Dominique Weis

High‐precision isotopic characterization of USGS reference materials by TIMS and MC‐ICP‐MS

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.

Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean

Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998–99) and four from Legs 119 and 120 (1987–88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ~110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatile-rich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet–biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet–biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate.

High‐precision Pb‐Sr‐Nd‐Hf isotopic characterization of USGS BHVO‐1 and BHVO‐2 reference materials

The recent development of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and increasing use of the technique have created the need for well-characterized rock standards, especially for isotopic systems where no internal fractionation correction can be applied. This paper presents a careful leaching experiment on the U.S. Geological Survey (USGS) reference materials BHVO-1 and BHVO-2 (Hawaiian basalts) and documents the evidence for contamination of the rock powders during processing. This contamination accounts for the difference in Pb isotopic ratios of BHVO-1 and BHVO-2 as well as for their lack of homogeneity both in Pb isotopic compositions and in some trace element contents. Copyright 2005 by the American Geophysical Union.

Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano.

The present work reports multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) measurements of the isotopic compositions of Hf and Pb in the first 3 km of the deep core retrieved by the Hawaii Scientific Drilling Project. The measurements cover all the samples from the standard geochemical reference set, glasses from the deep hole, and replicates from the pilot hole. Both Hf and Pb are less radiogenic in Mauna Loa compared to Mauna Kea. The transition between Mauna Kea and Mauna Loa lavas in the deep core is progressive for eHf and 208Pb/204Pb, but a sharp discontinuity is observed for 208Pb*/206Pb*. There is no correlation between the alkalinity of the samples and isotopic composition. In detail, the Hf isotope compositions of samples from the pilot hole are not all identical to those of the HSDP-2 core for samples retrieved from a similar depth, suggesting that steep topography existed at the time of emplacement or that a different eruptive sequence was recorded. The strong correlation between 208Pb*/206Pb* and 3He/ 4He (He data from M. D. Kurz et al. (Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project, submitted to Geochemistry Geophysics Geosystems, 2002)) requires the episodic incorporation of a component that resembles the basalts erupted by either Kilauea or the Loihi eruptive centers (this component is referred to as K/L). The data suggest that some 500 kyr ago, Mauna Kea was tapping a mantle source similar to that tapped by Kilauea today. Isotopic variability of Pb and He cannot be accounted for by radiogenic ingrowth in a closed system, but requires the mixing of mantle source components with distinct outgassing histories. The time series of isotopic and concentration data in Mauna Kea samples spanning about 350,000 years of age indicate the recurrence of geochemical patterns in the melting column. Ignoring the most recent alkalic samples, we find that the dominant fluctuations of eHf and 207Pb/204Pb correspond to a period of 50,000 years. For La/ Yb, Zr/Nb, 87Sr/ 86Sr, 206Pb/204Pb, 207Pb/ 206Pb, and 208Pb/206Pb, a dominant period of ca. 18,000 years is obtained. Once provision is made for the existence of harmonics, the consistency between the isotopic spectrum of the pilot hole and the HDSP-2 core is very good. The input of the K/L component does not seem to be periodic. We use these recurrence intervals in conjunction with the upwelling rate deduced from buoyancy flux and seismic evidence of the maximum dimension of scatterers to constrain the radius of the Hawaiian plume conduit to be in the range of 10-50 km and the upwelling velocity to be in the range of 0.13-3 m/yr. Plausible vertical length scales of heterogeneities in the conduit are 6.5-160 km.

Cogenetic silica-rich and carbonate-rich melts trapped in mantle minerals in Kerguelen ultramafic xenoliths: Implications for metasomatism in the oceanic upper mantle

In an attempt to characterize metasomatic agents for the oceanic upper mantle, we have undertaken a study of melt and fluid inclusions trapped in metasomatized peridotite nodules (anhydrous spinel lherzolites and harzburgites) from the Kerguelen Islands (southern Indian Ocean). These xenoliths contain three types of genetically related inclusions hosted by olivine, clinopyroxene and orthopyroxene. These are silicate melt inclusions, carbonate-rich inclusions and CO2 fluid inclusions. These inclusions are secondary in nature and form trails along fracture planes in the sheared peridotites. Heating experiments conducted on silicate melt inclusions give an estimation of the entrapment temperatures ( ≈ 1250°C) and indicate that there is no genetic relationship between the inclusions and their host minerals. The chemical composition of the silicate melt inclusions is characterized by normative quartz and feldspar components, with SiO2 ≈ 60wt%, Al2O3 ≈ 20wt%, Na2O and K2O each ≈ 4–5wt%, FeO and MgO 1000 ppm, H2O ⩾ 1.2%, and oversaturation of the melt with CO2. The trace element signature is characterized by LREE enrichment, negative HFSE (Ti and Zr) anomalies and a TiZr value of 17. The trapped melt has crystallized the following minerals: K-rich amphibole (kaersutite), diopside, rutile, ilmenite and carbonate (magnesite). Carbonate-rich inclusions, interpreted as trapped carbonate melt, have crystallized calcite. The carbonate-rich inclusions are often physically connected with the silicate melt inclusions, indicating the former existence of a homogeneous melt which later unmixed into two separate melts. Cogenetic relationships between CO2 inclusions and both carbonate melt inclusions and silicate melt inclusions yield a minimum trapping pressure for all types of inclusions of 12.5 kbar at 1250°C, corresponding to upper mantle depths. Based on their daughter mineral types, their chemical composition and high volatile element contents, the silicate-carbonate melt inclusions trapped in the ultrabasic xenoliths of the Kerguelen Islands are interpreted as small amounts of a metasomatic melt phase. These melt inclusions cannot result from melting of the anhydrous peridotite assemblages in which they have been trapped. They must represent an exotic, migrating metasomatic melt phase in the oceanic lithosphere below the Kerguelen Islands.

