The Galápagos and Hawai'i archipelagos are formed by mantle plumes originating at the large low shear velocity province (LLSVP) boundary. We report new high‐precision Pb, Sr, Nd, and Hf isotopic ...analyses on 83 Galápagos samples and compare them with those of Hawai'i. The data confirm that like Hawai'i, Galápagos is a bilaterally asymmetric plume whose compositional boundary trends NW‐SE. On their northeast sides, the plumes share a common source, Pacific lower mantle, whose intermediate isotopic signature may be common to many plumes. The Hawaiian and Galápagos plumes' southwestern sides are anchored in the Pacific LLSVP and are compositionally distinct; in Hawai'i, Loa trend lavas reflect contributions from the EM1 mantle end‐member, whereas in Galápagos, HIMU is dominant, suggesting that the Pacific LLSVP is compositionally heterogeneous and includes different types of recycled material. Furthermore, the surficial expression of a bilaterally asymmetric plume is strongly influenced by its tectonic setting: (a) Thick Hawaiian lithosphere supports a volcano evolution process, including rejuvenated volcanism, whereas the thin Galápagos lithosphere inhibits Hawai'i‐style rejuvenated‐stage eruptive activity, instead causing extended, widespread volcanism; (b) the proximity of the Galápagos to a mid‐ocean ridge causes entrainment of the depleted upper mantle, overwhelming depleted material intrinsic to the plume and affecting volcanoes' magmatic architecture; and (c) the geometric relationship between the LLSVP boundary and plate motion influences geochemical patterns at the surface. Thus, despite striking differences in surficial expression of the Galápagos and Hawai'i plumes, they share a common generation mechanism, supplied by the Pacific LLSVP and the lower mantle.
Plain Language Summary
The Galápagos and Hawaiian Islands are formed by mantle plumes, which provide an opportunity to document the compositional structure of the lower mantle. The Hawai'i plume ascends from the core–mantle boundary along the interface between two major mantle reservoirs, the Pacific large low shear velocity province (LLSVP), and the surrounding lower mantle. The LLSVP is a region at the core–mantle boundary that seismic velocities suggest is denser and hotter than the rest of the mantle. The Galápagos is also located along the LLSVP‐lower mantle interface, in the eastern Pacific. Prior to this study, the Galápagos plume was thought to be quite different from the one supplying Hawai'i, primarily because the distribution of geochemical compositions across the islands forms two parallel volcanic chains in Hawai'i, a pattern not observed in the Galápagos. Our new geochemical data from 83 lavas across the Galápagos indicate that the Galápagos plume also exhibits bilateral compositional asymmetry, but it is expressed as two broad zones that cross the archipelago instead of parallel chains. We propose that the Hawai'i and Galápagos plumes are generated by the same mechanism, consisting of parallel filaments of compositionally distinct material rising from the boundary of the LLSVP with the lower mantle.
Key Points
The Galápagos and Hawai'i plumes are bilaterally asymmetric in composition, sourced at the LLSVP‐lower mantle interface
The Galápagos and Hawai'i plumes share a common mantle source, the Pacific lower mantle
The Pacific large low shear velocity province is heterogeneous, with a variety of ancient recycled material (HIMU‐ and EM1‐type)
Controlling the accuracy and precision of geochemical analyses requires the use of characterized reference materials with matrices similar to those of the unknown samples being analyzed. We report a ...comprehensive Pb‐Sr‐Nd‐Hf isotopic and trace element concentration data set, combined with quantitative phase analysis by XRD Rietveld refinement, for a wide range of mafic to ultramafic rock reference materials analyzed at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. The samples include a pyroxenite (NIM‐P), five basalts (BHVO‐2, BIR‐1a, JB‐3, BE‐N, GSR‐3), a diabase (W‐2), a dolerite (DNC‐1), a norite (NIM‐N), and an anorthosite (AN‐G); results from a leucogabbro (Stillwater) are also reported. Individual isotopic ratios determined by MC‐ICP‐MS and TIMS, and multielement analyses by HR‐ICP‐MS are reported with 4–12 complete analytical duplicates for each sample. The basaltic reference materials have coherent Sr and Nd isotopic ratios with external precision below 50 ppm (2SD) and below 100 ppm for Hf isotopes (except BIR‐1a). For Pb isotopic reproducibility, several of the basalts (JB‐3, BHVO‐2) require acid leaching prior to dissolution. The plutonic reference materials also have coherent Sr and Nd isotopic ratios (<50 ppm), however, obtaining good reproducibility for Pb and Hf isotopic ratios is more challenging for NIM‐P, NIM‐N, and AN‐G due to a variety of factors, including postcrystallization Pb mobility and the presence of accessory zircon. Collectively, these results form a comprehensive new database that can be used by the geochemical community for evaluating the radiogenic isotope and trace element compositions of volcanic and plutonic mafic‐ultramafic rocks.
