Trace Element Systematics Research Articles

U-decay series and trace element systematics in the 1978 eruption of Ardoukoba, Asal rift: timescale of magma crystallization

230Th– 238U, 226Ra– 230Th, 231Pa– 235U disequilibria, Th and Sr isotopes, trace and major elements were measured in a tholeiitic basalt series erupted within a single week in November 1978 at the Ardoukoba Volcano (Asal rift, Djibouti). ( 230Th/ 238U) and ( 231Pa/ 235U) activity ratios are constant for all the lavas, within analytical uncertainties, and are equal to 1.42 and 2.20, respectively. ( 226Ra/ 230Th) activity ratios in the whole rocks decrease with Th content from 1.93 to 1.35. Trace elements measured in plagioclase and groundmass show that all the lavas display a similar proportion of accumulative plagioclase (∼30%). Once corrected for plagioclase accumulation, the trace element variations can be modeled as a single fractional crystallization series which requires 30% crystallization (49% plagioclase, 39% olivine and 12% clinopyroxene). However, this fractional crystallization cannot explain the whole range in ( 226Ra/ 230Th). 226Ra radioactive decay must also be taken into account. Consequently, this basaltic series also represents an age series evolving from the older magma (#544) to the most recent magma (#546). To constrain magma crystallization ages from ( 226Ra/ 230Th) data, two end-member models are proposed. The first one corresponds to several magma batches crystallizing continuously as closed systems. The second model corresponds to a zoned chamber crystallizing continuously, and regularly replenished with fresh parental magma. The fraction of crystallized magma per unit of time is 3.3×10 −4 yr −1 for the open-system, and 3.6×10 −4 yr −1 for the closed-system. Model ages of crystallization of 1880 yr (±80 yr) and 870 yr (±30 yr) are inferred from the open- and the closed-system model, respectively.

Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle

The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt, and Pd were determined in 18 mantle-derived basalts from a variety of tectonic settings and six komatiites from three locations. All analyses were performed using isotope dilution, Carius tube digestion, and the precise technique of multiple collector inductively coupled plasma mass spectrometry. Multiple analyses of two samples indicate external reproducibilities, based upon separate dissolutions, of approximately 2–9% in the ppt to ppb concentration range. Mid-ocean ridge basalts from the Kolbeinsey Ridge, tholeiites from Iceland and alkali basalts from the Cameroon Line define three individual sample suites that are characterized by distinct major, trace, and platinum-group element systematics. All three-sample suites display correlations of the PGE with MgO, Ni, and Cr. The new analytical results are employed to constrain the geochemical behavior of the PGE during the formation and differentiation of mantle–derived melts. The PGE are inferred to be compatible in sulfides during partial melting with sulfide-silicate melt partition coefficients of ∼1 × 10 4. The fractionated PGE patterns of mantle melts are a consequence of the incompatibility of Pd in nonsulfide phases, whereas Ir and Ru must be compatible in at least one other mantle phase. Model calculations indicate that PGE alloys or spinel may be responsible for the higher compatibility of the latter elements during partial melting. It is further demonstrated that the shape of the melting regime has a profound effect on the PGE systematics of mantle magmas. The systematic trends of the three sample suites in plots of PGE against Ni and Cr are the result of magma differentiation processes that involve fractional crystallization of silicate minerals and the concurrent segregation of an immiscible sulfide liquid. The behavior of the PGE during magma fractionation indicates that the segregated sulfides probably equilibrate with >90% of the silicate magma and that PGE scavenging by sulfides is best described by a combination of batch and fractional equilibrium partitioning.

