Radiogenic Lead Isotope Ratios Research Articles

S, Pb, and Fe isotope compositions of sulfides in middle and southern Okinawa Trough: Implying the complicated hydrothermal systems in back-arc spreading centers

Sulfur, lead, and iron isotopic systematics of sulfides from the Iheya North hydrothermal field in middle Okinawa Trough and the Yonaguni Knoll IV hydrothermal field in southern Okinawa Trough were investigated to understand the origin of ore-forming material and isotope fractionation during sulfide precipitation in back-arc spreading centers. The δ34S values (from 8.60‰ to 9.68‰) of sulfide minerals in Iheya North field suggest sulfur was derived from the leaching of basement rocks and the reduction of seawater sulfate. But an additional light sulfur source from the dissolution of biogenic sulfides in sediments is indispensable for the observed low δ34S values (between 4.78‰ and 4.97‰) of Yonaguni Knoll IV sulfides. Lead isotope compositions of all sulfides vary in a relatively wide range (206Pb/204Pb = 18.432 to 18.563, 207Pb/204Pb = 15.579 to 15.671, and 208Pb/204Pb = 38.539 to 38.979, respectively) which involves the mixing of mantle-derived and sediment-derived lead in varying proportions. Unusually high radiogenic lead isotope ratios of Yonaguni Knoll IV sulfides (207Pb/204Pb = 15.628 to 15.671, and 208Pb/204Pb = 38.854 to 38.979) imply the presence of old crustal fragments beneath southern OT. The hydrothermal fluids in OT are characterized by extremely low calculated δ56Fe values (−1.00‰ to −1.32‰) which is attributed to reducing sediments. The paired Fe-isotope compositions of pyrite and chalcopyrite revealed significantly different iron isotope compositions of hydrothermal fluids in study areas and extremely complicated ore-forming processes. Rapid precipitation of pyrite from FeS precursors accompanying iron isotopic disequilibrium moves from FeS-fluid equilibrium to pyrite-fluid equilibrium during pyrite recrystallization. The combination of sulfur, lead, and iron isotopic data suggests that hydrothermal systems in back-arc spreading centers related to plate subduction are generally affected by mantle, overlying sediments as well as potential old continental crust fragments, which results in various hydrothermal fluids and complicated mineralization processes.

Lead isotope and metal source of Shang bronzes: a response to Sun et al.’s comments

In this brief response to Sun et al.’s () comments on our paper, we re‐emphasize that archaeological, chemical and isotopic evidence are all relevant to the discussion about the metal source of the Shang period with highly radiogenic lead isotope ratios. The southern African bronzes have much lower lead contents and quite different lead isotopic signatures than the Shang bronzes. More importantly, there was no metallurgy of any kind in southern Africa before c.200 ce, so southern Africa cannot possibly be the source of Shang bronze, which date to about 1500 to 1000 bce.

Geochemical zoning of volcanic chains associated with Pacific hotspots

Lavas erupted from volcanoes in the north of the Hawaiian volcanic island chain have a different geochemical signature from those in the south. Analysis of the geochemistry of lavas erupted in the Samoan and Marquesas volcanic chains reveals similar trends, implying that the lowermost mantle beneath the southern Pacific is isotopically enriched. Recent Hawaiian volcanism is manifest as two geographically and geochemically distinct groups of volcanoes1, the Loa trend in the south and the Kea trend in the north2,3. The differences between the Loa and Kea lavas are attributed to spatial variations in the geochemical structure of the underlying Hawaiian mantle plume4,5,6,7,8,9. In turn, the Hawaiian plume structure is thought to reflect heterogeneities in its mantle source7,8. Here we compile geochemical data10 from the Hawaiian and two other volcanic ocean island chains—the Samoan and Marquesas—that formed above mantle plumes upwelling beneath the Pacific plate. We find that the volcanoes at both Samoa11 and the Marquesas12 show geographic and geochemical trends similar to those observed at Hawaii. Specifically, two subparallel arrays of volcanoes exist at both locations. In each case, the southern trend of volcanoes has higher radiogenic lead isotope ratios, 208Pb*/206Pb*, and lower neodymium isotope ratios, ɛNd, than those of the corresponding northern trend. We suggest that geochemical zoning may be a common feature of mantle plumes beneath the Pacific plate. Furthermore, we find that the pattern repeats between island chains, with the highest 208Pb*/206Pb* and the lowest ɛNdfound at Samoa in the south and the lowest 208Pb*/206Pb* and the highest ɛNd observed at Hawaii in the north. We infer that isotopically enriched material is preferentially distributed in the lower mantle of the Southern Hemisphere, within the Pacific low seismic velocity zone.

