Interpretation of fluid inclusions in quartz deformed by

Earth and Planetary Science Letters 417 (2015) 107–119

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Interpretation of fluid inclusions in quartz deformed by weak ductile shearing: Reconstruction of differential stress magnitudes and pre-deformation fluid properties Larryn W. Diamond a,∗ , Alexandre Tarantola b,a a b

Rock–Water Interaction Group, Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, CH-3012 Bern, Switzerland Université de Lorraine, CNRS, CREGU, GeoRessources lab., BP 239, F-54506, Vandoeuvre-lès-Nancy Cedex, France

a r t i c l e

i n f o

Article history: Received 2 December 2014 Received in revised form 4 February 2015 Accepted 12 February 2015 Available online 6 March 2015 Editor: A. Yin Keywords: fluid inclusions quartz ductile deformation differential stress Central Alps

a b s t r a c t A well developed theoretical framework is available in which paleofluid properties, such as chemical composition and density, can be reconstructed from fluid inclusions in minerals that have undergone no ductile deformation. The present study extends this framework to encompass fluid inclusions hosted by quartz that has undergone weak ductile deformation following fluid entrapment. Recent experiments have shown that such deformation causes inclusions to become dismembered into clusters of irregularly shaped relict inclusions surrounded by planar arrays of tiny, new-formed (neonate) inclusions. Comparison of the experimental samples with a naturally sheared quartz vein from Grimsel Pass, Aar Massif, Central Alps, Switzerland, reveals striking similarities. This strong concordance justifies applying the experimentally derived rules of fluid inclusion behaviour to nature. Thus, planar arrays of dismembered inclusions defining cleavage planes in quartz may be taken as diagnostic of small amounts of intracrystalline strain. Deformed inclusions preserve their pre-deformation concentration ratios of gases to electrolytes, but their H2 O contents typically have changed. Morphologically intact inclusions, in contrast, preserve the pre-deformation composition and density of their originally trapped fluid. The orientation of the maximum principal compressive stress (σ1 ) at the time of shear deformation can be derived from the pole to the cleavage plane within which the dismembered inclusions are aligned. Finally, the density of neonate inclusions is commensurate with the pressure value of σ1 at the temperature and time of deformation. This last rule offers a means to estimate magnitudes of shear stresses from fluid inclusion studies. Application of this new paleopiezometer approach to the Grimsel vein yields a differential stress (σ1 –σ3 ) of ∼300 MPa at 390 ± 30 ◦ C during late Miocene NNW–SSE orogenic shortening and regional uplift of the Aar Massif. This differential stress resulted in strain-hardening of the quartz at very low total strain (4% strain, most of the precursor inclusions became dismembered into an irregularly-shaped relict surrounded by a cluster of neonate inclusions. Fig. 3k shows such a cluster viewed down σ1 . The neonates and the branches emanating from the relict inclusion are arrayed on a plane, resulting from healing of a disc-shaped microcrack. This planar microstructure is more evident when viewed perpendicular to σ1 (Fig. 3l). These morphological features are essentially identical to those of the natural inclusions from Grimsel (compare with Fig. 3g, h, i). (2) Fluid inclusion composition

During the experiments all the inclusions behaved as closed systems with respect to their gas and electrolyte components (e.g., the CO2 /NaCl concentration ratio was conserved) but some inclusions lost H2 O to the surrounding quartz and hence their salinities increased. Fig. 5 shows salinities in terms of equivalent NaCl concentration in the aqueous phase. In the experimentally deformed samples, most of the intact inclusions preserved their pre-deformation salinity or became slightly more saline (compare with precursor field, Fig. 5b). The neonates systematically lost H2 O and thus became markedly more saline, whereas the relict inclusions show slightly more scattered salinities than the precursors. The pattern of salinity distributions and relative trends between the intact, relict and neonate inclusions is remarkably similar to that in the natural samples (Fig. 5a). (3) Fluid inclusion density In the particular kind of fluid inclusions under study, the molar volume of the carbonic phase, V m (car), is inversely proportional to the bulk density of the inclusion (Diamond et al., 2010). The value of V m (car) is readily determined from homogenization temperatures of the carbonic liquid and