Key Points:
Pb‐Sr‐Nd‐Hf isotopic and trace element data set of mafic‐ultramafic rock reference materials
First report of isotopic compositions for many reference materials
Assessment of leaching effects on basalts
To evaluate metallurgical processing as a source of Zn and Cd isotopic fractionation and to potentially trace their distribution in the environment, high-precision MC-ICP-MS Zn, Cd and Pb isotope ...ratio measurements were made for samples from the integrated Zn–Pb smelting and refining complex in Trail, B.C., Canada. Significant fractionation of Zn and Cd isotopes during processing of ZnS and PbS ore concentrates is demonstrated by the total variation in
δ
66/64Zn and
δ
114/110Cd values of 0.42‰ and 1.04‰, respectively, among all smelter samples.
No significant difference is observed between the isotopic compositions of the Zn ore concentrates (
δ
66/64Zn
=
0.09 to 0.17‰;
δ
114/110Cd
=
−
0.13 to 0.18‰) and the roasting product, calcine (
δ
66/64Zn
=
0.17‰;
δ
114/110Cd
=
0.05‰), due to ∼
100% recovery from roasting. The overall Zn recovery from metallurgical processing is ∼
98%, thus the refined Zn metal (
δ
66/64Zn
=
0.22‰) is not significantly fractionated relative to the starting materials despite significantly fractionated fume (
δ
66/64Zn
=
0.43‰) and effluent (
δ
66/64Zn
=
0.41 to 0.51‰). Calculated Cd recovery from metallurgical processing is 72–92%, with the majority of the unrecovered Cd lost during Pb operations (
δ
114/110Cd
=
−
0.38‰). The refined Cd metal is heavy (
δ
114/110Cd
=
0.39 to 0.52‰) relative to the starting materials. In addition, significant fractionation of Cd isotopes is evidenced by the relatively light and heavy isotopic compositions of the fume (
δ
114/110Cd
=
−
0.52‰) and effluent (
δ
114/110Cd
=
0.31 to 0.46‰). In contrast to Zn and Cd, Pb isotopes are homogenized by mixing during processing. The total variation observed in the Pb isotopic compositions of smelter samples is attributed to mixing of ore sources with different radiogenic signatures.
Oceanic sediments deposited at high rate close to continents are dominated by terrigenous material. Aside from dilution by biogenic components, their chemical compositions reflect those of nearby ...continental masses. This study focuses on oceanic sediments coming from the juvenile Canadian Cordillera and highlights systematic differences between detritus deriving from juvenile crust and detritus from old and mature crust. We report major and trace element concentrations for 68 sediments from the northernmost part of the Cascade forearc, drilled at ODP Sites 888 and 1027. The calculated weighted averages for each site can then be used in the future to quantify the contribution of subducted sediments to Cascades volcanism. The two sites have similar compositions but Site 888, located closer to the continent, has higher sandy turbidite contents and displays higher bulk SiO2/Al2O3 with lower bulk Nb/Zr, attributed to the presence of zircons in the coarse sands.
Comparison with published data for other oceanic sedimentary piles demonstrates the existence of systematic differences between modern sediments deriving from juvenile terranes (juvenile sediments) and modern sediments derived from mature continental areas (cratonic sediments). The most striking systematic difference is for Th/Nb, Th/U, Nb/U and Th/Rb ratios: juvenile sediments have much lower ratios than cratonic sediments. The small enrichment of Th over Nb in cratonic sediments may be explained by intracrustal magmatic and metamorphic differentiation processes. In contrast, their elevated Th/U and Nb/U ratios (average values of 6.87 and 7.95, respectively) in comparison to juvenile sediments (Th/U~3.09, Nb/U~5.15) suggest extensive U and Rb losses on old cratons. Uranium and Rb losses are attributed to long-term leaching by rain and river water during exposure of the continental crust at the surface. Over geological times, the weathering effects create a slow but systematic increase of Th/U with exposure time.
► The continental source of Cascadia basin sediments has juvenile characteristics. ► Modern sediments eroded from juvenile crust and cratons have different compositions. ► High Th/U and Th/Rb characterize old, mature and weathered continental crust. ► Weathering leads to massive U and Rb losses in the continental crust. ► Th/U and Th/Rb increase with exposure time at the surface of continents.
