Sr Pb Isotope Research Articles

Geochemistry of the paleoproterozonic Nanying granitic gneisses in the Fuping complex: implications for the tectonic evolution of the Central Zone, North China Craton

The Nanying granitic gneisses represent a major ∼2.0 Ga magmatic event in the Fuping Complex of the Central Zone of the North China Craton. They occur in the Chengnanzhuang shear zone, which is chiefly developed along the contact between the Neoarchean Fuping tonalite-trondhjemite-granodiorite (TTG) gneisses and the Wanzi paragneisses. The Nanying granitic gneisses were derived from monzogranite and syenogranite, with minor amounts of quartz diorite and granodiorite, and belong to the metaluminous to peraluminous, high-potassium calc-alkaline series, correlations between major and trace elements, and similarities in chondrite-normalized rare-earth-element (REE) patterns, support field relationships (e.g. gradational contacts) indicating that the Nanying granitic gneisses originated from similar magmas. The granodioritic, monzogranitic and syenogranitic gneisses have similar ranges of initial 87Sr/ 86Sr values (0.7030–0.7093, 0.7021–0.7193 and 0.7049–0.7111, respectively), although these variations are partly attributable to local resetting of the isotopic system during late deformation/hydrothermal alteration. The whole-rock 208Pb/ 204Pb, 206Pb/ 204Pb and 207Pb/ 204Pb values of the Nanying granitic gneisses are 37.32–76.8, 15.24–23.82 and 14.92–17.92, respectively. These Sr–Pb isotope systematics, together with previously published Nd isotope compositions, suggest that the Nanying granitic gneisses were derived from partal melting of the ∼2.50 Ga Fuping TTG gneisses, with local assimilation involving the Wanzi paragneisses. The Nanying granitic gneisses are geochemically similar to the C CA and C CL groups of granitoids, except for a few samples, which plot in the H LO group. The Nanying granitic gneisses exhibit depletion in Rb, Sr, Ba, Ti and U, and enrichment in Th, Nb, Zr, Hf and REE, similar to those of syn-shear, high-potassium calc-alkaline granitoids and late-shear alkaline granitoids; they therefore represent the product of a syn-collisional to post- collisional magmatic event. This collisional event, which is significantly older than the ∼1.84–1.80 Ga event that has been proposed for the juxtaposition of the Eastern and Western Archean continental blocks in the final stabilization of the North China Craton, supports a tectonic model involving multiple collisional events for the amalgamation of the internal lithotectonic domains within the Central Zone and the assembly of the North China Craton as a whole.

Nd–Sr–Pb isotopes of Tengchong Cenozoic volcanic rocks from western Yunnan, China: evidence for an enriched-mantle source

Tengchong Cenozoic volcanic activity in the southeastern Himalayan belt of western Yunnan is considered to be rift-related after the Indo-Eurasia collision. In this study, Nd–Sr–Pb isotopic compositions were analyzed in order to understand the genesis of the Cenozoic volcanic rocks. Five basalts and andesitic basalts are characterized by high 87Sr/86Sr ratios (0.7057–0.7081), low ϵNd values (−1.1 to −5.7), and particularly high 208Pb∗/206Pb∗ ratios (1.08–1.12). Twenty samples of rocks, which include granite, granodiorite, and amphibolite from the same region, were analyzed to evaluate the role of contamination and assimilation in the volcanic rocks. Granites have high 87Sr/86Sr ratios and low ϵNd values, indicating a crustal origin. Granodiorites have 87Sr/86Sr ratios of 0.7069–0.7100 and ϵNd-values of −2.9 to −7.6 that indicate contribution of a mantle component. Amphibolites that are characterized by high ϵNd-values of 9.4 to −1.7 may represent remnants of obducted oceanic crust of the Neo-Tethyan basin. The volcanic rocks are distinguished from the granitoids and amphibolites in terms of Pb isotopic compositions. This observation probably implies that crustal contamination played a minor role in the origin of the volcanic rocks. Isotopic evidence for the volcanic rocks indicates that they probably originated from melting of an enriched-mantle source, that is, ascribed to assimilation of subducted oceanic crust and sediments.