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vapour phases, either via bubble–point transitions, T bub (car), or dew-point transitions, T dew (car), as plotted on the lower x-axes of Fig. 5a and b. Fig. 5b shows that, in the experiments, intact inclusions preserve their pre-deformation density. In contrast, neonates exhibit much higher densities than the precursors, reflecting a trend towards equilibration with respect to the high imposed σ1 (Diamond et al., 2010). This trend supports other proof provided by Tarantola et al. (2010, 2012) and Diamond et al. (2010), that the healed microcracks in which the neonates are hosted did not form during elastic loading or unloading of the samples. The relict inclusions, on the other hand, either maintain or decrease their densities compared to the precursors. In the natural samples (Fig. 5a), the densities of the various kinds of inclusions (intact, relict and neonate) show essentially the same trends and relative distributions as in the experiments. (4) Alignment of planer fluid inclusion clusters In the experiments, all the thousands of planar clusters of dismembered inclusions lie parallel to each other, defining a distinct, penetrative cleavage throughout the sample. Within a given crystal, the orientations of the disc-shaped clusters all plot on the same crystallographic plane. Depending on how the monocrystal was oriented in each experiment, this plane was either a rhomb (Fig. 4c; Tarantola et al., 2010) or a prism (Tarantola et al., 2012). Similarly, measurements of the natural inclusions reveal that the planar clusters all lie within one crystallographic cleavage plane per crystal, in this case a prism (Fig. 4a, b). (5) Angle between quartz cleavage and principal stress axes In the experiments the cleavage plane that hosts the dismembered inclusions always lies at a very high angle (80–85◦ ) to the direction of σ1 (e.g. Fig. 4c, in which σ1 has been rotated from vertical to the same angle as in Fig. 4a, b to facilitate comparison). For a given monocrystal orientation, the rule is that the cleavage develops along the particular lattice plane (rhomb, prism or presumably basal) that lies closest to perpendicularity with respect to σ1 (Tarantola et al., 2010, 2012), i.e. along the plane with the lowest resolved shear stress. As explained by Tarantola et al. (2010), this orientation is in stark contrast to tensile (mode-I) fractures that form parallel to σ1 at lower confining pressures (such as those that host the secondary fluid inclusions at Grimsel). At the high confining pressures of the experiments, tensile stresses are absent (Paterson, 1978) and so microcracks propagate subparallel to the weakest confinement direction, namely σ3 . Applying the above rule, a dashed field has been constructed in Fig. 4a and b showing the range of possible orientations of σ1 that could have produced the observed microcracks. During all relevant stages of deformation at Grimsel, σ1 was directed NNW–SSE with a subhorizontal plunge (Fig. 1c; Pfiffner et al., 1990). As expected from the experimentally derived rule, the regional σ1 direction indeed intersects the dashed field at a high angle to the prism plane on which the naturally deformed inclusions are aligned. Only prism planes were found in the 2 crystals measured in the Grimsel vein because the crystals by chance have similar orientations (compare positions of c-axes in Fig. 4a, b), attributable to subparallel comb-growth during their original hydrothermal precipitation. Different host planes are expected in crystals oriented differently with respect to σ1 . (6) Deformation microstructures in host quartz As in the Grimsel vein, the single-crystal experiments exhibit patchy undulose optical extinction, deformation lamellae and bands of discrete c-axis misorientation along basal and prism planes. All these features are diagnostic of intracrystalline de-