Corals at their latitudinal limits: laser ablation trace element systematics in Porites from Shirigai Bay, Japan

The rapid analytical technique of laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) was used to measure the trace elements B, Mg, Sr, Ba and U in a high-latitude coral colony ( Porites lobata) taken from Shirigai Bay, Japan (32°N). A wide range of sea surface temperatures (SSTs 14.5–28°C) and upwelling events influenced this coral. Cold winter SSTs caused a decrease and/or cessation of skeletal extension. Measurements of U/Ca and Sr/Ca indicate an approximately linear response to SSTs above 18°C and a non-linear response below 18°C. Mg/Ca and B/Ca measurements both showed annual cycles broadly consistent with SST variations, but also exhibited intra-annual fluctuations not associated with temperature, suggesting that the incorporation of Mg and B into the coral skeleton was not simply regulated by temperature. It is shown that Ba/Ca ratios provide a proxy for wind-induced seasonal upwelling. This is inferred from the strong correlations between the strength of zonal winds ∼1 month prior to the SST minimum and the Ba/Ca maximum. Secondary upwelling events occurred during the summers of 1982, 1987, 1991 and 1992. These summers were cooler than average and were associated with El Niño Southern Oscillation events.

Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions

The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary eruptive vents in addition to the large, composite volcanoes of Popocatepetl, Iztaccihuatl, and Nevado de Toluca. The eruptive vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous in the Sierra Chichinautzin that forms the southern terminus of the Valley of Mexico. The Chichinautzin volcanic field (CVF) is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites, andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO, and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements. Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust. In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub-alkaline basaltic melts.

Rare earth element systematics in scheelite from hydrothermal gold deposits in the Kalgoorlie-Norseman region, Western Australia

Scheelite (CaWO 4 ) is a widespread accessory mineral associated with hydrothermal veins in the Archean gold deposits of the Kalgoorlie-Norseman region of Western Australia. Rare earth element (REE) and other trace element abundances in scheelite have been determined in situ by excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) in order to constrain the composition and sources of the mineralizing fluids. The scheelites can be grouped according to the two distinct types of chondrite-normalized REE (REE N ) patterns which they exhibit: hump-shaped (type I) and flat (type II). Type I scheelites have higher total and Na concentrations than type II samples and can be further subdivided into type Ia which has maximum REE N concentrations between Sm and Gd, and type Ib which has maximum REE N concentrations displaced toward Dy. Type I scheelites are dominant at Coolgardie, Kalgoorlie, and Kambalda, whereas type II samples are most abundant at Norseman. A few scheelite grains exhibit both type I and type II characteristics.The two types of REE N patterns with different Na abundances suggest that the trivalent REE substituted into the Ca site of scheelite by two different mechanisms: 2Ca (super 2+) = REE (super 3+) + Na (super +) for the type I scheelites and 3Ca (super 2+) = 2REE (super 3+) + Ca (where Ca is a Ca site vacancy) for the type II samples. The parabolic shape of REE N patterns in the type I scheelites can be used to calculate the ionic radius of the REE (super 3+) that preferentially substitutes into the scheelite structure. When type I REE patterns are normalized to account for variations in light REE abundance, the average value obtained is 1.060 Aa for type Ia and 1.055 Aa for type Ib scheelites. These values are smaller than the value of 1.12 Aa for the Ca site but are in excellent agreement with the value of 1.06 Aa predicted by the coupled Na (super +) -REE (super 3+) substitution mechanism, assuming size as well as charge compensation. This result implies that Na (super +) and REE (super 3+) substitute into adjacent sites in the crystal structure of type I scheelites. In contrast, the incorporation of REE into type II scheelites appears to be independent of size. It is suggested that in this type of scheelite, involvement of a Ca site vacancy relaxes the size constraint on the substitution of the REE (super 3+) . The influence of REE speciation in the hydrothermal fluid on the systematic difference between scheelite-fluid partition coefficients for type I and II scheelites is of secondary importance compared with crystal chemistry effects.The distinctive REE N patterns and high Na contents of type I scheelites imply that they crystallized from hydrothermal fluids with higher Na activities than those that formed type II samples. Type I scheelites exhibit no changes in the size of the Eu anomaly with REE concentration, implying a predominance of Eu (super 3+) and crystallization under relatively oxidized conditions. Type II scheelites have variable Eu anomalies and trivalent REE concentrations and thus appear to contain mostly Eu (super 2+) and to have formed under reduced conditions. Type Ia and II scheelites have (Ce/Lu) N > 1 and are interpreted to have crystallized from light REE-enriched fluids, whereas type Ib scheelites with (Ce/Ln) N < 1 formed from light REE-depleted fluids.The fluid characteristics inferred from the REE and trace element systematics in the scheelites can be attributed to the rock types and estimated physical conditions in the Kalgoorlie-Norseman region at the time of hydrothermal activity. High Na activities are consistent with buffering of fluids by greenschist facies mafic rocks, whereas low fluid Na activities are a product of fluid-rock reactions at higher metamorphic grades. Oxidized fluids are consistent with near-surface derivation, whereas reduced fluids are indicative of generation at elevated temperatures and pressures. The range and distribution of fluid (Ce/Ln) N ratios are similar to those of the major rock types in the region.

