George A. Jenner

ICP-MS — A powerful tool for high-precision trace-element analysis in Earth sciences: Evidence from analysis of selected U.S.G.S. reference samples

Abstract Inductively coupled plasma-mass spectrometry (ICP-MS) is a powerful analytical technique, with considerable application to geochemistry. In this paper, we demonstrate the power of ICP-MS as a tool in petrogenetic studies. Data for 28 elements in seven U.S.G.S. basalt and andesite reference samples (AGV- 1 , BCR- 1 , BHVO- 1 , BIR- 1 , DNC- 1 , W- 1 and W- 2 ) are presented. Accuracy and precision (standard deviation) for 26 of these elements is either excellent ( A detailed description of the ICP-MS technique we have developed is given. A simple HF-HNO 3 dissolution in screwtop Teflon® bombs was used for sample preparation. ICP-MS instrumental sensitivity was measured by external solutions, with surrogate calibration for 4 elements. Matrix correction was made by use of standard addition. Naturally occurring internal standards (Rb, Y, Ce and Pb) were used to correct for instrumental drift between sample and spiked-sample measurements. We report data for the elements: Li, Cs, Rb, Sr, Ba, Zr, Hf, Nb, Ta, Y, Sc, U, Th, Pb and all 14 of the REE. In addition, the procedure acquired data for Be, Mo, W, Tl and Bi. Many elements in this array are crucial for the evaluation of petrogenetic models and source tracing. Herein lies the real power of ICP-MS — one technique, one dissolution, small sample size, good limits of detection, good to excellent accuracy and good precision for many geochemically important elements.

Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas

Fractionation of some or all of the high field strength elements (HFSE) Nb, Ta, Zr, Hf, and Ti relative to other trace elements occurs in igneous rocks from convergent margins and in the average continental crust, and is generally attributed to a process occurring during subduction. The experimental partitioning of an extensive array of trace elements between rutile/melt pairs is presented which enables the effect of rutile during melting in subduction zones to be directly assessed. DNb and DTa are in the range 100–500, DZr and DHf are about 5, whereas all other trace elements analyzed have Drutile/melt less than 0.1. Published D patterns for Nb and Ta between rutile and water-rich fluids are similar to those for melt, whereas the values for Zr and Hf are significantly higher. DNb and DTa values for clinopyroxene and garnet are much lower than for rutile, and cannot cause the fractionation of HFSE from other elements seen in island arcs. The presence of rutile in the subducted slab residue during dehydration may be essential in the production of the geochemical signatures of arc magmas, whereas that of the continental crust, including higher Zr/Sm, may be produced by melting of eclogite.

Inductively coupled plasma-mass spectrometric analysis of geological samples: A critical evaluation based on case studies

Inductively coupled plasma-mass spectrometry (ICP-MS) is a relatively new and promising analytical technique, with the potential to be an excellent analytical tool in earth sciences. In this paper, we provide an overview of the use of ICP-MS in earth sciences, based on our experience with this technique over the last five years. This paper discusses a variety of calibration techniques, chemical separation and preparation procedures, followed by various data acquisition protocols to determine a variety of elements in samples ranging from mineral separates, Fe formations, to ultramafics. The procedures evaluated include methods for the determination of 33 trace elements using a modified standard addition procedure and its adaptation to mg quantities of mineral separates; a procedure for the determination of Y, all the rare-earth elements (REE), and Th using a Na2O2 sinter, with quantification using internal standards; and techniques for the determination of very low-level REE in ultramafic samples using ion-exchange concentration. To solve dissolution difficulties involved in Fe formation samples a procedure using oxalic acid to complex the Fe is demonstrated on the reference material IF-G. Precious-metal determination of all the Pt-group elements along with Re and Au is summarised. The application of ICP-MS to isotope ratios is discussed with reference to the determination of 147Sm144Nd and Pb-Th-U isotope ratios. From the results of these studies, it is clear that the future for ICP-MS in earth sciences is very promising.

