Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)

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Basic working principles

The secondary ion mass spectrometry is an analytical technique with the ability to quantitatively detect in situ all the elements (especially for light elements, e.g., H, Li, Be, B) in the chemical periodic table with detection limits reaching part per million or below. The instrument mainly consists of a primary ion source, a mass spectrometer and a secondary ion detection system (Fig. 3-2). In present, the obstacle to the extensive application of SIMS is the dependence on standards to quantify the matrix effects strongly dependent on the chemical compositions of the analyzed materials, and the high cost for this equipment.
The primary ions, generally O- or Cs+ ions for geological materials (Fig. 3-3), are extracted from the sources and focused by a series of electrostatic lenses (L1, L2, L3 and L4 in Fig. 3-4a) onto the sample surface. These lenses can be adjusted to get the appropriate intensity and focusing for measuring. During an entire analytical session, the intensity of the primary ion beam usually fluctuates. If it deviates too much from the initial value, it should be adjusted again because its variation has a significant influence on the instrumental matrix fractionation (Fitzsimons et al., 2000; Hauri et al., 2006a). When the primary ion beam bombards the sample surface, ions will partly transfer their high energy (4 to 20 keV) to the matrix atoms by collision. The atoms that obtain energy in excess of their binding energy will break away from the matrix and be sputtered (Fig. 3-3). Many of the released atoms are neutral while parts of them are charged as ions (Fig. 3-3) that can be accelerated by an electric field and transferred to the mass spectrometer. These secondary ions subsequently can be selectively positive or negative depending on the polarity of the accelerating voltage.
Here, it’s worth noting that not only are the target ions sputtered by the primary beam, but also ions of the other elements froming the matrix and even complex ions formed by combination of two or more atoms of different elements. Commonly, complex ions with mass close to that of target ions can be produced and go through the entrance slit, which requires a mass spectrometer at high mass resolution to distinguish the ions concerned from the interference complex ions. It is achieved by combination of an electrostatic filter and a magnet sector, forming a double-focusing mass spectrometer (Fig. 3-2). The mass resolution can be controlled by adjusting the width of the entrance slit and the exit slit, which will significantly influence the intensities of secondary ions ultimately reaching the collectors. Some technical ways can be adopted to raise the secondary ion transmission (details referring to Hilton (1995)).

Standards for Li isotopes measuring by SIMS

To better calibrate the instrumental fractionation during measuring Li isotopes in peridotite minerals by SIMS, a series of Ol, Cpx and Opx have been analyzed by electron probe and ion probe and been identified to have the potential as standards for in-situ Li isotope measurements (Table 4-1; Su et al., 2015). These standard mineralsare from mantle xenoliths and have a wide compositional range, which can mitigate the matrix effects during calibration. Meanwhile, large quantities of measurements on different grains of each sample as well as on different positions in a single grain suggest homogeneous distributions of major elements, Li and its isotopes for each sample.

Water content by SIMS

For the samples AL32, AL47P1 and AL56, it was not possible to get thick sections for FTIR measurements. Therefore, the water content of minerals in these samples were measured on thin sections by means of IMS 1280 ion probe (Fig. 4-3c) following a procedure similar to that of Füri et al. (2014). Several standards with known water content covering a wide range were used to calibrate the instrumental matrix effect and monitor the moisture in the sample chamber: synthetic forsterite (<0.4 ppm water); synthetic quartz (<1 ppm water); pyrope (56 ppm water); StHs glass (250 ppm water). Rims along with cores in several grains of each phase in individual samples were analyzed to check the intra-grain water homogeneity. During analysis, the spectrometry was set to detect the signals of secondary ions, 17O, 16O1H, 18O, 27Al, 28Si, and 30Si, sputtered by 10 keV primary Cs+ ion beams of intensities 2.5-3nA with the spot size of 20μm in diameter. The electron gun has been always kept open during analysis for charge compensation. Given very low hydrogen
concentrations in samples, the moisture in the background is a crucial factor to influence the accuracy of ultimate analytical results (Koga et al., 2003). A few ways  have been used to diminish it. Through every measurement, the pressure of the sample chamber was always lower than 10-9 mbar and a liquid nitrogen trap was used to eliminate the moisture in the sample chamber. The samples were put in the airlock usually two days before being analyzed to remove water contamination on sample surface. When beginning measuring everyday morning, the degree of drying in the sample chamber was checked by operating the mass scan on the samples with the electron gun on solely. The everyday measured 16O1H/30Si ratios of synthetic forsterite (<0.4 ppm water) were never higher than 0.0030. Each measurement lasted for about 15 minutes, composed of 60s pre-sputtering, 20 cycles with counting times of 2.96s for 17O, and 18O, and 27Al, and 28Si, and 30Si, 12s for 16O1H, 4s for the backgrounds at dummy mass 16.7 and 26.5. The mass resolving power (MRP; M/ΔM) was set at 7000 to separate the peak of 16O1H from that of 17O. An energy offset of 50±10 eV was applied to minimize the matrix effect. The H2O/SiO2 ratio of concerned samples can be calculated using an equation established by polynomial fitting between known H2O/SiO2 and currently measured 16O1H/30Si of these standards (Fig. 3-6). The SiO2 content of each sample is from EMPA results. The estimated errors for all the results are less than 40 ppm (±2σ).

