Quantitative compositional mapping of mineral phases by electron probe micro-analyser

Lanari, Pierre; Vho, Alice; Bovay, Thomas; Airaghi, Laura; Centrella, Stephen

doi:10.1144/sp478.4

Cited by 113 publications

(98 citation statements)

References 98 publications

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“…This result is in line with small equilibration volumes, which would, for example, be smaller than hand specimen sample used to estimate the bulk rock composition. The quantitative mapping strategy employed in this study helps to ensure that the composition of each textural domain is accurately determined (Lanari & Engi, ; Lanari et al., ). This would not be the case by using only qualitative maps and EPMA spot analyses.…”

Section: Discussionmentioning

confidence: 99%

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Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

Laurent

Lanari

Naïr

et al. 2018

Journal Metamorphic Geology

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High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.

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Section: Discussionmentioning

confidence: 99%

“…The X‐ray compositional maps were processed using the program X MapTools 2.2.1 (Lanari et al., , ). The classification assigned each pixel to a mineral, which was then standardized using the high‐resolution spot analyses as internal standards.…”

Section: Methodsmentioning

confidence: 99%

Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

Laurent

Lanari

Naïr

et al. 2018

Journal Metamorphic Geology

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“…The fundamental idea behind our quantitative electron probe chemical mapping technique follows internal standardization methods (De Andrade et al 2006;Lanari et al 2014Lanari et al , 2018. Thus, our technique requires referential spot analysis of all mineral phases within an area to be mapped, X-ray mapping of the area (both using EPMA), and additional data processing by computer.…”

Section: Methodsmentioning

confidence: 99%

“…Previous studies have proposed other means to construct quantitative X-ray maps within a reasonable time frame (Tracy et al 1976;Clarke et al 2001;Tinkham and Ghent 2005;De Andrade et al 2006;Lanari et al 2014Lanari et al , 2018. Among them, XMapTools (Lanari et al 2014(Lanari et al , 2018 provides a reliable analytical procedure by combining mapping results obtained at a higher current intensity with referential spot analysis measured within a mapped area. XMapTools addresses matrix effects by implementing (1) classifying mineral phases based on mapping data and (2) converting X-ray intensities into calibrated mass concentrations for each mineral phase by comparing the X-ray map to referential spot analysis within a mapped area.…”

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confidence: 99%

A rapid and precise quantitative electron probe chemical mapping technique and its application to an ultrahigh-pressure eclogite from the Moldanubian Zone of the Bohemian Massif (Nové Dvory, Czech Republic)

Yasumoto

Yoshida

Kuwatani

et al. 2018

American Mineralogist

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Quantitative X-ray mapping using an electron probe enables quantitative evaluation of inhomogeneities within rocks. Recent studies have proposed methods to construct quantitative chemical maps by combining X-ray maps with referential spot analyses within a mapped area. These approaches address matrix effects by assuming each pixel in the mapped area represents a single phase. In such cases, the spatial resolution of the X-ray maps must be sufficiently high to separate mineral phases. This study proposes a new procedure to reliably quantify centimeter-scale X-ray maps even if the maps contain an ineligible number of pixels analyzing multiple phases because of a large mapping probe diameter. Such multi-phase pixels are statistically classified into their constituent phases by introducing a distribution-based clustering analysis. Furthermore, based on referential spot analyses, we implemented corrections for matrix effects and the backgrounds of single-and multi-phase pixels. Our technique, termed QntMap, was developed as an open source R package and distributed on a social coding platform, GitHub (https://github.com/atusy/qntmap).We applied QntMap to calculate local bulk compositions within an ultrahigh-pressure eclogite from Nové Dvory, Czech Republic. The studied sample is a garnet-rich bimineralic eclogite that includes a 3 mm thick pyroxene-rich layer. A mapped area is approximately 3 × 1 cm in size and oriented normal to the layer. A profile normal to the layer shows increases in Cr 2 O 3 (0.0 to 0.3 wt%) and X Mg [Mg/(Fe+Mg) = 0.5 to 0.8] from the garnet-rich matrix toward the pyroxene-rich layer. A large variation in X Mg and high-Cr 2 O 3 contents in the pyroxene-rich layer are inconsistent with a cumulate origin. We suggest that the pyroxene-rich layer was derived from a pyroxenitic melt that intruded the eclogite.

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“…For each phase, an average of 10 spot analyses were acquired along several transects, at high angle from the grain boundaries, to take into account the mineral heterogeneities. These spot analyses were used as internal standards to obtain the quantified maps (element concentration or phase component proportion), using the software solution XMapTools 2.4.3 (Lanari, Vho, Bovay, Airaghi, & Centrella, ; Lanari et al, ). The mineral abbreviations used below are from Whitney and Evans ().…”

Section: Control Of the Host Rock On The Brittle Precursor Mineralogymentioning

confidence: 99%

On the petrology of brittle precursors of shear zones – An expression of concomitant brittle deformation and fluid–rock interactions in the ‘ductile’ continental crust?

Leydier

Gonçalves

Lanari

et al. 2019

Journal Metamorphic Geology

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The inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings.

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Quantitative compositional mapping of mineral phases by electron probe micro-analyser

Cited by 113 publications

References 98 publications

Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

A rapid and precise quantitative electron probe chemical mapping technique and its application to an ultrahigh-pressure eclogite from the Moldanubian Zone of the Bohemian Massif (Nové Dvory, Czech Republic)

On the petrology of brittle precursors of shear zones – An expression of concomitant brittle deformation and fluid–rock interactions in the ‘ductile’ continental crust?

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