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formation by bending and slip of the lattice along various glide systems (e.g. Hirth and Tullis, 1992). No subgrains were observed in the experiments but they are common in the Grimsel vein. The development of subgrains and their eventual rotation and recrystallization results from accumulation of dislocations in tilt walls, and so it typically follows development of undulose extinction and bands of discrete c-axis misorientation (e.g. Trepied et al., 1980). Thus, these additional features in the Grimsel case may be due to (a) locally higher strain than in the experiments or (b) differences in orientation of the causative stresses (simple shear in nature versus pure shear in the experiments) or (c) the presence of crystal aggregates in nature versus a single crystal in the experiments or (d) lower strain rates in nature than in the experiments, the greater time available in nature allowing dislocations to reorganize themselves into dislocation boundaries (annealing). Another difference compared to the experiments is the stronger linkage of the microcracks that emanate from the relict inclusions in the Grimsel vein. This linkage produces the anastomosing network of rough fractures that divides some of the quartz crystals into lens-shaped compartments. The roughness of the fractures suggests their contribution to the total strain was limited to the volume increase associated with their opening, without any significant slip along their surfaces. The linkage of the microcracks in the Grimsel vein is probably due to the closer initial spacing of precursor inclusions, and it most likely occurred after development of the other intracrystalline deformation features. No markers are present to measure the overall strain within the relict hydrothermal crystals, but the lack of any perceivable crystal elongation suggests it was small (e.g. L/L < 5%), largely comparable to that in the experiments. The similarities between the experimentally and naturally deformed fluid inclusions with respect to the above six features are extremely strong. Moreover, there are no additional features of the experimentally deformed inclusions that are not found in the natural examples. The fact that the experimental strain rates and P –T conditions were far from natural values seems to be unimportant. Presumably this means that the extreme conditions in the experiments simply accelerate the deformation-induced modifications of the inclusions, which in nature develop at slower, geological rates. 5.2. Deformation regime indicated by microstructures The combination of intracrystalline microstructures described above for the experimental and natural samples, and the occurrence of subgrain rotation recrystallization in the natural case, are collectively diagnostic of deformation in the plastic regime at T > 350–400 ◦ C (Lloyd and Freeman, 1994; Stipp et al., 2002; Law, 2014). On the other hand, the presence of microcracks emanating from the relict fluid inclusions could be interpreted as semibrittle behaviour. However, in both the naturally and experimentally deformed crystals, virtually no shear strain was accommodated by the microcracks, as their orientations are subperpendicular to σ1 . Other slip planes with greater resolved shear stresses remained inactive. Therefore, semibrittle behaviour, which involves appreciable mode-II fracturing at high P conf (Hirth and Tullis, 1994), can be ruled out. The experiments of Tarantola et al. (2010, 2012) showed that the σ1 -perpendicular microcracks formed during application of differential stress. They are a peculiarity of deformed fluid inclusions that has not been previously treated in microstructural studies (perhaps because they are only readily visible in non-standard thick sections), yet they appear to be an integral part of low-strain intracrystalline deformation at least in the low-T portion of the plastic field.

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Fig. 6. (a) Schematic stress–strain curve for quartz (based on Hirth and Tullis, 1994, and Haertel and Herwegh, 2014) at greenschist-facies conditions corresponding to deformation stages 2 and 3 at Grimsel Pass (approx. 360–420 ◦ C at P lith ∼ 250–340 MPa). The small extent of intracrystalline strain in relict hydrothermal quartz crystals in the studied vein is deduced to have occurred near the strain-hardening peak, where the shear stress is highest. (b) Schematic summary of selected microstructures in the Grimsel vein. Large hydrothermal quartz crystals display crystal–plastic deformation features and a crystallographically-controlled penetrative cleavage defined by arrays of deformed fluid inclusions. Contacts of large crystals are marked by subgrain rotation recrystallization. Not shown are anastomosing fractures and late tensile fractures with secondary fluid inclusions (Fig. 3e).

To account for the low strain in the natural and experimental crystals, we propose that fluid inclusion deformation occurred along the transient strain-hardening segment of the stress–strain function of quartz, where differential stresses are highest (Fig. 6a). Higher strains along the margins of the hydrothermal crystals in the Grimsel vein are manifested by subgrain rotation recrystallization (presumably transient strain-softening rather than steadystate flow), which obliterated all the pre-existing fluid inclusions at those sites. 5.3. Estimation of shear stresses and P–T–t evolution of the Grimsel vein Based on their experimental results, Diamond et al. (2010) and Tarantola et al. (2012) proposed that the density of the densest neonate inclusions can be used to reconstruct the P –T conditions of maximum stress achieved during fluid inclusion deformation. This is because the neonate inclusions tend to adopt densities commensurate with the imposed σ1 rather than with the mean of the imposed stresses, (σ1 + σ2 + σ3 )/3. This somewhat counterintuitive behaviour is consistent with the finding of Hirth and Tullis (1994), that coesite forms in quartz deformation experiments at points of stress concentration where σ1 reaches the thermodynamic pressure stability of coesite, even when the mean stress is below the coesite stability threshold. Provided fluid inclusions are trapped in the homogeneous state, their P fluid –T conditions of entrapment are constrained to lie somewhere along the isochore specific to their composition and density (e.g. Diamond, 2003). The isochores of the fluid inclusions in the present case are essentially straight lines in P –T space (Tarantola et al., 2012). The green fan in Fig. 7a shows the range of isochores defined by the precursor and intact inclusions in an example experiment from Diamond et al. (2010). Similarly, the blue fan shows the range of isochores for the neonate inclusions. Since the neonate inclusions are notably denser than the intact and precursor inclusions, the blue fan lies at significantly higher pressure. The same relative positions of isochores is observed in the naturally deformed inclusions at Grimsel (Fig. 7b), consistent with all the other similarities between the experimental and naturally deformed samples enumerated in Section 5.1. In Fig. 7a the green fan of precursor isochores passes through the pressure value of σ3 . This is because the experimental T –σ3

