Thermal Ionization Mass Spectrometry Technique Research Articles

Direct Rubidium-Strontium Dating of Hydrocarbon Charge Using Small Authigenic Illitic Clay Aliquots from the Silurian Bituminous Sandstone in the Tarim Basin, NW China

Illitic clay is ubiquitous in clastic hydrocarbon reservoirs, and the host for several radiometric isotopes such as the potassium-argon (K-Ar) and rubidium-strontium (Rb-Sr) systems. This study applied the isotope-dilution thermal ionization mass spectrometry technique for small samples (3–4 mg) to conduct illite Rb-Sr isotope dating of five illitic clay samples from the Silurian bituminous sandstone (SBS) intersected by five drillholes in the Tarim Basin, NW China. The 87Rb/86Sr ratio of clays is fractionated mainly by the addition of Rb during the illitization of mixed-layer illite/smectite (I/S), which is the dominant clay species in the Tarim Basin samples. The subsample-scale Rb/Sr isotope values suggest that each subsample may contain I/S particles of slightly variable degrees of illitization. Three of the analyzed samples (H6, KQ1 and TZ67) generated Rb-Sr isochron ages of 141 ± 61 Ma, 332 ± 32 Ma and 235 ± 8 Ma (errors quoted at 2σ), respectively. These results are similar to the corresponding K-Ar ages (125 Ma, 389 Ma and 234 Ma). The isotopic ages are consistent with the timing of hydrocarbon charge which varies in different drillholes as constrained by basin modelling, indicating that a closed-system behavior is attained by the hydrocarbon charge that inhibits the illitization of I/S. The Rb-Sr isotope analyses of the other two samples (YM35-1 and Q1) that did not yield isochron ages suggest the conditions for producing isochrons were not satisfied, which may be caused by disturbance of the isotope system by a post-charge hydrothermal event. The outcomes of this study show the robust potential of Rb-Sr clay subsample geochronology for cross-checking isotopic ages yielded by other systems (e.g. K-Ar system) and constraining the timing of hydrocarbon charge.

Ancient-depleted and enriched mantle lithosphere domains in northern Madagascar: geochemical and isotopic evidence from spinel-to-plagioclase-bearing ultramafic xenoliths

Mantle xenoliths hosted in Cenozoic alkaline rocks of northern Madagascar (Massif d'Ambre and Bobaomby volcanic fields) are spinel lherzolites, harzburgites and rare websterites. Petrography, electron microprobe, LA-ICP-MS and thermal ionization mass spectrometry techniques allowed to recognize domains characterized by variable degree of partial melting and extent of re-enrichment processes: 1) refractory spinel-to-spinel+plagioclase-lherzolites, with clinopyroxenes having marked LREE (Light Rare Earth Elements) depletion ((La/Yb)N~0.2) and very high 143Nd/144Nd (0.513594), which represent a limited and shallow portion of old mantle that suffered low degree partial melting (2–3%) and was later accreted to the lithosphere. These lherzolites acted as a low-porosity region, being, in places, percolated by small volumes of melts shortly before eruption; 2) lherzolites and harzburgites that suffered variable degrees of partial melt extraction (up to 15%), assisted and/or followed by pervasive, porous flow infiltration of alkaline melts in a relatively large porosity region, leading to the creation of a wide area rich in secondary mineral phases (i.e. olivine, clinopyroxene and pargasitic amphibole), enriched in incompatible elements (e.g., LaN/YbN in clinopyroxene up to 15) and having radiogenic Sr and unradiogenic Nd; 3) websterites and wehrlite-bearing samples that record differentiation processes of alkaline melts highly enriched in Th, U and LREE, not yet documented in the erupted volcanics of northern Madagascar. The mantle xenoliths of northern Madagascar show a regional decrease of the equilibration temperature from to SW (up to 1180°C, Nosy Be Archipelago) to the NE (up to 900°C, Bobaomby district). A significant lithologic and geochemical variation of the shallow lithospheric mantle beneath northern Madagascar is noted, in contrast with the relatively uniform geochemical and isotopic composition of the host alkali basalt and basanite lavas.

