Mike Tubrett

U–Pb dating of detrital zircons for sediment provenance studies—a comparison of laser ablation ICPMS and SIMS techniques

Abstract New developments in U–Pb dating of zircons by laser ablation (LA) ICPMS are described and, for the first time, a direct comparison of detrital zircons dated by LA ICPMS and SIMS methods is presented. True real-time mass bias correction is made by aspirating a Tl/U tracer at the same time as laser ablation. The method is similar to that described in Horn et al. (2000) , except that enriched 233U rather than 235U is used in the tracer solution. Correction for laser-induced Pb/U elemental fractionation is based on a mathematical treatment of time-resolved data that is independent of laser ablation characteristics and does not require external standardisation. Internal corrections for mass bias and elemental fractionation eliminate the effects of variable sample matrix on isotopic ratios and improve the accuracy of U–Pb dating by laser ablation ICPMS. With the proper error propagation, the precision of U–Pb age determinations is only slightly worse than SIMS-based ion probe dating. However, LA ICPMS is capable of much more rapid analysis of the large number of zircons required for sediment provenance studies. There is excellent agreement between concordant laser ablation ICPMS and SIMS analyses of detrital zircons extracted from lower Silurian metasandstone from the Ulven Group (Skarfjell Formation) in the west Norwegian Caledonian nappes. Both LA ICPMS and SIMS U–Pb zircon ages indicate that sedimentary detritus of the Ulven Group was supplied from a terrain containing zircons of Archean, Proterozoic and early Ordovician age.

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.

A North American provenance for Neoproterozoic to Cambrian sandstones in Tasmania

Abstract Tasmania forms an enigmatic province within the Neoproterozoic to Cambrian history of Australia. It lies at the boundary between Australia and North America in most Rodinia reconstructions but no reliable lithostratigraphic correlations have been reported with either mainland Australia or North America. We used detrital zircon age spectra, measured by LAM-ICP-MS, of Neoproterozoic and Cambrian sandstones in Tasmania to search for evidence of correlations with these two continental blocks during the time slice critical to Rodinia breakup. The Tasmanian sandstones are dominated by 1600–1900 Ma and 1200–1500 Ma age zircons. There is little evidence for Grenville (∼1100 Ma) and Ross (∼550 Ma) Orogen sources in these sandstones, in contrast to detrital zircon age spectra of similar age rocks in South Australia. The detrital zircon age spectra of Tasmanian sandstones are different from age spectra reported from British Columbia. They are very similar to age spectra reported from Cambrian sandstones of Nevada, supporting Rodinia reconstructions that place southwestern USA near to Tasmania in the Neoproterozoic.

Testing the reproducibility of Mg/Ca profiles in the deep-water coral Primnoa resedaeformis: putting the proxy through its paces

Two samples of the calcitic deep-sea coral Primnoa resedaeformis have been analysed for Mg/Ca ratios by micro-beam methods ( laser-ablation ICP-MS and electron microprobe). Continuous profiles of Mg/Ca have been studied with the aim of establishing the reproducibility of the variations in different parts of the coral, and therefore the potential use of Mg/Ca as a paleoceanographic or paleoclimatic tracer.

Neon Inductively Coupled Plasma for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

The use of a neon inductively coupled plasma (Ne ICP) in place of an argon inductively coupled plasma (Ar ICP) for laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) is demonstrated. Gas blank mass spectra were obtained comparing the signals obtained with an Ar ICP with the Ne ICP. Sensitivity (signal/concentration) data were also obtained for a number of analytes using solid reference materials. The sensitivity data is complicated by significant changes in optimum Ne ICP parameters (especially nebulizer gas flow) for analytes of varying mass. The sensitivity of the Ne ICP is in general also lower than that obtained for the Ar ICP. The potential for the Ne ICP to reduce the formation of argides, both in the background and from sample induced interferences, is demonstrated. Clearly shown is the reduction of the interferences of 63Cu40Ar and 65Cu40Ar on 103Rh and 105Pd in a Cu2S sample, as well as the reduction of the interferences of 58Ni40Ar and 60Ni40Ar on 98Ru and 100Ru in a NiS sample.

Chemical and structural composition of Atlantic Canadian moose (Alces alces) incisors with patterns of high breakage.

Analysis of mammalian teeth can provide information regarding local environmental conditions. For example, a high incidence of breakage and wear within a population may indicate poor food quality. Individuals consuming a diet causing high mechanical stress on their teeth, and/or lacking the appropriate minerals for proper development, could experience degradation of tooth condition. Previously, we documented a high rate of incisor tooth breakage, with age, in two genetically distinct moose populations in Atlantic Canada. In this study, multi-element (11B, 63Cu, 64Zn, 75As, 85Rb, 88Sr, 111Cd, 118Sn, 137Ba, 208Pb, 232Th, and 238U) analyses using laser ablation ICP-MS were performed on moose incisors from multiple North American regions. The purpose was to determine whether the elemental composition of moose incisors varies among regions, and whether that variation is related to tooth degradation among Atlantic Canadian populations. A principal components analysis revealed that nearly 50% of the elemental variation in the inner enamel matrix of moose teeth was explained by three groupings of elements. The element groupings revealed differences among geographic regions, but did not explain the variation between incisors that were broken and those that were not. Regression models indicate that the elemental group which includes Cu, Pb, and Zn is related to decreases in incisal integrity. It is likely that other environmental factors contribute to the occurrence of increased incisor breakage in affected populations. The relationship between food resource quantity and quality, as a function of moose density, is hypothesized to explain loss of tooth integrity.