Influence of deformation and fluids on the Ti exchange in quartz

2021 ◽  
Author(s):  
Michel Bestmann ◽  
Giorgio Pennacchioni ◽  
Bernhard Grasemann ◽  
Christoph Schrank
Keyword(s):  
Dynamic Recrystallization ◽  
Eastern Alps ◽  
Critical Examination ◽  
Strain Partitioning ◽  
Ambient Conditions ◽  
Heterogeneous Distribution ◽  
Rock Interaction ◽  
Crystallographic Preferred Orientation ◽  
Quartz Veins ◽  
Ti In Quartz

<p>In the last 10 years, many attempts have been mad to use the titanium-in-quartz geothermobarometer (TitaniQ) to constrain the ambient conditions during mylonitization of quartz in metamorphic rocks. However, most of the studies have shown that the TitaniQ is not readily applicable. First, the application of the TitaniQ calibrations<sup>1-2</sup> is possible if two of the relevant variables (temperature, pressure and Ti activity) can be fixed. But the results of both calibrations can deviate by >100°C. Secondly, several studies have shown that deformation/recrystallization processes, the availability of aqueous fluids, the amount of strain and the duration of deformation result in microstructures with a heterogeneous distribution of Ti concentrations [Ti]. Therefore, in most cases, homogenous and complete equilibration of the [Ti] at the ambient conditions of deformation does not occur. In quartz mylonites, the microstructure is commonly complex as result of strain partitioning and total accumulated strain. For such a complex rock the challenge for applying TitaniQ is to identify those domains where Ti re-equilibration to the syn-kinematic ambient conditions, did possibly occur. Identifying such domains requires the strict integration of correlated high-resolution analysis by optical microscopy, SEM-CL, EBSD and Ti-in-qtz analysis using secondary ion mass spectrometry (SIMS). This integrated information especially provides a robust interpretative tool for the interplay between grain-scale deformation, fluid-rock interaction, geochemical exchange and the evolution of the crystallographic preferred orientation during progressive strain.</p><p>We present the study of the deformation microstructures of quartz veins (Schober Group, Eastern Alps) as key example of such an integrated data collection to unravel characteristic deformation processes responsible for the partial or complete resetting of the Ti-in-quartz system under retrograde conditions. The Schober quartz veins developed at amphibolite facies conditions (510-590 °C, 0.5-0.6 GPa) and were overprinted by deformation at lower greenschist facies. Subgrain rotation (SGR) recrystallization was the dominant recrystallization mechanism during mylonitization. During deformation complete resetting of the initial [Ti] of 3-4 ppm down to 0.2-0.6 ppm occurred in domains (e.g. pressure shadows) where sufficient fluids were available and could percolate through the microstructures. High strain and pervasive quartz dynamic recrystallization did not necessarily result in homogeneous and complete re-equilibration of the [Ti]. Our study reveals that subgrain boundaries were locally pathways for partial [Ti] reset.</p><p>Using the example of mylonitized quartz veins from the Schober Group in the Austroalpine domain of the Eastern Alps, we aim at showing that the initial Ti-in-qtz and corresponding CL signature of the quartz vein is reset to different degrees even at high strains and pervasive dynamic recrystallization, depending on the availability of fluids and its repartitioning.</p><p> </p><p>(1) Huang, R., Audétat, A., 2012. The titanium-in-quartz (TitaniQ) thermobarometer: a critical examination and re-calibration. Geochim. Cosmochim. Acta 84, 75–89.</p><p>(2) Thomas, J.B., Watson, E.B., Spear, F.S., Shemella, P.T., Nayak, S.K., Lanzirozzi, A., 2010. TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz. Contrib. Mineral. Petrol. 160, 743–759.</p><p> </p>

Microstructural evolution of amphibolite-eclogite facies quartz veins under low greenschist facies deformation condition

2020 ◽  
Author(s):  
Michel Bestmann ◽  
Benjamin Huet ◽  
Bernhard Grasemann ◽  
Giorgio Pennacchioni
Keyword(s):  
Deformation Temperature ◽  
Chemical Exchange ◽  
Shear Zones ◽  
Deformation Condition ◽  
Slip Systems ◽  
Rock Interaction ◽  
Crystallographic Preferred Orientation ◽  
Quartz Veins ◽  
Deformation State ◽  
Eclogite Facies