Using precise CA-TIMS ages of volcanic air-fall tuff beds in correlating the Walloon Coal Measures of the Surat Basin, Australia

The Upper Jurassic Walloon Coal Measures of the Surat Basin is one of Australia’s largest and most productive gas provinces. Despite the drilling of over 8500 wells and numerous publications, the stratigraphic framework is poorly defined. The laterally discontinuous nature of the sedimentary facies, including coals and fluvial channel sandstones, makes correlation difficult. The abundance of volcanic air-fall tuff beds within strata across the basin provides a unique opportunity to independently verify existing stratigraphic frameworks. Using the high-precision chemical abrasion thermal ionisation mass spectrometry technique, zircon grains from 28 tuff beds have been successfully dated within an error margin of less than 100 kyr. These dates substantially revise biostratigraphic and lithostratigraphic frameworks. Lithostratigraphic units are diachronous across the basin. In addition, the sparsity of key spore–pollen taxa limits the application of biostratigraphy. The complex interplay of climate and subsidence on facies distributions can now be documented over a time frame of ~4 Ma. Syntectonism played an important role in variable palaeodrainage patterns across the basin, the frequency of fluvial avulsions and preferential sites of peat accumulation through time. The new stratigraphic framework should aid in future exploration for coal seam gas in the area. Dating tuff beds using high-precision dating techniques should also assist in correlation of non-marine strata elsewhere in the world.

Petrological, geochemical and isotopic characteristics of the lithospheric mantle beneath Sardinia (Italy) as indicated by ultramafic xenoliths enclosed in alkaline lavas

Mantle xenoliths hosted in Miocene-Quaternary mafic alkaline volcanic rocks from Sardinia have been investigated with electron microprobe, laser ablation microprobe-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry techniques. The xenoliths are anhydrous clinopyroxene-poor lherzolites and harzburgites, plus very rare websterites and olivine-websterites. Glassy pods having thin subhedral to euhedral microlites of olivine, clinopyroxene and spinel have been found in harzburgites and websterites. Clinopyroxene shows trace element variability, with values of (La/Yb)N ranging from sub-chondritic (0.01) to supra-chondritic (8.6). The Sr–Nd isotopic ratios of the clinopyroxenes fall mostly in the field of the European lithospheric mantle xenoliths (87Sr/86Sr from 0.70385 to 0.70568 and 143Nd/144Nd ranging from 0.512557 to 0.512953). The geochemical characteristics of the Sardinian xenoliths testify to the variable degrees of earlier partial melt extraction, followed by metasomatic modification by alkaline melts or fluids. Websterites are considered to represent small lenses or veins of cumulitic (i.e. magmatic) origin within the mantle peridotite.

Effect of Microstructure of Graphite on the Nonreductive Thermal Ion Emission in Thermal Ionization Mass Spectrometry

The emission behavior of polyatomic ions in the ionization source of thermal ionization mass spectrometry (TIMS) was investigated. The results suggest that the presence of a graphite promoter plays a key role for the formation and stable emission of polyatomic ions, such as M(2)X(+), M(2)BO(2)(+), Cs(2)NO(2)(+), and Cs(2)CNO(+). Our data further implied that the intensity of M(2)X(+) and M(2)BO(2)(+) increases and the emission temperature decreases with increasing cationic and anionic radius. During the boron isotopic measurement using the Cs(2)BO(2)(+)-graphite-PTIMS method, the isobaric interference ion Cs(2)CNO(+) cannot be transformed from nitrate or organic compounds containing an amide group but can be induced by the existence of trace amounts of boron because of its special electron-deficiency property (B(3+)). Characterization on the planar crystalline structure of various graphite samples with SEM, TEM, and Raman spectroscopy confirmed the relationship of the emission capacity of polyatomic ions and the crystal microstructure of graphite and provides direct evidence that graphite with a perfect parallel and equidistant layer orientation shows a beneficial effect on the emission of polyatomic ions in TIMS. The mechanism study on the formation of polyatomic ions opens the possibility to establish high precision methods for isotopic composition analysis of more nonmetal elements with the TIMS technique.