Evaluating the Contribution of Long-Range Transport of Heavy Metals from the Asian Continent to Their Concentrations in Sediment Cores from Lake Shinji, Western Japan

The historical trend of heavy metal pollution recorded in sediment cores from Lake Shinji, western Japan, was investigated to evaluate the contribution of increasing long-range transport of heavy metals from the Asian continent in recent years. The concentrations of Cd, Co, Cr, Cu, Ni, Pb, Sb, and Zn and lead isotope ratios were determined for sediment cores collected at two sites in the lake. Among the metals, Cd, Sb, and Zn showed markedly high concentrations since the 1970s. Moreover, a high Pb concentration and less radiogenic lead isotope ratios have been observed since the 1980s in the core from a site close to the mouth of a major river. Air masses from the Asian continent, including China, Russia, and South Korea, have less radiogenic lead isotope ratios than those from Japan. This suggests that the recent increase in Pb concentration in the sediment core is primarily due to the long-range transport of heavy metals from the Asian continent, followed by their deposition in the catchment area of the river. The concentration ratios of Pb/Cd, Pb/Sb, and Pb/Zn of the sediment around 2000 were calculated on the basis of the metal concentrations in excess of those before 1940. They were then compared with the volume-weighted annual average concentration ratios of Pb/Cd, Pb/Sb, and Pb/Zn of rain samples collected on the shore of the lake for 1999–2001. The result showed that the ratios of the former to the latter are 1.0 for Cd, 0.69 for Sb, and 0.31 for Zn. Thus, it is likely that the long-range transport of Cd and Sb from the Asian continent also contributes significantly to the recent increase in the concentrations of these metals in the sediment core from Lake Shinji. For Zn, however, the contribution from the Asian continent was evaluated to be small, suggesting the importance of local sources such as effluent discharges.

Metal sources of the large Nezhdaninsky orogenic gold deposit, Yakutia, Russia: Results of high-precision MC-ICP-MS analysis of lead isotopic composition supplemented by data on strontium isotopes

A collection of galena from the Nezhdaninsky gold deposit (62 samples), as well as galena from the Menkeche silver-base-metal deposit and the Sentyabr occurrence and K-feldspar from intrusive rocks of the Tyry-Dyby ore cluster have been studied using the high-precision (±0.02%) MC-ICP-MS method. Particular ore zones are characterized by relatively narrow variations of isotope ratios (no wider than σ6/4 = 0.26%). Vertical zoning of Pb isotopic composition is not detected. Variation in Pb isotope ratios mainly depends on the type of mineral assemblage. Galena of the gold-sulfide assemblage dominating at the Nezhdaninsky deposit is characterized by the following average isotope ratios: 206Pb/204Pb = 18.472, 207Pb/204Pb = 15.586, and 208Pb/204Pb = 38.605. Galena from the regenerated silver-base-metal assemblage is distinguished by less radiogenic lead isotope ratios: 18.420, 15.575, and 38.518, respectively. In lead from the Nezhdaninsky deposit, the component, whose source is identified as Permian host terrigenous rocks, is predominant. The data points of isotopic composition of lode lead make up a linear trend within the range of μ2 = 9.5-9.6. K-feldspar of granitic rocks has less radiogenic and widely varying lead isotopic composition compared to that of galena. The isotopic data on Pb and Sr constrain the contribution of Late Cretaceous granitic rocks as a source of gold mineralization at the Nezhdaninsky deposit. The matter from the Early Cretaceous fluid-generating magma chamber participated in the ore-forming system of the Nezhdaninsky deposit. The existence of such a chamber is confirmed by the occurrence of Early Cretaceous granitoid intrusions on the flanks of the Nezhdaninsky ore field. The greatest contribution of magmatic lead (∼30%) is noted in galena from the silver-base-metal mineral assemblage. This component has isotopic marks characteristic of lower crustal lead: the elevated 208Pb/206Pb ratio relative to the mean crustal value and the lower 207Pb/204Pb ratio. Taken together, they determine a high Th/U ∼ 4.0 in the source and μ2 = 9.37–9.50. This conclusion is consistent with the contemporary tectonic model describing evolution of the South Verkhoyansk sector of the Verkhoyansk Foldbelt and the Okhotsk Terrane.

