Isotopic Ratios Of Nd Research Articles

Weathering process at the natural fission reactor of Bangombé

The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found. Most of them are mined out. The reactor zone of Bangombe, is only 10–11 m below the surface. This site has been influenced by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombe during the course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns, the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced by the anomalous isotope ratios of REE because of the fission process. The 149Sm/147Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of 149Sm and subsequent transmutation, whereas 147Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE. Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model.

Country-rock contamination of marginal mafic granulites bordering the Nain Plutonic Suite: implications for mobilization of Sr during high-grade contact metamorphism

The marginal mafic granulites that locally border the Nain Plutonic Suite (NPS) have a range of initial Nd-isotope ratios that overlap with that of the NPS anorthosites and associated Nain dykes. The similarity in Nd-isotope data suggests that gneissic Archaean country rocks have contaminated all the anorthosites, marginal mafic granulites, and dykes. Sr-isotope data for the mafic granulites and dykes support a country rock contamination scenario but preclude wholesale assimilation of rocks such as the host Archaean tonalite gneisses as the sole contaminant. Initial epsilonSrvalues of +10 to +403 and +0.9 to +242 for the mafic granulites and dykes, respectively, are significantly higher than values for NPS country rocks examined thus far. The elevated initial εSrvalues are therefore interpreted to result from the introduction of radiogenic Sr into the granulites and dykes via Sr-rich fluids, generated by the breakdown of Rb-rich mineral phases such as biotite in the country rocks during NPS.

Elemental and isotopic (Sr, Nd, and Pb) characteristics of Madeira Island basalts: evidence for a composite HIMU - EM I plume fertilizing lithosphere

The within-plate volcanic island of Madeira is considered a hot-spot originating from a mantle plume. Lavas range from basanites to mugearites. The majority of primitive lavas are nepheline normative and characterized by strongly fractionated rare-earth elements, where (La/Yb)n = 12-25, and by pronounced normalized negative K but positive Nb-Ta anomalies, similar to high 238U/204Pb (HIMU) type basalts. Madeira basalts are highly depleted in terms of Sr- and Nd-isotope ratios, which are negatively correlated and define a trend steeper than the mantle array. Pb-isotope ratios are moderately high (206Pb/204Pb up to 19.582), with almost all samples plotting close to the Northern Hemisphere Reference Line. Radiogenic isotope systematics demonstrate that the mantle source of Madeira basalts is the result of a multicomponent mixture where HIMU and enriched mantle I (EM I) plume components are diluted by a depleted component (DMM) which represents more than 85% of the source volume. The presence of DMM is considered to result from entrainment of asthenospheric depleted material by the plume. There is trace-element evidence for liquids having equilibrated with garnet and amphibole, the presence of the latter mineral reflecting lithospheric involvement in the genesis of the magmas. Such was done via assimilation, by plume (plus entrained asthenosphere) magmas, of melts generated from oceanic lithosphere and leaving amphibole as residue.

Geochemical and isotopic structure of the early Palaeozoic active margin of Gondwana in northern Victoria Land, Antarctica

The regional distribution of geochemical and isotopic compositions of granitoid rocks from a Gondwana continental margin is studied to highlight its structure and geodynamic evolution. The intrusive rocks emplaced during the early Palaeozoic Ross Orogeny in northern Victoria Land (Antarctica) constitute a high-K calc-alkaline association. The geographic patterns of isotope and geochemical data on granitoid rocks allow the distinction of two portions of the continental margin, separated by a sharp discontinuity. The portion towards the palaeo-Pacific Ocean (Oceanward Side) displays strongly regular inland increase of Sr- and decrease of Nd-isotope ratios, coupled with analogous variations in major and trace elements; on this basis we infer a NW-SE-trending margin affected by SW-directed subduction. The portion towards the East Antarctic Craton (Continentward Side) shows a similar regular variation only for Nd isotope compositions, consistent with a hypothesis of a N-S-striking margin with west-ward subduction. In the Oceanward Side, isotope and trace-element characteristics suggest that the granites were generated by extensive interaction of mantle-derived magmas with high-level crustal melts. The origin of Continentward Side intrusives is compatible with a process of interaction between mantle-derived melts and a mafic granulite lower crust. The granitoids of the two crustal sectors share the same Proterozoic Sm-Nd model ages, suggesting that they both belong to the same crustal province. We interpret this arrangement of crustal segments as due to the shift and rotation of a forearc sliver of the Gondwana margin. This movement was likely enhanced by oblique subduction under an irregular margin weakened by the presence of a magmatic arc.

