John N. Christensen

APPLICATIONS OF MULTIPLE COLLECTOR-ICPMS TO COSMOCHEMISTRY, GEOCHEMISTRY, AND PALEOCEANOGRAPHY

Multiple collector-inductively coupled plasma mass spectrometry (MC-ICPMS) is a new technique for the measurement of isotopic compositions at high precision, and is of great relevance to planetary, earth, ocean, and environmental sciences. The method combines the outstanding ionization efficiency of the ICP source with the superior peak shapes achievable from the ion optical focal plane of a large dispersion magnetic sector mass spectrometer, utilizing simultaneous multiple collection to achieve the most precise isotopic measurements yet made for many elements—particularly those with high first ionization potential. The addition of a laser facilitates studies for which spatial resolution is required. This method is still in its infancy, yet diverse applications have already led to a number of important scientific developments. Here we review some of these accomplishments and the potential for further work. The Lu-Hf isotopic system, for many years considered analytically challenging, is now relatively straightforward and offers great promise in fields as diverse as garnet geochronology, hydrothermal fluxes to the oceans, and crustal evolution. The age of the Earth’s core, the Moon, and Mars have been measured using a new short-lived chronometer 182Hf-182W. Other such new chronometers will follow. High precision isotope dilution measurements of the Earth’s inventory of many poorly understood elements such as In, Cd, Te, and the platinum group elements are providing tests for models for the accretion of the inner solar system. The small natural isotopic variations in elements such as Cu and Zn, produced by mass dependent fractionation, are now measurable at high precision with this method, and entirely new fields of stable isotope geochemistry can be developed. Similarly, measurements of small nucleosynthetic isotopic anomalies should be made easier for some elements. Measurements of U and Th isotopic compositions at very high sensitivity and reproducibility are now possible, allowing the development of higher resolution Quaternary geochronology. Finally, using laser ablation, the first precise in situ Sr, Hf, W, and Pb isotopic measurements have been made in natural materials, opening up a range of microanalytical isotopic studies in petrology and marine geochemistry. MC-ICPMS offers exciting times ahead in areas well beyond the bounds of geochemistry. Indeed, MC-ICPMS is likely to become the method of choice for many isotopic measurements because it is a more user friendly and efficient method for the acquisition of high precision data. It is also much more versatile, permitting elements to be measured that were previously considered intractable, and allowing the acquisition of data in situ, all with a single mass spectrometer. The limiting factor on the sensitivity is the transmission which is ≤2% for all instruments thus far designed. If it is found possible to improve the transmission still further, thermal ionization mass spectrometry, the technique that has, thus far, provided the high precision measurements necessary for most of the vast field of radiogenic isotope geochemistry, may be relegated to specialized applications.

Recent developments in inductively coupled plasma magnetic sector multiple collector mass spectrometry

Abstract This paper describes advances in isotopic measurements that have been made with an inductively coupled plasma source magnetic sector multiple collector mass spectrometer and presents results of new experiments aimed at further evaluating the instruments capability. It is shown using standard solutions that trace element ratios such as Rb/Sr can be measured precisely without isotope dilution by comparison with reference solutions of known composition. Similarly, using a new wide flight tube, Pb isotopic compositions and U/Pb ratios can be accurately measured simultaneously without isotope dilution. The effects of deliberately including changes in the running conditions (r.f. power) are shown to be significant for measuring trace element ratios but not for mass bias and interference-corrected isotopic compositions. Finally, it is demonstrated that precise and accurate isotopic compositions of elements as refractory as W can be determined relatively easily by solution nebulization and even by direct laser ablation of complex silicates. Spectral interferences in such experiments are negligible. These experiments serve to highlight the remarkable potential that this new field offers for hitherto difficult isotopic measurements in nuclear, earth, environmental and medical sciences. Isotopic measurements can be made that are reproducible at high precision through a range of running conditions, even in the presence of isobaric interferences. The ability to correct for mass discrimination accurately using a second element of similar mass, the very high sensitivity for elements that are otherwise difficult to ionize, the demonstrated capability for laser ablation work and the ability to measure through a wide mass range simultaneously give this instrument major advantages over other more traditional techniques of isotopic measurement.

Correlation by Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window, Eastern Alps

In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67−0.13+0.19mm/million years and 0.88−0.19+0.34mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7−0.7+1.2×10-14 s-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at ∼30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4−2+3mm/year and 2±1 mm/year, respectively.

