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The picrite of Ölfusvatnsárgljúfur
  
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Rei




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PostPosted: Apr 25, 2015 11:25    Post subject: The picrite of Ölfusvatnsárgljúfur  

So, short summary... picrite is a high magnesium, deep-sourced lava generally containing olivine phenocrysts and xenoliths rich in olivine and feldspars. It's normally associated with certain types of "oceanic" volcanoes, but otherwise is relatively rare. It forms when very deep, magnesium-rich magma begins to cool, forming crystals that can only form at high pressure, with the cooling suddenly being interrupted by eruptive activity that brings it to the surface.

Iceland's lavas are rather diverse, but picrite exists in several places in the country, and one of them is Ölfusvatnsárgljúfur (try saying that five times fast ;) ), about 45 minutes from Reykjavík (when the direct route is open) and just south of Ţingvallavatn. The lava is from a relatively young eruption at Tjarnarhnjúkur far to the south-southwest, but most of it has since been covered up by glacial till and other debris; however, in Ölfusvatnsárgljúfur a river has carved its way down and through the flow.

One can hike on either the west or east side of the river. The east side is the "easy" side; it's lower and has "easier" (although still often very difficult) access to the river, doesn't have as many obstacles that require weaving around, and has a mountain road that parallels the river. The west side has the advantage of being less traveled, but has the aforementioned disadvantages - with most of the added height coming in the form of talus slopes.

My journey involved a travel down on the west side and a return on the east. While I didn't find either dense encrustings of olivine or particularly large crystals like I would have loved, it was still a neat hike and I found some interesting lesser specimens.



16638381194_295ead2ae0_k.jpg
 Description:
Ölfusvatnsárgljúfur
Arriving as close as possible to Ölfusvatnsárgljúfur. The canyon doesn't start until about 2 1/2 kilometers from the road. However, if you had an offroad vehicle you could probably drive not only up to it but pretty far along the east side.
 Viewed:  9519 Time(s)

16638381194_295ead2ae0_k.jpg



17234876106_49b7c1779a_k.jpg
 Description:
There's sort of a trail, but it doesn't always go where you want it to go. Although that's partly because if you try to follow the river, even from the ridge above, you'll get dead-ended quite often.
 Viewed:  9508 Time(s)

17234876106_49b7c1779a_k.jpg



16638379944_d28b90349f_k.jpg
 Description:
Lots of beautiful crystals... unfortunately most were of the non-metallic-cation variety, if you catch my drift ;)
 Viewed:  9512 Time(s)

16638379944_d28b90349f_k.jpg



17073043988_e0abe674c5_k.jpg
 Description:
The canyon begins. Doesn't look too difficult, good!
 Viewed:  9507 Time(s)

17073043988_e0abe674c5_k.jpg



17073045058_4475be2448_k.jpg
 Description:
Little cliff caves. A good number of rocks are covered in clear/white crystalline phenocrysts but no olivine or anything else interesting..
 Viewed:  9499 Time(s)

17073045058_4475be2448_k.jpg



16640598893_159b61fbdc_k.jpg
 Description:
A giant boulder forced me to divert a bit up a slippery talus slope. "A bit" quickly evolved into "a couple hundred meters"
 Viewed:  9516 Time(s)

16640598893_159b61fbdc_k.jpg



17073018788_c78de9a387_k.jpg
 Description:
The boulder - is that a hyaloclastite breccia?
 Viewed:  9512 Time(s)

17073018788_c78de9a387_k.jpg



17074637349_eefce4491c_k.jpg
 Description:
Suddenly I realize that I won't just be going down the other side...
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17074637349_eefce4491c_k.jpg



17260797745_ff5e4f3554_k.jpg
 Description:
Yep - it's morphed into your Standard Iceland Terrorcanyon, where the river often has no bank, but rather directly abuts steep cliffs or slippery talus slopes, forcing one to travel the upper ridges to have to avoid going up and down talus dozens of times. Since the ridges are just glacial till, this wasn't particularly appealing.
 Viewed:  9503 Time(s)