Isotopic constraints on the influence of the Icelandic plume

Thermally buoyant mantle, in the form of a plume, rises beneath Iceland creating a major topographic anomaly along the Mid-Atlantic Ridge and in the surrounding ocean basin. However, the influence of the Iceland plume on the composition of lavas erupted on adjacent ridges remains a contentious issue. Trace element systematics and radiogenic isotope ratios of Sr, Nd and Pb suggest that the plume influences a region 1200 km in length. In contrast, the 3He anomaly associated with Iceland closely corresponds to the 2400 km ridge section affected by thermal uplift. We present evidence that the Sr, Nd and Pb isotope signature of the Iceland plume is in fact as widespread as its thermal and 3He anomalies. Results imply that much of the source of North Atlantic ridge basalts has been contaminated by lateral outflow of asthenosphere from the Icelandic plume. Consequently, estimates of the average composition of mid-ocean ridge basalt (MORB) sources are likely to be biased by including data from plume-contaminated regions. True MORB values, and perhaps upper mantle geochemistry, can be constrained only by considering data untainted by plume asthenosphere.

A strontium and neodymium isotopic investigation of the Laramie anorthosites, Wyoming, USA: Implications for magma chamber processes and the evolution of magma conduits in Proterozoic anorthosites

A strontium and neodymium isotopic investigation of plagioclase-rich cumulate rocks in the 1.43 Ga Laramie anorthosite complex (LAC) provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of Proterozoic anorthosites. The 725 km 2 LAC intruded the boundary between Archean rocks of the Wyoming Province in the north and Proterozoic island arc terranes in the south, the Cheyenne belt, at pressures of 3–4 kilobars. Initial strontium and ε ND isotopic ratios from the three large composite anorthositic intrusions (Poe Mountain, Chugwater, and Snow Creek) and late troctolitic intrusions range from 0.7034–0.7055 and +0.9 to −4.9, respectively. The isotopic ratios in the cumulates vary as a result of (1) the isotopic composition of the parental mantle-derived high-Al gabbroic magmas, (2) the amount of contamination by Archean rocks during ascent through the crust, and (3) mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement. Strontium isotopic and normative An variations across the 5–7 km thick stratigraphic section of layered plagioclase-rich cumulates in the 200 km 2 Poe Mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in Proterozoic anorthosites. The earliest and stratigraphically lowest intrusion was emplaced as a plagioclase-rich magma that crystallized large volumes of compositionally homogeneous anorthosite. At least three subsequent chamber-wide episodes of magma replenishment, followed by mixing, are indicated by abrupt shifts in I Sr and/or normative An in the stratigraphically higher levels of the Poe Mountain anorthosite. Strontium and neodymium isotopic disequilibrium between a high-Al clinopyroxene megacryst and the layered cumulates is consistent with a high-pressure origin for Al-rich pyroxene megacrysts in Proterozoic anorthosites. The megacryst crystallized in a magma chamber at pressures of 10–12 kilobars from relatively uncontaminated basaltic parent magma (high-Al gabbro) and was transported through the crust in progressively contaminated anorthositic magma. The range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high-Al gabbroic dikes in the LAC, supporting the proposition that high-Al gabbros are parental to anorthosite (Mitchell et al., 1995). The isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by Archean orthogneisses and/or metapelitic rocks. Decreasing I Sr and increasing ε Nd with a relative decrease in age of the intrusions (Snow Creek → Poe Mountain/Chugwater → troctolites) indicates that the magma conduit became increasingly insulated from crustal contamination over time. This study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal/upper mantle pressures, during the ascent of magma diapirs, and within individual magma chambers in Proterozoic anorthosites, and underscores the need for stratigraphic control when addressing the origin of plagioclase-rich cumulates.