Namurian bentonites in the Pennine Basin, UK – origin and magmatic affinities

Nine Namurian clay bands retrieved from boreholes in the northern part of the Pennine Basin are, on the basis of their petrography, mineralogy and geochemistry, shown to be volcanic in origin and are therefore bentonites. The bentonites, which have a fragmental texture, are normally graded and show rare preservation of shard textures, representing vitric tuff deposits that have been altered subsequently to clay‐dominated horizons. Crystals are a minor component of the bentonites, but biotite, in particular, is concentrated at the base of the beds. A clay mineral assemblage of mixed‐layer illite–smectite with subordinate kaolinite identifies most of the samples as K‐bentonites, but kaolinite dominates two samples that can be classed as tonsteins. Temporal variation of salinity within the depositional basin is suggested to explain these different clay assemblages. The major element geochemistry of the bentonites reflects their clay mineralogy and the compositions of diagenetic minerals present, the latter including pyrite, carbonates and hydroxyapatite. Enrichment of the bentonites in some trace elements (including Ba, Sr, Pb, Cu and Ni) can be related to the presence of the diagenetic minerals, but the extent to which the elements are added from external sources as opposed to being redistributed within the ash is unclear. Immobile trace element systematics suggest a rhyodacite/dacite composition for the original ash and derivation from the collision of plates, this being supported by evidence provided by the rare earth elements (REE) in one group of samples. However, in another group of samples, variations in REE concentrations may be caused by mobility of these elements during alteration. The chemistry of the Namurian bentonites contrasts markedly with that of the local Carboniferous volcanics but is comparable, in some respects, with one group of Westphalian tonsteins, although the latter are more rhyolitic in character. It is suggested that the Namurian bentonites and the Westphalian tonsteins of acid affinities originated from volcanic activity associated with a destructive plate margin in the Variscan externides and that the observed compositional trend may reflect magma evolution possibly related to the progressive east–west closure.

Trace element systematics of ultramafic and mafic volcanic rocks from the 3 Ga North Caribou greenstone belt, northwestern Superior Province