Crustally contaminated komatiites and basalts from Kambalda, Western Australia☆

Archean high-Mg basalts at Kambalda, Western Australia, are geochemically different from associated komatiites and tholeiites. The komatiites and tholeiites are moderately to strongly depleted in incompatible elements [e.g., (LaSm)N = 0.63–0.73, ZrY = 2.4] while the overlying basalts are strongly enriched in these elements [(LaSm)N = 1.2–2.6, ZrY = 2.9–3.9]. Interpretation of the magmatic evolution of the volcanic suite is complicated by element mobility during hydrothermal alteration and metamorphism, and during the formation of felsic ocelli in the basalts. The latter process caused large variations in SiO2, FeOtot and MgO, and lesser variations in other elements. Fractional crystallization also played a role in generating a range of element concentrations and rock types, but does not explain the differences between komatiites and basalts. The major influence on the trace-element characteristics of the volcanic suite was contamination of komatiite magma by crustal rocks. Quantitative modelling suggests that: (a) the lower komatiite was contaminated during eruption by thermal erosion and assimilation of a mixture of sediment and tholeiite; and (b) the compositions of the Kambalda high-Mg basalts result from up to 25% contamination of komatiite, at depth, by material with the composition of modern upper continental crust. More mafic compositions such as those estimated for lower-crustal rocks or Archean continental crust are not appropriate.

Determination of partition coefficients for trace elements in high pressure-temperature experimental run products by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS)

Abstract This paper reports the first trace element partition coefficients measured on experimentally produced products (clinopyroxene, garnet, rutile, and glass) by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). A 266 nm (UV) laser microprobe was used to improve ablation characteristics and to achieve a fourfold reduction in ablation pit diameter compared to the previously used 1064 nm beam. Results are compared with PIXE analyses on the same experimental products, and literature values, where available, for similar systems, and include the first simultaneously measured partition coefficients for Zr, Nb, and Ta between rutile and glass. Advantages of the LAM technique include rapid results and simultaneous determination of a wide range of major and trace elements, thus ensuring sampling integrity through time-resolved analysis of the sampled material.

The Sm-Nd age of Kambalda volcanics is 500 Ma too old!

Although Sm-Nd isotopic analyses of volcanics from Kambalda, Western Australia, form a 3.2 Ga linear array, Pb−Pb analyses of the same suite give an isochron age of 2.73±0.03 Ga and a 2.7 Ga model age for the associated sulphides. We suggest that the 3.2 Ga age is incorrect and that the Sm-Nd array results from mixing between depleted mantle and either older continental crust or enriched mantle.

Trace element partition coefficients for clinopyroxene and phlogopite in an alkaline lamprophyre from Newfoundland by LAM-ICP-MS

Mineral/rock matrix partition coefficients have been determined for clinopyroxene (Cpx) and phlogopite from a Mesozoic alkaline lamprophyre from Newfoundland, Canada, by Laser Ablation Microprobe (LAM-ICP-MS). Values for twenty-one elements were obtained for Cpx, whereas only eleven were possible for phlogopite due to REE abundances below detection limits ( 1 ppm). Ablation pits of 40–50 μm diameter enabled investigation of zonation in phenocryst phases. In general, phenocrysts exhibit little trace element zonation except in the outermost overgrowths of Cpx. In these, a fourfold to fivefold increase in many trace element abundances correlates strongly with increasing Ti and Al contents, in agreement with recent experimental studies. Only Li shows appreciable zonation in phlogopite, being enriched in the rims. Comparison of the partition coefficients determined by in-situ laser analysis with those obtained from apparently pure mineral separates by solution ICP-MS indicates that, for several elements, considerable differences exist, e.g., DBa 0.0006 vs. 0.0255. These differences are attributable to the inclusion of trace element-rich overgrowths, and zones of trace element enrichment and micro-inclusions, in addition to the possibility of small amounts of matrix or glass (1–2%) in the nominally clean clinopyroxene mineral separates. Partition coefficients for Cpx are lower than most published values for basaltic rocks, but are comparable to experimental values for basalt determined by SIMS. Comparison with experimental values from lamproite discounts a strong bulk compositional effect, so that the discrepancy with earlier values for basalt should probably be attributed to problems with mineral separates, emphasizing the need for high quality partitioning determinations with in-situ microbeam methods. The phlogopite data extend considerably the published range; the new values are also generally lower than published values, although the discrepancy here may be due to bulk compositional effects, as many published values are from more silicic systems. The lamprophyre values are within the range of sparse experimentally determined values.

