DIFFARG Modeling Results

Using the event history developed above, modeling shows (Figure 4.7 B) that heating feldspars to 200°C causes only trivial Ar loss (<0.1%), and thus does not affect ⁴⁰Ar/³⁹Ar ages (with respect to the reported age uncertainties), in all but the smallest diffusion domains (ca. 25µm) where a small (ca. 5%) loss is indicated. Less significant heating (<100 °C) yields no Ar loss, regardless of domain size. A ca. 300°C heating event releases all of the Ar held in small diffusion domains (<90 µm), thus resetting the ⁴⁰Ar/³⁹Ar chronometer, and also significantly effects larger domains with varying partial loss of Ar (Figure 4.7 B). The Ar that is lost from a sub-grain is preferentially lost from the outer portion of the sub-grain as the diffusion pathways are shorter (see Figure 4.8). This would create an apparent age profile across the effective radius of each of the feldspars subgrains (see Figure 4.7 C). If a diffusion domain was completely reset, Ar ingrown after the heating event will produce a flat apparent age spectrum across the grain radius (Figure 4.7 C). If diffusion domains of various sizes exist within a sample, some grains with small diffusion domains can be completely reset while others with larger domains retain evidence for complex thermal histories.

The model reasonably indicates that if the diffusion domains are small enough (ca.

<90μm), the temperature high enough (>300 °C), or the heating event is long enough (>10Ma), all grains could be completely reset.

4.7.4 ⁴⁰Ar/³⁹Ar data interpretation with respect to modeling results

The nine plateau ages span the range of integrated ages, 293 to 321 Ma, with plateaux consisting of at least 3 contiguous steps, >50% of total ³⁹Ar, and a probability cutoff of 95%. The numerical modeling and petrographic information described above are critical in interpreting these ⁴⁰Ar/³⁹Ar data. The loss of Ar by diffusion due to heating in geological settings is analogous to the diffusion induced by laser incremental heating analysis (although there is a considerable difference in rates between laboratory and natural heating). With each progressive heating step, Ar gas diffuses from more centrally located sites within each diffusion domain (i.e., subgrain). The increase in age congruent with increased laser power, evident in a majority of the data, indicates that the edge of the diffusion domain has a relatively young apparent age, with older apparent ages towards the center of each domain, see Figure 4.4 b,c,d,e; Figure 4.5 a,c,e,f and Figure 4.6 c,d. This type of profile can be seen in the DIFFARG model from a grain that has undergone partial thermal resetting, (ca. 200 µm radius, Figure 4.7 C). This is analogous to measurements of Pb diffusion within apatite grains, where the grain represents a single diffusion domain (Cochrane et al., 2014, see figures 3b and 4). If the domain is large enough (e.g. radii of 400 µm), it is possible that the magmatic age could be retained within most retentive sites (Figure 4.7 C). The age of the Navan syenite is known to be older than the erosion surface, which the Red Beds overlie. It is believed to be Devonian in age (see above), and no apparent ages for any step of the experiments obtained herein approach this age (ca. 400 to 418 Ma). This is likely due to the lack of sufficiently large diffusion domains to retain Ar ages from this event. It should also be noted that the model indicates that the metal-bearing ore fluid, thought to be ≤200 °C, could not have fully reset any modeled diffusion domain, and thus should not have significantly affected ages presented here.

More generally, many age spectra yield plateau ages, which are typically interpreted to represent the age of a specific geological event. However care should be taken as DIFFARG modeling indicates that a diffusion domain size that is just large enough to avoid complete resetting will result in a nearly flat age profile relative to diffusion domain radius of a geologically meaningless age, (e.g. 100 µm radius Figure 4.7 C). Such apparent plateau ages in this study are masked by the several Ma analytical uncertainty for each individual step. Grains 2, 6, 8, 9 and 10 are believed to be represented by this apparently flat but actually variable profile. The second youngest plateau age, grain 6, yields a pattern similar to the ideally reset spectrum, although the last step (Figure 4.6 c step J) may be slightly older (but is within analytical uncertainty). The numerical model does indicate that if the diffusion domain is sufficiently small, the anticipated heating event can reset the diffusion domain completely (Figure 4.7 b 25µm). This completely reset diffusion domain faithfully records the last time it was at this high temperature, and yields a flat apparent age relative to domain radius (e.g. 25 µm radius, Figure. 4.7 c). Thus the youngest grain or grains that do not yield increasing ages with increased laser power in an incremental heating experiment should represent the most recent heating event. The effect of combining various subgrain sizes on apparent Ar ages is illustrated in Figure 4.8. Grain 29 (Figure. 4.6 e) fits these criteria, as it yields an apparently flat age spectrum (after the first two steps) with the youngest age seen in this study at 293 ±3Ma. This grain is interpreted to have small diffusion domains that have been completely reset.

We show that the majority of grains analyzed here yield age spectra that are indicative of partial thermal resetting from a moderately hot fluid (ca. 300°C) and degassing of varying sub-grain sizes with differing diffusion domain sizes (Figure 4.8). Many grains clearly record a rising age spectrum that can be interpreted to represent the partial retention of Ar and can thus only indicate that thermal disturbance is younger than the apparent age. The thermal modeling shows that no other geological event in this region was likely to be faithfully recorded in the grains. The complications of micro textures on Ar systematics

and step heating analyses have been investigated by others, with doubt being cast on the ability to extract multiple ages and create a detailed thermal history from a single grain (Parsons et al., 1999). Note that here we attempt only to use the youngest age to constrain the timing of the most recent heating event, and make no attempt to construct more detailed thermal histories. Other grains that appear to yield flat profiles (Figure 4.4 b Figure 4.5 b and Figure 4.6 a,b,c,) likely actually have unresolvably rising profiles. The youngest and completely flat plateau age recorded in grain Grain 29 (Figure 4.6 e) likely represents a grain with small patch perthite textures that have been completely reset and thus record a thermal event (Figure 4.7 b), as modeled in the 25 µm radius diffusion domain.