High precision trace element data from ultramafic and mafic volcanic rocks from two units of the ∼3 Ga North Caribou greenstone belt in the northwestern Superior Province reveals new evidence for the contamination of early Archæan komatiites. Spinifex-textured komatiites (MgO=20−-27 wt.%) of the Opapimiskan–Markop unit are characterised by flat to moderately enriched light rare earth element (LREE) trace element patterns (La/Sm n=1–2) in conjunction with negative Nb and Ti anomalies (Nb/La pm=0.48–0.59; Ti/Ti*=0.6–0.8), consistent with contamination of a primary Munro-type Al-undepleted ultramafic melt by felsic material compositionally similar to Archæan tonalitic-trondhjemitic-granodioritic (TTG) suites. The trace element characteristics of these komatiites are distinct from both Proterozoic and Phanerozoic arc and rift picrites. Mg-tholeiites intimately associated with the komatiites display variable rare earth elements (REE) (La/Sm n=0.8–1.5) and Nb and Ti anomalies also consistent with variable degrees of contamination (Nb/La pm=0.58–0.98; Ti/Ti*=0.6–1.0). An intermediate suite of high-Mg andesites (54–60 wt.% SiO 2) associated with both the tholeiites and the komatiites can be derived by fractional crystallisation of a melt with the trace element characteristics of the most contaminated komatiites. Compositionally uniform Mg- to Fe-tholeiites of the South Rim unit are believed to overlie the Opapimiskan–Markop unit, possibly in stratigraphic contact. They are characterised by flat trace element patterns (La/Sm n=0.9–1.0), are uncontaminated, and comparable with modern day oceanic plateaux, such as Ontong Java. The two units likely represent the eruptive products of a single mantle plume that entrained tholeiitic melts from the upper mantle before impinging upon the Archæan crust at ∼3 Ga.

Trace element systematics of Mg-, to Fe-tholeiitic basalt suites of the Superior Province: implications for Archean mantle reservoirs and greenstone belt genesis

Magnesian-, to Fe-rich tholeiitic basalts with near flat REE patterns, locally associated with komatiites, are the prevalent rock type in several areally extensive volcanic sequences of the 2.9–2.7 Ga Archean greenstones belts of the Superior Province. In each of five greenstone belt sequences in the Uchi, Wabigoon, Wawa and Abitibi subprovinces, there is a continuous trend in compositions from high to low Mg# (total range, 65 to 21) accompanied by decreasing Cr and Ni contents, but increasing contents of Fe, Th, Nb, Zr, Hf, REE and Y. The total range of La/Sm n is 0.60 to 1.40 and HREE patterns are generally flat. Collectively the basalts have a total range of Nb/La pm from 0.60 to 1.95, and Th/La pm from 0.61 to 2.36; in three of the sequences there is a negative correlation of Nb/La pm with La/Sm n. Alteration, crustal contamination, thermal assimilation of altered ocean crust, melting processes, and fractional crystallization can all be ruled out as the cause of the spectrum of Nb and Th anomalies. Rather, the anomalies are interpreted in terms of a heterogeneous multi-component mantle plume: positive anomalies represent a residual slab component from oceanic crust processed through a subduction zone and recycled into the mantle, whereas the negative anomalies reflect a recycled slab-derived lithosphere component. These components were likely reactivated in a mantle plume, and/or entrained in a plume. Consequently, greenstone belt formation was preceded by ocean floor spreading, arc formation and recycling into the mantle. These tholeiitic basalt sequences are directly analogous to basalts of Phanerozoic ocean islands or ocean plateaus, such as Iceland and Ontong Java, many of which also possess ranges of Nb/La pm and Th/La pm interpreted as mixing of different mantle components. In two of the belts there is clear evidence for intercalated coeval plume and calc-alkaline arc magmatism. Some Mg- to Fe-tholeiitic basalts in these belts have pronounced negative Nb-anomalies where Nb/La pm<0.6; compositionally these basalts are similar to Phanerozoic primitive arc, or back-arc tholeiites. Jamming of plume-related plateaus into primitive arcs may govern the transition to calc-alkaline sequences.

Major- and trace-element systematics and isotope geochemistry of Cenozoic mafic volcanic rocks from the Vogelsberg (central Germany): Constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources

The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.

Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera

A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle. The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle. In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes [Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the lithospheric mantle.

