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Some don’t like it hot: accounting for Ne diffusion during exposure in hot deserts

April 22, 2020

Another guest author! Michal Ben-Israel is a Ph.D. from the Hebrew University of Jerusalem, spending her time attempting to apply cosmogenic Ne-21 to quantify rates of surface processes in the deep geological past, to variable degrees of success.

It has been pointed out before that cosmogenic Ne-21 is, simply put, a pain in the neck. As a stable cosmogenic nuclide, Ne-21 has undeniable potential, especially when it comes to exposure dating past 10^7 yr. Unfortunately, cosmogenic Ne-21 comes with an impressive list of issues that need to be considered when applying it over such timescales. Issues such as interferences from non-cosmogenic Ne-21 trapped in the crystal lattice (of atmospheric, nucleogenic, or other sources), inherited cosmogenic Ne-21, post-burial muon produced Ne-21, not to mention the overall scarcity of preserved sediments/surfaces that are old enough to justify getting into this whole Ne-21 mess. 

In defense of cosmogenic Ne-21, it has been shown to be useful in dating exposure of very slow eroding surfaces that have been exposed beyond the reach of radioactive cosmogenic nuclides, like in Antarctica or the Atacama. These two cold deserts constitute the perfect case-study for exposure dating with cosmogenic Ne-21: uninterruptedly exposed, slowly eroding, and with plenty of quartz to sample. But these are not the only locations that check those Ne-21 boxes, parts of Australia, the southwest US, and the Levant all make good candidates. Except, that when we consider hot deserts, we find ourselves faced with one more disadvantage of Ne-21: diffusion. 

Thermally activated diffusion of Ne from quartz is yet another thorn in the side of the application of cosmogenic Ne-21 over geological timescales. Neon, being a noble gas, is inert and so it readily diffuses out of the quartz lattice, which like all diffusive processes, depends on temperature and time. Diffusion can be useful, and a few experimental studies into diffusion kinetics of Ne in quartz led the way to some cool applications of diffusion, such as paleothermometry. However, when it comes to extended periods of surface exposure, diffusion is a foe. 

This thought experiment started with a thought provoking discussion with Marissa Tremblay (see referee comment – RC3) wherein she wondered about possible diffusion in the reported Miocene chert pebbles, sampled from the Negev Desert. While diffusion from these pebbles is most likely insignificant, the question arose – what would happen to exposure dating in uninterruptedly exposed, slow eroding, quartz covered surfaces in hot deserts.

The first thing to note about hot deserts is that they get very very hot. Air temperatures in the Sahara, for example, can reach 56ºC (~134ºF for the metrically challenged). Surface temperatures and particularly temperatures of exposed dark rocks such as chert or patina covered quartz/quartzolite can be significantly hotter. In fact, Mcfadden et al. proposed solar heating as a mechanism for physical weathering of rocks in arid landscapes, leading to preferential vertical cracking in exposed boulders and pebbles. So it would be reasonable to assume that a dark rock in a hot desert during mid-day in the summertime would reach temperatures of up to 80ºC (this might be on the high end of the scale but is probably still within reason). The question I’ll examine next is, would such temperatures affect surface exposure dating and by how much?

On the left are fragments of what used to be a chert cobble from the hyperarid Eilat area. On the right is a well-developed desert pavement composed of dark fragments of cherts and quartzolites located in the northern Negev Desert.

For the diffusion calculations, we can assume that only during Apr-Oct and only between 10 am to 4 pm, rock temperatures get sufficiently hot. We can simplify these calculations by assuming that during [(6/12)*(6/24)] of the time each year, dark rocks at the surface reach a steady temperature of 80°C. One more assumption we need to make is regarding the effective mineral radius. In the presented calculations, I used a 250 and 850 micrometers diameter (the commonly used grain-size sampling range for cosmogenic nuclide analysis), and 1 cm mineral diameter (a typical size for desert pavement fragment). [sidenote: mineral radius is one of the variables in the diffusion equation, which might have the most uncertainty, as mineral radii can range between tens of microns to a few centimeters. It is also important to note that mineral diameter isn’t necessarily equal to grain size, but that is a whole other question that I am not sure I know how to answer, and I definitely don’t want to get into here.]

Now we can calculate the time-integrated diffusion over 100yr intervals for a total span of 10 Myr. For this imaginary scenario, I scaled production for a site in the Negev Desert, where some very slowly eroding surfaces have been reported. This calculation could have also been normalized to production rate or calculated for SLHL production rates, but I chose this site for no other reason other than I wanted to use data for an existing location.

This figure shows how diffusion affects cosmogenic Ne-21 calculated exposure ages. On the left, we can see how the concentration of cosmogenic Ne-21 deviates (dashed lines) from the cosmogenic Ne-21 produced (continuous line), depending on grain size radius. On the right is the actual exposure age versus the calculated exposure age, with the continuous line representing no diffusion and the dashed lines representing diffusion depending on grain size.

What we learn from this figure is that for sand size range, diffusion becomes significant after ~2-3 Myr of exposure, and beyond 5 Myr, the difference between apparent exposure time and calculated exposure time gets problematic. So right around the time that exposure can no longer be quantified using Al-26 and Be-10, and Ne-21 gets useful, is when cosmogenic Ne-21 starts to falter and we need to start considering the effects of diffusion. To sum it all up, I guess we can add diffusion during exposure (in hot deserts) to the long list of nuisances that come with cosmogenic Ne-21 exposure dating. 

At the base if this text hangs the question, is cosmogenic Ne-21 worth bothering with for exposure dating? There is now even more evidence to suggest the answer is – no, but I don’t think the answer is this clear-cut. At this point in time, Ne-21 remains the ONLY available cosmogenic nuclide that could be applied to quantify rates of surface processes throughout the geological record, at least until another stable or slow decaying nuclide comes along (I’m looking at you, Mn-53…)

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