ANOMALOUS COSMOGENIC 3 HE PRODUCTION AND ELEVATION SCALING IN THE HIGH HIMALAYA
2.5 Discussion
2.5.3 Limited Importance of Non-Cosmogenic 3 He
Because production rates in this study are higher than previously observed (Farley et al., 2006), we must consider whether we have somehow underestimated non-spallogenic sources of
3He. The strong linear relationship between uncorrected 3Hetot in zircon and apatite and 10Be in quartz, and the near-zero intercepts of the fitted lines demonstrate that 3He in these samples is primarily produced by spallation (Figures 2.2b and 2.3b). This conclusion is supported by the absence of a correlation between Li content and uncorrected 3Hetot/10Be ratio, either within mineral groups or between them. Likewise, no correlation was discovered when a step-wise multiple linear regression model was constructed in which Li content, exposure age, (U-Th)/He closure age, lithology, and grain size were sequentially added as predictor variables and regressed against 3Hetot/10Be ratio. The lack of correlation between Li content and uncorrected 3Hetot/10Be ratio is particularly important for garnet, because its large grain size and high Li content make its corrected 3He concentration insensitive to the assumed Li concentration of the host mineral.
Likewise, the lack of correlation between Li content and 3Hetot/10Be ratio for zircons in quartzite is important because the homogenous quartzitic lithology leads to roughly constant implanted
3Hecn and 3Henuc components among our samples (Figure 2.5).
Another piece of evidence supporting a limited contribution of non-cosmogenic 3He is the relatively similar uncorrected 3Hetot/10Be ratios of kyanite and garnet, despite the fact that garnet has three orders of magnitude higher Li contents. Because grain size and composition are similar between the two mineral phases, and they are cohosted in three different moraine
boulders, their spallogenic 3Hec production rates should be similar. Because the non-spallation
3He component in kyanite is negligible due its low Li content, the production rate in kyanite can also be used to validate the corrected production rate calculated in garnet. For example, the element-specific production rates of both Masarik (2002) and Kober et al. (2005) predict that the production rate in the Fe-rich garnets used in this study should be ~14% lower than in kyanite.
Using this value and the observed production rate in kyanite, we expect a production rate in our garnet of ~152 at g-1 yr-1. This agrees well with our corrected production rate of 153 at g-1 yr-1, which reflects the average 14% correction calculated for Li produced 3He. Taken together, these
observations suggest that our estimates of Li produced 3He are correct and strongly support the conclusion that the 3He production rates we infer are in fact cosmogenic.
A puzzling observation of our study is that production rates in kyanite and garnet are significantly lower than in apatite and zircon, despite predictions based on element-specific production rates that suggest they should be higher. The difference between kyanite and zircon is particularly compelling, because both minerals have very small non-cosmogenic components.
We can use our observed production rate in kyanite to calculate element-specific production rates for Al, Si, and O if it is assumed that the ratio of production rates between these elements matches those predicted by Kober et al. (2005) and Masarik (2002). This is an appropriate assumption because the two models agree well, predicting that 14-16% of production derives from Si, 29- 31% from Al, and 55% from O. Using these values, we predict 153, 159, and 197 at g-1 yr-1 for Si, Al, and O.
If estimates for Si and O are combined with the observed production rate in zircon, an element-specific production rate of ~270 at g-1 yr-1 is calculated for Zr. The production rate from this element has not been established, but our value is far higher than other elements which have been tabulated (Kober et al., 2005; Masarik, 2002) and seems implausible. One possible
explanation is implantation of spallation produced 3He from adjacent minerals, which would affect smaller grain sizes (e.g. apatite and zircon) more severely. However, this explanation is unlikely for three reasons. First, grain-size experiments run on samples 259 and 431 show no grain-size dependence, except for very small grains. No significant difference in 3Hetot
concentration was detected between samples with average grain sizes of ~65 and 168 m for sample 431, and only a ~15% difference was observed between average grain sizes of ~36 and 98
m for sample 259 (Figure 2.6). Second, Farley et al. (2006) did not see elevated 3He production in apatite or zircon of only slightly larger size than used in this experiment, and observed only
~10% increase in 3He concentration over a two-fold range in grain size. Most importantly, even
if all of the 3He in our zircon and apatite samples were implanted by spallation in adjacent crystals, the observed production rate is still much higher than the ~177 at g-1 yr-1 we would expect for quartz using our estimates for Si and O given above. Production rates in other rock- forming minerals would not be expected to differ from quartz by more than ~5% (Farley et al., 2006). As discussed below, the higher production rate in zircon than in garnet is most likely a result of an unexpected correlation of 3Hec/10Be ratio with elevation coupled with the fact that on average the garnets were sampled at a lower elevation than the zircons.