Geochemistry of post-Acadian, Carboniferous continental intraplate basalts from the Maritimes Basin, Magdalen Islands, Québec, Canada

Abstract The Magdalen Islands are located in the Gulf of St. Lawrence, near the centre of the late Devonian to Carboniferous composite Maritimes Basin, which opened in response to extensional tectonism following continental collision during the Acadian orogeny. Composed essentially of cap rocks above salt diapirs, these islands expose some of the youngest mafic volcanic rocks in the Maritimes Basin. The Magdalen Islands volcanics are geochemically heterogeneous, tholeiitic to alkalic basalts. The alkalic basalts are generally more enriched in incompatible trace elements than the tholeiites, having higher Zr, Th, Ta, Hf, LREE, Ti and P abundances, average [La/Sm]CN of 3.0±0.8 and [Tb/Yb]CN of 1.6±0.2, and eNd values of +2.0 to +6.3 (at 330 Ma). In the group of tholeiites, average [La/Sm]CN is 1.6±0.9, average [Tb/Yb]CN 1.3±0.2, and eNd values range from −3.0 to +7.0 (at 330 Ma). The incompatible trace element signature of the least enriched tholeiites can be produced by different degrees of partial melting (≈7 to 15%) of a depleted normal mid-ocean ridge basalt (N-MORB) type to slightly enriched asthenospheric mantle source. The genesis of the alkalic basalts necessitates the involvement of a much more enriched, asthenospheric or lithospheric, mantle component having affinities with the source of HIMU-OIB; i.e., high eNdT, Ta/LREE and Nb/LREE values. Tholeiites enriched in incompatible trace elements display compositional shifts toward the alkalic basalts, suggesting various degrees of magma mixing between the two types of melts. Crustal contamination appears as a fairly limited process and may be invoked for only a few samples, explaining nevertheless the low eNd value of −3.0 obtained for one of the tholeiitic basalt. The available geochemical data cannot preclude the involvement of a deep mantle plume in the genesis of the Magdalen basalts. However, based on regional geological considerations (e.g., the relatively small volume of lavas and their dominantly alkalic character) we favor the alternative hypothesis that melting was caused by decompression melting and passive asthenospheric mantle upwelling associated with lithospheric extension, which in turn induced partial melting of the metasomatized subcontinental lower lithospheric mantle, generated small volumes of melts with some HIMU-OIB characteristics, and ultimately the alkalic basalts. The episode of metasomatism considered here is tentatively associated with an earlier phase of alkalic volcanism which occurred circa 550 Ma on the southeastern margin of the North American plate, and was related with the opening of Iapetus. One may suppose that alkalic melts were then trapped at the base of the lithospheric mantle, and that their isolation from the convecting asthenosphere for some 220 Ma led to the isotopic compositions of the most enriched tholeiites and alkalic basalts sampled on the Magdalen Islands.

Isotopic and trace element composition of volcanic glasses from the Akaki Canyon, Cyprus: implications for the origin of the Troodos ophiolite

