Enstatite Chondrites

Let's take a break from radar for a moment and talk about some of my past work.

In the summer of 2015, I had the opportunity to take part in the Misasa International Summer Internship Program in Misasa, Japan.  The program has two branches: Geochemistry and Geophysics.  I took part in the Geochemistry program, hosted by the Pheasant Memorial Laboratory. Six of us worked as a team to analyse the compositions of three meteorite samples, and interpret any relationships between mineral phases and degree of thermo-metamorphism. All three were enstatite chondrites, the rarest type of meteorite, constituting only 2% of discovered falls. Our samples are as follows:

Things are getting pretty hot right over here

"E" designates that they are enstatite chondrites. H/L refers to them containing relatively high iron or low iron.  The number refers to the degree of thermo-metamorphism, with 7 being the highest (totally melted). Eagle shows the highest degree of thermo-metamorphism of our samples, and Sahara has the least.  The differences between them are striking.  Sahara is completely heterogeneous with abundant condrules, whereas Eagle's minerals are equigranular. Sahara is so primative, that not only is there an abundance of perserved glass matrix, but we even found a Calcium-Aluminium Inclusion! (CAI)  For those unaware, CAIs are the oldest known particles in the solar system, so this is a pretty incredible find. In the middle, NWA-1222 is also heterogenous, but does not have any condrules.  What is most interesting about NWA-1222 is that it seems to show intermediate exsolution phases of Cr and Fe-S minerals as the sample was heated up, but not to the extent or duration of Eagle (whose phases have completely exsolved).

Our objectives are to determine:

- The mineralogical composition of the samples using in situ analysis
- The chemical composition of the samples using whole rock geochemistry
- How the distribution of elements changes between mineral phases as metamorphism increases

- Age date the samples

The Pheasant Memorial Laboratory has every beautiful piece of equipment you could ever want.

Using both the in-situ and whole rock geochemistry techniques together allows us to build a broader picture of how these samples evolved through time and space (pun intended). In summary, we looked at mineral shapes and probed their composition with the scanning electron microscope-electron microprobe fancy hybrid machine, as well as crush up and dissolve bits of the meteorites in acid to find out what the powder was made of.  My favourite part was using the ICP-MS, where I got to blast little holes in our meteorite samples for element analysis.

I'm pipetting something dangerous!

The resulting products were pie charts to show the distribution of elements in different mineral phases (not shown).  

Introductory analytical geochemistry in a nutshell.  I have not shared any elemental analysis pie charts in this post, but you know they exist and what they show.

What we found:

EH3 (Sahara)
- Intergrowths of kamacite (Fe0.9Ni0.1) and troilite (FeS), forming a granophyric-like texture.  These two minerals are fully separate phases in the EL5 and EL6 meteorite.
- CAI contains many complex mineral phases that are not found elsewhere in the sample
- An unknown interstitial "glass" phase contains ample amounts of alkali elements.

- RAMAN identified christobalite (>1470oC silica polymorph)

EL5 (NWA-1222) - Elise's favourite

- Mineral shapes are irregular
- Fantastic exsolution texture between a chromium-bearing phase (proto-daubreelite (FeCr2S4) and troilite (FeS). There is also exsolution of a Mn-rich mineral out of troilite.
-RAMAN identified tridymite (870oC - 1470oC silica polymorph)

I feel fuzzy, proud, and excited every time I look at this figure.  We were getting anomalously high chromium readings for the troilite minerals (2%).  Upon closer inspection, there are small minerals of daubreelite with high Cr (10%) exsolving out of the troilite.  Also the same is happening with manganese, but that is covered by the watermark and isn't discussed as much in this post.

EL6 (Eagle)

- Exsolution of daubreelite (FeCr2S4) and troilite (FeS) is near completion!

- Crystals are (sub) euhedral

- "Mystery" SiO2 phase -> used RAMAN spectroscopy to detect sinoite (Si2N2O), which previous literature explicitly states is not in Eagle.  It's always a good day when you prove someone wrong.

Big picture:

From the exsolution of Cr-phases between the EL5 and EL6, we can constrain the temperature of metamorphism.  The transition of Cr-Troilite to Troilite + Daubreelite occurs below 800oC. SO COOL! (because it's below 800oC? Get it?) I have a ternary diagram that illustrates this, but this post already has quite a few technical figures.  I was the ternary diagram-shishou (master) in my Misasa MISIP cohort. 