Sources of continental magmatism adjacent to the late Archean Kolar Suture Zone, south India: distinct isotopic and elemental signatures of two late Archean magmatic series

Conspicuous Nd, Sr and Pb isotopic differences exist between the Archean gneiss terranes adjoining the suture at the Kolar Schist Belt, south India. These gneisses, which are the deformed equivalents of plutonic and volcanic rocks, have known or inferred igneous ages of 2630 to 2530 Ma. Initial isotopic ratios of Nd, Sr and Pb suggest that metaplutonic gneisses west of the Kolar Schist Belt were emplaced into, and variably contaminated by, an evolved continental crust that formed prior to 3200 Ma. Felsic metaigneous gneisses that occur as slivers on the western margin of the schist belt have an isotopic character similar to that of the metaplutonic rocks on the same side of the Kolar Schist Belt. On the east side of the Kolar Schist Belt the isotopic evidence suggests that the 2530 Ma granitic gneisses were not derived from or contaminated by an older continental crust. Their source probably evolved with a Nd isotopic composition similar to that of typical Archean mantle, but became light rare earth element enriched after 2900 to 2700 Ma. The inferred tectonic setting for the west side of the Kolar Schist Belt is an Andean continental magmatic arc. For the east side of the Kolar Schist Belt, a possible Phanerozoic analog is an evolved island arc, such as Japan.

Exchange of neodymium and its isotopes between seawater and small and large particles in the Sargasso Sea

The concentrations and isotopic ratios of Nd were measured in small (1–53 μm) and large (> 53 μm) in situ filtered particles, sediment trap material, sediment, and aerosol samples, collected during oligotrophic and productive conditions in the Sargasso Sea. Atmospheric input is the main source of exogeneous Nd into the Sargasso Sea. Based on Nd Al ratios in trapped material, we suggest that up to 50% of atmospheric Nd could dissolve at the air/ sea interface. The concentration and isotopic composition of Nd vary with both depth and the particle size. Neodymium concentrations of small and large particles increase with depth (from 2.9–12 μg/g and 5.8–11.8 μg/g, respectively). At 40 m, the Nd signature of large particles is significantly more negative ( ϵ Nd(0) = -13.1 ± 0.2) than that of the small ones ( ϵ Nd(0) = -11.7 ± 0.3 and the particles filtered deeper. Below 237 m, the ϵ Nd(0) values for small particles match the seawater, whereas the ϵ Nd(0) values of large particles remain less radiogenic. These differences are related to the different behaviors of the particulate fractions. Small particles apparently exchange Nd with seawater and then sink rapidly to depth as biologically formed aggregates (large particles). These aggregates isolate the small particles, slowing the exchange with the water to produce different signatures between large particles and seawater. Since these exchange processes modify the particulate Nd signature, the shallow radiogenic ϵ Nd(0) signature observed in the surface and subsurface waters in the NW Atlantic is not due to external contributions and is likely of advective origin. At depth below 800 m, the magnitude of the aggregation/ disaggregation process varies significantly with season; the fraction of small particles repackaged into large aggregates can vary from 0–8% to 80–100% in productive and oligotrophic conditions, respectively. This seasonal variation may correspond to sinking particles of different zooplanktonic origin. Vertical fluxes of Nd into and out of the Sargasso Sea, as well as exchange fluxes in the water column are examined. About 50% of the particulate flux from the surface is remineralized at depth; a net scavenging flux of 0.8 10 −4 g/m2/y is required. This value corresponds to a scavenging Nd residence time of ∼ 1900 y, considerably longer than previous estimates of global oceanic residence time of Nd on the order of 400 y.

