Modification of fluid inclusions in quartz by deviatoric stress. II: experimentally induced changes in inclusion volume and composition

Abstract

Fluid inclusions in quartz are known to modify their densities during shear deformation. Modifications of chemical composition are also suspected. However, such changes have not been experimentally demonstrated, their mechanisms remain unexplained, and no criteria are available to assess whether deformed inclusions preserve information on paleofluid properties. To address these issues, quartz crystals containing natural CO₂–H₂O–NaCl fluid inclusions have been experimentally subjected to compressive deviatoric stresses of 90–250 MPa at 700°C and ~600 MPa confining pressure. The resulting microcracking of the inclusions leads to expansion by up to 20%, producing low fluid densities that bear no relation to physical conditions outside the sample. Nevertheless, the chemical composition of the precursor inclusions is preserved. With time the microcracks heal and form swarms of tiny satellite inclusions with a wide range of densities, the highest reflecting the value of the maximum principle stress, σ ₁. These new inclusions lose H₂O via diffusion, thereby passively increasing their salt and gas contents, and triggering plastic deformation of the surrounding quartz via H₂O-weakening. Using microstructural criteria to identify the characteristic types of modified inclusions, both the pre-deformation fluid composition and syn-deformation maximum stress on the host mineral can be derived from microthermometric analysis and thermodynamic modelling.

Appendices

Appendix 1: Measurement of volume fraction of carbonic phase in a precursor inclusion

Following the method of Bakker and Diamond (2006), the volume fraction of the carbonic phases in an example precursor inclusion can be determined from the two data plots in Fig. 12. Figure 12a displays triplicate measurements of the total projected area of the inclusion as a function of rotation angle of the spindle stage. Figure 12b displays triplicate measurements of the area fraction of the carbonic phase as a function of rotation angle of the spindle stage. The maximum in the data array in Fig. 12a lies at the rotation angle 12.5° (marked by an arrow). This angle, when intersected with the area-fraction curve in Fig. 12b, indicates the area fraction (0.193) which most closely approximates the numerical value of the volume fraction. The method carries an uncertainty of ±4% relative. Thus, the result for this example is φ(car) = 0.193 ± 0.008.

Fig. 12

Estimation of the volume fraction of the carbonic phase, φ(car), at room temperature in an example inclusion using the method of Bakker and Diamond (2006). a Projected area of the inclusion versus angle of rotation in the spindle stage. b Area fraction of the carbonic phase versus angle of rotation in the spindle stage. See text for explanation

Appendix 2: Calculation of relationship between V _m(car) and φ(car) for an expanding fluid inclusion

To test the hypothesis that the relict fluid inclusions have expanded their volumes at constant composition, the relationship between the observable parameters V _m(car) and φ(car) can be calculated as a function of volume expansion. For a fluid inclusion of fixed composition, the mole fraction of carbonic phase can be obtained by rearranging Eq. 5:

$$ X_{\text{car}}^{\text{tot}} = 1 - {\frac{{X_{{{\text{H}}_{2} {\text{O}}}}^{\text{tot}} }}{{X_{{{\text{H}}_{2} {\text{O}}}}^{\text{aq}} }}}. $$

(10)

This value can be inserted into a rearrangement of Eq. 2 to yield the molar volume of the carbonic phase:

$$ V_{\text{m}} ({\text{car}}) = {\frac{{V_{\text{m}} ({\text{tot}}) - V_{\text{m}} ({\text{aq}})}}{{X_{\text{car}}^{\text{tot}} }}} + V_{\text{m}} ({\text{aq}}), $$

(11)

where for simplification V _m(aq) is held constant. Input values of V _m(tot) are generated by multiplying the initial (measured) V _m(tot) by an arbitrary factor to simulate expansion of the inclusion volume. For example, 10% expansion increases an initial V _m(tot) of 22 cm³ mol⁻¹ to a final V _m(tot) of 24.2 cm³ mol⁻¹. Solution of Eq. 10 for a range of V _m(tot) yields the model relationship between V _m(aq) and the hypothesized volume expansion (curve in Fig. 11b).

Inserting the appropriate constant V _m(aq) and the V _m(car) value calculated from Eq. 10 into the following expression (Eq. 5 in Diamond 2001):

$$ \varphi ({\text{car}}) = {\frac{{\left( {{\frac{{V_{\text{m}} ({\text{aq}})}}{{V_{\text{m}} ({\text{tot}})}}} - 1} \right)}}{{\left( {{\frac{{V_{\text{m}} ({\text{aq}})}}{{V_{\text{m}} ({\text{car}})}}} - 1} \right)}}}, $$

(12)

yields the model relationship between V _m(car) and φ(car) (curve in Fig. 11a), which can be compared with the observations.