conditions were chosen purposefully so that the internal pressure of the inclusions would match the external confining pressure on the quartz crystal (where P conf = σ3 ). This choice was necessary because preparatory hydrostatic experiments without shear stresses had shown that the inclusions can withstand only about 150 MPa differential pressure (| P incl − P conf |) without changing their shapes and densities (Diamond et al., 2010). In contrast to the precursor isochores, the P –T positions of the neonate isochores were not knowingly constrained a priori. It was therefore a surprise that, for each of the 9 deformation experiments, the a posteriori calculated isochore of the densest neonate was found to run through the experimentally imposed pressure of σ1 at the temperature of the experiment (Tarantola et al., 2012). The aim of the following discussion is to apply this experimental rule to the isochores of the Grimsel inclusions (Fig. 7b) and estimate the paleo-stress values of σ1 and σ3 . Identification of the exact points on the isochores where the Grimsel vein formed and where it was subsequently deformed requires independent estimates of pressure or temperature valid for the time of those events. The necessary independent information is obtained by overlaying the P lith –T –t path of the host rocks of the vein, as given in Fig. 2, onto the isochore plot. Fig. 8 shows the result of this overlay. The first assumption in this treatment is that P fluid during formation of the Grimsel vein was equal to or lower than the lithostatic load. This implies that the P lith –T –t path must cut across the green fan of isochores to explain the range of densities of the intact inclusions. The green rhomb in Fig. 8 shows the unique segment of the path where this condition is fulfilled, suggesting hydrothermal quartz growth and entrapment of the primary CO2 –H2 O–NaCl inclusions during decompression from 510 to 330 MPa, accompanied by cooling from ∼460 to 430 ◦ C. This segment of the P lith –T –t path lies within the stability field of biotite, in accord with field observations, and it overlaps the transition between regional deformation stage 1 (dashed segment of path, see label in Fig. 2) and stage 2 (continuous segment of path, cf. Fig. 2), as deduced from the structural observations. The second assumption is that, during subsequent stage 2 and 3 dextral shearing, lithostatic pressure (i.e. σvertical ) equalled σ2 . Thus, the range of likely T –σ2 conditions during vein deformation are marked by white dots in Fig. 8. The corresponding values of σ3

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Fig. 7. (a) P –T plot of isochores of precursor and intact inclusions (green band) and of neonate inclusions (blue band) in experiment Def-3 (one of 9 equivalent experiments in Diamond et al., 2010; Tarantola et al., 2010, 2012). (b) P –T plot of isochores of natural intact and neonate inclusions in the Grimsel vein. Position of solvus is from Gehrig (1980). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

can be estimated from the experimentally determined constraint (on essentially identical inclusions) that the intact inclusions cannot have experienced more than 150 MPa of differential pressure (| P incl − P conf |) at any time in their history. This pressure can be subtracted from the densest intact isochore (green dashed line, Fig. 8) to provide a lower bound on the possible values of σ3 following vein formation (red dashed line, Fig. 8). Accordingly, feasible T –σ3 conditions during vein deformation are shown by the red dots in Fig. 8. The deformation of the fluid inclusions is therefore interpreted to have occurred between 420 and 360 ◦ C with σ3 between 305 and 220 MPa, some 30 MPa below the corresponding σ2 values. Now applying the experimental rule to estimate σ1 , the blue isothermal arrows in Fig. 8 link each example σ3 value (red dots) with a corresponding σ1 value on the densest neonate isochore (blue dots). Thus, σ1 is estimated to have been between 615 and 510 MPa, implying a range in shear stresses (σ1 –σ3 ) between 310 and 290 MPa (an example of 305 MPa is labelled in Fig. 8). Secondary fluid inclusions were trapped in the quartz vein during subsequent NNW-striking tensile fracturing (brown segment of P –T –t path in Fig. 8). As some of these inclusions were also dismembered into clusters of neonates surrounding relicts, ductile shearing is deduced to have overlapped (i.e. alternated) with the onset of brittle fracturing. Over the final segment of the exhumation path (t < 5 Ma), fluid pressures fell below lithostatic, reaching hydrostatic values below 200 ◦ C, as recorded by entrapment of the last secondary fluid inclusions at 3.3 Ma (Hofmann et al., 2004).