In situ Sr isotopic analysis by laser ablation

We demonstrate that the Sr isotopic compositions of geological materials can be measured in situ through laser ablation using an ICP multiple-collector double focusing magnetic sector mass spectrometer (MC-ICPMS). This provides rapid, texturally sensitive, high precision (0.004%) Sr isotopic analysis without the need for chemical preparation of samples. The system used employs a Q-switched Nd-YAG infrared laser. The laser-ablated material is carried by Ar flow to an ICP torch. The resulting ions are extracted from the plasma and focused with d.c. quadrupole lenses and an electrostatic analyzer prior to entering a magnetic sector analyzer and collection in an array of Faraday collectors. A feldspar megacryst, with independently known 87Sr86Sr of 0.703117 ± 13, and a modern gastropod shell, with independently known 87Sr86Sr of 0.709170 ± 10, were used evaluate the method. Both have ∼ 2000 ppm Sr. Laser power was adjusted to give an average 88Sr of 2 × 10−11 amps during analysis to replicate the beam size during typical thermal ionization mass spectrometry (TIMS). Within-run precisions in 87Sr86Sr of < ± 4 × 10−5 compare favorably with the precision from TIMS. Thirteen analyses of the Cameroon feldspar megacryst yielded a weighted average 87Sr86Sr of 0.703106 ± 22, in perfect agreement with the TIMS measurement. Similarly, 22 analyses of the modern gastropod shell gave a weighted average of 0.709182 ± 22, identical to the known value. No memory effect was observed when switching from the Cameroon feldspar sample (run for several hours) to the modern shell material. The power of this technique is illustrated with a preliminary study of systematic differences in 87Sr86Sr of ∼ 0.0004 between two feldspar crystals of a Long Valley basalt. One feldspar has identical 87Sr86Sr to the host basalt, confirming the lack of any matrix effect with this method.

Actual timing of neodymium isotopic variations recorded by FeMn crusts in the western North Atlantic

Hydrogenetic ferromanganese (FeMn) crusts from the western North Atlantic record variations in the Nd and Pb isotopic composition of Cenozoic deep water preserved during their growth. The timing and cause of the most striking change have been the subject of debate. Some have proposed that the shift took place after 4 Ma in response to the closure of the Panama gateway. Others have argued that the major change in isotope composition occurred as early as 8 Ma. This study presents high-resolution Nd isotope records for crusts previously dated using 10Be/9Be chronology. These data confirm that the shifts in Nd occurred after 4 Ma, consistent with a likely relationship with the closure of the Central American Isthmus and intensification of Northern Hemisphere Glaciation, and in accordance with changes seen in other physical and chemical records. These results illustrate the need for both a robust chronological framework and high-resolution records before a reliable paleoceanographic interpretation can be made of the variations recorded by FeMn crusts.

High-precision in situ 238U–234U–230Th isotopic analysis using laser ablation multiple-collector ICPMS

We have developed a method for the rapid, in situ measurement of U–Th isotopic compositions at the semimicro scale using laser ablation sampling, combined with multiple collector ICP magnetic sector mass spectrometry (MC-ICPMS). The system uses a Q-switched and frequency quadrupled 266 nm Nd:YAG laser to ablate samples containing 100 ppm levels of U at 150 μm scale resolution, corresponding to 1–4 ng 238U, ∼70–200 fg of 234U and 20–60 fg of 230Th consumed per analysis. Synthetic glass standards and naturally occurring samples of zircon and opal, with U contents of 460, 260, and ∼200 ppm, respectively, were used to assess the precision and accuracy of our laser ablation technique. Our initial experiments used argon as the plasma support gas. Thirty-seven laser analyses on the glass and 29 on the zircon give respective mean [234U/238U]act of 0.17114 ± 0.00022 and 1.0018 ± 0.0014 (2σM), indistinguishable from the MC-ICPMS solution nebulization values of 0.17094 ± 0.00006 and 1.0011 ± 0.0009 (2σM), respectively. The usual within-run precision obtained for both glass and zircon is ±3‰ at the 2σM level. An additional 12 laser analyses on the opal give a mean [234U/238U]act of 0.9997 ± 0.0034, in excellent agreement with the expected secular equilibrium value of unity and a typical within-run precision of ±8‰ (2σM). Our Nd:YAG laser, coupled with an all Ar gas system, produces large elemental fractionation effects between U and Th. Both 238U/232Th and [230Th/238U]act can be measured at the per mill level, but Th ion beams are suppressed relative to U. As a result, the Th/U ratios are systematically lower, and the apparent 238U–234U–230Th ages are systematically younger than the true values. The U–Th fractionation is primarily controlled by ionization conditions in the plasma, transportation efficiency of ablated particles, and the composition of the sample matrix. The use of helium instead of Ar in the ablation cell significantly improves the relative sensitivity of Th, and entirely eliminates the elemental fractionation between U and Th, while retaining accuracy and precision in U isotope measurement. With He, mean values for [230Th/238U]act of 0.996 ± 0.013 and 238U/232Th of 1.625 ± 0.092 were determined for the zircon standard, in excellent agreement with the solution nebulization values of 1.0042 ± 0.0016 and 1.6288 ± 0.0006 (2σM), respectively. In an unknown sample, it is possible to determine correct values for [230Th/238U]act and 238U/232Th, with respective within-run uncertainties as good as 7‰ and 2‰, by monitoring the isotopic composition of a well characterized, matrix-matched standard. For high-U material, the combined uncertainties in [234U/238U]act and [230Th/238U]act routinely translate to 2σM errors in the 238U–234U–230Th age of better than ±2,500 years in 100,000-year-old samples.

Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts

Abstract High-resolution Hf isotopic records are presented for hydrogenetic Fe–Mn crusts from the North Atlantic and Indian Oceans. BM1969 from the western North Atlantic has previously been shown to record systematically decreasing Nd isotopic compositions from about 60 to ∼4 Ma, at which time both show a rapid decrease to unradiogenic Nd composition, thought to be related to the increasing influence of NADW or glaciation in the northern hemisphere. During the Oligocene, North Atlantic Hf became progressively less radiogenic until in the mid-Miocene (∼15 Ma) it reached +1. It then shifted gradually back to an ϵHf value of +3 at 4 Ma, since when it has decreased rapidly to about −1 at the present day. The observed shifts in the Hf isotopic composition were probably caused by variation in intensity of erosion as glaciation progressed in the northern hemisphere. Ferromanganese crusts SS663 and 109D are from about 5500 m depth in the Indian Ocean and are now separated by ∼2300 km across the Mid-Indian Ridge. They display similar trends in Hf isotopic composition from 20 to 5 Ma, with the more northern crust having a composition that is consistently more radiogenic (by ∼2 ϵHf units). Paradoxically, during the last 20 Ma the Hf isotopic compositions of the two crusts have converged despite increased separation and subsidence relative to the ridge. A correlatable negative excursion at ∼5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan main central thrust. Changes to unradiogenic Hf in the central Indian Ocean after 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reduction of Pacific flow through the Indonesian gateway. In either case, these results illustrate the utility of the Hf isotope system as a tracer of paleoceanographic changes, capable of responding to subtle changes in erosional regime not readily resolved using other isotope systems.

Direct dating of sulfides by RbSr: A critical test using the Polaris Mississippi Valley-type ZnPb deposit

Abstract The RbSr dating of sphalerites is a powerful method for directly determining the age of base metal deposits and testing models for large-scale fluid flow. However, the uncertainty over the exact host phases for the trace amounts of Rb and Sr and the causes of variability in Rb Sr have caused concern over the reliability of the method. Here we show that the Polaris MVT deposit, with a geologically well-constrained age of formation, confirmed by paleomagnetic measurements, and hosted in significantly older sedimentary rocks, yields a consistent RbSr age of 366 ± 15 Ma, providing the first unequivocal vindication of the reliability of the method.

Indium and tin in basalts, sulfides, and the mantle

The geochemistry of In and Sn are poorly understood, in part, because of difficulties in obtaining accurate concentrations for these elements in geological materials. Furthermore, InSn ratios in sulfides could be sufficiently high to facilitate the use of 115In115Sn geochronology, if the separation and precise measurement techniques were available. In this paper we describe methods for the separation of In and Sn from silicates and sulfides. Indium can be measured by thermal ionization mass spectrometry (TIMS) at very high sensitivity ( > 13%). However, its mass fractionation is difficult to correct reliably. Tin is more difficult to measure by TIMS because of its higher ionization potential. Both elements can be measured effectively using the new technique of MC-ICPMS, since the ionization efficiency is extremely high, molecular interferences are negligible, and mass fractionation in spiked In and Sn can be corrected by monitoring the mass bias in admixed Pd and Sb, respectively. Using these techniques, it is demonstrated that In and Sn concentrations can be measured reliably for silicates and sulfides. Indium and Sn data for international silicate rock standards are in excellent agreement with recommended values. The SnSm ratios determined for ocean island basalts (0113) are within the same range as those recently reported, where Sn was measured by spark source mass spectrometry. Indium is very uniform in OIB and behaves as a slightly incompatible trace element, comparable in bulk distribution coefficient to the heavy rare earths or Y. InY in 0113 is very uniform, averaging 0.0028 ± 0.0005 (Iσ), but is weakly related with PbCe, implying that these ratios may be partly controlled by sulfide at small degrees of partial melting. The similarity in average InY between OIB, N-MORB (0.0025) and the continental crust (0.0025), together with the similarity in SnSm in MORB, OIB, and continental crust contrasts with chalcophile/lithophile and siderophile/lithophile element ratios such as PbCe and WBa, which are high in the continental crust because of decoupling in the subduction environment. The overall behavior of both In and Sn within the silicate Earth is dominated by lithophile affinity. The primitive mantle is estimated to have InY = 0.003 ±0.001, both higher and lower than previous estimates and corresponding to an In concentration of 14 ppb. Ignoring any In that may have been partitioned into the core, the corresponding total Earth concentration of >10 ppb corresponds to <85% depletion relative to CI chondrites. This is less depleted than anticipated by at least a factor of 2, given the supposed volatility of In based on assumed condensation temperatures and depletions in volatile lithophile elements. There is no evidence that In has been segregated into the Earths core. This can be explained if, during the earlier stages of accretion, under reducing conditions, In was too volatile to be transferred into the core. During the later stages of accretion, conditions may have been relatively oxidising such that In behaved as a lithophile element with higher condensation temperature rather than as a volatile chalcophile element. Hence, the InY ratio of Earths primitive mantle may be representative of the mixture of volatile depleted and undepleted material that accreted in the inner solar system. SNC meteorites have a similar range of InY to the silicate Earth, suggesting Mars accreted from a similar mixture of material already depleted in In, and presumably other moderately volatile elements. In contrast, the InY ratio in lunar basalts ranges through four orders of magnitude from silicate Earth values in lunar soils to extremely In-depleted compositions. This is unlikely to be caused by heterogeneous distributions of extreme In depletion on the Moon as a result of volatile depletion. Rather, the more reducing conditions appear to result in In behaving as a relatively compatible trace element during lunar melting and differentiation. Although their behavior on Earth is strongly lithophile, In and Sn are sometimes enriched in sulfides and InSn can be sufficiently high in some sphalerite, chalcopyite, and tetrahedrite that the predicted 115Sn excess caused by decay of 115In in ancient sulfide deposits should be measurable with MC-ICPMS.