A cross-calibration of chlorine isotopic measurements and suitability of seawater as the international reference material

A collection of 24 seawaters from various worldwide locations and differing depth was culled to measure their chlorine isotopic composition ( δ 37Cl). These samples cover all the oceans and large seas: Atlantic, Pacific, Indian and Antarctic oceans, Mediterranean and Red seas. This collection includes nine seawaters from three depth profiles down to 4560 mbsl. The standard deviation (2 σ) of the δ 37Cl of this collection is ±0.08‰, which is in fact as large as our precision of measurement (±0.10‰). Thus, within error, oceanic waters seem to be an homogeneous reservoir. According to our results, any seawater could be representative of Standard Mean Ocean Chloride (SMOC) and could be used as a reference standard. An extended international cross-calibration over a large range of δ 37Cl has been completed. For this purpose, 13 geological fluid samples of various chemical compositions and a manufactured CH 3Cl gas sample, with δ 37Cl from about −6‰ to +6‰ have been compared. Data were collected by gas source isotope ratio mass spectrometry (IRMS) at the Paris, Reading and Utrecht laboratories and by thermal ionization mass spectrometry (TIMS) at the Leeds laboratory. Comparison of IRMS values over the range −5.3‰ to +1.4‰ plots on the Y= X line, showing a very good agreement between the three laboratories. On 11 samples, the trend line between Paris and Reading Universities is: δ 37Cl Reading=(1.007±0.009) δ 37Cl Paris−(0.040±0.025), with a correlation coefficient: R 2=0.999. TIMS values from Leeds University have been compared to IRMS values from Paris University over the range −3.0‰ to+6.0‰. On six samples, the agreement between these two laboratories, using different techniques is good: δ 37Cl Leeds=(1.052±0.038) δ 37Cl Paris+(0.058±0.099), with a correlation coefficient: R 2=0.995. The present study completes a previous cross-calibration between the Leeds and Reading laboratories to compare TIMS and IRMS results (Anal. Chem. 72 (2000) 2261). Both studies allow a comparison of IRMS and TIMS techniques between δ 37Cl values from −4.4‰ to +6.0‰ and show a good agreement: δ 37Cl TIMS=(1.039±0.023) δ 37Cl IRMS+(0.059±0.056), with a correlation coefficient: R 2=0.996. Our study shows that, for fluid samples, if chlorine isotopic compositions are near 0‰, their measurements either by IRMS or TIMS will give comparable results within less than ±0.10‰, while for δ 37Cl values as far as 10‰ (either positive or negative) from SMOC, both techniques will agree within less than ±0.30‰.

Analysis of 232Th/ 230Th in volcanic rocks: a comparison of thermal ionization mass spectrometry and other methodologies

