National Centre for Earth Science Studies

Instrumentation

Teledyne CETAC LSX-213 G2+ laser ablation system: The Teledyne CETAC LSX-213 G2+ laser ablation system offering a 5ns shot pulse is capable of generating > 3 mJ/pulse of laser energy with a spot size range between 4-200 μm. This allows improved absorption of energy and less transmission of the beam into the underlying epoxy. This aids in retarding the instability and U-Pb fractionation during spot analysis which indeed reduces the stabilization period allowing a short warm-up time. The laser is characterized by a flat-topped beam energy profile with pulse width < 5 nsec, enabling a repetition rate up to 20 Hz. The system is equipped with a HeLEx II sample ablation cell allowing automated stage translation with a precision of ±2–3 μm for pre-setting of analytical points. The HeLEx II with an internal volume cup enables effective sample transportation to the ICPMS through the articulated gas inlet/outlet arm. The ablated material from the sample is pushed out of the chamber by purging helium gas which later mixes with the carrier argon gas and gets transported into the plasma. The fast washout of the laser ablation system allows rapid measurement of elements with a minimal fractionation effect.

Agilent 7800 Quadruple ICPMS: The Agilent 7800 Quadrupole ICPMS is equipped with a dual-mode discrete dynode electron multiplier as an ion detector. The quadrupole mass analyzer is a 3 MHz hyperbolic rod having a mass range 2–260 u and variable mass resolution from 0.3 u to 1.0 u. This enables a sensitivity ≤ 5 × 10-7 counts for low masses and ≤ 1 × 10-7 counts for higher masses. The Scott-type double-pass quartz spray chamber with High Matrix Introduction system tandem with 500 - 1600W RF generating capability allows complete ionization of the introduced sample. The Octopole Reaction System (ORS) with He gas in the cell interacts with the ions from the sampling cone to avoid the interference of polyatomic molecules.

Nu Plasma III Multi collector ICPMS: Nu-PLASMA-III is a double-focusing Multi Collector ICP Mass Spectrometer (MC-ICP-MS) designed to provide a high precision isotope ratio of elements. The MC-ICPMS is equipped with 16 Faraday detectors, 2 Daly detectors and 4 Ion counters. The instrument retains the unique, patented, variable dispersion Zoom lens enabling the simultaneous measurement of the isotopes of elements from lithium to the actinide series on its static collector arrays. This allows comprehensive coverage of different isotopic applications over the full mass range.

Calibration strategy

U-Pb geochronology using LA-ICPMS: The ICPMS is tuned for U-Pb geochronology by ablating a raster over NIST 610 (Pearce et al., 1997) standard glass reference material to get the maximum signal intensity and stability for analyzed masses. The ICPMS parameters along with nebulizer and laser gas flows are adjusted to achieve maximum counts for the monitored masses keeping U/Th ratio ~1. Plasma oxidation level is monitored by Th/ThO ratio and double charged ion ratio was monitored using Ca/Ca2+. The He flow is optimized to get maximum counts and to minimize fractionation during sample transport. Spot analysis on accessories is conducted at a spot size of 30 microns with a laser fluence of ~4 J/cm2 on the sampling surface and a repetition rate of 10 Hz in Q-switch mode. LIEF correction is applied using zero ablation time intercepts of least-squares linear regression lines fitted to the time-resolved isotopic ratio data using the protocol of Košler and Sylvester (2003) whereas inter-element fraction and instrumental mass discrimination are corrected by normalization to a matrix-matched primary reference material, which is analyzed during the analytical sessions under the same conditions as the samples. In-situ trace element determination is carried out using NIST 612 and NIST 610 (Pearce et al., 1997) as reference standards for time-drift correction and quality monitoring. Off-line data reduction is carried out using Iolite 4.1, ICPMS DataCalc 12.0 and Glitter software.

Lu-Hf measurements in zircons using LA-MC-ICPMS: Zircons analyzed for U-Pb dating will be reanalyzed and the laser spots positioned as close as possible to the previous crater. Before integrating with the LA, pure 25 ppb JMC 475 solution is analyzed for calibration and peak selection. For hafnium measurement, the laser is operated at a spot size of 50 μm at 10 Hz repetition rate in Q-switch mode, imparting a laser fluence of ~3 J/cm2 in the sample surface. Data are reduced off-line and calculated isotopic ratios and elemental concentrations are processed using Iolite 4.1. Isotopic fractionation of Hf is determined by measuring interference-free 179Hf/177Hf and βHf is determined using a constant 179Hf/177Hf value of 0.7325 (Patchett et al.,1981). Similarly, isotopic fractionation of Yb is corrected by calculating βYb using the measurement of interference-free 173Yb/171Yb using a constant 173Yb/171Yb value of 1.132685 (Vervoort et al., 2004). Correction for the interference of 176Yb on 176Hf is carried out by measuring the interference-free 173Yb and using the constant 176Yb/173Yb value of 0.796218 (Chu et al., 2002).

Detailed analytical protocol for all these techniques is according to “Dev, J. A., Tomson, J. K., Sorcar, N., & Nandakumar, V. (2021). Combined U-Pb/Hf isotopic studies and phase equilibrium modelling of HT-UHT metapelites from Kambam Ultrahigh temperature Belt, South India: Constraints on tectonothermal history of the terrain. Lithos, 106531 (DOI; https://doi.org/10.1016/j.lithos.2021.106531) .”

Sample Preparation

Accessory minerals are analyzed in both mineral separates as well as in thin sections. Accessory minerals are separated from bulk rock samples using conventional Wilfley table and magnetic separation methods in the Thin section and Sample preparation Laboratory at NCESS. The handpicked grains are mounted in epoxy-filled blocks of 2.5 cm diameter and subsequently sectioned and polished. Grain mounts are later cleaned in different steps with ethanol and deionized water in an ultrasonic bath to remove surface contamination during sample preparation before insertion into the laser sample cell. This allows skipping pre-ablation during LA analysis even though pre-ablation is carried out in rare cases if surface contamination is noticed. The internal structure of targeted minerals is studied using backscattered electron (BSE) and cathodoluminescence (CL) images taken either from SEM or EPMA laboratories at NCESS.

Figure: SEM images of different natural reference standards. A. 91500 Zircon, B. Plesovice Zircon, C. BB11 Zircon, D. R632 Rutile, E. Diamantina Monazite, F. IN1 Baddeylite