Among volcanic hot spots globally, Hawaii has the highest magma flux, yet there is significant controversy surrounding the composition of the mantle sourcing Hawaiian lavas. In order to place ...constraints on the source lithologies of Hawaiian lavas, we explore relationships between major elements and radiogenic isotopes in tholeiitic, shield‐building lavas. Olivine‐fractionation corrected lava compositions reveal clear trends between radiogenic isotopes and major elements. Individual data points exhibit remarkable trends and there is no need to average the data by volcano. Data form arrays that are anchored by Koolau lava at one end (with high 87Sr/86Sr, 187Os/188Os, SiO2, and Na2O/TiO2, and low 143Nd/144Nd, 206Pb/204Pb, TiO2, CaO and CaO/Al2O3) and by Kea and Loihi lavas at the other (with low 87Sr/86Sr, 187Os/188Os, SiO2, and Na2O/TiO2, and high 143Nd/144Nd, 206Pb/204Pb, TiO2, CaO and CaO/Al2O3). FeOtotal, Al2O3 and Na2O concentrations do not correlate with radiogenic isotopes. The Hawaiian data set exhibits correlations that mirror the best correlations between major elements and radiogenic isotope in the global ocean island basalt (OIB) database. We suggest that the mechanism driving the correlations in Hawaii illustrates, in microcosm, a larger global process that generates major element variability in mantle plumes. Like the global arrays, the Hawaiian lavas with radiogenic Pb and SiO2‐poor lavas are sourced by a SiO2‐poor mafic component (pyroxenite) admixed with peridotite, while Hawaiian lavas with unradiogenic Pb and high SiO2 are sourced by a SiO2‐rich mafic component (eclogite). The variable SiO2 in the mafic component may result from different degrees of SiO2‐extraction from the slab during subduction.
Key Points
Major elements correlate with radiogenic isotopes in Hawaiian lavas
The correlations help constrain the source lithologies in the Hawaiian plume
The correlations in Hawaii follow, in microcosm, the larger global correlation
Hawaiian volcanoes record 6 Ma of potentially deep mantle chemistry and form two parallel volcanic chains that are geochemically unique, named Loa and Kea. Loa volcanoes erupt lavas with isotopically ...enriched compositions thought to reflect the presence of recycled material in the deep mantle source of the Hawaiian plume. Variations in stable thallium (Tl) isotopes have been used to trace recycled pelagic ocean sediment from subduction to eruption in arc and intraplate lavas. Previous work attributed heavy Tl isotopic compositions in eight Loa samples to recycled sediments in their source. We reexamined this hypothesis using a large sample set (n = 34) of shield‐stage, tholeiitic basalt from 13 Hawaiian volcanoes representing the entire range of isotopically enriched and depleted compositions along the Hawaiian chain. Samples were acid‐leached prior to isotopic analysis to remove post‐eruption alteration and resulting ε205Tl values show statistical differences between Loa and Kea volcanoes. Corresponding isotopic data and re‐analyzed trace element concentrations suggest that the ε205Tl values are primary magmatic signatures. Possible co‐variations between heavy ε205Tl and oxygen isotopes in samples from Kea‐trend volcanoes could reflect the presence of ancient, recycled pelagic sediment on the Kea side of the Hawaiian plume, which samples the average deep Pacific mantle. As such, the deep mantle source of both Loa and Kea Hawaiian volcanoes may contain recycled materials of different natures and recycling histories, which supports work from both geophysical and geochemical studies suggesting that the Earth's lower mantle is chemically heterogeneous on multiple spatial scales.
Plain Language Summary
The Hawaiian volcanoes form two parallel geographic and geochemical trends, named Loa and Kea, that are produced by a deep mantle plume originating at the core‐mantle boundary. Volcanoes from the Loa trend have more “enriched” isotopic compositions, indicative of recycled surface materials in their source, whereas volcanoes from the Kea trend tend to have average Pacific mantle compositions. Thallium (Tl) isotopes (205Tl and 203Tl) are unequally distributed across Earth's chemical reservoirs and can show large concentration contrasts, for example between pelagic sediments (>>100 ng/g) and the Earth's mantle (<1 ng/g). We measured the Tl isotopic composition in Hawaiian samples and found that, among other indicators, the heavier Tl isotopic compositions measured in some volcanoes of the Kea geochemical trend might co‐vary with oxygen isotopes, suggesting that their Tl compositions could result from recycled surface materials in their source. This shows that the mantle source of both the Loa and Kea geochemical trends likely contains materials recycled through the mantle, which is significant because thus far the Kea volcanoes have shown fairly uniform isotopic compositions representative of the average, deep Pacific mantle.