Mid-Ocean Ridge Melting: Constraints from Lithospheric Xenoliths at Oahu, Hawaii

by melting a lherzolitic source. Instead, they reflect mixing between One plagioclase–spinel lherzolite and four spinel lherzolite xenoliths two components: (1) the pooled melt derived from lherzolitic source from Oahu, Hawaii, contain clinopyroxene grains that show homoin a passive melting regime with an Fmax of 30% (~80% of the geneous rare earth element (REE) abundances and smooth REE mixture) and (2) melt derived from garnet pyroxenite by 40% patterns with systematic depletion of light REE (LREE). These fractional melting (~20% of the mixture). This model produces five xenoliths are mid-ocean ridge basalt (MORB) magma-depleted 7·1 km of crust. residues with compositions that were not modified by later metasomatism. Trace element systematics of these xenoliths were used to investigate the melt production rate (dF/dP) within a 90-my-old residual mantle column (RMC). Such rates were calculated as

Mid-Ocean Ridge Melting: Constraints from Lithospheric Xenoliths at Oahu, Hawaii

One plagioclase–spinel lherzolite and four spinel lherzolite xenoliths from Oahu, Hawaii, contain clinopyroxene grains that show homogeneous rare earth element (REE) abundances and smooth REE patterns with systematic depletion of light REE (LREE). These five xenoliths are mid-ocean ridge basalt (MORB) magma-depleted residues with compositions that were not modified by later metasomatism. Trace element systematics of these xenoliths were used to investigate the melt production rate (dF/dP) within a 90-my-old residual mantle column (RMC). Such rates were calculated as ratios of the difference in extents of depletion (dF) to the difference in equilibrium pressures (dP) between two xenolith samples. The extents of melting were modeled from REE, Sr and Zr abundances in clinopyroxene; and equilibrium pressures were inferred from the two-pyroxene geobarometer of Mercier et al. (1984, Contributions to Mineralogy and Petrology, 85, 391–403). Equilibrium pressures range from 21 to 7 kbar and extents of melting vary from 2 to 8%. Together, these data constrain the maximum extent of melting in the garnet lherzolite stability field to be <2% and a dF/dP of 0.43%/kbar within the stability field of spinel lherzolite. Uncertainties in the estimates of equilibrium pressure and extent of depletion lead to a slightly broader range of dF/dP values (0.26-0.78%/kbar). These values are significantly lower than that of ∼1.2%/kbar suggested by most previous studies. With the best estimated mean dF/dP of 0.43%/kbar, only 3.9 km of crust could have been generated by melting lherzolite in the pressure range of 26–7 kbar. The thickness and composition of the crust that overlies the 90 Ma Oahu RMC require a higher extent of melting. Based on the REE abundances, most samples from the 90 Ma crust and East Pacific Rise can be explained as pooled melts derived from the lherzolitic source in a passive melting regime with a maximum extent of melting (Fmax) of 30%. This model produces a dF/dP of 4.4%/kbar in the pressure range of 7–2 kbar and generates an additional 1.8 km of crust. In detail, the high [Sm/Yb]DM ratios in most East Pacific Rise samples cannot be explained by melting a lherzolitic source. Instead, they reflect mixing between two components: (1) the pooled melt derived from lherzolitic source in a passive melting regime with an Fmax of 30% (∼80% of the mixture) and (2) melt derived from garnet pyroxenite by 40% fractional melting (∼20% of the mixture). This model produces 7.1 km of crust.

Komatiite-Tholeiite and Boninite Series Volcanic Associations of the Abitibi Greenstone Belt: Plume-Protoarc Interaction, and Crustal Growth

