formation waters, Alberta Basin, Western Canada Sedimentary Basin. II. Isotope systematics and water mixing

Isotopic measurements (Sr, O, D) on formation waters from the Alberta Basin have been made, covering a stratigraphic range from Devonian to Upper Cretaceous. These measurements, combined with chemical compositional trends, give evidence for two distinct water regimes. One hydrological regime is composed of waters hosted in Devonian-Lower Cretaceous reservoirs, the other waters from Upper Cretaceous and younger sedimentary rocks. The two regimes are separated by a regional transgressive shale in the Colorado Group, the Second White Speckled Shale Formation. The waters within the Devonian-Lower Cretaceous regime exhibit a large range in 87Sr/ 86Sr values (0.7076–0.7129), but have similar Sr concentrations, regardless of host lithology. Bulk rock and late-stage diagenetic cements are less radiogenic than present brines. Importantly, brines from Devonian carbonates possess the most radiogenic Sr isotopic signatures of the waters examined. Devonian shales and/or Cambrian shales may be sources of high 87Sr/ 86Sr ratios in the carbonate-hosted waters. Waters from the Upper Cretaceous clastic units, which have ratios as low as 0.7058, and diagenetic cements from Upper Cretaceous clastic units appear to have precipitated from fluids similar in Sr isotopic value to modern brines. High Sr concentrations in the Cretaceous clastic waters and sedimentary rocks and correspondingly low 87Sr/ 86Sr ratios suggest that volcanism in Montana during the Cretaceous may have provided a source of sediments to the study area. Cross-formational upward water migration, superimposed on lateral fluid flow, is required to explain the geochemistry and isotopic systematics in the brines from Devonian-Lower Cretaceous reservoirs. Strontium isotope ratios and Sr contents suggest a two component mixing relation for these waters. This system of waters also exhibits δD values characteristic of meteoric values in the Neogene, reflecting post-Laramide flushing of Tertiary waters throughout the basin, with subsequent hydrochemical isolation from more modern waters. In contrast, waters in Upper Cretaceous reservoirs have O and D isotopic compositions similar to those of present day rainfall; these, in conjunction with very dilute Sr concentrations and low Sr ratios, suggest hydrological isolation from the stratigraphically lower system.

Geochemical evolution of the New England seamount chain: Isotopic and trace-element constraints

Samples collected by dredge hauls and submersible dives from the New England Seamount (NES) chain, the most prominent of six seamount groups in the Northwest Atlantic Ocean, have been analyzed for Sr, Nd and Pb isotopic compositions and rare-earth and other trace-element contents. Unleached and HCl-leached whole-rock samples, and hornblende and clinopyroxene separates were analyzed in order to obtain isotopic data from the old and altered NES samples. Rare-earth and other trace-element contents were measured on unleached whole-rock samples. The results demonstrate that rimary (unaltered) geochemical data can be derived from older altered oceanic volcanics. The NES have isotopic and trace-element characteristics typical of alkaline volcanics from ocean islands, in particular those characterized by radiogenic Pb isotopic signatures (NES 206 Pb 204 Pb values range up to 20.65). The seamounts exhibit considerable interseamount variation, while intraseamount chemical heterogeneity appears to be substantially less. An increase in radiogenic Pb along the chain towards the southeast is suggested. No other simple geographic trends in chemistry are apparent. The isotopic characteristics of the NES necessitate at least three distinct source components, with substantial involvement of a “radiogenic Pb” type mantle. The isotopic similarities between the NES, Canaries, Azores and Ahaggar imply some type of systematic geographic distribution of this “radiogenic Pb” component.

Isotopic composition of lead in the Austinville-Ivanhoe Pb-Zn district, Virginia

Lead isotopic studies from the Austinville-Ivanhoe district of southwest Virginia demonstrate that galenas of different generation have distinct lead isotopic compositions. Within a single orebody paragenetically early galenas have lower lead isotopic ratios than later galenas. In addition, smaller orebodies characteristically show a more radiogenic isotopic signature than larger ore lenses. Mixing of lead from the enclosing sedimentary sequence and basement rocks is suggested as a possible mechanism of generating the isotopic composition of lead near Austinville. Accordingly, the slope of a regression line is thought to have no age significance.