Petrogenesis of Mesoproterozoic (Subjotnian) rapakivi complexes of central Sweden: implications from U–Pb zircon ages, Nd, Sr and Pb isotopes

Petrogenesis of Mesoproterozoic (Subjotnian) rapakivi complexes of central Sweden: implications from U–Pb zircon ages, Nd, Sr and Pb isotopes

Petrogenesis of Mesoproterozoic (Subjotnian) rapakivi complexes of central Sweden: implications from U–Pb zircon ages, Nd, Sr and Pb isotopes

ABSTRACTU-Pb zircon geochronology of Mesoproterozoic (Subjotnian) rapakivi complexes in central Sweden yields: 1526 ± 3 Ma (Mullnäset), 1524 ± 3 Ma (Mårdsjö), 1520 ± 3 Ma (Nordsjö) and 1497 ± 6 Ma (Rödön). Together with complexes further S in Sweden, they constitute the westernmost, youngest (1·53−1·47 Ga) belt of rapakivi magmatism in the Fennoscandian shield.The low initial εNd values (−8·9 to −4·8) of all studied Subjotnian basic, intermediate and silicic rocks, require an input from an old (Archaean) low-radiogenic source component, as evidence for Palaeoproterozoic protoliths in the age range 2·5−2·1 Ga is lacking in this region. Crustal, early Svecofennian + Archaean (roughly 30−40%) sources are suggested for the Subjotnian A-type granites and syenites, where the granites derive from undepleted, granodioritic, and the syenites from monzodioritic (±depleted crustal) protoliths. The basic rocks originate from a depleted mantle acquiring the enriched Nd isotopic signatures during interaction with an Archaean lower crust (20−40%), largely depleted after rapakivi melt extraction. Pb isotope data from feldspars (207Pb/204Pb to 15·018−15·542) support the presence of Archaean components in the magmas.The results indicate that an Archaean basement is underlying relatively wide areas of Svecofennian formations in central Sweden. This old basement section was most likely rifted off the Archaean craton in the NE in Palaeoproterozoic times.

Mantle reservoir geochemistry from statistical analysis of ICP-MS trace element data of equatorial mid-Atlantic MORB glasses

We report Inductively Coupled Plasma Mass Spectrometer (ICP-MS) trace element concentration data for 21 elements in 51 basaltic glasses recovered from 58 stations along a 1500-km segment of the equatorial mid-Atlantic Ridge (MAR; 5°N to 5°S). Isotope systematics of these glasses revealed mixing between two isotopically distinct (Schilling, J.-G., Hanan, B.B., McCully, B., Kingsley, R.H., 1994. Influence of the Sierra Leone mantle plume on the equatorial MAR: a Nd–Sr–Pb isotopic study. J. Geophys. Res., 99, 12005–12028.) HIMU and DM mantle sources. Trace element data presented here describe three distinct mantle reservoirs. Based on our new trace element data, we identified three, and possibly four, end-member mantle reservoirs beneath the equatorial MAR. HIMU-type signatures dominate the glasses from the northern equatorial MAR (5°N to 0°) with La/Ba, La/Th and rare earth element (REE) values similar to P-type mid-Ocean Ridge Basalts (MORB; 0.1, 9.11, (La/Yb) N =5.5, respectively). The basaltic glasses from the southern region (0° to 5°S) are mildly radiogenic in Pb and light REE depleted with N-type MORB trace element values for La/Ba, La/Th and REE (0.8, 40.0, (La/Yb) N <0.6, respectively). We estimate the mantle magma-source characteristics and identify the relative contributions of the mantle end-member components in the glasses erupted along the 1500-km transect using the trace element concentration data. Specifically, we recognize the presence of a statistically and geochemically distinct mantle reservoir depleted in Th and Ba relative to average depleted MORB between the Romanche and the Chain fracture zones. To reach the full potential of this data set, we model mantle reservoir compositions using a modification of Q-MODE factor analysis. Multivariate classification and reduction of these data clearly distinguish trace element groups each reflective of mantle source heterogeneities present in this region of the MAR. Statistical analysis of these new trace element and available Nd–Sr–Pb isotope data for the same samples defines four statistically significant distinct end-member mantle components. The binary mixing model developed previously from geochemical data that invoked the mixing of two mantle reservoirs, HIMU and N-MORB, cannot adequately explain the variance in these trace element data. Incompatible element concentrations and ratios best describe the modeled end-members reservoirs, which are HIMU (HPM), transitional N-MORB (TNM), depleted N-MORB (DNM) and an fourth incompatible element-enriched component (EPM). The statistically determined end-members explain 100% of the geochemical variance in the population but their modeled compositions may be related to process controlled variation. We clarify the relationship between statistical and geochemical reservoirs and conclude that a multiple mantle component model involving three of the identified reservoirs is clearly necessary to describe, specifically, the trace element characteristics of the equatorial mid-Atlantic MORBs. Our statistical methods yield tabulated trace element concentration data for each of these the modeled mantle components and provide insights into the MAR system, unavailable by any other technique, by allowing an estimation of the relative contributions of these end-member components in each of the 51 basaltic glass samples.