Komatiite-tholeiite sequences and boninite series volcanic rocks are spatially associated at several localities along a 300 km strike sector of the late Archaean (2.7 Ga) Abitibi greenstone belt, Canada. Examples include the Kidd volcanic complex (KVC) and volcanic sequences in Tisdale and Whitney Townships near Timmins, Ontario, as well as volcanic sequences near Val d'Or, Quebec. In the field, the different types of flows interdigitate on a scale of 10's to 100's metres. Komatiites include Munro, or Al-undepleted type, and Barberton, or Al-depleted type. Spinifex textured Al-undepleted komatiites at the KVC and Whitney localities, like Munro counterparts, are characterized by near-chondritic A1203/TiO2, Ti/Zr, and Zr/Y ratios. They have (1) near-fiat HREE, (2) variable L R E E depletion (La/Smn 0.4 to 0.8), and (3) small positive Hf anomalies relative to Sm, with Zr/Hf 400 km, whereas Al-undepleted komatiites reflect melt segregation at shallow levels. Mg-, Fe-tholeiites are spatially associated with the komatiites. They have near-flat R E E patterns over a continuous range from 4 to 20x chondritic abundance. Mg#, Cr, and Ni decrease continuously with increasing REE, Th, Nb, Zr, Hf and Y. The most primitive have near-chondritic interelement ratios. This magma series is interpreted to represent dynamic partial melting in the outer envelope of the mantle plume from which the komatiitic liquids were derived. Collectively, the tholeiites have a continuous range from negative to positive anomalies of Nb relative to Th and La, similar to Phanerozoic and Recent ocean plateau basalts and some ocean island basalts (Saunders et al., 1988). Alteration and crustal contamination can be ruled out as the source of the anomalies. The negative anomalies are consistent with recycling of subduction-influenced subarc mantle, or continental crust, into the mantle, and the positive anomalies recycling of ocean crust processed through a subduction zone (Saunders et al., 1988; McDonough, 1991). Consequently, the plume from which the komatiites and tholeiitic basalts were derived was multi-component, as demonstrated from trace element and isotope systematics on Phanerozoic and Recent plume-related basalts (Mahoney et al., 1993). Compositionally, the boninite series flows range from Mg# = 81-45, TiO2 = 0.17-0.41, and Ni = 1020-310 ppm. They are characterized by high A1203 (12-16 wt.%) contents, and A1203/TiO2 (60-84) ratios. In common with Phanerozoic boninites, these flows have negatively fractionated H R E E (Gd/Ybn = 0.4-0.6), Zr/Y (1.4-2.2) less than the primitive mantle value of 2.4, and variably positive Zr and Hf anomalies, where Zr/Hf 53% and Mg# > 60 given by Crawford et al. (1989). Collectively, the boninite series overlap spinifex texatred Al-undepleted komatiites in terms of TiO2 and Zr contents, but are much more aluminous, possess higher A1203/ TiO2 ratios and Sc and Yb contents, but lower Gd/ Ybn and Zr/Y ratios. The boninites are interpreted to have formed in a protoarc from second stage metasomatic enrichment of a mantle source highly

Noble Gas Systematics of Iceland: Small Scale Investigation, Large Scale Interpretation

Iceland presents a unique opportunity for the study of mantle domain interactions and crustal evolution. The spatial and temporal concurrence of the Icelandic mantle plume with the North Atlantic spreading centre has produced a sub-aerial topographic high culminating in oceanic crust 9-16 km thick. Plume interaction with the North Atlantic N-MORB source is however debated and the presence of a depleted NMORB signature in Icelandic magmatism is not universally accepted. Traditional Sr-Nd-Pb and trace element systematics offer conflicting viewpoints to this question.

Subsolidus REE partitioning between pyroxene and plagioclase in cumulate eucrites: An ion microprobe investigation

In an attempt to elucidate the relationship between the cumulate eucrites and the noncumulate (main series) eucrites, we have examined the trace-element systematics of plagioclase grains that coexist with inverted pigeonites of the cumulate eucrites. Analyses were done by secondary ion mass spectrometry (SIMS). Plagioclase compositions are uniform in trace-element abundances within grains from each cumulate eucrite, but are variable between the four analyzed: Moama, Moore County, Serra de Magé, and Binda. Shapes of CI-normalized trace-element abundance patterns of the plagioclase grains of the cumulates are typical of plagioclase, showing a LREE-enriched pattern with positive Eu anomalies reflecting the site preference of Eu 2+ in the feldspar structure. Using trace-element concentrations of the pyroxenes (orthopyroxene, pigeonite, augite; Pun and Papike, 1995) and plagioclase (this study), we calculate the subsolidus partition coefficients (Ds) for the REEs and Y between plagioclase and pyroxenes in the examined cumulate eucrites. Trace-element systematics suggest that igneous trace-element signatures have been altered by subsolidus exchange for both the plagioclase and pyroxene. Calculated hypothetical parental-melt patterns determined from pyroxene and plagioclase are different and can differ by a factor of 4–5 for individual REEs.