<p>Quartz veins in poly-metamorphic settings often accommodate the latest deformation state and therefore can provide important information. Identification of microfabric (microstructure and crystallographic preferred orientation, CPO) evolution of quartz during mylonitization, and especially of the grain-scale interplay between brittle and crystal-plastic processes, has different relevant implications: e.g., on understanding the efficiency of fluid mobility through deforming quartz that can dramatically influence the rheology and the degree of chemical exchange. However, in order to interpret the microstructure and the related deformation processes it is necessary to relate these especially to the deformation temperature. Particularly the CPO and the Ti-in-qtz geothermometry is used to constrain the deformation temperature. However, both methods have to be applied with great caution because even when many times used some fundamental processes are not fully understood yet.</p><p> </p><p>Here we present results from deformed quartz veins from the Prijakt Nappe (Autroalpine Unit, Schober Mountains, Central Eastern Alps). These veins localized ductile shear and eventually seismic faulting (recorded by the occurrence of pseudotachylytes) within Eo-Alpine eclogite-facies shists. The veins formed shortly after the eclogitic peak, but the temperature of their deformation remains unconstrained. CL imaging reveals critical details for understanding the role of microfracturing and fluid-rock interaction during initial stages of shear localization, the onset of dynamic recrystallization and the resetting of the Ti-in-quartz geochemistry. Even when optical-light-microscopy and EBSD analysis indicate crystal plastic deformation by subgrain rotation CL and orientation contrast (OC) imaging gives evidence of brittle stage of deformation at least for some of the deformation microstructure. Microshear zones show a bulk dark-CL, but still bright tones in cores of new recrystallized grains similar to the CL signature of the host coarse quartz crystals. CL dark tones also match with the pattern of subgrain boundaries. This reflects fluid permeability pathways along subgrain and grain boundaries (identified by widespread fluid inclusions) and the associated partial resetting of Ti concentrations. The CPO of the new grains within the micro-shear zones rotate with the sense of shear around the kinematic Y-axis and cannot be related to the activity of specific slip systems. In contrast the partial single girdle of c-axis within the ultramylonite with its elongated substructured grains and its characteristic layered microstructure can be related to the activity of several slip systems. Misorientation axis analysis indicates that prism</p><p> </p><p> </p><p> </p>

scholarly journals Recrystallization processes, microstructure and crystallographic preferred orientation evolution in polycrystalline ice during high-temperature simple shear

2019 ◽  
Vol 13 (5) ◽  
pp. 1495-1511 ◽  
Author(s):  
Baptiste Journaux ◽  
Thomas Chauve ◽  
Maurine Montagnat ◽  
Andrea Tommasi ◽  
Fabrice Barou ◽  
...  
Keyword(s):  
High Temperature ◽  
Grain Boundary ◽  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Simple Shear ◽  
Shear Plane ◽  
Preferred Orientation ◽  
Dislocation Slip ◽  
Crystallographic Preferred Orientation ◽  
Polycrystalline Ice

Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 Tm) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.

scholarly journals New orogenic type gold occurrences in the Uyanga ore knot (Central Mongolia)

2019 ◽  
Vol 98 ◽  
pp. 08020
Author(s):  
Ariunbileg Sodov ◽  
Olga Gaskova ◽  
Altansukh Gankhuyag ◽  
Dagva-Ochir Lkhagvasuren ◽  
Otgonbaatar Dorjsuren ◽  
...  
Keyword(s):  
Microprobe Analysis ◽  
Gold Deposits ◽  
Gold Content ◽  
Rock Interaction ◽  
Central Mongolia ◽  
Quartz Veins ◽  
Central Asian ◽  
Mineral Assemblages ◽  
Metallogenic Zone ◽  
Water Rock Interaction

The Khangay-Khentey belt is located in central Mongolia (Central Asian Orogenic Belt). The Uyanga ore knot district of the Khangay metallogenic zone are hosted by the lower-middle Devonian volcanogenic-sedimentary Erdenetsogt formation. The new Burgetei, Ult and Senjit gold occurrences were studied. The rocks of the Erdenetsogt formation have an irregular gold content: 0.96 g/t Au is determined in quartz vein (BG-7/16), Au content is highest up to 3.5 g/t in the quartzite-jasper (Ult-7/16 and Ult-9/16) cut by quartz veins in the Ult occurrence. The Senjit occurrence represents Au-Hg-Sb epizonal level of orogenic gold deposits structure with highest Hgand Sb content up to 8.5 ppm and 39 ppm respectively. The Au content of arsenic pyrite of the Burgetei and Ult is below the detection limit by electron microprobe analysis. The Au content of arsenopyrite of the Ult occurrence is highest (up to 238 ppm). The ore-mineral assemblages in the new gold occurrences reflect the differences between three explored sites, formed in the course of fluid evolution during the water-rock interaction. Variable concentrations of indicative elements (As, Te, Sb, Hg) and their ratios confirm this fact.

Cyclic brittle-ductile behaviour recorded in exhuming high-pressure continental units of the Northern Apennines.

