Derek Vance

The significance of monazite U–Th–Pb age data in metamorphic assemblages: a combined study of monazite and garnet chronometry

Abstract In this study, a coupled U–Th–Pb isotopic and EMP chemical study was carried out in situ on monazite (micro-inclusions within garnet as well as matrix grains) from rocks recovered from two chronologically well-constrained areas of the Himalayan orogen. Monazite inclusions within garnets from three samples yield ages of ∼44–36 Ma, whereas matrix grains within one sample are typically younger (30–26 Ma). Y depletion of these younger matrix grains indicates that they grew after garnet had crystallised. The recognition of episodic monazite growth during regional metamorphism, first at greenschist facies (before garnet growth) and secondly at upper amphibolite facies (post-dating garnet growth), allows relative growth ages of the occluding garnet to be calculated. These are in excellent agreement with Sm–Nd garnet ages from surrounding units. This approach not only provides a ready means of obtaining porphyroblast growth ages but also allows the combination of precise U–Th–Pb data from metamorphic monazite with thermobarometric information obtained from rock-forming minerals.

An assessment of mass discrimination in MC-ICPMS using Nd isotopes

The stability of mass discrimination in multiple-collector magnetic-sector inductively coupled mass spectrometry (MC-ICPMS)—with time, between elements and between samples—makes it potentially much simpler to deal with than in TIMS. However, while the stability of mass bias across limited areas of the mass spectrum is critical to the derivation of precise isotope ratios, the fundamentals of mass bias behaviour of MC-ICPMS instruments are still incompletely characterised. In this paper, we present Nd isotope data for standards and samples with the aim of using the well-known Nd isotopic system to obtain systematic information on the nature of the mass bias in MC-ICPMS. n nAn extensive Nd dataset was obtained on a Micromass IsoProbe and more limited data on two different Nu Instrument machines. The IsoProbe data were obtained over 18 months between March 2000 and September 2001. The standard approach of using 146Nd/144Nd to normalise other Nd isotope ratios leads to both inaccurate (by around 100 ppm in the case of the exponential-law normalised 143Nd/144Nd ratio) and relatively imprecise (2 rsd=45 ppm for 143Nd/144Nd) results. On the IsoProbe, this is due to the fact that the magnitude of the exponential mass bias itself varies, albeit by a small amount, over limited mass ranges such as that for Nd. The result is that the inaccuracy is much greater for isotope ratios that have an average mass further away from that of the normalising ratio—for example, >500 ppm for 150Nd/144Nd vs. <30 ppm for 145Nd/144Nd. Both accuracy and precision increase dramatically if a normalising ratio is used that is close in mass to the ratio to be normalised. 143Nd/144Nd (average mass=143.5) normalisation with 145Nd/142Nd (average mass=143.5) yields a value identical to TIMS. An alternative approach is to use post-normalisation linear correlations between isotope ratios to do a secondary mass bias correction. Such an approach with the 143Nd/144Nd ratio using 142Nd/144Nd yields a value identical to the TIMS value and a long-term reproducibility of 14–20 ppm. This compares with a reproducibility of 45 ppm using simple normalisation to 146Nd/144Nd. We have tested both these approaches on standards and samples with 143Nd/144Nd up to 30 e units different from our in-house standard and identical results to TIMS are obtained. n nPost-normalisation correlations between isotope ratios obtained on the Nu Instruments MC-ICPMS are qualitatively very similar to those obtained on the IsoProbe and suggest a common cause. This, despite the very different physical characteristics of the various instruments. Furthermore, it also appears that qualitatively very similar effects, though at much smaller magnitude, are observed in TIMS. The data suggest that the quasi-empirical exponential law does not perfectly correct for mass discrimination on any mass spectrometer. This inadequacy becomes important, for precise isotope ratio analysis, when dealing with the large mass discriminations inherent in MC-ICPMS.

Erosion and Exhumation in the Himalaya from cosmogenic isotope inventories of river sediments