Phenocryst and melt inclusion chemistry of near-axis seamounts, Valu Fa Ridge, Lau Basin: insight into mantle wedge melting and the addition of subduction components

Phenocryst assemblages, and mineral and melt inclusion compositions of magmas erupted at near-axis seamounts on either side of Valu Fa Ridge provide a hitherto unprecedented insight into the complexity of magma generation in this back-arc basin tectonic setting. Two fundamentally different primitive primary melt compositions are identified based on melt inclusion compositions, olivine phenocryst chemistry, and the early co-crystallisation of either magnesian clinopyroxene (Mg# to 93) or magnesian orthopyroxene (Mg# to 93.5) with magnesian olivine (to Fo 94) and Cr-rich spinel (Cr# = 0.78–0.87). One magma type is a H 2O-rich (∼ 2.5 wt%), high-CaO (∼ 14 wt%), low-Al 2O 3 (∼ 8 wt%) magnesian basalt, variants of which occur in both the eastern and western seamounts. The other is a low-Ca boninite-like magma that only occurs as a component of the western seamount magmas. Large and systematic variations in incompatible trace-element compositions of melt inclusions trapped in primitive olivine phenocrysts, reflect an integration of diverse but geochemically related melt fractions to produce the magmas at each seamount. Trace-element systematics require the variable addition of a LILE-, Pb-, and Cl-rich component to the mantle wedge source with increased influence toward the Tofua arc. This component, as invoked in most models of arc magma genesis, is likely to be a supercritical aqueous fluid released by dehydrating subducting ocean crust beneath the volcanic arc front. We propose that southward propagation of the back-arc basin spreading center mantle provided heat necessary to generate both magmatic suites by decompression melting of refractory hydrated sub-arc lithosphere, probably veined by clinopyroxene-rich dykes in the case of the high-CaO magma series. These near-ridge seamount lavas are very similar to those drilled at ODP Site 839 in the Lau Basin, and we suggest that the Site 839 basalts, as well as other Lau Basin seamount arc-like magmas, were produced from sub-arc lithosphere during southward propagation of the Eastern Lau Spreading Center ∼ 2–3 Ma.

Th–Sr–Nd–Pb isotope and trace element evidence for the origin of the São Miguel, Azores, enriched mantle source

A suite of young alkali basalts from across the island of São Miguel has been analyzed for major and trace element concentrations and Sr, Nd, Pb and Th isotopic compositions in order to document the intra-island variation, and to constrain the processes involved in the formation of the enriched mantle sources of these lavas. As shown in previous studies, large isotopic and trace element variations correlate with geographic location across São Miguel. New Th isotope data show a similar correlation, with ( 230Th)/( 232Th) varying from 1.094 to 0.877. These variations are attributed to two-component mixing, where the western end of the island is similar to the source of the Azores Platform tholeiites, and the eastern end of the island has a more enriched, EMII-type source. Trace element systematics indicate that the São Miguel enriched mantle source is enriched relative to the predominant Azores plume source in elements including K, Rb, Th, U, Pb and Zr, but not Ba, Nb or Ti. Incorporation into the Azores plume of approximately 5% subduction-processed terrigenous sediment can explain many of the trace element and isotopic characteristics of the São Miguel enriched mantle source. However, this mechanism cannot easily explain the constant Cs/Rb ratios across the island, the concave downward trend of Ba/Th vs. Ti/Zr, and the recent decrease in Th/U of the São Miguel enriched mantle source documented by Th and Pb isotope systematics. An alternative model is that the São Miguel enriched mantle source is subcontinental lithospheric mantle. This is supported by the similarity of the trace element signatures of the São Miguel enriched mantle source to those of hydrous metasomatized mantle xenoliths. In particular, the low Ba/Rb signature of the São Miguel enriched mantle, which is unusual for oceanic basalts, is a characteristic of many hydrous metasomatized mantle xenoliths and magmas believed to be derived from such sources. We propose that this subcontinental lithospheric mantle resided originally beneath northwestern Africa or Iberia but was delaminated during rifting upon the opening of the Atlantic Ocean basin, and that it is presently a shallow, localized contaminant that is melted due to an influx of heat from the Azores plume.