We can also constrain the temperature of metamorphism further to being between ~600-800oC by the relative abundances of magnesium sulphides and manganese sulphides.  I made a ternary diagram for that, too. It is nice when your minerals agree with one another.

There is a continuous depletion of sodium from the least to most metamorphosed.  This is evinced by feldspar composition.  K and Na depletion in feldspars may indicate loss of low melting point material with increasing temperature.  I will bless you with at least one of my ternary diagrams.

Age dating

Couldn't age date Sahara (EH3) because we only had a thin section and no bulk sample.
Both Eagle and NWA-1222 showed Sm/Nd isotopes dating close to the age of formation of the solar system, and Rb/Sr ages close to the late heavy bombardment.  It is possible that the Sm/Nd age pertains to the age of formation, while the Rb/Sr ratio dates the age of metamorphism.

In conclusion:

The exsolution of Fe/Mn/Cr phases consistently occur ~600-700oC.  The exsolution lamallae are indicative of slow cooling. This texture constrains peak metamorphism temperature, which likely occurred during the late heavy bombardment period.

We used RAMAN to identify different silica polymorphs.  These minerals likely formed prior to chondrite accretion, but this constrains our temperatures of formation, which was likely during the formation of the solar system. Further, we found sinoite in Eagle, which wasn't supposed to be there.

This post ended up way longer than I expected.  I admit that it is still a quick synopsis of this project, with a plethora of important details and measurements missing.  If you're interested, feel free to contact me and I'm happy to discuss our enstatite chondrite study with you!

Radarclinometry

In starting to write the SOAR-E grant proposal to attain RADARSAT-2 images over Iran, Catherine e-mailed me her 2008 paper that uses an Iranian salt diapir as a case study.  Originally the article was just so I could get the diapir coordinates, but I started to read it and found it quite interesting.

Radar topography of domes on planetary surfaces by Neish et al. (2008) introduced me to the technique of radarclinometry, which is to use radar images to produce topographic information.  By measuring variations in radar image radiance, you can find relative elevations using "shape-from-shading" techniques.  One of the largest controls on radar-backscatter values is the incidence angle at the surface.  If you imagine that a structure is homogenous, you can attribute changes in backscatter to variations in incidence angle, i.e. slope.  Once we know the slope, we can produce a topographic profile for the feature!  Neat, eh?

There are a couple of methods for this that you can use.

A one-dimensional method determines the amount of slope on a surface by measuring the brightness of lines of pixels to make a topographic profile.  This method doesn't give you any information perpendicular to your slope profile. There is a two-dimensional photoclinometry method that fits a digital topographic model to an image.  This incorporates two-dimensions, but is both slow to process and is sensitive to artifacts in the radar backscatter images.  Another method, which is employed here, uses a larger scale image of a specifically shaped feature, and adjusts the height accordingly.  This is less vulnerable to backscatter variations than the other 2-D method.

The purpose of this paper is to determine if you can use radioclinometry to measure the heights and shapes of "viscously emplaced domes", with a specific interest in studying features on Saturn's moon Titan.  The altimetry data for Titan is both sparse and poor resolution, so exploring alternative methods of gaining elevation information can have large impacts on how we study Titanean features.

What do shield volcanos and salt diapirs have in common?  They are both viscously emplaced domes! That is, they both form by the slow movement of thick, soft, deformable material, and make roughly circular shapes.  So, this study assumes a dome-shaped profile, and uses the aforementioned radarclinometry technique to measure the height profiles of viscously emplaced domes across the solar system.  The authors use an Iranian salt diapir of known elevation, and pancake domes on Venus to test their methodology before applying the technique to Ganesa Macula - a 180 km across, circular, radar-dark feature on Titan.  Ganesa Macula is suspected to have volcanic or cryovolcanic origins.

How did they do?

Well, after comparing different models, the authors found that they did a pretty good job of fitting the radarclinometry data they produced to the available topographic data available for the Iranian salt diapir. The terrestrial case study demonstrates a good proof-of-concept for this technique. In contrast, the radarclinometry profiles made for the Venusian pancake domes were consistent, but a little less than the altimetry.  Nonetheless, the heights they measured for Ganesa Macula (2.0-4.9 km) fits previous constraints, so the results are promising. They also estimate the volume of Ganesa Macula ato 30,000-40,000 km^3.  This is a significant volume, because if Ganesa Macula is made of a volcanic lava, based on Titan's heat production rates it would have to becomparable to the duration of Earth's Deccan Traps.  Perhaps Ganesa Macula is the result of a rare event, which would explain why it is such a unique feature on Titan.