Key Points
Thallium isotopic compositions measured in a sample set of Hawaiian shield lavas represent primary magmatic signatures
Heavier isotopic values in some Kea‐trend volcanoes suggest the presence of ancient pelagic sediment in the Kea source of the Hawaiian plume
The deep mantle source of both Loa and Kea Hawaiian volcanoes contains recycled materials of varying lithologies, histories, and ages
Environmental monitoring and remediation require techniques to identify the source and fate of metals emissions. The measurement of heavy metal isotopic signatures, made possible by the advent of the ...MC-ICP-MS, is a powerful new geochemical tool, which may be used to trace the source of these metals in the environment. In a multi-tracer study, Cd, Zn and Pb isotopic compositions (MC-ICP-MS) and elemental concentrations (HR-ICP-MS) are used to distinguish between natural and anthropogenic sources of these metals in bivalves collected from western Canada (British Columbia), Hawaii, and the USA East Coast.
Variability in the δ
114/110Cd values of bivalves (−1.20‰ to −0.09‰) is attributed to differences in the relative contributions of Cd from natural and anthropogenic sources between sites. Cadmium isotopic compositions (δ
114/110Cd
=
−0.69‰ to −0.09‰) identify high Cd levels in B.C. oysters as primarily natural (i.e., upwelling of Cd rich intermediate waters in the North Pacific), with some variability attributed to anthropogenic sources (e.g., mining and smelting). Variability in the δ
66/64Zn values exhibited by the B.C. bivalves is relatively small (0.28–0.36‰). Despite the low Pb levels found in B.C. oysters, Pb isotopes are used to identify emissions from industrial processes and the consumption of unleaded gasoline and diesel fuel as significant metal sources. Although the Cd concentrations of the USA East Coast bivalves are primarily lower than those of B.C. oysters, their relatively light Cd isotopic compositions (δ
114/110Cd
=
−1.20‰ to −0.54‰) indicate the significance of anthropogenic Cd sources and are attributed to the high prevalence of industry on this coast. The δ
114/110Cd values of USA East Coast bivalves include the lightest ever reported, with the exception of values reported for extraterrestrial materials. In addition, the Pb isotopic compositions of bivalves from the USA East Coast indicate Pb emissions from the combustion of coal are an important source of Pb, consistent with the high consumption of coal for power production on this coast.
This study demonstrates the effective use of Cd and Zn isotopes to trace anthropogenic sources in the environment and the benefit of combining these tools with Pb “fingerprinting” techniques.
Kauaʻi shield‐stage lavas are central to understanding the origin of the distinct Kea and Loa Hawaiian geochemical trends in Hawaiian basalts. These trends reflect two geochemically distinct sides in ...the Hawaiian plume, with Loa to the southwest and Kea to the northeast. The geochemistry and Sr‐Nd‐Hf isotopic compositions of shield‐stage lavas from Kauaʻi show a transition from Kea to Loa across the island with the Loa mantle source becoming dominant as the volcano grew. This geochemical transition is gradual from west to east Kauaʻi and supports the hypothesis that the Kauaʻi volcano sampled both sides of the bilateral Hawaiian plume, a phenomenon that is unusual for a Hawaiian volcano. Notably, Kauaʻi marks the arrival of progressively larger volumes of Loa compositions within the Hawaiian mantle plume. The new data from Kauaʻi, combined with an updated and comprehensive database of Hawaiian shield‐stage major element oxides, trace element concentrations, and isotopic compositions normalized to the same standard values, allows for the Pb‐Sr‐Nd‐Hf isotopic compositions of the Average Loa (‘ALOA’) common geochemical component to be estimated. Despite the bilateral Loa‐Kea geochemical trend beginning at Molokaʻi, Loa compositions dominate the erupted volume of Hawaiian volcanoes younger than 3 Ma, validating the volumetric importance of the Loa source in the lower mantle portion of the Hawaiian plume.
Plain Language Summary
Hawaiian volcanoes are arranged along two parallel geographic trends named Loa and Kea. Volcanoes belonging to either trend have distinct geochemical compositions that are linked to their deep mantle sources as sampled by the Hawaiian mantle plume. The Kea composition has been present in shield‐stage basalts for ∼81 Ma, however the Loa composition is relatively new and has mainly been measured in volcanoes formed since 3–4 Ma. We used the geochemistry and isotopic compositions of shield‐stage basalts from the island of Kauaʻi to show that Loa compositions began to appear in larger amounts in the Hawaiian plume around 5 Ma. These new data, combined with a large and carefully curated geochemical data set of Hawaiian samples, has allowed us to estimate the average composition of Loa and its associated isotopic end‐member compositions. This work demonstrates that Loa was an important mantle source for the older Hawaiian volcanoes such as Kauaʻi and dominates shield lavas along the Hawaiian chain. Notably, the geochemistry of Kauaʻi’s volcanic rocks represents the long‐term establishment of Loa compositions in the Hawaiian plume.