Kerguelen Archipelago revisited: geochemical and isotopic study of the Southeast Province lavas

In order to constrain an isotopically extreme mantle component involved in the genesis of oceanic basalts, we studied some of the youngest lavas of the Kerguelen Archipelago. According to the model developed by Gautier et al. [1], these young lavas were derived from the Kerguelen mantle plume, with little, if any, depleted MORB component. The alkaline lavas in the Southeast Province of the Kerguelen Archipelago correspond to two main volcanic phases: (a) a 20–22 Ma old lower Miocene series with basalts and trachytes belonging to a mildly alkaline series comparable to the mildly alkaline series of Gautier et al. [1] and (b) a 6.6–10.2 Ma old [2] upper Miocene series with basanites, tephri-phonolites and phonolites, forming a highly alkaline series. The lower Miocene lavas haveeNd,87Sr/86Sr and Pb isotopic ratios overlapping with those of Kerguelen transitional and mildly alkaline series, i.e.eNd from +2.2 to −0.6 and87Sr/86Sr between 0.70493 and 0.70522. The upper Miocene lavas have higher87Sr/86Sr ratios (> 0.7054) and lowereNd ( ≪ 0), i.e. comparable to the highly alkaline series of Gautier et al. [1]. In contrast, compared to other lavas from the Kerguelen Archipelago, the upper Miocene lavas have distinctly less radiogenic206Pb/204Pb ratios. They plot to the left of the Indian OIB field in Pb-Pb diagrams, i.e. in a direction opposite to that expected for contamination with sediment or young continental crust. In both series, the evolved lavas and associated basalts/basanites overlap in isotopic ratios, although incompatible element abundance ratios in the evolved lavas were modified by fractionation of amphibole, Fe-Ti oxides, apatite and sphene. We propose that the isotopic characteristics of the upper Miocene lavas correspond to the pure Kerguelen plume signature, which is:87Sr/86Sr= 0.7054−0.7058,eNd = −0.2 to −2.9,206Pb/204Pb= 18.06−18.27,207Pb/204Pb= 15.54−15.58 and208Pb/204Pb= 38.68−39.16. The Kerguelen plume therefore has an intermediate isotopic composition and corresponds neither to the EM I nor the EM II component [3]. A continuum between EM I and EM II is expected for mantle contaminated by recycled continental crust or continent-derived material.

Petrology and geochemistry of the Kerguelen Archipelago basalts (South Indian Ocean): evolution of the mantle sources from ridge to intraplate position

Kerguelen basic lavas belong to three magmatic series: transitional, mildly alkaline and highly alkaline, showing from the first to the latter, a general increase in alkali and incompatible element contents as well as some of the incompatible trace element ratios [(Ce/Yb)n,Nb/Zr,Nb/Y,Zr/Y,Ti/Zr].Ba/Nb,La/Nb and all ratios involving Th are in the range of those observed for Dupal type OIBs although subtle differences exist mainly reflecting a slight Th enrichment in the Kerguelen basalts relative to other OIBs. New Sr, Nd and Pb isotope data strengthen the previously identified Dupal signature of Kerguelen rocks. While the transitional series, older than 26 Ma, shows slightly depleted Sr and Nd isotopic signatures (87Sr/86Sr 0), the alkaline series, between 26 and about 8 Ma, show a slightly enriched signature which is strengthened in the highly alkaline series (87Sr/86Sr up to 0.70056), younger than 12 Ma. Pb isotope ratios do not show any significant difference between these three series. These isotopic data coupled with trace element geochemistry indicate a mixing process between a depleted MORB component and an enriched OIB-type component, the Dupal plume. The Pb isotope characteristics indicate the presence of an old recycled component (continentally derived sediment or crust itself) incorporated into the deep plume source-region which may be responsible for this enrichment. The Kerguelen Islands show a geodynamic evolution from an early ridge centered stage, above or close to the South East Indian Ridge (SEIR) 45 Ma ago, to the present day intraplate setting. The geochemical and isotopic variations reflect this geodynamic evolution and correspond to variable degrees of mixing between the two extreme components (SEIR, MORB and the Kerguelen plume) whose relative proportions evolve with the archipelago position, i.e. with the distance from the ridge at the time of eruption.