17260797745_ff5e4f3554_k.jpg



17053418317_0f6595c4ba_k.jpg
 Description:
Considered turning around here... cold and windy and not able to hike where I wanted to.
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17053418317_0f6595c4ba_k.jpg



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 Description:
 Viewed:  9510 Time(s)

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17053383057_a662f27526_k.jpg
 Description:
Columnar basalt layer
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 Description:
 Viewed:  9506 Time(s)

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17259016502_8a13c1f034_k.jpg
 Description:
Had to divert down into a deep side canyon. I'm quite thankful that I searched for an easier way down rather than taking the steep mud slope that first presented itself to me.

It probably would have been faster, in the sense that I probably would have ended up tumbling all the way down ;)
 Viewed:  9500 Time(s)

17259016502_8a13c1f034_k.jpg



17260261871_48543b5809_k.jpg
 Description:
Walking back to the river
 Viewed:  9511 Time(s)

17260261871_48543b5809_k.jpg


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PostPosted: Apr 25, 2015 11:45    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Part 2


16638391334_d97f59ad88_k.jpg
 Description:
Found several of thes giant, totally out of place boulders. I know there's supposed to be volcanic bombs in the area, and that'd explain the out-of-place rocks. But they look like breccias or something. So I don't know.
 Viewed:  9505 Time(s)

16638391334_d97f59ad88_k.jpg



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 Description:
 Viewed:  9502 Time(s)

17073235750_0f59e785c7_k.jpg



17073026798_e2e4f8c451_k.jpg
 Description:
Here is where I first started finding olivine crystals in the rocks.
 Viewed:  9515 Time(s)

17073026798_e2e4f8c451_k.jpg



17259047742_46b65459ca_k.jpg
 Description:
 Viewed:  9499 Time(s)

17259047742_46b65459ca_k.jpg



17260290981_0b434e8d3e_k.jpg
 Description:
Between this and my last picture I found the neatest samples I recovered (pictures after a bit)
 Viewed:  9511 Time(s)

17260290981_0b434e8d3e_k.jpg



17234838956_482c271c00_k.jpg
 Description:
What's this? An easy way down? An actual riverbank?
 Viewed:  9498 Time(s)

17234838956_482c271c00_k.jpg



17053400937_79a8471a3c_k.jpg
 Description:
 Viewed:  9495 Time(s)

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17053406397_9a0ef36521_k.jpg
 Description:
So... still hyaloclasite?
 Viewed:  9501 Time(s)

17053406397_9a0ef36521_k.jpg



17260779425_8d7743c981_k.jpg
 Description:
Inside the larger dark rock in the last picture... no clue what the green thing is, but I couldn't reach it.
 Viewed:  9494 Time(s)

17260779425_8d7743c981_k.jpg



17074627999_36b75ef58b_k.jpg
 Description:
Looks crossable... and hey, I brought these rubber boots...
 Viewed:  9494 Time(s)

17074627999_36b75ef58b_k.jpg



17234810246_8518ebd241_k.jpg
 Description:
Beginning to regret this... the moving ripples and uneven pressure are throwing me off my balance...
 Viewed:  9499 Time(s)

17234810246_8518ebd241_k.jpg



16640612893_674ce14497_k.jpg
 Description:
Didn't fall :) Going up the other side.
 Viewed:  9500 Time(s)

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16640608483_df4dfa4da2_k.jpg
 Description:
Standard Icelandic Fluffymoss.
 Viewed:  9487 Time(s)

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17259070422_9cca240bf6_k.jpg
 Description:
Trying to follow a Standard Icelandic Trail. That is, half the time there's no recognizeable trail, half the signposts are missing, and sometimes they try - like here - to trick you into walking to your death ;) "Yes yes, that's a trail! No, what do you mean that's just a rift in a snowbank? No, you're seeing things..."
 Viewed:  9492 Time(s)