Highly unradiogenic lead isotope ratios from the Horoman peridotite in Japan

Peridotites from the Horoman massif have the least radiogenic lead isotope ratios reported from any mantle material, and unlike any inferred from the compositions of mid-ocean ridge basalts. These data hint at the existence of ancient mantle domains that are not sampled by mid-ocean ridge basalts. Basalts at mid-ocean ridges are generated by partial melting of the Earth’s upper mantle. As a result of this process, the upper mantle has become depleted over time in elements that are preferentially removed by melting1,2,3. Although mid-ocean-ridge basalts have traditionally been thought to reflect the chemical composition of such depleted mantle2,3,4,5,6,7, recent work has revealed the existence of domains in the upper mantle that are apparently not sampled by the basalts8. Here we present the lead (Pb), neodymium (Nd) and hafnium (Hf) isotope compositions of peridotites from the Horoman orogenic massif in Japan, which is considered to represent the residues of melting of the upper mantle. These peridotites exhibit the lowest Pb isotope ratios reported from any known mantle material, along with high Nd and Hf isotope ratios. These data suggest that chemical depletion of the peridotites occurred around a billion years ago, and that they represent ancient mantle domains that have escaped convective stirring and homogenization. We suggest that such domains—if abundant in the mantle—may constitute a hitherto unrecognized reservoir with highly unradiogenic lead.

U-series, SrNdPb isotope and trace-element systematics across an active island arc-continent collision zone: Implications for element transfer at the slab-wedge interface

We present U-series, SrNdz.sbnd;Pb isotope and trace-element results of a regional study of geochemical systematics across an island arc-continent collision zone in the East Sunda Arc of Indonesia. Samples from four active volcanoes exhibit a striking compositional range from low-K tholeiitic to ultrapotassic, but all are characterised by high 87 Sr 86 Sr (0.7053–0.7067), radiogenic lead isotope ratios ( 206 Pb 204 Pb = 18.99–19.15), low ( 230 Th) ( 232 Thz) (0.66–0.85), and low 143 Nd 144 Nd (0.51255–0.51272), except for high 143 Nd 144 Nd (>0.51286) at the volcanic front. Low ( 230 Th) ( 232 Th) ratios are also found in terrigenous sediments in front of the arc, which, in combination with Srz.sbnd;Ndz.sbnd;Pb isotopic constraints, indicates that subducted continental material contributes to magma sources in this arc sector. The volcanoes close to the trench show a large excess of 238U over . 230Th (up to 80%) and of 226Ra over 230Th (up to 800%). In addition, they are enriched in elements thought to be mobile in hydrous fluids during slab-wedge transfer, such as Ba, Pb, and Sr. In contrast, Uz.sbnd;Thz.sbnd;Ra systematics are close to equilibrium in the volcanoes behind the front. Abundance patterns of incompatible trace elements in these rocks are similar to those of the terrigenous sediments, so that, in comparison with the arc-front lavas, they possess low Ba/La, Ba/Th, La/Th, Pb/Ce, and Zr/Nb. Higher concentration levels and less interelement fractionation form conspicuous differences with the front volcanics. Our combined isotopic and trace element data are consistent with three-component mixing whereby a slab-derived hydrous fluid and a siliceous melt are both added to the sub-arc mantle source. The hydrous fluid largely controls the input in the shallow part of the subduction zone, whereas the siliceous melt dominates the flux at deeper levels. Sedimentary material is considered to be the primary source of both. The large U-Th-Ra disequilibria at the front suggests that element transfer is a currently active process associated with present-day subduction of continental material.

Lead isotope variations across the central Peruvian Andes

Lead isotope values of ore and host-rock samples from 18 deposits of nine mining districts in an Andean transect between latitudes 9 degrees and 14 degrees S are reported. The ore deposits discussed in this paper comprise a relatively narrow age span of about 200 m.y. (Upper Triassic to early Pliocene). The lead isotope ratios from different ore deposits correlate closely with their geotectonic settings. There is a systematic increase of radiogenic lead isotope ratios from west to east, i.e., from ore deposits located near the coast (in the Lower Cretaceous marginal basin of central Peru) to ore deposits in the carbonate platform at the margin of the Brazilian shield. For the coastal deposits Ral, Condestable, and Los Icas the lead may be mantle derived and was supplied via the Casma volcanics. The more radiogenic lead of the Leonila Graciela-Santa Cecilia ores may be related to lateral facies variations within the marginal basin. It is assumed that here a mantle-derived component was mixed with a crustal component. The lead isotope ratios from ore deposits in the Western Cordillera mainly indicate a contribution from the orogen reservoir. Leaching of the Paleozoic basement is probably the main source. Igneous activity may have facilitated remobilization processes during younger tectonic and magmatic cycles. Mixing between upper crust and orogen reservoirs with participation of magmatic lead can explain the lead isotope ratios of the ore deposits in the Atacocha district. The lead isotope signature of the Shalipayco ores is interpreted in relation to the Permo-Triassic Mitu volcanism. Leaching of Permian rift-related volcanics could be responsible for the less radiogenic lead in this deposit. The ores of the San Vicente deposit are characterized by highly radiogenic values and negative model ages, typical of upper crustal leads in cratonic regions. The ore lead of San Vicente could have been derived by leaching clastic sediments derived from Precambrian upper crust (Brazilian shield).

