Monday, May 7, 2012

Is that mantle on the mantle?

There was a time when I thought these rocks contained “messages from the mantle” -- the shiny olive-green “alien inclusions” visible below at closer range.  Now I’m not so sure.  Is this olivine really relic mantle material?  Was it truly torn from upper mantle rocks through which the magma moved?  Or did it crystallize out as the magma cooled?

Olivine is a mineral uncommon at the earth’s surface but abundant in the upper mantle.  Gem quality olivine is called peridot.  Olivine is thought to crystallize early in the formation of basalt, as in this example (left) from Hawaii.

Here are some photos of my olivine in situ.  These are phenocrysts -- large crystals in a fine matrix; the largest that I found were a bit over an inch across.  Apparently the phenocrysts do not weather out intact; I found only small pieces of olivine in searching through coarse debris in sand-filled depressions in the lava field.
This lava flow with olivine phenocrysts was produced during the second phase in the life of the cinder cone visible behind the flow below.  In the first phase, explosive activity blew cinders into the air, building up the cone and depleting the magma of gases.  Later eruptions produced molten rock that flowed out of vents and breached the southwest side of the cone to create the Black Rock lava flow, the youngest feature (ca 20,000 years old) of the Lunar Crater Volcanic Field in central Nevada.
Black Rock lava flow breached the southwest side of Black Rock Crater.
The Black Rock lava flow; aerial from ArcGIS Online.


Vitaliano and Harvey (1965; figure to right, click to view) studied the basalt of Black Rock Summit, which they characterized as large crystals of olivine, pyroxene and plagioclase in a matrix of alkali basalt. They suggested that the phenocrysts were the result of crystallization during early phases of magma production, although some observations were not consistent with knowledge of sequence and environments of crystallization.  [For details, see the original paper.]



In contrast, my geo-guide considers these olivine phenocrysts to be inclusions carried up from the upper mantle by magma (Orndorff et al. 2001).  DeCourten (2003, page 217) also characterizes Quaternary magma of this region as rich in iron and magnesium, often with fragments of upper mantle rock.  If olivine crystallizes out during magma production, why is the olivine of the Black Rock flow considered to be mantle inclusions?  Alas, I don’t have an answer.

I ended up at the Black Rock lava flow last week thanks to Geology Underfoot in Central Nevada (Orndorff et al. 2001), which features it in Vignette 17 -- “Message from the Mantle”.  This is a spectacular place, with dark fresh-looking lava sculptures stark against pale sand and shrubs of the high desert.  Both chunky aa and ropy pahoehoe textures are in the mix.  Wind is filling depressions in the lava flow with sand -- an eolian mantle under construction -- and plants, lizards, horned toads and rabbits make their homes there.
Chunky aa lava, with developing eolian mantle (sand).
Ropy pahoehoe lava is less common; boot for scale.
The surrounding landscape is surreal -- wide-open, dark, with many graceful cinder cones.
Sparky admires the Quaternary volcanic landscape.
I was eager to find the mantle material the authors described, and spent quite a bit of time taking photos and looking through coarse debris in the pockets of sand.  Plagioclase and pyroxene were common (left and right below; identifications based on Orndorff et al.), but olivine was rare.  Some mischievous herbivore had left promising little green balls that repeatedly caught my eye.










Fool's olivine, above and below.
I ended my visit with just a few olivine treasures, now displayed on the mantle.


Sources

DeCourten, F.L.  2003.  The broken land; adventures in Great Basin geology.  Salt Lake City: University of Utah Press.

Orndorff, R.L., Wieder, R.W. and Filkorn, H.F.  2001.  Geology underfoot in central Nevada.  Missoula, MT:  Mountain Press Publishing Co.

Vitaliano, C.J. and Harvey, R.D.  1965.  Alkali basalt from Nye County, Nevada.  The American Mineralogist 50: 73-84.

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