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.

Site 23

After six days amidst the maroon, red, pink, orange, tan and white sedimentary rocks that dominate the landscapes of  the Colorado Plateau Province, I woke up this morning to something very different.

Monday morning with Navajo sandstone at the Wedge Overlook, San Rafael Swell, Colorado Plateau.

Tuesday morning with Sevier River hoodoos, Tushar Mountains, eastern Basin and Range Province.

My neighbors here in the Castle Rock Campground are hoodoos eroded from the mid-Tertiary Sevier River Formation, a mix of sedimentary strata derived from volcanic rocks upstream.  The volcanics are products of the Mt. Belknap Caldera ca 10 miles to the south, part of the massive volcanic conflagration in the Basin and Range Province in mid-Tertiary time.

Volcanic material was eroded off the highlands and deposited in lake beds, canyons and river channels, including the one now occupied by Castle Rock Campground.  Then, with subsequent Basin and Range uplift, streams were rejuvenated and the river deposits themselves were eroded -- forming these wonderful hoodoos.  The layered texture reflects sediment diversity, ranging from fine to quite coarse, with some cobbles up to several feet across.

Bands of cobbles deposited during more energetic river flow ... perhaps flash floods?

Ash layers (white bands) indicate there was ongoing volcanic activity in the area during deposition.  Note the gradation of color above the lower ash layer with deposits containing progressively less ash (vs. the sharp contrast below).  A younger layer is visible near the top of the highest castle in upper right of photo.  These two layers have been dated at 13.8 and 6.9 Ma.

Most of the material of the Sevier River Formation was derived from the Joe Lot tuff, which covers much of the area.  The contact between the two rises above the upstream end of the campground.  Rises above???  That’s because the formations are severely tilted, even though Sevier River strata are nearly horizontal just a short distance downstream.

How cool!  Here you can view results of both Tertiary volcanics and Tertiary extension, for Basin and Range tectonics are featured as well.  A short distance south is a normal fault along which the northern block, containing the campground, has dropped.  The resultant deformation is easy to see at the upstream end of the campground.

Sevier River Formation, dipping north on left, nearly horizontal on right.

In this photo looking east across the drainage from the slope above Site 23, the red arrow points to Sevier River beds tilted very steeply, and appearing to lean against the Joe Lott tuff.  At the blue arrow to the north, beds dip much less (white ash layer is a good marker), and they become nearly horizontal downstream.

Here are the same "leaning" slabs (red arrow above) but viewed from below.  This area can be accessed via a trail from Site 17, marked "No ATVs".

The Sevier River Formation is not as photogenic as the more richly-colored sedimentary rocks of the Colorado Plateau; the hoodoos are a bit washed out in comparison, but the patterns are striking, especially in the shady alcoves.

Right:  vertical drainages (slots) develop on Sevier River outcrops, from several inches across to wide enough to enter.  Do hoodoos come about because of this erosional tendency?

The back walls of the slots have beautiful horizontal ripples (below, dog for scale).

Left:  in an especially shady alcove there is a bit more color and warmth in photos ... thanks to the flash.

Below:  the artistic handiwork of erosion.

The Sevier hoodoos and Joe Lott tuff at Castle Rock Campground are featured in Vignette 12 in Geology Underfoot in Southern Utah (Orndorff, Wieder and Futey, 2006), and in the Utah Geologic Survey’s Geosights.  Both sources mention Site 23 as an excellent location to view rocks and structures of interest.  From the west side of the campground loop, informal trails lead into the hoodoo neighborhoods above.

Below:  both the Sevier River Formation (right) and Joe Lott tuff (left) are visible at Site 23. Here at the contact they dip to the north, due to a fault a short distance upstream.

How to get there:

Castle Rock Campground is just south of Interstate 70, about 20 miles west of Richfield, Utah, and 17 miles east of the junction with Interstate 15.  Fremont Indian State Park, close by on the other side of the highway, is worth a visit as well.  Take exit 17 and go south 1.2 miles to the campground.  Sites are $13 per night (2012), and include water, dumpsters, bathrooms, a creek, plenty of shade, a hiking trail up Joe Lott Creek, entry to the FISP Museum, and of course great entertainment for geo-geeks.  Famous Site 23 is on the west side of the loop at the upstream end of the campground ... but be prepared for hordes of geo-pilgrims ;-)