Friday, 12 September 2014

Day 6 - Sample, Sample, Sample!!!

Observations of the LandSat data courtesy of NASA and the United States Geological Survey, several individual lava flows can be identified. The aim today was to concentrate on sampling the lavas from the young flows that cross the centre of the caldera.
The literature states that these eruptions were fissure eruptions. Observations made from elevations show lobing, suggesting flow. But close up the lavas are very blocky with no evidence of the classic pahoehoe or a'a structures. The lavas originate from a cone situated central to the caldera and by and large have the characteristics of scoria style eruptions.  These lavas are the youngest in the caldera and according to the geophysics results, are situated over the highest temperatures. Furthermore, it is thought that lavas have exploited an incomplete ring fault, that formed during a phase of caldera collapse.

The samples will be cut into thin sections and small 1 inch cubes set in epoxy resin. Using a range of lab methods, the thin sections can be used to identify mineral composition can be easily identified, alterations and causes, any melt rock reactions may also be identifiable. The resin blocks will be used to identify the presence of fluid inclusions that can be analysed. The composition of the fluids will help establish the depth source of the lavas and further aid the understanding of meteoric versus historical contributions to the fluids.

You may have noticed I have been referring to the flows as lava and not by its actual name. Literature states that the lavas are dominated by trachytes, however in the field it is often difficult to make an accurate identification of some units because the grain sizes are to small to identify individual minerals. This is where thin sections come. I am not in anyway contesting the repeated identification of trachyte, however the samples collected are unusually dark for a trachyte. You may recall from Day 4, that I was fortunate enough to observe drill cuttings under the microscope at well MW-09. The samples were very clearly dominated by feldspars, confirming that the sample was indeed a trachyte. Some of you may think the unusual dark appearance is associated with alteration. But I have specifically hunted out sample sites away from areas of alteration and have observed fresh surfaces.

So on to the sampling!

Top of lava flow 2. Flow 3 is in there somewhere.

The first sample site was from flow 1 in the below image. The sample site was fairly easy to access with a short scramble through the vegetation clapping at the snakes and Jenny banging the rock hammer with the chisel! I think if Jenny saw a snake should would run faster than Road Runner in the opposite direction. 
The second sample site was from flow 3 and developed in to an interesting task. From the base of the flow we identified a location that seemed very easy to get to with a reasonable scramble. How wrong could we have been?
The climb was deceivingly steep and a lot further than expected. But by the time we had got to the chosen site, we were close to the top of the flow. 

From the top of the lava flow, looking down. If you can make out the tiny white fleck in the middle of the image, that is a field ruck sack.

We decided to carry on going to the top, hoping we would find the flow top with evidence of a'a and pahoehoe. Wrong again! Just dense vegetation, preventing us from sampling flow 3 at this  So back to the sample site. After completing the sampling, it was precarious decent with lots of noise from people a short distance away. It was only when we got back to the car that our driver (who had accompanied us on the climb) told us that they were hunters and we had been mistaken for gazelle.  
The vegetation at the top of the flow was too thick to sample from flow 3 at this location.
Bags were checked after being left on the floor just in case a snake had decided to call it home.

Lala Salama from Kenya!

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