RbSr ages and Nd isotopic compositions of melt inclusions from the Bishop Tuff and the generation of silicic magma

RbSr ages of melt inclusions in quartz have the potential to provide a unique and illuminating record of the differentiation and accumulation history of silicic magmas. Here we report the first Sr and Nd isotopic measurements of melt inclusion bearing quartz (MIBQ), extracted from the Bishop Tuff (BT), representing what is arguably the worlds most controversial yet classic zoned silicic magmatic system. Early erupted BT airfall pumice, representing material from the top of the magma chamber, yields individual quartz crystals with Rb/Sr > 70 and variable apparent differentiation ages of 1,420 ± 80 ka, 2,100 ± 100 ka, 2,150 ± 150 ka and 2,500 ± 200 ka. A bulk quartz separate has an apparent age of 1,900 ± 300 ka, while a quartz separate from an individual airfall pumice has an apparent age of 1,330 ± 80 ka. These ages are similar to the RbSr differentiation ages (2,047-1,894 ka) and k-Ar eruption ages (2,100-1,300 ka) of the adjacent early pre-caldera rhyolites at Glass Mountain (GM), indicating that the upper portions of the BT magma chamber included components that episodically differentiated over the same time interval as early Glass Mountain volcanism. However, at this stage the magmatic systems were separate, as indicated by eNd of −1 for airfall melt inclusion, similar to the BT and late GM rhyolites, compared to eNd of −3 for early GM rhyolites. In contract to BT airfall, melt inclusions from intermediate and later erupted BT ignimbrite pumice, from deeper levels of the magma chamber, give younger apparent differentiation ages: 1,040 ± 140 ka (late erupted), 1,180 ± 80 ka (intermediate erupted clast 1), and 1,060 ± 110 ka, 1080 ± 110 ka (intermediate erupted clast 2, two quartz size fractions). These ages are indistinguishable from the RbSr differentiation age of the late pre-caldera GM rhyolites, 1,140 ± 80 ka. The eNd of intermediate and late erupted melt inclusions is −1, the same as BT airfall melt inclusions. These data are consistent with an early episodic development of the evolved upper portions of the BT magma chamber. However, we find no evidence that most of the differentiation took place at around 2 Ma, as recently suggested on the basis of 40Ar39Ar dating. Most of the Bishop Tuff magma, particularly the later erupted portions, differentiated in major events at around 1.0–1.2 Ma. Early differentiated melts repeatedly accumulated in the upper portions of the chamber concomitantly with differentiation, accumulation and eruption in the nearby Glass Mountain magma reservoir, which possibly represented a separate cupola to a larger complex system. By ∼ 1.2 Ma the Bishop Tuff magmatic system expanded to include the Glass Mountain region. We find no evidence for differentiation younger than ∼ 1.0 Ma in any samples from the Bishop Tuff (or Glass Mountain), confirming the view that over the > 300,000 years that elapsed before eruption of the Bishop Tuff, the stratified magmatic system remained largely stable.