Techniques for measuring Th isotopic compositions of igneous rocks are evaluated using thermal ionization mass spectrometry (TIMS) data collected at the University of Hawaii (UH). A new single collector method for unspiked 232Th/ 230Th measurement is presented and compared to UH results using other published methods (multicollector and “split-analysis” single collector, a.k.a. “bridge”). Analyses of a synthetic thorium standard (UCSC-Th-A, n=245 over 4 years), ionization from single and triple filaments, instrumental parameters affecting analytical quality, and analyses using two secondary energy filters (a 30-cm electrostatic analyzer, ESA, and a Micromass WARP©) are discussed. The TIMS technique with the best combination of accuracy (42 analysis mean=0.12%>gravimetric), precision, reproducibility and analyte size was single collector ion-counting analysis of 50-ng single filament loads. Static multicollector analysis (500 ng triple filament loads) resulted in somewhat lower overall precision at similar accuracy (with small but measurable differences between the ESA and WARP data). Bridge analyses (50-ng single filament loads) were the least precise and accurate. Low-Th-abundance volcanic rocks (10 duplicates, 2 triplicates) were also analyzed for 232Th/ 230Th by the single collector technique. Chemical blanks were demonstrably negligible and all replicate sets agreed to within the precision of the standards data. Generalizations are made about the state of the art (analysis techniques and the standards themselves) using a compilation of synthetic and natural standards data from 25 publications (15 laboratories worldwide) collected by TIMS, high resolution α-spectrometry, and other mass spectrometry types. Precision and accuracy are variable between data sets (by more than 10×), regardless of instrumentation. Not all publications give uniform analytical meta data, making it difficult to assess the causes of this variability. Perhaps contrary to popular conception, traditional α-spectrometry data can be highly accurate and only moderately imprecise relative to mass spectrometry data. Inter-laboratory means for commonly used standards (UCSC-Th-A, Table Mountain Latite, TML, and AThO) are calculated. The mean 232Th/ 230Th of the 146 UH analyses of UCSC-Th-A is 170,790 (2 σ=1.4%, 2 σ m=0.11%, 0.11%>gravimetric). Combining with available literature data (unfiltered) yields 232Th/ 230Th=170,760 (255 analyses, 2 σ=1.2%, 2 σ m=0.08, 0.09%>gravimetric).

Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions

Reduction of selenium oxyanions by microorganisms is an important process in the biogeochemical cycling of selenium. Numerous bacteria can reduce Se oxyanions, which are used as electron acceptors during the oxidation of organic matter in anoxic environments. In this study, we used a double spike ( 82Se and 74Se) thermal ionization mass spectrometry technique to quantify the isotopic fractionation achieved by three different species of anaerobic bacteria capable of accomplishing growth by respiratory reduction of selenate [SeO 4 2− or Se(VI)] or selenite [SeO 3 2− or Se(IV)] to Se(IV) or elemental selenium [Se(0)] coupled with the oxidation of lactate. Isotopic discrimination in these closed system experiments was evaluated by Rayleigh fractionation equations and numerical models. Growing cultures of Bacillus selenitireducens, a haloalkaliphile capable of growth using Se(IV) as an electron acceptor, induced a 80Se/ 76Se fractionation of −8.0 ± 0.4‰ (instantaneous ϵ value) during reduction of Se(IV) to Se(0). With Bacillus arsenicoselenatis, a haloalkaliphile capable of growth using Se(VI) as an electron acceptor, fractionations of −5.0 ± 0.5‰ and −6.0 ± 1.0‰ were observed for reduction of Se(VI) to Se(IV) and reduction of Se(IV) to Se(0), respectively. In growing cultures of Sulfurospirillum barnesii, a freshwater species capable of growth using Se(VI), fractionation was small initially, but near the end of the log growth phase, it increased to −4.0 ± 1.0‰ and −8.4 ± 0.4‰ for reduction of Se(VI) to Se(IV) and reduction of Se(IV) to Se(0), respectively. Washed cell suspensions of S. barnesii induced fractionations of −1.1 ± 0.4‰ during Se(VI) reduction, and −9.1 ± 0.5% for Se(IV) reduction, with some evidence for smaller values (e.g., −1.7‰) in the earliest-formed Se(0) results. These results demonstrate that dissimilatory reduction of selenate or selenite induces significant isotopic fractionation, and suggest that significant Se isotope ratio variation will be found in nature.

Spectroscopic study of thorium using continuous-wave resonance ionization mass spectrometry with ultraviolet ionization