A Strontium and Neodymium Isotopic Investigation of the Fongen--Hyllingen Layered Intrusion, Norway

The Fongen—Hyllingen Intrusion, situated ∽60 km SE of Trondheim, Norway, is a synorogenic layered mafic intrusion of Caledonian age 426-2+8⁠. The intrusion is divided into four evolutionary stages based on cryptic variations: Stage I—a basal reversal; Stage II—unchanged mineral chemistry or slight normal evolution; Stage III—a gradual regression; Stage IV— a strong normal fractionation trend Magma replenishment dominated during most of the crystallization, i.e. during Stages I, II and III Replenishing magma was more dense than resident, evolved magma, and continuing influx eventually caused a compositionally stratified magma column to form. Cryptic lateral variation is an important feature in the southern part of the complex and formed by in situ crystallization from a stratified magma along an inclined floor, where modal layering formed parallel to the crystallization front. Initial Sr- and Nd-isotopic ratios in the cumulates vary as a result of assimilation of country rock and subsequent mixing between uncontaminated, replenishing magma and contaminated, resident magma. The parental magma had a moderately depleted isotope composition, relative to Bulk Earth, with εNd=5·84 and Sri=0·70308, whereas the main contaminant was a partial melt of metapelitic country rock with εNd=-8·74 and Sri=0·7195 (Sri is the initial 87Sr/86Sr). Sri in the analysed cumulate whole-rock samples ranges from 0·70308 to 0·70535 and initial εNd ranges from. 1·58 to 5·84. There is a strong correlation between mineralogical composition and isotopic trends in most of the cumulates: the most primitive samples are the least contaminated, as reflected by relatively high ed and low Sr,, and more evolved samples have progressively lower eNi and higher Sry A gradual regression of several hundred metres thickness characterizes Stage III; stratigraphically upwards mineral compositions become more primitive and isotope compositions more depleted (higher εNd and lower Sri), implying a process of. progressive mixing-in of replenishing, primitive and uncontaminated magma. Magma influx in Stage III took place by fountaining, whereas magma addition was more tranquil in the earlier stages. The fountaining influx entrained resident, relatively evolved and contaminated magma, resulting in a hybrid magma which ponded at the floor. During prolonged magma addition with concomitant crystallization, the lowermost magma layer was replaced by progressively more primitive hybrid magma, creating a gradual regression in the crystallizing cumulate sequence. A detailed two-dimensional study reveals lateral variations in mineral compositions both at the base and top of Stage III, whereas lateral variations in Sr- and Nd-isotopic compositions are present at the top, but not at the base. This implies that the lowest crystallizing part of the magma column was essentially isotopically homogeneous, but compositionally stratified, before influx in Stage III. Isotopic gradients in the magma were strong close to the roof, where most of the assimilation occurred, and decreased downwards, merging into isotopically homogeneous magma. This stratified system was destroyed by turbulent mixing between replenishing and resident magma during fountaining influx in Stage III, and a new stratification was established with both an isotopic and a compositional gradient. After the final influx, crystallization continued in an essentially closed system, in which the remaining magma column eventually became homogenized, as magma layers mixed when their densities converged owing to release of buoyant, residual liquid during fractional crystallization.

Ce isotope ratios of N-type MORB

Isotope ratios of Ce, Nd and Sr, and light rare-earth elements (La, Ce, Nd and Sm) concentrations were measured for sixteen N-type MORB samples from DSDP Sites 420, 421, 429, 597B and 597C (East Pacific Rise), 504B (Costa Rica Rift), and 395, 519A, 522B, 562 and 564 (Mid Atlantic Ridge). All the samples were confirmed as N-type MORB based on Nd and Sr isotope ratios, and LREE patterns. The Ce isotope ratios ranged from 0.0225616 (ϵCe = − 1.6) to 0.0225633 (ϵCe = − 1.0). Averages of ϵCe and ϵNd of N-type MORB were − 1.4 and 9.5, respectively. The average of ϵCe is quite different from previous estimates. We propose that ϵCe = − 1.4 should be used as the typical MORB Ce isotope ratio in future studies.