I expect that next time I write I'll have fixed the registration issue with my radar images.  I look forward to showing you my PALSAR acquisitions!

Cheers!

From one desert to another

With the intent of returning to the image misregistration issue once I'm back from Québec, I've been switching gears into proposal-writing mode.

Catherine and I have discussed the possibility of extending our work to compare and contrast our Axel Heiberg Island diapirs to those in the Zagros Mountains, Iran. I have frequently referred to Axel Heiberg Island as "having the second highest concentration of salt diapirs in the world".  Well, Iran has the first-highest concentration of salt diapirs in the world.  These two sites would be interesting to compare for multiple reasons.

1. They are both desert environments, meaning that radar signals are less likely to be affected by soil moisture or obstructed by vegetation.
2. They are different types of desert environment.  Axel Heiberg Island is a polar desert, where cryoturbation is the predominant weathering mechanism.  Iran is a hot desert, with aeolian processes, gravity-driven mass movements, and periodic flashflooding as predominant weathering mechanisms. Different erosion mechanisms may lend to observable differences in surface roughness of the diapirs and surrounding rock units.
3. The diapirs have different compositions.  The salts on Axel Heiberg Island are predominantly anhydrite (CaSO4) weathering to gypsum (CaSO4 * H2O), whereas the salts in Iran are more classic halite rock salt (NaCl). Both salts are soft and soluble, but have different crystal structures which may lend to different radar responses and erosion patterns.

To initiate this change of pace, Catherine has asked me to write a draft proposal for the CSA's SOAR-E program.  RADARSAT-2 images aren't free, so this program let's us pitch our wonderful science ideas to the CSA and request access to their data.  I have not written a proposal before, so this is a slightly intimidating, but beneficial, experience.

Unfortunately there do not appear to be any PALSAR-1 Quad-Pol acquisitions over Iran, so we will not be able perform the same comparison between C-band and L-band for the Iranian diapirs.

PALSAR Hiccups

Hi hi~

Last post, I mentioned that we had acquired (or at least were in the process of acquiring!) quad-polarized images from the Phase Array L-Band Synthetic Aperture Radar (PALSAR) on Japan's Advanced Land Observing Satellite (ALOS).  The images have been downloaded, and processing near completed!  I've made multilooked and terrain corrected HH Intensity images and CPR images.

However, there is a bit of a problem.

If you look closely at some of the acquisitions, you can see that there is some kind of image artifact running East-West through some of them.  This artifact does not exist in the raw level 1.1 images, and it only seems to appear after the terrain correction step.

I consulted Mike, an experienced post-doc in the Neish lab research group, about the artifacts and he suggested I start troubleshooting by looking at the DEM I'm using for terrain correction.  This is the also same DEM that I've been using for the RADARSAT-2 images, and I've subsequently noticed the same error in these images as well.  Despite the DEM being a mosiac, there does not appear to be any visible seam in the image which could have caused the trouble.  Catherine thinks there is a misregistration issue, which could be manually resolved if other troubleshooting methods fail.  We might also try terrain correction with a different DEM.

I'm hesitant to report specific CPR data analysis until after this is fixed, but we can make some preliminary observations from what we have.

Salty surfaces are seem to be a little bit rougher in PALSAR's L-band radar than in RADARSAT-2's C-band radar, with salt domes having average CPR values of 0.45 (average CPR of domes - taking the average of the mean CPR for measured salt domes) or 0.51 (average pixel value for all measured salt domes).  In comparison, the RADARSAT-2 has average CPR values of ~0.40 (for both average of domes, and average per pixel).  The difference between CPRs of 0.4-0.5 might not be too significant, but we can at least see that the diapirs are appearing rough over a range of scales.

The average CPR for remobilized salt deposits appears significantly rougher in L-band than C-band. This, however, appears to be attributed to two anomalous deposits. Of 14 measured deposits measured in L-band, two appear to have average CPR > 1, whereas the remaining deposits have a diverse range of averages from 0.14-0.56.  In comparison, salt deposits in C-band show far more constrained average values between 0.23-0.38, with the mean of average deposits (0.28) being closer to mean pixel value (0.26).  This might mean that a few of the deposit areas are not smooth sands as previously interpreted, but are actually filled with cobble-sized clasts that area being noticed at the longer L-band wavelength and not the shorter C-band.  It would be interesting to field check these different deposits to validate this hypothesis.

I'm out of town next week (wooo!  I'm visiting Québec City for the Winter Carnival!) but we will return to this problem once I'm back.