The outward erosional flux is a key factor in the tectonic evolution of mountain belts and there is much debate about the feedbacks between tectonics, erosion and climate. Here we use cosmogenic nuclides (10Be and 26Al) analysed in quartz from river sediments from the Upper Ganges catchment to make the first direct measurements of large-scale erosion rates in a rapidly uplifting mountain belt. The erosion rates are highest in the High Himalaya at 2.7±0.3 mm/yr (1σ errors), fall to 1.2±0.1 mm/yr on the southern edge of the Tibetan Plateau and are 0.8±0.3 to <0.6 mm/yr in the foothills to the south of the high mountains. These relative estimates are corroborated by the Nd isotopic mass balance of the river sediment. Analysis of sediment from an abandoned terrace suggests that similar erosion rates have been maintained for at least the last few thousand years. The data presented here, along with data recently published for European river catchments, demonstrate that a log–linear relationship between relief and erosion rate holds over three orders of magnitude variation in erosion rate and between very different climatic and tectonic regimes. n nThe erosion rate estimates from cosmogenic nuclides correlate well with exhumation rates calculated from previously published apatite fission track ages in the Indian Himalaya. This confirms that much of the exhumation in the Himalayan mountain chain is now balanced by erosion. However, exhumation rates calculated from high blocking temperature systems, such as 40Ar/39Ar in muscovite, imply lower exhumation rates. Rocks presently at the surface must have undergone a three- to six-fold increase in exhumation rate within the last few million years. We show how this could be explained either by climatic forcing of erosion rate changes or by tectonics. Published evidence for equally rapid changes of exhumation rate in the past and the probable diachroneity in the time at which the present exhumation rates accelerated imply that tectonics has moderated at least some of the change in exhumation rates.

Mass discrimination correction in multiple-collector plasma source mass spectrometry: an example using Cu and Zn isotopes

Multiple-collector magnetic sector ICP-MS affords the possibility of applying instrumental mass discrimination corrections using a reference isotope ratio of an element (dopant) other than the analyte. Much attention has focused on the use of this approach for lead isotope analysis using a thallium dopant and the technique has also been applied to copper–zinc isotope analysis. The most successful applications of the doping approach have established empirical mass bias relationships between dopant and analyte isotope ratios but this often has to be done for single analytical sessions. Insufficient intra-session variation in mass discrimination often leads to poor constraints on these relationships. Moreover, with the Tl–Pb system there is some doubt over whether samples and standards exhibit the same relationship. Here we show that for the Cu–Zn system, two improvements on previous approaches lead to precise and accurate isotope ratios for unknowns. Firstly, addition of Sr to mixed Cu–Zn standard solutions generates extreme variation in mass bias so that empirical mass bias relationships between analyte and dopant are much better constrained. Secondly, we show that inadequate chemistry, specifically the inefficient removal of matrix Fe and Ti, seriously compromises the Cu–Zn isotope analysis of samples but that with clean chemistry, samples with complex matrices demonstrably yield similar mass discrimination relationships between Cu and Zn isotopes to standards. We also document previously unreported aspects of Cu–Zn isotope analysis: (1) that Cu–Zn mass bias relationships depend critically on the Cu/Zn ratio of the solution; (2) that for an introduction system with a desolvating membrane, the behaviour of standards is highly variable, perhaps due to variations in the oxidation state of Cu in the solution, and that this can be overcome by the passage of standards through the ion exchange procedure used to purify samples. Finally, we document chemical separation and mass spectrometric techniques that permit the isotopic analysis of order of magnitude smaller samples than previously achieved and report values for BCR-1 basalt standard of δ66Zn = 0.20 ± 0.09‰ n (n n = 12) and δ65Cu = 0.07 ± 0.08‰ n (n n = 6) at the 95% confidence level.

New neodymium isotope data quantify Nile involvement in Mediterranean anoxic episodes

The development of widespread anoxic conditions in the deep oceans is evidenced by the accumulation and preservation of organic-carbon–rich sediments, but its precise cause remains controversial. The two most popular hypotheses involve (1) circulation-induced increased stratification resulting in reduced oxygenation of deep waters or (2) enhanced productivity in the surface ocean, increasing the raining down of organic matter and overwhelming the oxic remineralization potential of the deep ocean. In the periodic development of deep-water anoxia in the Pliocene–Pleistocene Mediterranean Sea, increased riverine runoff has been implicated both as a source for nutrients that fuel enhanced photic-zone productivity and a source of a less dense freshwater cap leading to reduced circulation, basin-wide stagnation, and deep-water oxygen starvation. Monsoon-driven increases in Nile River discharge and increased regional precipitation due to enhanced westerly activity—two mechanisms that represent fundamentally different climatic driving forces—have both been suggested as causes of the altered freshwater balance. Here we present data that confirm a distinctive neodymium (Nd) isotope signature for the Nile River relative to the Eastern Mediterranean—providing a new tracer of enhanced Nile outflow into the Mediterranean in the past. We further present Nd isotope data for planktonic foraminifera that suggest a clear increase in Nile discharge during the central intense period of two recent anoxic events. Our data also suggest, however, that other regional freshwater sources were more important at the beginning and end of the anoxic events. Taken at face value, the data appear to imply a temporal link between peaks in Nile discharge and enhanced westerly activity.