17259070422_9cca240bf6_k.jpg



16638321734_b36672e2db_k.jpg
 Description:
The high "plains" where I turned around.
 Viewed:  9501 Time(s)

16638321734_b36672e2db_k.jpg


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PostPosted: Apr 25, 2015 11:53    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Part 3


16640596723_5043c9c93d_k.jpg
 Description:
 Viewed:  9506 Time(s)

16640596723_5043c9c93d_k.jpg



17260770625_8d91230498_k.jpg
 Description:
A sizeable horizontal fault between layers.
 Viewed:  9501 Time(s)

17260770625_8d91230498_k.jpg



17260297571_90f939b61c_k.jpg
 Description:
 Viewed:  9486 Time(s)

17260297571_90f939b61c_k.jpg



17234846606_1a1e833a47_k.jpg
 Description:
The terrain on the far side that kept me from approaching the deepest parts of the canyon. It would have been easier from the east side that I was walking back on, but I felt (perhaps rightly, perhaps wrongly) that I had seen most of what the rocks were like here, and I was getting frostbite.
 Viewed:  9494 Time(s)

17234846606_1a1e833a47_k.jpg



17260296331_192e186cc6_k.jpg
 Description:
The boulder that forced me out of the canyon to begin with.
 Viewed:  9513 Time(s)

17260296331_192e186cc6_k.jpg



17259033832_d0d9910326_k.jpg
 Locality:
Keriđ Volcano, Grímsnes, Iceland's Western Volcanic Zone, Iceland
 Description:
Ready for the final kilometers to the car
 Viewed:  9509 Time(s)

17259033832_d0d9910326_k.jpg



17074604519_8ae1ef3d96_k.jpg
 Locality:
Iceland
 Description:
But first, a quick slide down the snow ;)
 Viewed:  9512 Time(s)

17074604519_8ae1ef3d96_k.jpg


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PostPosted: Apr 25, 2015 11:56    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Okay, now for the minerals!


s1500001.jpg
 Description:
I almost completely ignored them, just thinking that they were just some sort of rusty basalt... but I quickly noticed that they were not only glimmering, but that the glimmers were irridescent. A still pic doesn't show irridescence well, but in person in sunlight it's really beautiful. Too bad they're so small.
 Viewed:  9486 Time(s)

s1500001.jpg



s1500009.jpg
 Description:
In the beginning I only noticed the irridescence, but up close there's all sorts of colourful crystals. The greens are surely olivine, but I don't know about the reds.

Oh how I wish the crystals in these samples were bigger! :)
 Viewed:  9468 Time(s)

s1500009.jpg



s1500003.jpg
 Description:
These white crystals were more common than the brilliant green olivine, although I don't know what they are. I love the shiny, perfectly-bubbling black surface texture from a lot of these specimens, although I don't know what it is.
 Viewed:  9466 Time(s)

s1500003.jpg



s1500007.jpg
 Description:
 Viewed:  9475 Time(s)

s1500007.jpg



s1500010.jpg
 Description:
Some sort of inclusions
 Viewed:  9483 Time(s)

s1500010.jpg



s1500012.jpg
 Description:
Olivine phenocrysts
 Viewed:  9468 Time(s)

s1500012.jpg



s1500013.jpg
 Description:
 Viewed:  9460 Time(s)

s1500013.jpg



s1500014.jpg
 Description:
 Viewed:  9461 Time(s)

s1500014.jpg



s1500016.jpg
 Description:
Even small, I just love olivine. Hard to believe it's the most common mineral on Earth.
 Viewed:  9467 Time(s)

s1500016.jpg



s1500019.jpg
 Description:
 Viewed:  9462 Time(s)

s1500019.jpg



s1500021.jpg
 Description:
Some splatter from a viscous, gas-rich lava? Not sure how to interpret that "coated" look
 Viewed:  9468 Time(s)

s1500021.jpg



s1500023.jpg
 Description:
Love that texture.
 Viewed:  9471 Time(s)

s1500023.jpg



s1500024.jpg
 Description:
Almost perfectly spherical cavity here - any clue what might have been there? Maybe...
 Viewed:  9478 Time(s)

s1500024.jpg



s1500027.jpg
 Description:
Almost perfectly spherical interior material. Again, any clue what it is?
 Viewed:  9469 Time(s)

s1500027.jpg


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PostPosted: Apr 25, 2015 18:15    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