Elemental and Nd–Sr–Pb isotope geochemistry of flows and dikes from the Tapi rift, Deccan flood basalt province, India

The Deccan Traps are a large rift-associated continental flood basalt province in India, parts of which have been studied extensively in terms of geochemistry, palaeomagnetism and stratigraphy. However, the basalts of the Tapi rift in the central part of the province have been little-studied thus far. Two ENE–WSW-trending tectonic inliers of the Deccan basalts in this region, forming ridges rising from younger alluvium, are made up of basalt flows profusely intruded by basaltic dikes. Both of these ridges lie along a single lineament, although they are not physically continuous. The flows are aphyric, plagioclase-phyric and giant-plagioclase basalts, and the dikes are aphyric or plagioclase-phyric. We consider the two inliers to have been originally continuous, from the presence of bouldery remnants of a major dioritic gabbro dike along both. Samples of this dike from both ridges have previously yielded typical Deccan ages of 65.6±0.5 Ma and 65.6±0.6 Ma by the 40 Ar– 39 Ar incremental heating technique. Initial 87 Sr/ 86 Sr ratios and ε Nd( t) values, and present-day Pb isotopic ratios of most dikes indicate that they are isotopically similar to lavas of the Mahabaleshwar and Panhala Formations of the Western Ghats, about 450 km to the south. Their mantle-normalized trace element patterns have small Pb and Ba peaks. One dike has a strong Bushe Formation affinity and a Nd–Sr isotopic composition more extreme than that of any other Deccan rock yet sampled, with ε Nd( t)=−20.2 and ( 87 Sr/ 86 Sr) t=0.72315 . Its mantle-normalized element pattern shows large Pb, Th and U peaks and large Nb–Ta troughs. Its elemental and isotopic chemistry reflects substantial continental contamination. The flows cut by the Mahabaleshwar-type dikes are isotopically similar to the Poladpur Formation lavas of the Western Ghats. Their mantle-normalized element patterns show modest peaks at Rb, Ba and Pb and rather low Nb and Ta relative to La, indicating that they have been contaminated to intermediate degrees. The mantle-normalized element patterns of all the flows and dikes show enrichment in the light rare-earth elements, with small or no Eu anomalies. The entire flow-dike sequence is similar to the Wai Subgroup of the Western Ghats, in terms of its elemental and isotopic chemistry and stratigraphic relationships. Wai Subgroup-like lavas (i.e., some of the younger magma types originally identified from the southern part of the Western Ghats) are previously known from the central, northern and northeastern Deccan, and many have been thought to be far-travelled flows erupted in the southwestern Deccan. Although at least the dikes, and probably the giant plagioclase basalt flows of our study area, are locally generated and emplaced, our new data extend the known outcrop area of these widespread magma types substantially, and these magma types indeed appear to have a nearly province-wide distribution.

The behaviour of platinum-group elements during magmatic differentiation in Hawaiian tholeiites.

Concentrations of platinum-group elements (PGEs) and gold in basalts from Kilauea and Mauna Loa, Hawaii, were determined by ICP-MS, using an improved fire-assay and tellurium coprecipitation technique for preconcentrating the metals. Major and trace elements, together with Pb-Sr isotope compositions of basalt samples confirmed that these samples are typical Hawaiian tholeiites. The abundances of the PGEs and Au in Hawaiian tholeiites are rather constant during magmatic differentiation processes, whereas MORBs show strong fractionation of those elements. The distinctive behaviour of these PGEs in Hawaiian tholeiites cannot be explained solely via. fractionation of those elements by separation of crystallizing phases such as chromite, olivine and clinopyroxene. Oversaturation of sulfur and separation of sulfides from magmas are required for elucidating PGEs and Au concentrations in Hawaiian tholeiites.