Isotopic constraints on the source of Argentinian loess – with implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima

We present rare-earth element (REE) and Sr–Nd isotopic data for Argentinian loess (28–38°S) with two aims: (1) to examine the source regions of Argentinian loess and the constraints that these put on palaeo-wind directions; (2) to further investigate the source of Antarctic ice-core dust and to test the hypothesis that some of it could be derived from a region to the north of Patagonia – into which the dry, dusty, westerly dominated Patagonian climate expanded during Quaternary glacial maxima. Sr–Nd isotopic data for Argentinian loess from north of 37°S are distinct from Patagonian loess compositions in that they have more radiogenic Sr (87Sr/86Sr=0.7059–0.7123) and less radiogenic Nd (ϵNd=−0.8 to −6.4). REE patterns and Sr–Nd isotopic values are relatively homogeneous for multiple samples taken from single loess sections but show significant differences between sections. In general, there is a northward change from Patagonia-like REE patterns and isotopic values away from volcanogenic signatures and towards those that are more like the continental crust. The latitudinal Nd isotopic pattern is remarkably similar to that for Andean volcanic rocks and suggests derivation of loess from the Andes by more-or-less direct westerly transport. For loess sections in the north, the data imply a contribution from Palaeozoic gneisses to the northwest in the Chilean Altiplano. Sr–Nd data for extra-Patagonian Argentinian loess north of 37°S do not support a significant role for this source region in supplying dust to Antarctica at the last glacial maximum. This conclusion contrasts with previous studies that suggest a significant northward shift in the climatic belts – and in particular the westerlies and the Antarctic Polar Front – during Quaternary glacial maxima. Very systematic relations between the Sr–Nd isotopic composition of loess and their Andean source highlights shifts in the Sr isotopic composition of loess to more radiogenic values and strongly suggests that the slight offset between Patagonian loess and ice-core dust identified by previous workers is due to grain-size differentiation effects.

Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation

Atmospheric CO2 fluctuations over glacial–interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. Leading hypotheses put forward to explain glacial–interglacial atmospheric CO2 variations invoke changes in deep-ocean carbon storage, probably modulated by processes in the Southern Ocean, where much of the deep ocean is ventilated. A central aspect of such models is that, during deglaciations, an isolated glacial deep-ocean carbon reservoir is reconnected with the atmosphere, driving the atmospheric CO2 rise observed in ice-core records. However, direct documentation of changes in surface ocean carbon content and the associated transfer of carbon to the atmosphere during deglaciations has been hindered by the lack of proxy reconstructions that unambiguously reflect the oceanic carbonate system. Radiocarbon activity tracks changes in ocean ventilation, but not in ocean carbon content, whereas proxies that record increased deglacial upwelling do not constrain the proportion of upwelled carbon that is degassed relative to that which is taken up by the biological pump. Here we apply the boron isotope pH proxy in planktic foraminifera to two sediment cores from the sub-Antarctic Atlantic and the eastern equatorial Pacific as a more direct tracer of oceanic CO2 outgassing. We show that surface waters at both locations, which partly derive from deep water upwelled in the Southern Ocean, became a significant source of carbon to the atmosphere during the last deglaciation, when the concentration of atmospheric CO2 was increasing. This oceanic CO2 outgassing supports the view that the ventilation of a deep-ocean carbon reservoir in the Southern Ocean had a key role in the deglacial CO2 rise, although our results allow for the possibility that processes operating in other regions may also have been important for the glacial–interglacial ocean–atmosphere exchange of carbon.

Fluid-enhanced melting during prograde metamorphism

Anatexis is a commonly recognized feature of high-grade metamorphism, but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path. A suite of small-volume, deformed, two-mica leucogranites has been recognized within the High Himalayan Crystalline Series of the Garhwal Himalaya. These granites are consistently more siliceous than minimum-melt granite compositions and are characterized by low Rb/Sr ratios, high Ba, low abundances of HFS elements and positive Eu anomalies. Such trace-element characteristics contrast strongly with the geochemistry of the well-studied Early Miocene leucogranites of the High Himalaya, derived from fluid-absent melting. Sm–Nd garnet dating of one deformed granite indicates a crystallization age of 39±3u2009Ma, c.u200915u2009Ma before the emplacement of the more voluminous High Himalayan leucogranites. Whilst some entrainment of restitic phases cannot be excluded, trace element signatures suggest a low temperature (<650°C) crustal melt formed under conditions of high H2O activity. Positive Eu anomalies and unusually low Rb/Sr ratios are indicative of rapid, disequilibrium melting. Fluid-enhanced melting may be a common feature of prograde upper amphibolite-facies metamorphism of orogenic belts, predating peak metamorphism by at least 15u2009Ma. These melts will only crystallize within this period if they segregate from their protoliths. Subsequent dating of long-lived melts would indicate erroneously young ages for the prograde melting events. However, melts formed in this way may be recognized by their distinctive trace-element chemistry. The persistence of early formed melts within an orogen provides insights into the prograde heating path, and may be critical in controlling the rheology of the middle crust, and hence its deformational history.