Petrogenesis of Mesozoic Potassic Magmatism of the Central Aldan: A Sr-Nd Isotopic and Geodynamic Model

The Mesozoic potassic magmatism of the Central Aldan (southeastern Siberian Platform) consists of volcanic and intrusive rocks of diverse composition, including highly potassic alkaline (leucitites, phonolites, lamproites, phologopite pyroxenites, shonkinites, feldspathoid syenites) and moderately alkaline (trachytes, gabbros, monzonites, syenites) rocks. They have a different level of alkalinity, as well as special petrochemistry and mineralogy. In addition, the distribution of incongruent elements in the rocks practically is identical: they are enriched in large-ion lithophile elements (LILE) and light rare-earth elements (LREE) and impoverished in high-field strength elements (HFSE) and TiO2. The Sr- and Nd-isotopic ratios in the potassic rocks of the Centra l Aldan vary considerably. They have low values of eNd(T) (−10.2 to −22.9) and (87Sr/86Sr)0 = 0.7048 0 t o 0.70723; that is, they differ from Chondriti c Uniform Reservoir (CHUR) values in th e field of an enriche d mantle source of enriched mantl ...

The relationship between alkaline magmatism, lithospheric extension and slab window formation along continental destructive plate margins

Abstract Two distinct groups of Late Cenozoic (15-<0.1 Ma), OIB-like basalts are recognized along the Antarctic Peninsula. Small volumes of undersaturated basanites, tephrites, alkali and olivine basalts and ‘within-plate’ tholeiites, which post-date the cessation of subduction as a result of ridge crest-trench interactions by 40 to <10 Ma, are scattered along much of the southern part of the peninsula. At James Ross Island, in the north of the peninsula, alkali basalts, hawaiites and rare mugearites were erupted synchronously with subduction at the South Shetland Islands trench to the west. Both groups of basalts are remarkably similar in terms of their geochemical and isotopic characteristics, although they apparently owe their origin to two distinct combinations of tectonic processes. The south Antarctic Peninsula basalts are causally related to the cessation of subduction, the formation of slab windows and upwelling and decompressional melting of sub-slab asthenosphere. Correlated trace element-isotope systematics demonstrate that the slab window-related basalts exhibit little evidence for interaction with subduction-enriched mantle or continental lithosphere and must have been derived from sub-slab mantle that had not recently been affected by subduction; LILE/HFSE ratios (e.g. Th/Ta 1.0–2.5, Rb/Nb, 0.25–1.5, Ba/Nb 2.5–7.0) and Sr- and Nd-isotope ratios ( 87 Sr/ 86 Sr 0.7027–0.7035, 143 Nd/ 144 Nd 0.51286–0.51296) are well within the range for ocean island basalts. The slab window-related basalts underwent rapid uprise from the mantle to the crust accompanied by limited fractional crystallization. The syn-subduction alkalic basalts of James Ross Island were generated during slab roll-back, probably related to the formation of the extensive slab window to the south. The James Ross Island Volcanic Group (JRIVG) exhibits evidence of polybaric fractionation including ponding at the base of the crust. Again, there is little evidence for interaction of these asthenospheric magmas with subduction-enriched mantle or continental lithosphere, although in detail, there are recognizable differences between the JRIVG and the slab windowrelated basalts. Melting at James Ross Island was facilitated by slab roll-back and associated lateral and vertical asthenospheric migration into the locus of a pre-existing area of attenuated lithosphere (‘thinspot’). This phase of extensional tectonism was probably originally initiated during the Late Jurassic, but other periods of extension may have taken place in latest Cretaceous times. Neither of these suites of basalts were generated as a result of significant lithospheric extension and passive asthenospheric upwelling on a regional scale. In addition, there is no evidence for the existence of a mantle hotspot beneath the region. The commonly held assumption that alkaline volcanism along consuming plate margins results from periods of significant inter-arc extensional tectonism is, therefore, not necessarily valid. The unique tectono-magmatic regime resulting from the formation of slab windows is probably the only setting in which small degree melts of MORB source asthenosphere not associated with a plume are generated and preserved in the geological record.

