Applications in Geosciences EPMA

Transcription

1 Applications in Geosciences EPMA

2 SX: Applications in Geosciences The frontier of geological applications for EPMA lies in precise and accurate trace element measurements. Recent years have seen the development of monazite geochronology requiring measurement of U, Th and Pb in complex spectra and with detection limits in the 10 s of ppm. In other recently proven applications, such as the geothermometers Ti in Quartz, and Zr in Rutile, detection limits may extend down to single ppms. CAMECA s fourth-generation EPMA was the platform of choice for the development of these and many other trace element applications. With the SXFive and its optimization for stable, high-current operation and reproducible, high-precision WDS, CAMECA continues to support emergent applications in the geological sciences. Mineralogy X-ray imaging & quantitative analysis of pyroxene Thanks to its high brightness Field Emission source, the CAMECA SXFiveFE electron column offers experimental conditions that combine low accelerating voltages with intense beam currents. Under such optimized conditions, the interaction volume between the electrons and the specimen is reduced, and high spatial resolution can be achieved. X-ray maps were acquired on exsolution lamellae of clinopyroxene in orthopyroxene. The lamalae are only a few hundreds of nanometers wide. Both phases could be analyzed with a fully focused spot, demonstrating the high spatial resolution capabilities of the SXFiveFE. The X-ray maps acquired on Ca and on Mg illustrate the instrument s unique ability to combine low beam voltage for spatial resolution optimization with high sensitivity crystals for X-ray detection optimization. Na 2 O MgO SiO 2 Al 2 O 3 CaO FeO MnO Total Cpx Opx

3 SX: Applications in Geosciences Quantitative analysis was then performed in two large phases of clinopyroxene (Cpx) and orthopyroxene (Opx) using a focused electron beam of 8kV and 20nA. The datasets below demonstrate the superior capabilities of the SXFiveFE to determine accurate quantitative elemental composition. Orthopyroxene quantitative analysis: Point Na 2 O MgO SiO 2 Al 2 O 3 CaO FeO MnO Total Average Orthopyroxene qualitative analysis: Point Na Clinopyroxene 2 O MgO SiO quantitative analysis: 2 Al 2 O 3 CaO FeO MnO Total 1 0, Average

4 SX: Applications in Geosciences Geochronology Minor element zonation in monazite Monazite (LREE phosphate) is a remarkably robust mineral for geochronology and is commonly measured by conventional isotopic methods. However, multiple ages are often recorded within a single monazite grain, and on a spatial scale smaller than can be resolved by even small spot isotopic instruments. Because monazite crystallizes with significant Th and U, and negligible original Pb, it is possible to use non-destructive EPMA analysis for accurate in situ measurement of Th, U and Pb in order to determine the crystallization age of subdomains on a submicron scale. The high spectral resolution and reproducibility of the CAMECA WDS brings unmatched precision and accuracy to the trace element measurements needed for this technique, and have allowed the development of a powerful new capability for geochronology at submicron spatial scales. Distribution of Y and Th in a monazite grain. Y ranges from 0.4 to 1 wt.%, and Th up to 0.7 wt.%. The bulk of this grain has been dated by CAMECA EPMA at 2554+/- 8 m.y. with the thin subvertical fracture-fill at 1837+/- 5 m.y. Data acquired with the CAMECA SXFiveFE at 6kV, 50nA. Courtesty M. Jercinovic, Univ. of Massachusetts, USA and R. Berman, Geological Survey of Canada. 4 2

5 SX: Applications in Geosciences Volcanology Phase analysis and distribution in rocks The sample is a laboratory-produced volcanic rock synthesized at high temperature and high pressure (1100 C, 4kbar). The experiment has been performed with 8 wt% H2O in an internally heated pressure vessel with controlled oxygen fugacity. Such experiments are conducted to simulate and understand the formation of minerals and crystallization path during volcanic eruption. The high-resolution x-ray maps obtenained with CAMECA SXFiveFe reveal the spatial distribution of geochemically important elements prior to quantitative analysis. Three phases are clearly shown: clinopyroxene (no K2O), glass (with 2% of K2O) phlogopite (with 5% of K2O). Experimental conditions for quantitative analysis: 8 20nA X-ray lines detected: Kα line of F, Na, Mg, Al, Si, Ca and Ti, Lα line for Fe Peak & background counting times: 60 s for Fe, 20 s for K, 40 s for the other elements Protocol of quantification: Measurements (n=8) in Phlogopite (K-rich area) Courtesy of Ida di Carlo, Institut des Sciences de la Terre d Orléans, France. 5

6 SX: Applications in Geosciences Geomicrobiology Studying the transformation of gold grains Electron microprobe quantitative maps of gold (Au, red) and silver (Ag, green) using the CAMECA SXFive provides information on the elemental distribution of Au grains by showing the stages of progressive transformation of Au by environmental cycling. (A) Adelaide Hills primary grains are composed of a homogenous Au Ag alloy (inset); Au grains from (B) the Old Pirate deposit (Northern Territory) show initial signs of secondary Au formation. Increasingly transformed Au grains are seen in (C) Fifield (New South Wales) and (D) Lively s Find (Arkaroola, South Australia). Courtesy of Maria Angelica Rea, Carla M.Zammit, and Frank Reith. Bacterial biolofilms on gold grains - implications of geomicrobial transformations of Gold. FEMS microbiology ecology 92, numero16 (2016): fiw

7 SX: Applications in Geosciences Meteorite Studies High-accuracy, non-destructive analysis of small crystals Recent analyses in our EPMA application lab of complex extraterrestrial samples such as the so-called «Paris Meteorite» demonstrate high resolution capabilities of the SXFiveFE Field Emission microprobe. A calcium-aluminium-rich inclusion (CAI) containing refractory phases formed by condensation of gas in the solar nebula was analyzed. X-ray element maps were recorded with the SXFiveFE at an 8 kev electron beam energy and 10 na beam current. The maps below clearly reveal the typical condensation sequence hibonite, perovskite and grossite encased in spinel. With these experimental conditions, the analytical resolution measured from Ti Kα line is better than 200nm. The high resolution Ca/Al/Ti X-ray map (right) also demonstrates zoning in hibonite, suggesting Ti supersaturation of the gas before perovskite could nucleate and accomodate it. Di=diapside; an = anorthite, Sp = spinel, Hib=hibonite, Pv=perovskite; Grss=grossite, Ge=gehlenite. Kα X-rays: Ca=geen, Al=blue, Mg=red (left); Ti = red (right) Thanks to its fine beam size, the SXFiveFE delivers high-accuracy quantitative analysis of these small crystals. MgO Al2O3 SiO2 CaO TiO2 Total Hibonite Grossite Perovskite Spinel Gehlenite Anorthite Diopside Sample courtesy of Dr.R.H. Hewins, Rutgers University, USA and Museum National d Histoire Naturelle, France. 8

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