References

Abstract

This paper reports detailed analyses of Nb and Ta concentrations of 19 eclogite samples and their principal mineral constituents from the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD) and nearby outcrops. We observe highly fractionated and overall suprachondritic Nb/Ta values in minerals, e.g., rutile (4.8-87), titanite (12-62) and amphibole (2.0-67). Amphiboles in amphibolites (retrograded from eclogite) can be classified into two groups: a low Nb/Ta group that bears higher Al contents and is thus of higher pressure origin, and a high Nb/Ta, lower pressure group. The former group was likely formed during subduction; the latter may have formed during exhumation in the presence of rutile and titanite. The significant Nb/Ta fractionation in rutile and other minerals may reflect early dehydration of the subducted slab at shallow depths before the formation of rutile, which occurs at depths =50 km. The dehydration, with amphiboles existing as the main Nb-Ta-bearing phase, would lead to Nb/Ta fractionation, i.e., forming subchondritic Nb/Ta ratios in the released fluids and, complementarily, suprachondritic Nb/Ta ratios in the residual phases. While a large proportion of the fluids may escape from the slab to the mantle wedge, considerable amounts of the fluids can be retained in hydrous minerals within the descending slab, thus forming hydrated cold eclogites with subchondritic Nb/Ta characteristics. As subduction continues to depths over 50 km, rutile appears and consequently controls the Nb-Ta budget. In the presence of rutile, melting of the hydrated cold eclogites with very low Nb/Ta ratios would form magmas with negative Nb, Ta anomalies and subchondritic Nb/Ta. Further dehydration of the continuously descending slab results in even more fractionated Nb/Ta ratios in subsequently released fluids and residues, providing a feasible explanation for the large Nb/Ta variation observed in the modern arc magmas and residual eclogites.

10.1016/j.chemgeo.2009.07.006

Abstract

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a microanalytical tool especially suitable for providing fast and precise U-Pb geochronological results on zircon grains. A new 193 nm excimer laser adapted to a micromachining workstation, equipped with a newly designed two-volume ablation cell and coupled with a quadrupole ICP-MS, is presented here. The system was tuned routinely to achieve sensitivities in the range of 3000 cps/µg g-1 for 238U (< 2% RSD), with a 34 µm spot size, at 5 Hz and ~ 8 J cm-2, while ablating the NIST SRM 612 glass reference material. The system was capable of providing fast (< 1.5 minutes each analysis) and precise (generally < 1.5% 1s errors) 206Pb/238U zircon ages. The ages of widely used reference material zircons (Plesovice, 337 Ma; Temora, 416 Ma; R33, 418 Ma; Sri Lanka, 564 Ma; 91500, 1065 Ma) could be precisely matched, with an accuracy on isotopic ratios that ranged from ~ 2 to ~ 6%, depending on the homogeneity of the natural reference materials.

10.1111/j.1751-908X.2009.00027.x

Abstract

We have determined the trace element compositions of coexisting fluid (carbonate-K-chloride-H2O) and single-phase mineral inclusions in peridotitic (Cr-diopside) and eclogitic (omphacite, garnet) inclusions in fibrous diamonds from the Panda kimberlite (Slave craton, Canada). These diamonds provide a unique insight into the nature of the metasomatic agent, the metasomatised minerals and the pre-metasomatic protolith. The fluid component is strongly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE). Co-existing peridotitic minerals record a melt extraction event (high Cr and Ni) in the protolith prior to the influx of the trapped metasomatic fluid. The silicate minerals are also strongly enriched in LREE. Calculated partition coefficients agree with experimentally determined values in the literature, despite the complex composition of the natural fluid. This indicates that the minerals have re-equilibrated with the metasomatic fluid. The trace element compositions of the mineral inclusions are comparable to many equivalent phases in monocrystalline diamonds. This suggests that the metasomatic fluid and the process recorded in these samples may also be responsible for the growth of some types of monocrystalline diamonds.

10.1016/j.epsl.2009.01.010

Abstract

In order to evaluate the comparative ablation behaviour and ICPMS performance of two excimer lasers with different pulse length, both an ATLEX-SI (ATL) and CompexPro 110 (Coherent) were coupled to the same laser beam delivery system and quadrupole ICPMS at RHUL (RESOlution M-50 and Agilent 7500ce). This allows for a better comparison of the relative merits of 4-6 ns (ATLEX) vs. 20 ns (CompexPro) laser-ablation of different geological and biological materials, since essentially all other remaining beam delivery parameters including laser ablation cell geometry remain unchanged. Initial data will be presented.

20090215_Muller_Comparison of short vs long_EWCPS.pdf [1.15 MB]

Abstract

A new custom-built excimer (193 nm) laser-ablation system with two-volume laser-ablation cell coupled to a quadrupole ICPMS is described, which combines rapid (<1.5 s for 99%) signal washout with full flexibility in sample size (50 x 50 x 25 mm) and high sensitivity (>10000 cps/ppm for mid-high m/z, 55 m, 5 Hz). An application of reconstructing medieval Pb exposure highlights the need for rapid signal washout in unravelling strongly varying Pb peaks in well-preserved archaeological tooth enamel.

10.1039/B805995K

Please contact Dr Müller if you would like a copy of this paper.

Abstract

We present a brandnew custom-built ArF excimer laser-ablation system coupled to a two-volume laser-ablation cell. The key feature of this LA cell is its fast signal washout (~2-3 s), while it retains full flexibility with respect to sample size (50 x 50 mm). Here we present - for the first time - design and initial data, obtained using an Agilent 7500ce quadrupole ICP-MS.

20080000_Muller_Rapid Washout_Winter.pdf [3.06 MB]