Key Points
Radiogenic (Sr‐Nd‐Hf, and Pb) isotopic compositions change from west to east across Kauaʻi and broadly correlate with age
Kauaʻi records the first large‐scale and long‐lasting occurrence of Loa‐trend Hawaiian compositions
The average Loa composition is constrained in Pb‐Sr‐Nd‐Hf isotopes and dominates compositions along the Hawaiian chain
Hawaiian volcanoes belong to two geographically and geochemically distinct trends, the Loa and Kea trends. The cause of this dichotomy is still strongly debated. One of the prevailing hypotheses is ...that the two trends originate in the deep mantle where the Hawaiian mantle plume straddles two geophysically and geochemically distinct domains at the core‐mantle boundary (CMB). New high‐precision multi‐isotopic (Pb, Hf, Nd, and Sr) compositions of lavas from three key volcanoes, Lō‘ihi, Kohala, and Haleakalā, show transitional signatures between Loa and Kea compositions that call into question the degree of physical independence between the two trends. Statistical analysis of multi‐collector inductively coupled plasma mass spectrometer or triple‐spike Pb (n > 800) and Sr, Nd, and Hf data (n > 400) for shield tholeiites from the entire Hawaiian Islands (<5.5 Ma) identifies six unique geochemical groups, and for the first time, documents large‐scale heterogeneities in the Kea trend. The spatial orientation of the six geochemical groups shows that the bilateral zonation of the plume source at the CMB is gradational, and that the Hawaiian mantle plume periodically entrains large‐scale ephemeral geochemical heterogeneities on million‐year, regional timescales. These geochemical heterogeneities are stretched vertically during transit of the plume to the surface and are observed in the lavas from the Hawaiian Islands. These results provide evidence that the large low shear velocity province in the deep Pacific is thermochemical and highly heterogeneous.
Key Points
Six unique geochemical groups are identified by statistical analysis of isotopic data of Hawaii shield basalts
The spatial orientation of the six geochemical groups shows that the bilateral zonation of the plume source at the core‐mantle boundary is gradational
The large low shear velocity province in the deep Pacific is thermochemical and highly heterogeneous
The Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia has undertaken a systematic analysis of the isotopic (Sr, Nd, and Pb) compositions and ...concentrations of a broad compositional range of U.S. Geological Survey (USGS) reference materials, including basalt (BCR‐1, 2; BHVO‐1, 2), andesite (AGV‐1, 2), rhyolite (RGM‐1, 2), syenite (STM‐1, 2), granodiorite (GSP‐2), and granite (G‐2, 3). USGS rock reference materials are geochemically well characterized, but there is neither a systematic methodology nor a database for radiogenic isotopic compositions, even for the widely used BCR‐1. This investigation represents the first comprehensive, systematic analysis of the isotopic composition and concentration of USGS reference materials and provides an important database for the isotopic community. In addition, the range of equipment at the PCIGR, including a Nu Instruments Plasma MC‐ICP‐MS, a Thermo Finnigan Triton TIMS, and a Thermo Finnigan Element2 HR‐ICP‐MS, permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the TIMS and MC‐ICP‐MS methods (e.g., Nd isotopic compositions). For each of the reference materials, 5 to 10 complete replicate analyses provide coherent isotopic results, all with external precision below 30 ppm (2 SD) for Sr and Nd isotopic compositions (27 and 24 ppm for TIMS and MC‐ICP‐MS, respectively). Our results also show that the first‐ and second‐generation USGS reference materials have homogeneous Sr and Nd isotopic compositions. Nd isotopic compositions by MC‐ICP‐MS and TIMS agree to within 15 ppm for all reference materials. Interlaboratory MC‐ICP‐MS comparisons show excellent agreement for Pb isotopic compositions; however, the reproducibility is not as good as for Sr and Nd. A careful, sequential leaching experiment of three first‐ and second‐generation reference materials (BCR, BHVO, AGV) indicates that the heterogeneity in Pb isotopic compositions, and concentrations, could be directly related to contamination by the steel (mortar/pestle) used to process the materials. Contamination also accounts for the high concentrations of certain other trace elements (e.g., Li, Mo, Cd, Sn, Sb, W) in various USGS reference materials.
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