Provenance of Heinrich layers in core V28-82, northeastern Atlantic: 40Ar/ 39Ar ages of ice-rafted hornblende, Pb isotopes in feldspar grains, and Nd–Sr–Pb isotopes in the fine sediment fraction

Several correlatable layers of sediment, rich in ice rafted grains, have been documented in the North Atlantic. The most notable within the last glacial cycle are the Heinrich layers, layers extremely rich in ice rafted detritus and generally barren of foraminifera within the North Atlantic ice rafted detritus (IRD) belt. The view of these layers is that they represent events where great armadas of icebergs were launched into the North Atlantic. The importance of the Heinrich layers lies in their connection with abrupt climate change in the North Atlantic, and perhaps globally. There is a growing number of published provenance studies of the Heinrich layers in the North Atlantic, based on a variety of methods. However, there is little overlap of methods applied to the same samples. In this contribution, we present a multi-component provenance study of Heinrich layers H1, H2, H4 and H5 from core V28-82 in the eastern North Atlantic. Our results indicate that virtually the entire inventory of terrigenous clastic detritus in Heinrich layers H2, H4 and H5 came from ancient continental sources surrounding the Labrador Sea. Although Heinrich layer H1 is similar in many respects, it appears to have some significant differences relative to the other three.

Geochemistry and water dynamics: II. Trace metals and Pb–Sr isotopes as tracers of water movements and erosion processes

A dynamic scheme for water movements over a flood in a small watershed was proposed in a previous paper [Ben Othman, D., Luck, J.M., Tournoud, M.G., 1997. Geochemistry and water dynamics: application to short-time scale flood phenomena in a small Mediterranean catchment: I. Alkalis, alkali-earths and Sr isotopes. Chem. Geol. [140] 9–28] based on major, alkali–alkali earth trace elements and Sr isotopes. In the present paper, metal contents and Pb isotope data are reported for the same samples, and compared to natural and anthropogenic local sources analysed for this purpose. Water treatment plants have stable 206Pb/ 204Pb but show relatively large 207Pb/ 204Pb variations, presumably related to yet unidentified (industrial?) Pb sources. Some Cu–Zn-rich chemicals used extensively on vineyards have unradiogenic 206Pb/ 204Pb around 17.7. Local rains sampled over two years show roughly similar low values. Pb from rocks is variable and more radiogenic ( 206Pb/ 204Pb ≈18.8–22.8). Except for the first hours, trace element concentrations in the dissolved load are similar to or slightly higher than those observed over the year, and similar to other moderately anthropogenic areas [Shiller, A.M., Boyle, E.A., 1987. Variability of dissolved trace metals in the Mississippi river. Geochim. Cosmochim. Acta [51] 3273–3277] for [Pb] (0.05–0.45 nM), [Zn] (10–75 nM) and [Cd] (0.03–0.18 nM). Dissolved [U] and [Co] show simple variations correlated to carbonate, related to local natural sources. Most trace elements in the particulate load are strongly correlated. Lead isotopes in the dissolved and particulate loads show ranges over the flood, again similar to those observed over the year ( 206Pb/ 204Pb ≈17.9–18.3), implying the same sources. The very good alignments observed in 207Pb/ 204Pb and 208Pb/ 204Pb vs. 206Pb/ 204Pb diagrams, especially for the particulate, are interpreted as mixing phenomena. Generally Pb isotopic signature of the dissolved load is less radiogenic than the particulate, indicating differences in sources or proportions and absence of isotopic equilibration with respect to the time of transfer. Pb isotopes shift regularly with time away from the road runoff endmember towards more radiogenic values. The strong negative correlation between Pb isotopes in the particulate load and Sr isotopes in the dissolved load, observed for the first time in a small watershed, probably reflects the local coupling between mechanical and chemical erosion, respectively.