Mg, Sr and Sr isotope geochemistry of a Belgian Holocene speleothem: Implications for paleoclimate reconstructions

In this study, variations in Mg/Ca, Sr/Ca and 87Sr/86Sr ratios in a Holocene Belgian speleothem (cave secondary carbonate deposit) are interpreted in terms of changes in water residence time and changes in weathering processes, possibly induced by changes in West European climate. A stalagmite from the Pere Noel cave (Belgium) was dated with the TIMS U/Th method and was deposited between ~ 13 and ~ 2 ka BP. The 1000 Mg/Ca ratio varies between 4.9 and 26, and displays short-term changes but no significant long-term trend. The 1000 Sr/Ca ratio varies between 0.09 and 0.31 and displays both short-term changes and a long-term decreasing trend from 12.9 to 3.5 ka. In parallel, the Sr isotopic composition of the speleothem decreases from 0.7090 at 12.9 ka to 0.7088 at 3.5 ka. The Sr isotope ratio is higher than expected from the overlying limestone (87Sr/86Sr = 0.7081) which implies an external source of radiogenic Sr, most probably from a silicate phase. Short-term and long-term changes in Mg/Ca, Sr/Ca as well as long-term changes of 87Sr/86Sr ratios can be explained by changes in the dissolution and precipitation processes of the host limestone. These processes are controlled by changes in water residence times linked to changes in the water excess (precipitation minus evapo-transpiration)

Evaluation of zinc, cadmium and lead isotope fractionation during smelting and refining

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 delta(66/64)Zn and delta(114/110)Cd values of 0.42 per thousand and 1.04 per thousand, respectively, among all smelter samples. No significant difference is observed between the isotopic compositions of the Zn ore concentrates (delta(66/64)Zn=0.09 to 0.17 per thousand; delta(114/110)Cd=-0.13 to 0.18 per thousand) and the roasting product, calcine (delta(66/64)Zn=0.17 per thousand; delta(114/110)Cd=0.05 per thousand), due to approximately 100% recovery from roasting. The overall Zn recovery from metallurgical processing is approximately 98%, thus the refined Zn metal (delta(66/64)Zn=0.22 per thousand) is not significantly fractionated relative to the starting materials despite significantly fractionated fume (delta(66/64)Zn=0.43 per thousand) and effluent (delta(66/64)Zn=0.41 to 0.51 per thousand). Calculated Cd recovery from metallurgical processing is 72-92%, with the majority of the unrecovered Cd lost during Pb operations (delta(114/110)Cd=-0.38 per thousand). The refined Cd metal is heavy (delta(114/110)Cd=0.39 to 0.52 per thousand) 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 (delta(114/110)Cd=-0.52 per thousand) and effluent (delta(114/110)Cd=0.31 to 0.46 per thousand). 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.

40Ar/39Ar geochronology of flood basalts from the Kerguelen Archipelago, southern Indian Ocean: Implications for Cenozoic eruption rates of the Kerguelen plume

The 6500 km2 Kerguelen Archipelago formed on the northern Kerguelen Plateau (NKP) (4×105 km2) which is a shallow submarine plateau belonging to the Kerguelen large igneous province in the southern Indian Ocean. Flood basalts make up 85% of the archipelago and are interpreted as the most recent volcanism (<40 Ma) from the Kerguelen hotspot which has erupted basalt for the last 115 million years. Based on 40Ar/39Ar incremental heating of acid-leached groundmass separates, we report isochron ages ranging from 29.26±0.87 Ma to 24.53±0.29 Ma for 15 basalts from five stratigraphic sections from diverse regions of the archipelago. The oldest dated basalt from the archipelago (∼29 Ma) is much younger than the ∼40 Ma age of conjunction between the hotspot and the Southeast Indian Ridge. Basalt eruption seems to have ceased shortly after ∼24 Ma although small volume, highly evolved lavas and plutons continued to form in the archipelago. The basalt age data suggest an average lava accumulation rate of ∼1.6±0.9 km/my during the Oligocene. The archipelago’s volumetric eruption rate (0.009 km3/yr) is lower than estimates made for the Cretaceous Kerguelen Plateau (1.7 km3/yr) and the Ninetyeast Ridge hotspot track (0.18 km3/yr), suggesting that the late Cenozoic extrusive activity of the Kerguelen plume is waning. Cenozoic volcanism attributed to the Kerguelen plume occurs over a diffuse area with Quaternary eruptions at Heard and McDonald Islands and within the Kerguelen Archipelago. The decreasing eruption rate and areally diffuse volcanism may be explained by the thick lithosphere of the Cretaceous Kerguelen Plateau overriding and insulating the plume. However, if the undated NKP, which underlies the archipelago, formed during the Cenozoic, then, the crustal production rate of the plume from 40 Ma to the present (∼0.25 km3/yr) would be similar to the crustal production rate (0.23 km3/my) previously estimated for the formation of the Ninetyeast Ridge (∼82–38 Ma).