Temperature micro-mapping in oscillatory-zoned chlorite: Application to study of a green-schist facies fault zone in the Pyrenean Axial Zone (Spain)
-
Vincent Trincal
,
Pierre Lanari
Abstract
Oscillatory compositional zoning in minerals has been observed in hydrothermal, magmatic, and metamorphic environments and is commonly attributed to chemical or physical cyclical changes during crystal growth. Chemical zoning is a common feature of solid solutions, which has been rarely reported in phyllosilicates. In this study, oscillatory zoning in chlorite is described in samples from the Pic-de-Port- Vieux thrust, a minor thrust fault associated to the major Gavarnie thrust fault zone (Central Pyrenees, Spain). The Pic-de-Port-Vieux thrust sheet comprises a 1-20 m thick layer of Triassic red pelite and sandstone thrust over mylonitized Cretaceous dolomitic limestone. The thrust fault zone deformation comprises secondary faults and cleavage affecting the Triassic pelite and sandstone. An important feature responsible to this deformation is a set of veins filled by quartz and chlorite. Chlorite is present in crack-seal extension veins and in shear veins; both structures opened under the same stress conditions. In some shear veins, chlorite occurs as pseudo-uniaxial plates arranged in rosette-shaped aggregates. These aggregates appear to have developed as a result of radial growth of the chlorite platelets. Oscillatory zoning has been imaged by backscattered scanning electron microscopy and by X‑ray quantitative micro-mapping. These oscillations correspond to chemical zoning with alternating iron-rich and magnesium-rich bands. The chlorite composition ranges from a Fe-rich pole to a Mg-rich pole. Fe3+/ΣFe values were measured in chlorite using μ-XANES spot analyses and vary from 0.23 to 0.44. The highest values are in the Ferich area. Temperature maps, built from standardized microprobe X‑ray images and redox state using the program XMapTools, indicate oscillatory variations from about 310 to 400 ± 50 °C during chlorite crystallization. These temperature variations are correlated with a Fe3+/ΣFe variation by Al3+Fe3+-1 and ditrioctahedral substitutions highlighted by Mg and FeTot contents (Fe-Mg zoning). Chemical variations could be then explained by alternation of cooling times and cyclical pulses of a fluid hotter than the host rock. It is however not excluded that kinetic effects influence the incorporation of Mg or Fe during chlorite crystallization.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- On understanding the structure and composition of crystals
- Stable and metastable silicate liquid immiscibility in ferrobasalts
- When was the Earth’s conveyor belt set in motion?
- Trace element partitioning into sulfide: How lithophile elements become chalcophile and vice versa
- Petrology on Mars
- Hf and Nd isotope systematics of early Archean komatiites from surface sampling and ICDP drilling in the Barberton Greenstone Belt, South Africa
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- Tracking the thermal decomposition of plasma-sprayed hydroxylapatite
- Experimental constraints on bubble formation and growth during magma ascent: A review
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- Crystal structure, high-pressure, and high-temperature behavior of carbonates in the K2Mg(CO3)2–Na2Mg(CO3)2 join
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- Micro- and nano-characterization of Zn-clays in nonsulfide supergene ores of southern Peru
- Mineralogical characterization of individual growth structures of Mn-nodules with different Ni+Cu content from the central Pacific Ocean
- The magma plumbing system of the Emeishan large igneous province and its role in basaltic magma differentiation in a continental setting
- Weathering of the Ethiopian volcanic province: A new weathering index to characterize and compare soils
- Recommended mineral-melt partition coefficients for FRTEs (Cu), Ga, and Ge during mantle melting
- Tetrahedral plot diagram: A geometrical solution for quaternary systems
- Investigations on alunogen under Mars-relevant temperature conditions: An example for a single-crystal-to-single-crystal phase transition
- Crystallization of calcium oxalate hydrates by interaction of calcite marble with fungus Aspergillus niger
- Direct and indirect evidence for free oxygen (O2–) in MO-silicate glasses and melts (M = Mg, Ca, Pb)
- Effect of fluid composition on growth rate of monazite in quartzite at 1.0 GPa and 1000 °C
- Determination of the full elastic tensor of single crystals using shear wave velocities by Brillouin spectroscopy
- Temperature dependence of the velocity-density relation for liquid metals under high pressure: Implications for the Earth’s outer core
- Thermal, compositional, and compressional demagnetization of cementite
- The MnCO3-II high-pressure polymorph of rhodocrosite
- Lanthanide tetrads in normalized rare element patterns of zircon from the Koktokay No. 3 granitic pegmatite, Altay, NW China
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