Lead isotopic patterns of galena in the Piedmont and Blue Ridge ore deposits, Southern Appalachians

Massive sulfide deposits of the southern Appalachians form two distinct groups of lead isotope ratios which correlate with their geologic settings. Piedmont massive sulfides are associated with volcanic rocks of predominantly Cambrian age. They have model lead ages which are too young, an effect of U/Pb enrichment of source rocks. Lead isotope ratios of Piedmont massive sulfide define a trend of decreasingly radiogenic values from the Mineral district, Virginia, to the Stone Hill deposit, Alabama, which probably reflects decreasingly ensialic volcanism from northeast to southwest. A regression through the data has a slope which corresponds to a secondary isochron age of about 3.7 b.y. The line is proposed to represent mixing between variable amounts of upper crustal lead (decreasing to the southwest) with lead from a source relatively depleted in U/Pb which has, however, experienced either continuous or episodic enrichment of uranium relative to lead. The occurrence of a similar trend in galenas from Kings Mountain belt ores is further evidence for such a source.Massive sulfides of the Blue Ridge geologic province occur in upper Precambrian metasedimentary sequences and are typically low in lead content. Lead isotope ratios of galenas from Ducktown and the Gossan Lead are highly enriched in 206 Pb compared to amounts expected for deposits of that age. The anomalously radiogenic component is probably the result of lead derived from the predominantly clastic sediments of the host sequences. The radiogenic component of the 206 Pb coupled with the narrow range of 207 Pb/ 204 Pb at Ducktown indicates that much older continental crustal rocks were not involved in the ore-forming process.Galenas from a vein in the Hamme tungsten district, North Carolina, form a tight grouping which is similar to lead in K-feldspars from postorogenic ( approximately 300-m.y.-old) granitic plutons. The mineralized veins of the Virginia gold pyrite belt have galenas with a wide spread of radiogenic lead isotope ratios and do not appear to be related to massive sulfide deposits of the central Virginia piedmont.

Isotopic composition of lead and concentrations of uranium, thorium, and lead in volcanic rocks from Dogo of the Oki Islands, Japan

The isotopic composition of lead and the abundances of lead, uranium, and thorium in alkali rocks from Dogo of the Oki Islands, and Taka-shima, West Japan, have been determined. The lead, uranium and thorium contents range from 2.4 to 21.7 ppm, 0.4 to 3.6 ppm, and 2.6 to 27.1 ppm, respectively. They show a slight increase with progress in magmatic differentiation and a large increase from trachyte to rhyolite. A range of μ0 (238U/204Pb) = 8.6∼8.7 is calculated for a primary growth curve from the leads of Dogo. The observed μ (238U/204Pb) values of the rocks fall in a range 8.4∼13.5. The observed κ (232Th/238U) values are in a range 4.8∼9.8. The observed μ and κ for the rocks of Dogo are higher than for tholeiites and highalumina basalts in Japan. The rocks from Dogo are relatively deficient in 206Pb compared with the calculated model, suggesting that at least some lead was derived from a source of low μ during the last 2∼3 b.y. Consequently these volcanic rocks show isochron model ages of around 0∼500 m.y., which are older than the known age of the rocks. In addition, olivine basalts are isotopically uniform and more radiogenic in 206Pb/204Pb than the trachybasalt and the alkali series differentiates. Either the trachybasalts were generated from a source different from the rest of the volcanic rocks or, if all the volcanic rocks were generated from a single source, they subsequently acquired some leads isotopically different from those of the source. Natural contamination can explain the peculiar results in the islands, where partial assimilation of pre-Tertiary basement rocks, probably Precambrian rocks resulted in the less radiogenic lead isotope ratios in the differentiates from the islands.