I enjoyed seeing these pictures, Rei, thanks for posting them. Those are good kinds of rocks!
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PostPosted: Apr 25, 2015 19:50    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Oh hey, just learned something! In the presence of water (which is in no short supply here ;) ), olivine readily weathers to iddingsite, "a combination of clay minerals, iron oxides and ferrihydrites". It's reddish orange. I'd be willing to bet that these red rocks I was randomly finding, such as the one above, are predominantly iddingsite from old olivine xenoliths. Does that pass muster?

If so, maybe I should make another trip out there some time in the future and see if I could find some unoxidized xenoliths.... or to another picrite site in the country maybe. Preferably one that's not an impassable Terrorcanyon ;)
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PostPosted: Apr 27, 2015 06:48    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Hi Rei. What a pleasure to view these pictures! I see a lake in the background. I bet there are some stunning fly fishing to be had in that streams. Thank you for sharing this outing with us.
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PostPosted: Apr 27, 2015 07:19    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Rei, your geological foot tour is captivating. Can't wrap my head around it's youth, and really brings home the notion of big time. A suggestion on some of your rock images would be to include a size notation on a few of them, maybe field of view. Thanks, and happy trails to you.
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PostPosted: Apr 27, 2015 08:26    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

@Vic: I'll try to remember to do that next time, thanks :)

@Pierre: That's Ţingvallavatn, the lake next to one of Iceland's most famous sites, Ţingvellir (the site of one of the world's oldest parliaments and an active spreading center of the Mid-Atlantic Ridge). In fact, the Mid-Atlantic ridge is very visible under that lake too - the canyon there is called Silfra, and while I've never dived it, the pictures are pretty stunning:

http://www.photocircle.net/eu/photos/thumbnails/zoom/11862-Silfra--by-boris-buschardt.jpg
http://www.wonderland.is/skrar/image/album/dive-silfra-1.jpg
(Links normalized by FMF)

As for the rivers, yeah, Iceland is a popular destination for fishing. The salmon rivers are particularly in demand, the British royals for example have come here pretty often for salmon fishing. But the countless smaller rivers like this, few people visit these locations - even though this is in the southwest where most of the population lives. A canyon like this probably averages only one visitor every several days day over the course of a year, less in the more remote stretches. In the highlands there are rivers this size that probably average a visitor only once every several decades. Some places being revealed by the melting glaciers have never borne witness to human footsteps before.

It's funny how 90% of the tourists all flock to the exact same touristed places (Blue Lagoon, Ţingvellir, Geysir and Gullfoss) when the whole country is packed full of natural pearls ;) For example, about four kilometers west of where I was hiking there is Hagavíkurlaugar:

http://www.google.is/search?site=&tbm=isch&source=hp&biw=2144&bih=1149&q=silfra&oq=silfra&gs_l=img.3..0l8j0i30l2.903.1457.0.1551.6.5.0.0.0.0.250.250.2-1.1.0.msedr...0...1ac.1.64.img..5.1.250.Gyd8l0_VElg#tbm=isch&q=hagav%C3%ADkurlaugar
(Links normalized by FMF)