Major, Trace Elements and Sr-Pb Isotopes in Rains: Constraints on Origin

Major, Trace Elements and Sr-Pb Isotopes in Rains: Constraints on Origin

Isotopic and chemical evolution of the post-caldera rhyolitic system at Long Valley, California

Post-caldera rhyolites of the Long Valley magmatic system are chemically less evolved than pre-caldera rhyolites or the initial phases of the Bishop Tuff and record temporal variations in composition which imply open-system magma differentiation involving magma replenishment. All post-caldera rhyolites lie on a well-defined Pb–Sr isotope mixing line between the ∼700 ka Resurgent Dome and the recent Inyo lava domes, precluding a simple cogenetic origin. Coherent temporal trends in Pb and Sr isotopes provide compelling evidence for the near-continuous addition of magma into a silicic magma chamber that was residual after Bishop Tuff eruption. Nd isotope ratios do not record such consistent variations, arguing against their use as a proxy for major additions of new magma and hence as a monitor of potential volcanic hazard. The lowering of 87Sr/86Sr with time demonstrates that there was little crustal interaction and that the Nd isotope composition of the added magmas was variable. The rhyolites of Mammoth Knolls are the most differentiated and define an Rb–Sr isochron of 277±124 ka. These data are consistent with stratification of the magma chamber and subsequent isolation of the upper, more evolved, sections at ∼275 ka. Post 400 ka, rhyolites become chemically more varied, supporting evidence of stratification. The high- and low-silica rhyolites record distinct temporal Sr isotope evolution, implying that the low-silica rhyolites sampled the bulk of the magma chamber whereas the high-silica rhyolites represent the upper isolated, parts of the magma chamber, where they resided for >30 and >300 kyr. After the eruption of the Bishop Tuff, the Long Valley magma chamber was well mixed for the first ∼400 kyr of its evolution and produced rhyolitic magmas at an average of ∼0.0001 km3/year. Upon stratification post 400 ka, tens of km3 of chemically more evolved magma were rapidly produced. Magma addition to the system was at a constant rate and more frequent than eruptions.

Nd, Sr, and Pb isotopic evidence for diverse lithospheric mantle sources of East African Rift carbonatites

Carbonatites may provide valuable information on mantle source compositions as their isotopic ratios are insensitive to crustal contamination. In order to place constraints on mantle sources, nineteen samples from three Miocene to Quaternary carbonatite areas in the East African Rift were analysed for their Sr, Nd, and Pb isotopic compositions. The samples from Kerimasi (northern Tanzania), Homa Mountain, and Wasaki Peninsula (both Lake Victoria, Kenya) as a whole show considerable variations in their isotope ratios (0.70327–0.70502 for 87Sr/ 86Sr, 0.51249–0.51283 for 143Nd 144Nd, 18.72–20.41 for 206Pb/ 204Pb, 15.52–15.78 for 207Pb/ 204Pb, and 39.22–40.47 for 208Pb/ 204Pb) that lie between the inferred compositions for HIMU (high 238U/ 204Pb mantle) and EM I (enriched mantle I) components in most isotope plots. The internal isotopic variations of the three carbonatite areas define distinct arrays and diverse trends in isotope diagrams. Although the isotope data define linear arrays in Sr Nd and Pb Pb diagrams, which suggest binary mixing between HIMU and EM I mantle components, neither the isotopic compositions of the carbonatites as a whole nor the compositional ranges for individual carbonatite occurrences can be explained by such a process. This clearly emerges from the absence of linear data trends in Sr Pb and Nd Pb isotope plots and from the lack of consistent endmember compositions. These features are also displayed by previously published isotope data for East African carbonatites. It is therefore suggested that carbonatite complexes within the East African Rift have isotopically distinct and small mantle sources that are probably not adequately described in terms of the mantle components defined for oceanic basalts. Most likely, these sources are located in a heterogeneous lithospheric mantle and were produced by enrichment and depletion processes at different times and degrees.

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.

Geochemical evolution of rift magmas by progressive tapping of a stratified mantle source beneath the Ross Sea Rift, Northern Victoria Land, Antarctica