Isotopic constraints on the timing of crustal accretion of the Bamble Sector, Norway, as evidenced by coronitic gabbros

This paper presents the first SmNd data for coronitic gabbros (“hyperites”) from the high-grade Proterozoic Bamble Sector, Norway. SmNd whole-rock isochron ages of the Vestre Dale Gabbro, 1.11±0.14 Ga, the Jomåsknutene Gabbro, 1.77±0.19 Ga, and the Flosta Gabbro, 1.64±0.23 Ga indicate that, contrary to earlier ideas, intrusion of these gabbros is not exclusively bound to the Sveconorwegian orogeny (1.2-0.9 Ga). Some of the gabbros were already emplaced at the onset of or during the Gothian Orogeny (1.75-1.5 Ga). Initial ϵ Nd values of about +5.5 of Jomåsknutene and Flosta Gabbros provide clear evidence of a depleted mantle reservoir beneath this part of the Southwest Scandinavian Domain. This highly positive value differs from the Nd-isotope evolution curve for the Southwest Scandinavian mantle as postulated by Mearns et al. (1986), which assumes that depletion of the mantle beneath this part of Scandinavia started at c. 1.74 Ga. However, the initial ϵ Nd value of the Vestre Dale Gabbro plots close to this curve, whereas 143Nd 144Nd ratios of the mantle sources of the Tromöy and Arendal Gabbros evolved with approximately chondritic 144Sm 144Nd ratios; their ϵ Nd (1.1 Ga) values range from +5.1±0.3 to +6.0±0.3. Gothian metasomatism of a depleted mantle source by LILE and LREE-bearing fluids provides a mechanism to obtain low Nd-isotope ratios in magmas represented by the Vestre Dale Gabbro. Examples of low Nd-isotope magmas have also been found in the Telemark Sector, and in southern Sweden. The marked depletion in Nb indicates that the metasomatic fluids were possibly related to Gothian subduction.

Genesis of the pliocene to recent bimodal mafic-felsic volcanism in the Debre Zeyt area, central Ethiopia: volcanological and geochemical constraints

The Debre Zeyt volcanic district is located about 40 km southeast of Addis Ababa, on the western shoulder of the Ethiopian Rift. Two main phases of volcanic activity are shown to have occurred in the area, both younger than the main episodes of rifting. Eruption of acid lavas and pyroclastics, between 4 and 1 Ma, brought to the formation of the central volcanoes of Yerer, Bede Gebabe and Zikwala. Younger activity generated basaltic cinder cones, maars and lava flows, all aligned along the main rift direction. A few intermediate and acid volcanics are associated with the younger basalts. Petrological and geochemical investigations have shown that the rocks from central volcanoes consist of peralkaline rhyolites and trachytes with a few trachyandesitic lava flows. All these rocks have high concentrations of Rb, Zr, LREE and other incompatible elements, and variable Ba and Sr contents. Measured Nd isotopic ratios are close to the bulk earth value, whereas Sr isotopic ratios are very variable, due to the combined effects of the high to extreme Rb/Sr values, variable ages and, possibly, different initial isotopic signatures. Basalts range from transitional to weakly alkaline in composition and display relatively homogeneous incompatible element contents and Nd and Sr isotopic ratios. Incompatible element ratios such as Ba/Rb are very variable in the basalts. The younger acid rocks have a large range of incompatible elements concentration with some rhyolites displaying very low values of some hygromagmaphyle trace elements (HYGE), such as Zr, Nb and LREE. The younger intermediate rocks have comparable HYGE contents as the basalts and define linear trends between basalts and low-HYGE rhyolites on several interelement variation diagrams. Geochemical modelling indicates that the major and trace element composition of the acid rocks from central volcanoes can be satisfactorily explained by a derivation from basaltic parents by fractional crystallization. Nd-isotopic ratios which, for the largest part, fall within the range of younger basalts suggest that, except for the Yerer rhyolites, the assimilation of the upper continental crust did not play a major role during magma evolution. The relatively constant HYGE contents of basaltic and intermediate younger volcanics exclude an evolution by fractional crystallization for this suite. The linear trends on inter-element diagrams suggest that mixing processes between basaltic magmas and an acid rock or liquid may be responsible for the generation of the intermediate rocks. This hypothesis is supported by Ba/Rb vs. Rb relationships revealing hyperbolic mixing trends between acid and basaltic end-members. This process also generated important geochemical variations within the basalts.