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Measuring procedure for water content

To ensure the spectra of high quality, samples are polished to wafers of 0.1 mm to 0.2 mm in thickness, much thicker than those for optical microscopy. During polishing, these sections were generally attached on glasses with epoxy, which has absorption bands at wavenumbers 2800-3000 cm-1 (Grant et al., 2007b). Therefore epoxy has to be thoroughly removed from sample wafers, to avoid any contribution of the epoxy on the mineral bands to calculate water content of analyzed minerals, especially for Opx. The sections were steeped in acetone for a few hours and then cleaned using ethanol for several times. Before formal measurements some spectra were collected in different zones of a wafer to check whether the epoxy remained. If did, the same washing procedure would be repeated until no bands of epoxy in the spectra randomly collected. Prior to measurements, the samples were scanned for a full image of their thin sections under a Bruker Hyperion 3000 FTIR-microscope.
Then twenty to forty grains for each phase (Ol, Opx and Cpx) were marked on the image, which will facilitate finding enough grains for analysis under the FTIR microscope.
After these preceding preparations, infrared spectra were acquired using a Bruker Hyperion 3000 FTIR-microscope attached to a Bruker Vertex 70 spectrometer (Fig. 3-7) equipped with a liquid nitrogen cooled MCT detector and a KBr beam splitter in the laboratory of infrared and Raman spectrochemistry (LASIR, Université Lille 1).

Table of contents :

1. Introduction
1.1 The lithospheric mantle
1.2 Lithium
1.2.1 Geochemical features
1.2.2 The current research state of Li in the lithospheric mantle
1.3 water in the NAMs
1.3.1 Water in the earth mantle
1.3.2 The current research state of water in the NAMs
1.4 Objective of this thesis
2. Geological background
2.1 The pre-Cenozoic tectonic evolution of the FMC
2.2 The Cenozoic volcanism in the FMC
2.3 The lithospheric mantle beneath the FMC
2.4 The sampling localities
3. Analytical methods
3.1 Sample preparations
3.2 Electron micro-probe analysis (EPMA)
3.3 Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)
3.4 Secondary ion mass spectrometry (SIMS)
3.4.1 Basic working principles
3.4.2 Standards for Li isotopes measuring by SIMS
3.4.3 Measuring procedure for Li isotopes by SIMS
3.4.4 Measuring procedure for water content by SIMS
3.5 Fourier transform infrared (FTIR) spectrometer
3.5.1 The rationale
3.5.2 Measuring procedure for water content by FTIR
3.5.3 Quantitative calculation procedure
4. Results
4.1 Petrological description
4.1.1 Mineral proportions and textures
4.1.2 Spongy textures in sample AL47P1
4.2 Mineral major element compositions
4.2.1 Ol
4.2.2 Opx
4.2.3 Cpx
4.2.4 Spl
4.2.5 Amp
4.2.6 AL47P1
4.3 Trace element compositions
4.3.1 Cpx in samples from Allègre
4.3.2 Cpx in samples from Mont Coupet
4.3.3 Amp in samples MC36 and MC53
4.3.4 Al, Ti, Cr and Ni in Ol from Allègre and Mont Coupet
4.4 Li concentrations and isotopic compositions in minerals
4.4.1 Samples from Allègre
4.4.2 Samples from Mont Coupet
4.5 Water content results measured by FTIR
4.5.1 Infrared spectra
4.5.2 Calculated water content from infrared spectra
4.6 Water content results from SIMS measurements
5. Thermobarometry
6. Discussion
6.1 Depletion by partial melting
6.2 Mantle metasomatism revealed by trace element compositions
6.2.1 Trace element variations in Cpx from Allègre
6.2.2 Trace element variations in Cpx from Mont Coupet
6.2.3 Trace element partition between Cpx and Amp in samples MC36 and MC53
6.2.4 Trace element compositions of metasomatic melts/fluids and melt-rock reaction kinetics
6.3 Li distribution and isotopic composition variations
6.3.1 intra- and inter-mineral Li and Li isotopes distribution in Allègre samples Intra-granular and inter-mineral lithium concentration disequilibrium Distinctive Li isotopic offsets among mineral phases induced by asynchronous metasomatic events
6.3.2 No obvious Li addition but large inter-sample Li isotopic composition variations in peridotite xenoliths from Mont Coupet Nearly equilibrated Li partitioning among minerals phases and no concentration zoning in grains Large isotopic composition variations caused by a recent metasoamtism
6.3.3 Origin of melts with exceptionally light Li isotopic compositions
6.3.4 Precedence relationship of the carbannatitic mantle metasomatism and percolation of the negative δ7Li melt
6.4. Water content of minerals in peridotite xenoliths from the French Massif Central
6.4.1 Assignment of bands in the infrared spectra
6.4.2 Water partitioning among mineral phases
6.4.3 The low water content in minerals from Allègre samples – equilibrated with degassed host magmas
6.4.4 Mineral water concentration variations in xenoliths from Mont Coupet Preservation of original water content during xenolith ascent The processes controlling the water content in peridotite xenoliths from Mont
Coupet Relationship between water content and trace element concentrations in Ol .
6.4.5 Water content in the lithospheric mantle beneath the French Massif Central


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