Source compositions of Neogene-Quaternary volcanic rocks from the McMurdo Volcanic Group of the Ross Sea Rift in Northern Victoria Land, Antarctica are constrained by NdSrPb isotopes and trace element ratios in near-primary basalts. The rocks erupted along the western rift margin (Victoria Land Basin) and the western rift shoulder (Transantarctic Mountains). Near-primary basalts show no evidence of crustal contamination, suggesting that their initial NdSrPb isotopes reflect the composition of their mantle sources. The initial isotope ratios of near-primary basalts range from about 87 Sr 86 Sr = 0.70281 to 0.70504 and 143 Nd 144 Nd = 0.51269 to 0.51291 ( ϵ Nd(t) = 1.3–5.5). The 206 Pb 204 Pb ratios vary between 19.3 and 20.1. Our results, in conjunction with data from the literature [1–3], suggest the involvement of three mantle source components (or their mixtures) during the formation of Ross Sea magmas: depleted MORB-type mantle, an enriched mantle component (EM), and a component akin to HIMU. The involvement of these mantle components during magma genesis correlates with tectonic setting: whereas MORB-type compositions are restricted to Recent within-rift basalts, EM and HIMU isotope signatures dominate off-rift magmas from the western rift shoulder and Marie Byrd Land respectively. Basalts from the western rift shoulder show temporal isotopic and trace element variations from EM towards HIMU-type signatures between 15 and 5 m.y. On the other hand, within-rift and Marie Byrd Land basalts changed from HIMU-type towards MORB-type compositions through time. These temporal geochemical variations together with the respective tectonic settings of magmatism suggest that the mantle beneath the Ross Sea Rift is stratified in the order MORB- to HIMU- to EM-type sources. It appears that during rift evolution the EM- and HIMU-type sources are consumed and depleted asthenospheric MORB-type mantle rises progressively into the melting region. This suggests that EM resides in the mantle lithosphere. The HIMU-type component may be related to the head of an active or, alternatively, ‘fossilized’ mantle plume attached to the base of the lithosphere beneath the Ross Sea area.

Geochemical characteristics and origin of the Jacupiranga carbonatites, Brazil

New major-, trace-element and Sr, Nd and Pb isotope data are presented for carbonatites and pyroxenites from the 130-Ma-old Jacupiranga complex in southern Brazil. Measured 87Sr 86Sr ranges from 0.7047 to 0.7055, 143Nd 144Nd from 0.51251 to 0.51264 and 206Pb 204Pb from 17.0 to 18.2. Most samples have high measured Th U (5–17), and high 208Pb 204Pb ratios (37.9–40.2) relative to 206Pb 204Pb , with time-integrated Th U > 4 . The pyroxenites have higher initial 206Pb 204Pb (17.63–17.67) and lower initial 87Sr 86Sr (∼ 07047) than the carbonatites (17.05–17.47 and 0.7049–0.7054, respectively). There is a broad negative PbSr isotope array between the carbonatites and the pyroxenites which precludes simple binary mixing, because of their very different Sr/Pb ratios. The isotope differences also appear to preclude models in which the carbonatites segregated by liquid immiscibility from magmas similar to those from which the pyroxenites crystallised, and models in which the isotope arrays are the product of crustal contamination processes. Rather, the initial Sr, Nd and Pb isotope ratios in the Jacupiranga complex, and the negative correlation of Pb and Sr isotope are similar to those in the Paraná high-Ti basalts, and the oceanic basalts of the Walvis Ridge and Tristan da Cunha. Thus, the initial isotope ratios of the Jacupiranga rocks are considered to have been inherited from the mantle source regions associated with incipient magmatism of the Tristan da Cunha hotspot and the opening of the South Atlantic. Finally, the observed variations are used to evaluate suggestions that certain element ratios in upper-mantle rocks, such as high Ca Al and La Yb and low Ti Eu , are features of carbonatite metasomatism. Mantle xenoliths altered by carbonatite metasomatism exhibit elevated Sr contents and low Rb Sr ratios, and most carbonatites have high U Pb , low Rb Sr and Sm Nd . Thus, the intial isotope ratios of the Jacupiranga infiltration and/or metasomatism by carbonatitic melts is one process which may have been responsible for the negative array of U Pb and Rb Sr inferred for the source of certain oceanic basalts.

Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S. 3He/ 4He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R A (R A is the atmospheric ratio of 1.39 × 10 −6 ), encompassing the range of previously reported values for MORB erupted away from high 3He/ 4He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in 3He/ 4He , Pb and Sr isotopes, and trace element ratios such as (La/Sm) N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in 3He/ 4He along the ridge can be used to refine the hotspot source-migrating-ridge sink model. MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by 87Sr/ 86Sr of ∼ 0.7022, Pb isotopes close to the geochron and with 206Pb/ 204Pb of ∼ 17.7, and 3He/ 4He of 8.6–8.9 R A. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between ∼ 20 and 24°S. The He Pb and He Sr isotope relations along the ridge indicate that the 3He/ 4He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of 3He/ 4He ratios in the island lavas. Details of the He Sr Pb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in the He/Pb ratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity. Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.

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.

