Stable isotopes

The oxygen isotope data for the three aliquots of feldspar yielded a mean δ¹⁸O = 8.2 ± 0.2‰; full results are reported in Table 4.1. The hydrogen isotope composition of included fluids within a large aliquot of the feldspar yielded a bulk δD value of -48‰, from six steps that were measured along the step-heated profile at increasing temperatures (Figure

4.3). The initial release of hydrogen was detected at over 300°C, with progressively more gas being released at higher temperatures (Figure 4.3 a). The peak release of hydrogen was between 580 and 650°C, which recovered approximately 100µmol. Hydrogen was still being recovered in the final step between 875 and 1200°C. The comparison of δD to temperature yields an apparent parabolic function that becomes heavier with increasing temperature (Figure 4.3 b). The continuous release of hydrogen does not mimic typical release from decrepitation of fluid inclusions, for example, in quartz, whose release is more typified by sudden rupture and almost instantaneous release, typically between 200-300°C (Barker and Robinson, 1984). The other three aliquots of feldspar run for bulk hydrogen isotopes yielded slightly lower values, resulting in an overall mean of -56 ± 5‰

(see Table 4.1). The total water content of the grains is measured at 1.0 to 1.1 wt.%, which, again, is not consistent with typical water concentrations from fluid inclusions (which are usually substantially less, ≤0.2 wt %; Gleeson et al., 2008).

4.6.3 ⁴⁰Ar/³⁹Ar geochronology

The majority of the ⁴⁰Ar/³⁹Ar data show complicated step heating profiles with isotope ratios indicative of intermittent multiphase gas release (e.g., VanLaningham and Mark, 2011). Fine control of the laser power was required to create equal release of gas in subsequent steps, and it was thus necessary during the experimental run to replace the digital laser controller with an analog controller to obtain higher precision (i.e., smaller step size) control over the laser power. The first five grains (Figures 4.4 a,b,c; 4.6 c,d) were run with the digital controller and thus have age spectra that include some undesirably large steps. With both laser controllers, the first few steps (at the lowest laser power) of some grains yield relatively old ages and have slightly elevated K/Ca. However, the K/Ca does also vary drastically (>8000) between steps in other portions of the age spectrum with no clear pattern or correlation with apparent age. The apparent age of the middle portion of the age spectra generally trends toward progressively older ages with increasing power (Figure. 4.4 b,c,d,e; Figure 4.5 c,d,e; Figure 4.6 c,d). Seven of the nine

plateau ages defined by MassSpec software include the last, highest power steps, and define an age that is older than the integrated age (Figure 4.4 b and Figure 4.6 a,b,c,d,e)).

Integrated ages (the mean age weighted by inverse variance, with uncertainties of standard error of the mean, multiplied by sqrt(MSWD) if MSWD>1) for all grains vary from 289 to 321 Ma, with the exception of a single grain with a calculated age of 356 ± 4 Ma (Figure 4.5 c); full results are reported in supplementary information (Table. 4.2). The plateau ages range from 293 to 321 Ma with an average total uncertainty of ± 3 Ma. The youngest plateau age yields no apparent variability in age after the initial decrease in age seen in the first two steps (Figure 4.4 e).

4.7 Discussion

Patch perthite textures result from the separation of a solid solution phase into its end-member constituents albite and anorthite in interwoven tube structures (Lee and Parsons, 2003). The observation of these textures within feldspar grains is consistent with post-magmatic alteration (Worden et al., 1990). In many cases, this alteration is likely related to deuteric alteration in the immediate post-magmatic phase of intrusion, but, in reality, the alteration could occur any time following initial magmatic cooling when a hot fluid passes through the feldspar. The alteration of the likely parent material, orthoclase, has significant implications for Ar diffusion and the closure temperature of the ⁴⁰Ar/³⁹Ar system due to the change in effective diffusion dimension (i.e., a grain size reduction) (Parsons et al., 1999). The diffusion of Ar within a crystal that is composed of a single diffusion domain with effective diffusion dimension equivalent to the radius of the crystal is governed by Fickian volume diffusion (McDougall and Harrison, 1999). In non-ideal crystals, inclusions and some defects can create fast diffusion pathways along which Ar can escape (Lee, 1995). For feldspars analyzed here, this fast diffusion would be prevalent along the boundaries of the sub-grains created by the patch perthite (e.g., Mark et al., 2008), as well as along the boundaries of other inclusions such as apatite (Figure 4.2) (assuming the patch

perthite microtexture is both micro-porous and micro-permeable). As a result, the reduction in diffusion domain size by patch perthite alteration means that Ar diffusion in this sample should be considered to be controlled by the size of sub grains, rather than total crystal size (Cassata and Renne, 2013, Parsons et al., 1999).

As discussed above, the formation of patch perthite textures is indicative of fluid-rock interaction. The fluid released from the patch perthite has an H isotope composition around -56‰. The evolution of these step-heated grains suggests that H may be lattice-bound, perhaps as OH groups, and that it is not H₂O trapped in fluid inclusions (Figure 4.3). Four repeat O isotope analyses of the silicate give a small δ¹⁸O range from 8 to 8.4‰. The δ¹⁸O value will have been imparted by the most recent fluid to recrystallize the feldspar. The

40Ar/³⁹Ar data indicate that these feldspars have been heated to temperatures in excess of 280°C, which is in agreement with the regional thermal state during the Variscan Orogeny (Jones, 1992). If we assume that this heating event was related to the same fluid which produced the perthite texture, then the fluid in isotopic equilibrium with the measured “K-feldspar” values has a range of δ¹⁸O from 1.9 to 2.3‰ (using the equation of O'Neil and Taylor, 1967); 3.2 to 3.6‰ is using the equation of Zheng, (1993). If end-member compositions of the patch perthite are used (albite and anorthite), then the range of calculated fluid δ¹⁸O, in equilibrium with these minerals at 280°C, ranges from 2.7 to 6.3‰ (using equations of Matsuhisa et al., 1979, Zheng, 1993). Whatever calculated fluid composition used, the character of the fluid is typical of crustally-equilibrated fluids (Sheppard, 1986) and consistent with fluids of a metamorphic character, with the implication that they were active during the Variscan Orogeny.