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.

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.

Unequilibrated eucrites and the equilibrated Juvinas eucrite; pyroxene REE systematics and major, minor, and trace element zoning

Other| December 01, 1996 Unequilibrated eucrites and the equilibrated Juvinas eucrite; pyroxene REE systematics and major, minor, and trace element zoning Aurora Pun; Aurora Pun University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, United States Search for other works by this author on: GSW Google Scholar James J. Papike James J. Papike Search for other works by this author on: GSW Google Scholar American Mineralogist (1996) 81 (11-12): 1438–1451. https://doi.org/10.2138/am-1996-11-1216 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Aurora Pun, James J. Papike; Unequilibrated eucrites and the equilibrated Juvinas eucrite; pyroxene REE systematics and major, minor, and trace element zoning. American Mineralogist 1996;; 81 (11-12): 1438–1451. doi: https://doi.org/10.2138/am-1996-11-1216 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract In unequilibrated eucrites, pyroxene is a sensitive recorder of variables that affect their crystallization histories. Therefore, major, minor, and trace element systematics of pyroxene were evaluated from several unequilibrated eucrites and an equilibrated eucrite, Juvinas, by EMP and SIMS techniques. The Cr, Al, and Ti zoning trends in pyroxene reflect its igneous crystallization history, including the sequence of crystallization of pyroxene and plagioclase. The primary substitutional couples in unequilibrated pyroxene among Cr, Al, and Ti are [6]Cr3+-[4]Al3+, [6]A13+-[4]A13+, and [6]Ti4*-2[4]Al31, with the [6]Ti4+-2[4]Al3+ couple dominating over [6]Al3+-[4]Al3+ following the onset of plagioclase crystallization. Reequilibration erases the primary igneous zoning trends of Cr, Al, and Ti, and the dominant substitutional couples in homogenized pyroxene are [6]Ti4+-2[4]Al3+ and [6]Cr3+-[4]Al3+ in equal proportions. Trace element compositions in unequilibrated eucrites are variable because of igneous zoning, but averaged analyses show core-rim differences that reflect their Ca-rich rims relative to Ca-poor cores. The pyroxene REE abundances in the unequilibrated samples are as much as ten times chondrite in their rims, whereas homogenized Juvinas pyroxenes have abundances between the core and rim values of pyroxene from unequilibrated eucrites.The ratio of REEs (homogenized pyroxene/bulk composition) for Juvinas is significantly higher than the ratio of REEs (pyroxene cores/bulk composition) for the unequilibrated eucrites. This is especially true for the HREEs. The higher observed REE concentrations in the equilibrated pyroxene result from the intracrystalline exchange between Ca-poor cores and Ca-rich rims and also from intercrystalline exchange with other phases, including plagioclase, phosphates, and mesostasis. Use of the equilibrated pyroxene REE concentrations and igneous D values leads to an incorrect estimate of the parental melt composition. The estimated chondrite-normalized melt REE concentrations are too high by a factor of two to four. These studies suggest that great care must be used to take into account subsolidus elemental redistributions when attempting to estimate parental melt compositions from measured mineral trace element concentrations. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.