This paper presents recent results on an improved method of ionization for the cw-RIMS process for thorium (Th). This method involves the application of a high power ultraviolet (UV) argon ion laser for the second step in a two-step (1+1) (photon-to-resonance plus photon-to-ionization) ionization scheme. Over 90 thorium transitions are identified for use in a (1+1) continuous-wave resonance ionization mass spectrometry (cw-RIMS) ionization scheme. The excitation cross-section of several strong transitions was determined to be 10 −13cm 2. The optimum cw excitation scheme was with the resonant laser tuned to the 384.08 nm (26 036 cm −1) transition when using the multi-line UV argon ion laser for signal enhancement. For thorium, the increase in ionization efficiency was documented to be a minimum of one order-of-magnitude improvement over that achieved by conventional thermal ionization mass spectrometry (TIMS). The measured total ionization efficiency (detected ion signal/sample atoms loaded) was as high as 0.41%, which easily provided signal levels for efficiency measurements on sample sizes down to 25 ng, and should provide sufficient signal for isotopic analysis of volcanic-like samples as small as 1–5 ng of thorium. Based on geometric overlap considerations, the cw-RIMS ionization efficiency within the laser focal volume approaches ~100%. This cw-RIMS ionization efficiency promises to provide ample signal for the 230Th/ 232Th isotope ratio analysis of nanogram volcanic-like samples. The ability to determine accurately and precisely the 230Th/ 232Th isotopic ratios for nanogram samples represents an improvement over the TIMS technique, and is anticipated to have a significant effect on uranium-series disequilibrium measurements important in geochemistry and geochronology.

Lead geochronology of zircon by LaserProbe-inductively coupled plasma mass spectrometry (LP-ICPMS)

LaserProbe-inductively coupled plasma mass spectrometry (LP-ICPMS) provides a sensitive, fast, and simple means to determine 207Pb 206Pb ages in single zircon grains. A Nd:YAG laser is used to irradiate the zircon surface and leaves a cylindrical pit of 30–60 μm, from which the vaporised materials are transported by argon gas to a Fisons-VG PQII+ ICPMS for analysis. No zircon abrasion, cleaning nor chemical separation procedures are required. The accuracy and the limitation of the method were evaluated by analyzing twenty-one zircon samples ranging in age from 2.7 Ga to 1.0 Ga, which have also been dated by the conventional U-Pb thermal ionization mass spectrometry technique (TIMS). The LP-ICPMS 207Pb 206Pb ages for zircons with grain size > 60 μm and 207Pb concentration > 3 ppm are within 1% of the TIMS ages. Smaller zircons (≤60 μm) and those with 207Pb concentration < 2 ppm yield inaccurate ages. Operating the LP-ICPMS at conditions that give a compromise between the ideal spatial resolution and instrument sensitivity, the limits of detection were found to be 0.2 ppm for 206Pb, 207Pb, and 208Pb. The precision of the 207Pb 206Pb ratio is generally 0.5–6% (1σ) from each sampling pit and is strongly dependent on the lead concentration. However, the precision for the average of the mean ratios from different pits in one grain or several grains of the same population are generally <1.5% (1σ). The results presented here demonstrate that the LP-ICPMS can be used to determine 207Pb 206Pb ages of zircons and that reliable ages can be obtained from high quality, limpid zircons with a simple Pb-loss history. Uses of the technique include screening of zircon populations from different rocks in areas of poorly known age relationships and provenance studies of detrital zircons from ancient and modern sedimentary sequences. Other applications include the study of growth zones and of inherited components in complex zircon populations. In comparison with the SHRIMP (Sensitive High Resolution Ion MicroProbe) technique, the LP-ICPMS has the disadvantage of partially destroying the sample and inability to obtain accurate and precise U Pb ratio at present. However, LP-ICPMS has better sensitivity, similar precision, but slightly worse spatial resolution for the measurement of 207Pb 206Pb relative to the SHRIMP. Low purchasing and operating cost for the LP-ICPMS provide a simple and economic alternative to the SHRIMP method.