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

Major- and rare-earth-element (REE) concentrations and U Th Pb, Sm Nd, and Rb Sr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China. The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) Nd Sr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) Sr Pb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) Nd Pb shows a positive correlation, and (4) the 207Pb/ 204Pb vs 206Pb/ 204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb/ 204Pb vs 206Pb/ 204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.

Mantle metasomatism: Isotope and trace-element trends in xenoliths from Kimberley, South Africa

Modal metasomatism in peridotite xenoliths from Kimberley, South Africa, is observed in the development of phlogopite, K-richterite, diopside and LIMA. The average bulk-rock minor- and trace-element contents of different peridotite types range up to 100× primitive mantle, and the bulk-rock compositions reflect the modal proportions of metasomatic minerals in individual samples. Present-day whole-rock 87 Sr 86 Sr =0.7045–0.7118 and 206 Pb 204 Pb =17.48–21.24 , and they have been little affected by interaction with the host kimberlite which has 87 Sr 86 Sr =0.7042 and 206 Pb 204 Pb =18.6–19.2 . The preferred interpretation is that these isotope ratios reflect Rb/Sr and U/Pb fractionation during the modal metasomatism ∼150 Ma ago, i.e. after Karoo magmatism at ∼190 Ma and prior to emplacement of the kimberlite at 90 Ma. Present-day ϵ Nd= −9.3 to +2.4, reflecting both the presence of low-Sm Nd Proterozoic mantle prior to metasomatism at 150 Ma, and the inferred shift to relatively high ϵ Nd during that metasomatism. Rb/SrU/Pb fractionation during metasomatism has resulted in a distinctive negative correlation between present-day 87 Sr 86 Sr and 206 Pb 204 Pb . This contrasts with positive SrPb isotope trends in MORB glasses and selected OIB, but it is similar to that observed in certain CFB provinces. The trend to relatively high Rb/Sr at low U/Pb appears to be linked to the development of a potassic phase, typically phlogopite, and as such it arguably offers the first evidence for a style of minor- and trace-element enrichment which is restricted to shallow levels in the Earth's mantle.

Pb isotope data from late Proterozoic subduction-related rocks: Implications for crust-mantle evolution

Assessment of the role of subduction in the generation of the continental crust and the creation of heterogeneities within the upper mantle is hindered by a lack of knowledge of the associated chemical fluxes. Here we evaluate further the relative fractionation of Th, U and Pb within subduction zones. Pb isotope ratios have been measured in a group of late Proterozoic subduction-related volcanics and intrusions from Saudi Arabia, and in feldspars from selected samples. Feldspar-whole rock pairs are used to calculate time-integrated μ, ω and κ ratios. Similar calculations are offered for the fine-grained rocks from which feldspar could not be separated, by assuming initial ratios based upon the feldspar data from other samples, and models of crust-mantle evolution. These data are used to estimate the U/Pb, Th/Pb and hence Th/U fractionation associated with subduction, and the most striking feature is the low-Th/U signature, which is similar to that measured in many recent island arc basalts. Thus, the data confirm the low-Th/U flux from mantle to crust above modern subduction zones. This contrasts sharply with the high Th/U of average continental crust, and with granites of similar age in the Damara belt of Namibia, which are largely derived from pre-existing continental crust, and have Pb isotope ratios that reflect higher U/Pb, Th/Pb and Th/U. It is concluded that these elevated ratios result from intracrustal differentiation processes. However, no clear correlation is observed between Th/U and Rb/Sr in a range of basalts to granites from the Central Andes, suggesting that variations in these ratios result, at least in part, from different processes. It has recently become popular to invoke subduction as a mechanism for the generation of isotope heterogeneity in mantle remote from present-day subduction zones. We explore this possibility for the Mesozoic flood basalts of Gondwana, which are characterised by high ϵ Sr and low ϵ Nd; a signature apparently derived largely from continental mantle lithosphere. They may be divided into low-Ti and high-Ti groups, and each has distinctive Pb and Sr isotope ratios. None suggest subduction-modified source regions that have evolved in response to a low-Th/U enrichment. However, the low-Ti basalts do have Sr-Pb isotope characteristics that are consistent with addition of subducted sediment to their mantle sources. The high-Ti basalts indicate source regions with high time-integrated Th/U ratios, similar to those of many Dupal OIB. Thus, there is further evidence for a link between the Dupal signature and processes thought to occur in the subcontinental lithospheric mantle sources of some flood basalts.