Some salty waters: Thenardite, halite, and gypsum

#StillWriting

In my drafts and writing revisions, I noticed that I had a few missing pieces of information.

To clarify a few questions I had regarding the chemistry of perennial springs on Axel Heiberg Island, I reached out to Mark Fox-Powell, who joined us in the field last year. Mark is a post-doctoral research fellow at the University of St. Andrews, with a background in microbiology. His current research is in astrobiology, with emphasis on geochemistry of natural waters.

On our trip, Mark sampled the water and precipitates in and around perennial springs. He is using these samples to analyse their water chemistry, and to produce visible and shortwave infrared spectra of the precipitates. Existing spectral databases of hydrated sulphate and chloride salts are derived from pure minerals produced in controlled laboratory conditions. By using the salts from perennial springs, his team will be able to measure the spectral signatures of impure, naturally occurring salts.

The ultimate goal is to use terrestrial salts as an analogue for "non-icy" materials on Jupiter's moon, Europa. Europa is an ocean world. Its surface is a shell of ice of unknown thickness, over an ocean thought to have the potential to support life. For this reason, Europa is the target of the next NASA flagship mission Europa Clipper which will launch in the early to mid 2020s, and will carry instruments to image its surface at higher resolution. While still unknown, the non-icy materials identified on Europa are hypothesized to be salt precipitates - by understanding the spectral properties of naturally occurring terrestrial salts under Arctic and European conditions, we may be able to better constrain which salts are occurring on Europa. By gaining insight into what chemical materials are present in Europa's waters, astrobiologists will have a better understanding of what kind of life could potentially inhabit these oceans.

So, what did Mark and the team at St. Andrew's find?

We visited three perennial springs during our 2017 Axel Heiberg Island field season. These were Lost Hammer Spring (north of Wolf Diapir), Stolz Springs (emerging from Stolz Diapir), and Colour Peak Springs (southern base of Colour Peak).

Aerial view of Lost Hammer Spring, north and downstream of Wolf Diapir. 

The main vent of Lost Hammer Spring is a >1 m high accumulation of mirabilite and thernardite. Halite precipitates in the surrounding white areas. There is evidence of seasonal layering within the vent.

Lost Hammer Spring has been previously studied by Western alumni Melissa Battler (2013). The water chemistry analysis falls in line with these data, with the spring water being dominated by sodium and chloride. Interestingly, it has the highest sulphate concentration of the springs we visited. Some of these sulphates are precipitating as mirabilite (Na₂SO. ₄· 10H₂O) or thernardite (Na₂SO₄₎  rather than gypsum (CaSO. ₄· 2H₂O) or anhydrite (CaSO. ₄), though. I'm planning on looking into sodium sulphates to see if they have similar or different spectral signatures than their calcium sulphate counterparts.

Segment of the very extensive perennial springs emerging from Stolz Diapir. According to Mark's analysis, the white minerals are dominated by halite and hydrohalite, whereas the darker, greyish minerals are predominantly mirabilite and thernardite

Stolz Springs has the highest concentration of chloride in its waters. This makes sense, given that Stolz Diapir has an outcrop exposure of halite at surface. Different parts of the spring deposits are dominated by halite, and others mirability/thernardite.

Perennial spring at the base of Colour Peak. The dark terraces are calcite+gypsum spring precipitates. The white minerals are halite forming at the edges of the springs.

Colour Peak has multiple spring outlets, which have appear to have lower chloride concentrations than the other sites. The dark terraces are only present at Colour Springs, and are made up of a combination of calcite and gypsum. There are also halite crystals precipitating on the soils adjacent to the terraces. If the terraces contain gypsum, then they certainly are contributing to the strong gypsum signature in our ASTER TIR images downslope of Colour Peak. The streams are very smooth compared to Colour Peak itself, which fits our hypothesis and radar observations!

One of the main takeaways here is that there are certainly more sodium-sulphates around Lost Hammer and Stolz springs than I thought.

Digging through some literature, Howari (2004) writes that thenardite has absorption features at 1.5, 2.0, and 2.3 µm due to the inclusion of water molecules. The latter two are very similar to the absorption features in gypsum at 1.9 and 2.2 µm. Similarly, although crystalline halite does not produce any notable spectral signatures, when aqueous it can also absorb at 2.0 µm from trapped water. Similarly, Howari et al. (2002) write that the SWIR signatures of thenardite can obscure that of gypsum when both are present in soils. This implies that thenardite, halite, and gypsum might look similar in our visible-near infrared and short-wave infrared and composite images. 

Stuff to consider.

I'm going to get back to writing.

Battler, M.M., Osinski, G.R., and Banerjee, N.R. 2013. Mineralogy of saline perennial cold springs on Axel Heiberg Island, Nunavut, Canada and implications for spring deposits on Mars. Icarus, 224: 364–381. doi:10.1016/j.icarus.2012.08.031.

Fox-Powell, M.G., Osinski, G.R., Gunn, M., Applin, D., Cloutis, E., and Cousins, C.R. 2018. Low-Temperature Hydrated Salts on Axel Heiberg Island, Arctic Canada, as an Analogue for Europa. In 49th Lunar and Planetary Science Conference. Lunar and Planetary Institute, Houston. p. Abstract #2564. Available from http://www.lpi.usra.edu/meetings/lpsc2018/pdf/2564.pdf.

Howari, F.M., Goodell, P.C., and Miyamoto, S. 2002. Spectral properties of salt crusts formed on saline soils. Journal of Environmental Quality, 31: 1453–1461. American Society of Agronomy, Crop Science Society of America, Soil Science Society.

Howari, F.M. 2004. Chemical and Environmental Implications of Visible and Near-Infrared Spectral Features of Salt Crusts Formed from Different Brines. Annali di chimica, 94: 315–323. Wiley Online Library.