Abstract We report major elements, K, Rb, Cs, Sr, Ba, Sc, Cr, Ni, Hf, Ta, Th, and REE concentrations and isotopic compositions of Sr, Nd, Pb and O of carefully handpicked volcanic glasses from the Akaki River section of the Troodos ophiolite complex. On the basis of Sr and O isotopic composition and Fe 2 O 3 /FeO ratios, nine of the ten glasses analyzed are considered to be fresh, even with respect to those elements that are easily affected by alteration. The glasses range in composition from basaltic andesite to dacite. Incompatible element concentrations in the mafic compositions are low (10 × chondritic) and the light REE are strongly depleted. However, the ratios of low field strength elements (K, Rb, Cs, Sr, Th) to high field strength elements (Ta, Hf, Ti) and rare earth elements are higher than those in N-MORB. Nd isotopic compositions show a very small range with e Nd(T = 80Ma varying from +6.5 to +7.5. Initial 87 Sr/ 86 Sr ratios range from 0.7033 to 0.7040. Pb isotope ratios have a restricted range with 206 Pb/ 204 Pb= 18.60–18.70, 207 Pb/ 204 Pb= 15.55–15.59 , and 208 Pb/ 204 Pb= 38.08–38.51 . The oxygen isotopic composition for fresh glasses falls within the range reported for fresh, mafic mantle derived volcanics, with δ 18 O varying from 5.4 to 6.5. All geochemical characteristics of the Troodos volcanic glasses are similar to those observed in supra-subduction zone volcanics, but we cannot distinguish between an early stage island arc or back-arc origin for the Troodos ophiolite on a geochemical basis only. However, considering our data and previously published geologic and geochemical data, we suggest that the Troodos ophiolite formed in the earliest stages of island arc development.

New ideas on the Proterozoic-Early Palaeozoic evolution of NW Iberia: insights from U-Pb detrital zircon ages

U–Pb ages were obtained on single detrital zircon grains separated from six samples of Neoproterozoic and Lower Palaeozoic sedimentary and volcanosedimentary rocks from NW Iberia using the laser ablation microprobe-inductively coupled plasma mass spectrometry (LAM-ICP-MS) method. Precambrian greywackes yielded abundant zircons with Neoproterozoic (800–640 Ma) and Mesoproterozoic (0.9–1.2 Ga) ages, and a smaller proportion of Palaeoproterozoic (1.8–2 Ga) and Archaean zircons. Palaeozoic samples (Lower Cambrian and Ordovician) yielded abundant zircons with younger Neoproterozoic (ca. 550 and 620 Ma) and Mesoproterozoic (0.9–1.2 Ga) ages. Palaeoproterozoic (1.8–2 Ga) and Archaean zircons were also found. This data set, used in conjuction with previous paleogeographic and isotopic studies sheds new light on the Precambrian-early Palaeozoic evolution of NW Iberia and is consistent with the following sequence of events: (1) Early Cadomian-Avalonian subduction and arc construction (ca. 800–640 Ma). This magmatic episode created the main arc edifice (Avalonia); (2) full development of a back arc basin upon which the Neoproterozoic sediments were deposited (ca. 640–600 Ma). The combined U–Pb ages of detrital zircons and Nd isotopic features of these sedimenary rocks suggest that they were mostly shed from the main magmatic arc. On the basis of the presence of Grenvillian age detrital zircons with short waterborne transport before incorporation in the sediment, we propose that the basin was possibly located in a peri-Amazonian realm close to West Avalonian terranes. These basins were developed upon a cratonic basement that possibly involved both Grenvillian (ca. 0.9–1.2 Ga) and Transamazonian (ca. 1.9–2.1) igneous rocks. The reported zircon ages suggest a long-lived subduction, starting at ca. 800 Ma and terminated by ca. 580–570 Ma with no geological record of a final collision event; (3) the continuation of extension gave rise to the undocking of Avalonia from the back-arc. Detrital zircon ages in Lowermost Cambrian strata suggest that the main arc edifice had drifted away by ca. 550–540 Ma and was no longer shedding detritus into the back-arc basin. (4) During the Lower Ordovician, further extension of an already thinned crust gave rise to the Lower Ordovician ‘Ollo de Sapo’ magmatic event (ca. 480 Ma). Coeval volcanism in neighbouring areas displaying within-plate geochemical signatures is consistent with an extensional setting for the generation of the Lower Ordovician igneous and sedimentary rocks. Detrital zircon ages and Nd isotopic features of the Ordovician greywackes reflect both an increase in the contribution from older crustal components and the addition of newly accreted crust. A progressively thinning crust is a likely scenario that would explain the simultaneous exhumation of lower crustal (Grenvillian+Transamazonian/Icartian) material and the generation of coeval magmatism. This latter scenario is consistent with models proposed for other circum-North Atlantic Avalonian-Cadomian terranes where repeated episodes of melting occurred in response to subduction and subsequent rifting events.