2020 ◽  
Author(s):  
Francesco Giuntoli ◽  
Giulio Viola
Keyword(s):  
High Pressure ◽  
High Strain ◽  
Shear Zones ◽  
White Mica ◽  
Northern Apennines ◽  
Strain Partitioning ◽  
Coseismic Slip ◽  
Quartz Veins ◽  
Mylonitic Foliation ◽  
History Of

<p>Exhumation of subducted high-pressure units is favoured by relatively narrow, high-strain shear zones, where most metamorphic and deformational processes occur. Unfortunately, these are commonly overprinted and/or partly or fully obliterated along the exhumation path by younger fabrics or by metamorphic re-equilibration. Their identification and characterization are, therefore, of primary importance when aiming at reconstructing the deepest (and thus earliest) tectonometamorphic history of high-pressure crustal units.</p><p>The Northern Apennines (Italy) offer the opportunity to study a unique setting where continental units (Tuscan Metamorphic Units) were subducted to high-pressure conditions and then exhumed and juxtaposed against non-metamorphic units (Tuscan Nappe). We have studied a well exposed section in the Monticiano-Roccastrada Unit of the Mid Tuscan Ridge (MTR), where a mesoscopic (~20 m length and 5 m high) compressional duplex deforms the Palaeozoic-Triassic quartz-rich metasandstones, metaconglomerates and minor metapelites of the Monte Quoio - Montagnola Senese Unit with a top-to-the-NE sense of shear (Arenarie di Poggio al Carpino Formation; Casini et al., 2007).</p><p>Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal severe strain partitioning within the duplex between metapelite levels, corresponding to 10-50 cm thick high-strain zones, and metasandstone levels, which form relatively strain-free metric horses. Early generations of quartz veins are highly transposed (sheath folds occur) parallel to the metapelitic high-strain shear zones. Veins are composed of iso-oriented quartz, forming up to several centimetre long single-grain ribbons, Mg-carpholite (XMg~ 0.65) needles and K-white mica marking the stretching lineation. Carpholite in the transposed veins invariably defines the stretching direction of shear zones. These high-P veins coexist with a later generation of less deformed, oblique quartz veins. The mylonitic foliation in the metapelites is defined by quartz, chloritoid, pyrophyllite and K-white mica forming a stretching lineation coherent with the one visible in the veins. Geometrical, cross-cutting and petrographic relations suggest that there has occurred cyclic deformation between brittle and viscous conditions, with the veins forming broadly syn-mylonitic shearing. Thermodynamic modeling results suggest >0.8 GPa and ~350°C for the formation of both the high-pressure veins and the mylonitic foliation.</p><p>Shear zones were subsequently folded about the NNW-SSE axis of the regional antiform associated with the MTR. Later brittle overprinting is represented by quart-filled tension gashes and localized C’ planes, mostly within the more competent metasandstone levels, indicating top-to-the-SW reactivation. In summary, our results suggest a cyclic brittle-ductile behaviour occurring at high pressure conditions. This could potentially reflect the repeated alternation between aseismic creep (viscous) and coseismic slip (brittle) during the first stages of the exhumation history of this portion of the northern Apennines, from lower to middle crustal levels in a compressional top-to-the-NE setting. Dating of K-white mica is ongoing to constrain the geodynamic scenario of such shear zone.</p><p> </p><p>Casini, G., Decandia, F.A., Tavarnelli, E., 2007. Analysis of a mesoscopic duplex in SW Tuscany, Italy: implications for thrust system development during positive tectonic inversion. Geol. Soc. London, Spec. Publ. 272, 437–446.</p><p> </p>

scholarly journals Oxygen isotopes in ophicalcites: an ever-lasting controversy?

2020 ◽  
Author(s):  
Daniel Bernoulli ◽  
Helmut Weissert
Keyword(s):  
Oxygen Isotope ◽  
Isotope Geochemistry ◽  
Regional Metamorphism ◽  
Hydrothermal Activity ◽  
Eastern Alps ◽  
Metamorphic Overprint ◽  
Rock Interaction ◽  
Sea Floor ◽  
Detachment Faults ◽  
Mid Atlantic Ridge

Abstract Tectono-sedimentary breccias, known as ophicalcites, overlie serpentinised peridotites at a Jurassic ocean–continent transition along the Penninic-Austroalpine transition in the Eastern Alps of Switzerland. Deformation of the exhumed mantle rocks and breccia formation occurred under decreasing temperatures and along low-angle detachment faults exposing the mantle rocks at the sea floor and was coupled with hydrothermal activity and carbonation of the serpentinites at shallow depth and/or at the sea floor. Carbon isotopes in the ophicalcites persistently show marine values; however, the interpretation of oxygen-isotope values remained controversial: are they related to Jurassic hydrothermal activity or do they reflect Alpine metamorphic overprint? Here we discuss recent interpretations that relate oxygen isotope values measured in ophicalcites exclusively to Jurassic hydrothermal activity; to this end we use data that we earlier obtained along a north–south profile across Graubünden (eastern Switzerland). We revisited the sites of controversial interpretation along a north–south profile in eastern Switzerland. Along this profile, oxygen isotope values in ophicalcites and overlying pelagic sediments, up to 25 my younger than the ophicalcites, show identical values and become systematically lower with increasing Alpine metamorphism; they strongly deviate from values in ophicalcites and pelagic sediments measured along the Mid-Atlantic Ridge or ancient Atlantic ocean-continent transitions as e.g. in the Iberia–Newfoundland transect. The oxygen-isotope values measured in Alpine ophicarbonates thus reflect isotopic resetting during the Alpine orogeny, related to fluid-rock interaction during regional metamorphism. Hydrothermal processes that accompanied the formation of ophicalcites are not disputed; however, they cannot be traced by oxygen isotope geochemistry.

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