6. Conclusions

To our knowledge, the six key features described in Section 5.1 are unique to fluid inclusions in quartz that has been weakly deformed by ductile shearing. Since the features of the experimental inclusions are identical to those in the Grimsel vein, the difference between pure and simple shearing appears inconsequential. The features differ clearly from those known to be induced by differential hydrostatic pressures. In settings where P incl  P conf , fluid inclusions implode and generate 3D haloes of neonates around the relict inclusions (Sterner and Bodnar, 1989), rather than the 2D disc-shaped clusters observed in the sheared samples from the Grimsel vein. In settings where P incl P conf , fluid inclusions decrepitate by generating microcracks with orientations that bear no direct relation to the orientation of the host crystal (Pêcher, 1981; Gratier and Jenatton, 1984). In samples deformed by nonhydrostatic shear stresses, by contrast, the microcracks emanating from fluid inclusions are aligned within a crystallographic plane that is oriented subperpendicular to σ1 . We propose that the observed one-to-one correspondence in features justifies interpreting the Grimsel samples – and by extension all other natural quartz samples affected by weak ductile deformation – in terms of the experimental results. Accordingly, we suggest that the following kinds of information can be obtained by applying the experimental rules to naturally deformed inclusions.

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and time of ductile deformation. By constructing the relevant fluid isochore, either the pressure value of σ1 or the temperature of deformation can be retrieved if the other variable is known independently. Application of this last rule to the Grimsel Pass quartz vein has yielded differential stresses of ∼300 MPa. Shear stresses comparable to or significantly higher than this have been estimated in other geological settings by applying steady-state flow laws to mylonites (e.g. 300 MPa by Küster and Stöckhert, 1997; 370–1170 MPa by Wightman et al., 2006; see also discussion by Mancktelow, 2008). We emphasize that our estimate for the Grimsel vein most likely represents the transient, strain-hardening segment of the stress–strain function of quartz at strains less than 5% L/L (Fig. 6). Thus, neonate inclusions are a novel paleopiezometer for this regime of weak ductile deformation in nature. Overall, the remarkable correspondence that we have found between the experimentally and naturally deformed samples provides a clear framework in which to interpret the significance, composition and density of fluid inclusions in quartz that has undergone weak ductile shearing. Further implementation of the above rules should provide new insight into processes of fluid–rock interaction within the ductile regime of the Earth’s crust. Acknowledgements

Fig. 8. Reconstruction of formation and deformation conditions of the studied vein at Grimsel Pass. The P lith –T –t path (black) and quartz brittle–ductile transition (grey box) are from Fig. 2. Grey C/B marks chlorite/biotite stability transition. Blue dots: maximum principal stress (σ1 ) during stage 2 and 3 dextral shearing; White dots: intermediate principal stress (σ2 ); Red dots: minimum principal stress (σ3 ). Application of the experimentally derived rules of fluid inclusion behaviour (Fig. 6; see text for explanation) yields ∼310 MPa for the differential stress (σ1 –σ3 ) during weak ductile shearing of the vein. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(1) Planar arrays of dismembered fluid inclusions defining a penetrative cleavage in quartz (Fig. 6b) are diagnostic of small degrees of intracrystalline plastic deformation. (2) All types of deformed inclusions (intact, relict and neonate) preserve the pre-deformation concentration ratios of gases to electrolytes. (3) The salinity and density of the central relict inclusion in each cluster cannot be used to determine the properties of the fluid that was trapped in the inclusions prior to deformation. Similarly, the systematically elevated salinity of the neonate inclusions is an artefact of the deformation and thus it provides no more than a maximum value for the salinity of the precursor inclusions. (4) Analysis of intact inclusions in deformed samples yields the pre-deformation properties of the fluid inclusions. Both the chemical composition and density of intact inclusions closely represent the properties of the precursors and hence the properties of the original parent fluid. (5) The orientation of σ1 at the time of ductile deformation can be derived from the pole to the cleavage plane within which the dismembered inclusions are aligned. This interpretation is the reverse of what would be deduced if the cleavage planes were to be interpreted as tensile (mode-I) fractures formed under low confining pressure in the purely brittle regime (cf. Boullier, 1999; Tarantola et al., 2010). (6) The density of the densest neonate inclusions, as obtained by appropriate microthermometric analysis and thermodynamic modelling, reflects the pressure value of σ1 at the temperature

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