Isotope and trace element evidence for three component mixing in the genesis of the North Luzon arc lavas (Philippines)

Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in 87Sr/86Sr (0.70327–0.70610), 143Nd/144Nd (0.51302–0.51229) and 208Pb*/206Pb* (0.981–1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high Δ8/4, and to a lesser extent high Δ7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5–5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS ‘subduction component’ interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (<5%) of subducted S China Sea terrigenous sediment. The 87Sr/86Sr ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with 143Nd/144Nd and 208Pb*/206Pb*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.

Ce and Nd isotope geochemistry on island arc volcanic rocks with negative Ce anomaly: existence of sources with concave REE patterns in the mantle beneath the Solomon and Bonin island arcs

138Ce/142Ce isotope ratios in Cenozoic island arc volcanic rocks are reported for the first time, together with isotope ratios of Nd and Sr and abundances of REE, Ba and Sr. The island arc volcanics studies here are boninites from Chichijima, the Bonin Islands, and basalts and andesites from the Solomon Islands. REE patterns of the island arc volcanic rocks from the Solmon Islands and the Bonin Islands are confirmed to have negative Ce anomalies. It is also disclosed that the majority of these island arc volcanic rocks show mainly positive values for both eCe and eNd. It is shown that these eCe and eCe values can hardly be interpreted by simple mixing between MORB and oceanic or continental crustal rocks; the former have positive eNd and negative eCe and the latter have negative eCe and positive or negative eNd. Existence of sources having positive eCe and eNd values is strongly suggested. If the sources are assumed to have been fractionated from CHUR (chondritic uniform reservoir) at the early or middle Precambrian era, the sources from which the volcanics were derived are concluded to have kept concave REE patterns with larger (La/Ce)N and smaller (Nd/Sm)N ratios than chondritic values over a substantial period of time, until the time of Cenozoic magmatism forming island arc volcanic rocks in question. During the periods of the Cenozoic magmatic activities and their related events, Ce anomalies are considered to have been created. From Ce and Nd isotope ratios, however, it is difficult to determine which of the following processes was responsible for the Ce anomaly; the incorporation process of subducted oceanic crust into magma at the mantle or the slab dehydration and metasomatism process. Nevertheless, so far as Ce and Nd isotopic ratios are concerned, incorporation of oceanic sediments did not take place to any clearly detectable degree.

Calibration of Faraday cup efficiency in a multicollector mass spectrometer

Faraday cup efficiencies (FCE's) in the multicollector mass spectrometer were examined with a static multicollection technique. The relative Faraday cup efficiency (RFCE) was estimated by measurement of Nd-isotope ratios with five different cup configurations. The application of RFCE correction to the static multicollection technique will increase the reliability of analytical results by eliminating the errors caused by differences in FCE's, and will also enable us to distinguish which Faraday cups are damaged and should be renewed.

Isotopic studies of processes in mafic magma chambers: II. The Skaergaard Intrusion, East Greenland

Isotopic ratios of Nd and Sr have been measured in a suite of samples spanning most of the exposed stratigraphy of the Skaergaard intrusion in order to detect and quantify input (such as assimilated wallrock and fresh magma) into the magma chamber during crystallization. Unlike δ18O and δD, Nd and Sr isotope ratios do not appear to have been significantly affected by circulation of meteoric waters in the upper part of the intrusion. Variations in initial 87Sr/86Sr and eNd suggest that the Skaergaard magma chamber was affected during its crystallization by a small amount (2%–4%) of assimilation of Precambrian gneiss wallrock (high 87Sr/86Sr, low eNd) and possibly recharge of uncontaminated magma. Decreases in eNd and increases in 87Sr/86Sr during the early stages (0%–30%) of crystallization give way to approximately unchanging isotopic ratios through crystallization of the latest-deposited cumulates. Modelling of assimilation-fractional crystallization-recharge processes using these data as constraints shows that the assimilation rate must have been decreasing throughout crystallization. In addition, the isotope data allow replenishment by an amount of uncontaminated magma equal to 20%–30% of the total intrusion mass, occurring either continuously or in pulses over the first 75% of crystallization. Comparison of the recharge models with published Mg/(Mg+Fe2+) data from Skaergaard cumulates shows that the modelled replenishment rates are not inconsistent with available major element data, although significant recharge during the final ∼25% of crystallization can be ruled out. The isotope data show that the Skaergaard magma could have incorporated only a small amount of the gneiss that it displaced from the floor of the chamber; assimilation appears to have taken place primarily across a partially molten zone that formed at the roof from the wallrock that was dislodged during emplacement. In the latest stages of crystallization (>75% crystallized), the Skaergaard magma may have become stratified into two separately-convecting layers, effectively insulating Layered Series cumulates from further contamination.