U-series ages of solitary corals from the California coast by mass spectrometry

The purpose of this study is to evaluate the feasibility of dating fossil solitary corals from Pleistocene marine strandlines outside tropical latitudes using the recently developed high sensitivity, high-precision U-series technique based on thermal-ionization mass-spectrometry (TIMS). The TIMS technique is much more efficient than conventional a spectrometry and, as a result, multiple samples of an individual coral skeleton, or different specimens from the same bed can be analyzed.Detached and well-rounded fossil specimens of the solitary coral Balanophyllia elegans were collected from relict littoral deposits on emergent marine terraces along the California coast at Cayucos terrace (elevation 8 m, previously dated at 124 and 117 Ky by α counting), Shell Beach terrace (elevation about 25 m, previously undated), Nestor terrace, San Diego (elevation 23 m, previously dated at 131 to 109 Ky ), Bird Rock terrace, San Diego ( elevation 8 m, previously dated at 81 Ky ). Attached living specimens were collected from the intertidal zone on the modern terrace at Moss Beach.Concentrations of 232Th in both living and fossil specimens are much higher than in reef-building corals (12 to 624 pmol/g vs. 0.1 to 1.6 pmol/g, respectively). However, because 230Th/232Th in Balanophyllia elegans are very low (2.22 × 10−3to 4.33 × 10−4), the high 232Th concentrations have negligible effect on the 230Th-234U dates. The high 232Th concentration in the living specimen (33.1 pmol/g) indicates that a significant amount of 232Th is incorporated in the aragonitic skeleton during growth, or attached to clay-sized silicates trapped in the skeletal material. The calculated initial 234U activities in the fossil specimens of Balanophyllia elegans are higher than the 234U activity in modern seawater or in the modern specimen. The higher initial activities could possibly reflect the influx of 234U-enriched continental water into Pleistocene coastal waters, or it could reflect minor diagenetic alteration, a persistent and fundamental problem in dating all corals.Samples from a compound specimen from the Cayucos terrace were subjected to different preparation procedures. Samples prepared by a standard acid washing procedure yielded 230Th-234U ages of 125, 123, and 122 Ky, whereas samples prepared by an abbreviated procedure without acid washing yield significantly lower ages of 113 and 112 Ky. Two other specimens from the same bed yielded 230Th-234U ages of 118 and 115 Ky. Also, two specimens from a stratigraphically higher bed yielded ages of 120 and 117 Ky, and three specimens from a lower bed yield ages of 115, 113, and 101 Ky. Nine of the twelve ages of the treated samples from the Cayucos terrace range from 125 to 113 Ky. However, the ages do not follow the stratigraphie order. Two possible interpretations are ( 1 ) the age of the terrace deposit is 125 Ky and all younger ages reflect variable diagenetic alteration or (2) the age of the terrace is 125 to 113 Ky and the ages reflect sediment reworking over a period of 12 Ky.Three specimens from a single bed on the Shell Beach terrace yield ages of 126, 122, and 121 Ky, similar to the older ages from Cayucos. The ages of solitary corals from the Cayucos and Shell Beach terraces are similar to ages of reef-building corals from terraces at numerous tropical localities. These are correlated with the last interglacial sea-level highstand, which probably stood 2 to 10 m above present sea level. The youngest ages and present elevations of the Cayucos and Shell Beach terraces yield tectonic uplift rates of 0.01 and 0.15 m/Ky, respectively, assuming the original elevation of each terrace was 7 m.Four specimens from the basal gravel on the Nestor terrace yielded ages of 145, 143, 137, and 133 Ky. The three oldest ages, however, are older than that associated with the last interglacial. The possible explanations for these older ages are ( 1 ) diagenic alteration or ( 2 ) the Nestor terrace deposits reflect in some way a poorly documented early phase of the last interglacial sea level high stand.

Radiocarbon dating accuracy improved

Scientists have extended the accuracy of carbon‐14 (14C) dating by correlating dates older than 8,000 years with uranium‐thorium dates that span from 8,000 to 30,000 years before present (ybp, present = 1950). Edouard Bard, Bruno Hamelin, Richard Fairbanks and Alan Zindler, working at Columbia University's Lamont‐Doherty Geological Observatory, dated corals from reefs off Barbados using both 14C and uranium‐234/thorium‐230 by thermal ionization mass spectrometry techniques. They found that the two age data sets deviated in a regular way, allowing the scientists to correlate the two sets of ages. The 14C dates were consistently younger than those determined by uranium‐thorium, and the discrepancy increased to about 3,500 years at 20,000 ybp.