They probably average maybe 1-2 visitors per day, if that. You know, picture how many people would visit that per day if it were in Yellowstone. Here it's just "yeah, yet another geothermal feature in the Hengill region" ;)
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PostPosted: Apr 27, 2015 11:59    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Hi Rei, Riana and I viewed the pictures and as she used to scuba dive, we both found them fascinating. This is a part of earth I would love to visit, but I have fishing in mind! Best regards.
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PostPosted: Apr 29, 2015 18:05    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Rei, many thanks for posting this thread with all the wonderful photo's of your exploration of the Icelandic Terror Canyon! I can understand why you didn't want to follow it too closely, especially at this time of year - how cold was it? We got those cold winds here in the UK a day or two later but it had warmed up a lot by the time it got here ;)
Serious stuff though; if you had got wet and had to struggle back to the car in that wind...(shudder). Maybe a return trip in late summer would be better. Being on your own as well would make the trip even more hazardous. Still, I would do the same, I've been in some pretty insane places on my own ;)
The olivine phenocrysts in the picrite rock are very fresh in places, and look very nice. I don't know what the white phenocrysts are, maybe the bytownite feldspar? Though I wouldn't have thought a rock composed of >90% olivine would contain that much calcium-feldspar...
I also don't know much about iddingsite.
I think the round, smooth hollows in the rock are just empty vesicles - gas bubbles - and doubt whether the lava would be old enough to allow secondary mineral formation in them... though obviously some of the olivine has been altered.
I have never encountered hyaloclastite; is that lava that has been extruded beneath glacial ice? Don't have much of that 'round these parts...

Great photo's and commentary. Love the landscape.

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PostPosted: Apr 29, 2015 20:29    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

The temperature probably wasn't too far below freezing, but the wind was probably at least 10 m/s (22mph) sustained, with gusts at least 20 m/s (45mph). It's enough to push you around when you're trying to keep steady on loose ground.

Worse than falling in the water would have been falling off one of those cliffs ;) No standing up after that... But, I try to be careful and assess the terrain I'm walking on. In all my years of hiking I've only ever had really one uncontrolled slide, and that was in an area where I considered an uncontrolled slide an acceptable risk. :)

I'm sure the hollows are just gas bubbles, but I love how tiny, glassy and regular they are. The lava on my land doesn't have as appealing of a surface texture (although I do have one area that has some bright red pahoehoe flows preserved with all their surface texture, which is kinda neat :) ).

So yeah, hyaloclastite is one of those things we have more of here than most places in the world. A lot of our big volcanoes are tuyas, that is, steep sides and then very flat tops:

http://www.google.com/search?site=&tbm=isch&source=hp&biw=1920&bih=935&q=tuya+volcano&oq=tuya+volcano&gs_l=img.3..0i19.314.2272.0.2510.16.13.1.0.0.0.223.1056.3j4j1.8.0.msedr...0...1ac.1.64.img..7.9.1057.1kSLaoBWo1Q
(link normalized by FMF)

They form when volcanoes erupt under a thick glacial ice cap. The lava that comes out is rapidly quenched by the meltwater, like an undersea eruption. If it's a slow, steady flow without much shattering, you get pillows; if it's powerful and fast explosive eruption, or if there's a lot of cold shock shattering of the pillow surfaces, you get hyaloclastite - the mixture of the rapidly cooled glassy stuff with the slower cooled granular stuff.

That's about the total extent of my knowledge on the subject, mind you ;)

Btw - unrelated - I don't remember if it was you, but someone posited that the large white veins I was seeing from a distance in the prior canyon I hiked in were dikes of rhyolitic lava rather than chert - which made me start to wonder whether the places on my land where I've found eroded rhyolite might be from such a dike rather than the rhyolite peak 15km away as I had long assumed. Well, yesterday evening I was picking up some wood that blew away from my property onto the neighbors' land, and what should I come across not more than 100 meters from the property line but another one of those veins, which I could look at right up close. And it indeed appeared to be rhyolite (or some other volcanic rock on the high silica side of the spectrum), not chert. So yeah, I think that's my explanation right there! Those dikes sure do get around...