The origin and evolution of magmas from the San Pedro-Pellado volcanic complex, S. Chile: multicomponent sources and open system evolution

The San Pedro-Pellado volcanic complex is located at 36° S in the Chilean Andes. The eruptive rocks of the complex record the development and collapse of a caldera, followed by voluminous, largely basaltic andesite, volcanism. At each stage of evolution, crystal fractionation was accompanied by variable degrees of contamination and mixing. Large variations in incompatible element ratios cannot be produced by closed system evolution. Correlations between indices of differentiation and incompatible element ratios, together with high δ 18O values, indicate that basaltic andesites have assimilated crust to generate the evolved volcanic rocks at San Pedro-Pellado. Even in the most mafic rocks, however, incompatible element characteristics are variable as a result of source heterogeneity and deep level processes. The restricted ranges in isotope ratios of Sr, Nd and Pb among San Pedro-Pellado rocks are due to the small contrast in isotopic compositions between magma and wallrock. Three source components are recognized as contributing to parental magmas at San Pedro-Pellado. Although the relative contributions of each cannot be quantified, the volumetrically dominant source component is the sub-arc asthenospheric mantle (MORB source). The major source of LILE is thought to be slab-derived fluids which modified the sub-arc mantle. Other incompatible elements may also have been enriched by interaction with the continental lithosphere (mantle and/or lower crust) during ascent.

Hydrogen and oxygen isotope variations in the high himalaya peraluminous Manaslu leucogranite: Evidence for heterogeneous sedimentary source

The Manaslu granite belongs to the High Himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting. δD and δ 18 O values have been determined for whole rock and coexisting minerals from the ~8 km thick Manaslu massif and its 50 km long dyke sheet, its country rocks and the Formation 1 (F1) paragneisses which are the source of the granite. For the granite, δD musc range from −70 to −85%. and δ 18 O W.R. from 10.9 to 12.8%. H and O-isotope fractionations among minerals are consistent with high temperature equilibrium and, for oxygen, closed system evolution. A few samples, coming mainly from the dyke swarm, have very low δD values (down to −188%.) and biotite-muscovite H-isotope fractionations indicative of disequilibrium; the D H ratios of associated magmatic tourmaline are essentially unmodified. From the distribution of δD values in the granite and its country rocks, circulation of very low deuterium meteoric hydrothermal waters was extremely localized. Because these waters are depleted in deuterium by up to 50%. relative to modern meteoric waters, the Manaslu area was either at an altitude substantially higher than that of today (2500–6000 m for the analyzed samples) or a mountain chain once existed to the south. The F1 gneisses have δ 18 O quartz between 12 and 14.3%. which confirms that the granite was generated from F1, but δD values are ≈20%. higher than in the granite. Such a difference can be a result of degassing of the magma and/or introduction of fluid in the melting zone. Infiltration of low δD fluid (≈−90%.) into the hot but dry F1 probably triggered partial melting; these fluids could have come from the dehydration of the Midlands sediments which are separated from the overlying F1 by the Main Central Thrust. The correlations among δ 18 O, ( 87Sr 86Sr) 20 Ma and ϵNd values in both F1 and the granite indicate that the variations of these isotopic ratios in the Manaslu are inherited from those in F1 at the time of melting. In turn, these ratios in F1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of Nd and O, and to the quantity of radiogenic Sr generated within the sediment, which is a function of age and Rb content (amount of phyllosilicate). Some other Himalayan leucogranites require either other crustal source rocks or the δ 18 O and 87Sr 86Sr ratios of F1 vary along the Himalaya.