Some time this summer I need to get out to the site of the latest volcanic eruption (Bárđarbunga / Holuhraun / Nornahraun), if the safety officials open it up. I don't expect to find any particularly interesting minerals there from a collection standpoint, but it was the highest volume eruption in Iceland since Laki's devastating 1783-1784 eruption, and I really can't wait to see the brand new "mountains" it's created :) Plus, I spent long enough having to breathe in those damned gases from the eruption, I think I deserve to get to see the source ;) Hmm, wonder if the witches' hair is still there...
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Mike Wood




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PostPosted: May 01, 2015 18:10    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

Thanks for the gooogle link Rei, some great info there. I happened to click on the central image, tenth row down (just right of the photo of the Icelandic hut), which linked to a fantastic website - containing everything about rocks one would want to know! It is very well organised and easy to navigate. I tried to post a reply yesterday including the direct link to that website but the damn spamfilter kept rejecting it, which is very frustrating!!

Lucky you finding the rhyolite dike on or adjacent to your land! Of course, it might not be rhyolite strictly speaking, but it could be some kind of acidic lava dike anyhow.
Chert, strictly speaking, is sedimentary in origin.

Good luck if you make it to Nornahraun this year; last I saw on the Icelandic Met Office website (re. Bardabunga updates) the restricted zone has shrunk to as little as 20m along the Northern edge of the lava flow, so you can't touch it but you could go and chuck rocks at it! (So long as the elves don't mind, of course ;-)

Cheers, Mike

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Rei




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PostPosted: May 09, 2015 03:17    Post subject: Re: The picrite of Ölfusvatnsárgljúfur  

So, I recently got back my mineral species test results.

The abundant white phenocrysts are anorthite crystals:

"Anorthite is the calcium-rich endmember of the plagioclase solid solution series ...
Anorthite is a rare compositional variety of plagioclase. It occurs in mafic igneous rock. It also occurs in granulite facies metamorphic rocks, in metamorphosed carbonate rocks and corundum deposits. ... It is more rare in surficial rocks than it normally would be due to its high weathering potential in the Goldich dissolution series ... It also makes up much of the lunar highlands; the Genesis Rock is made of anorthosite, which is composed largely of anorthite. Anorthite was discovered in samples from comet Wild 2, and the mineral is an important constituent of Ca-Al-rich inclusions in rare varieties of chondritic meteorites."

As for the rusty samples made of a number of different crystals, unfortunately he didn't analyze that which I found most interesting (the dark iridescent crystals). He did identify forsterite, which isn't surprising:

"Forsterite... is the magnesium-rich end-member of the olivine solid solution series.... Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites. In 2005 it was also found in cometary dust returned by the Stardust probe. In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.... Peridot is the gemstone variety of forsterite olivine. ... Forsterite-rich olivine is the most abundant mineral in the mantle above a depth of about 400 km ..."

... and he also identified celadonite:

"Celadonite is a mica group mineral, a phyllosilicate of potassium, iron in both oxidation states, aluminium and hydroxide.. usually forms massive aggregates of prismatic crystallites or in dull clay masses... Typically occurs as dull gray-green to bluish green masses. It forms vesicle fillings and linings in altered basaltic lavas."

Knowing about the anorthite, however, I can't help but wonder if the iridescence is more accurately labradorescence. Labradorite is a feldspar with a 50-70% anorthite percentage. And labradorescence matches the optical effect that one sees from the black crystals:

https://www.google.is/search?site=&tbm=isch&source=hp&biw=2144&bih=1149&q=labradorescence&oq=labradorescence&gs_l=img.3..0i19.368.2281.0.2582.15.9.0.0.0.0.302.566.2-1j1.2.0.msedr...0...1ac.1.64.img..13.2.564.6Mp_7Nth8jY&gws_rd=ssl
(link normalized by FMF)

But basically... when people say that some places in Iceland look like the surface of the moon... it's not just looks! Geologically, this canyon apparently has a lot in common with the geology of the lunar surface :) This is a result of, as was known, this being a very deep, mantle-sourced magma.
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