-Peridotite Xenolith- San Carlos, Arizona

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San Carlos, Arizona

Site Location: This particular peridotite xenolith sample is from San Carlos, Arizona. San Carlos is located in southeast/central Arizona, shown in Figure 1. San Carlos is part of the Central Highlands of Arizona, a transition between the Colorado Plateau to the north and the Basin Range to the south. San Carlos is a basin between the numerous mountain ranges in the region.

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Figure 1: Location of San Carlos in Arizona

Geologic Setting: Igneous and metamorphic rocks are well exposed in this region, the surrounding mountain ranges are dominated by granitic-gneiss features, and granitic plutons are commonly seen as intrusions in sedimentary rocks. Heat and water released from magmatic plutons metamorphosed local sedimentary rocks and created large concentrations of valuable copper ores. Thus, the region has been denoted for its exposure of magmatic exposure through xenoliths, igneous and metamorphic rocks, and ore deposits. San Carlos is a roughly 2.500 square mile region enclosed in a nationally protected Native American Reservation.Figure 2 below shows an exposed pluton of magma revealing masses of peridotite xenoliths, formed through rapid cooling.

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Figure 2: Exposed, uplifted magmatic intrusion revealing peridotite xenoliths in San Carlos, Arizona. Photo property of the Omnipressure Lab, Arizona State University.


Background: Peridotite is an ultramafic rock, consisting of mostly olivine and pyroxenes, rich in magnesium, and containing silica proportions below 45%. Peridotite contains relatively high proportions of forsteritic- olivine and other magnesium rich minor components like spine,l and is a commonly found xenolith. Mantle xenoliths are inclusions of foreign material in an igneous rock, and therefore can be analyzed as a direct proxy of the inaccessible mantle. Peridotite is the most common mineral in the mantle above 400 km depth, and is usually classified by percentage ratios of olivine and pyroxene using the CPX-OPX-Olivine naming triangle, shown in Figure 3. Below 400 km olivine is replaced by high pressure polymorphs, so peridotite does not occur. Olivine is also relatively unstable at shallow depths and frequently alters to serpentine through addition of water. The high amount of peridotite in the upper mantle or asthenosphere is a result of precipitation of olivine and pyroxenes, partial melting of upper mantle, and the original differentiation and accretion of the Earth. Peridotite is also often found in obducted ophiolite complexes, kimberlite pipes, acting as a reservoir for diamonds, talc, ores, and other natural resources.

The chemical formula for the components of peridotite:
  • Olivine: (Mg, Fe)2SiO4
  • Pyroxenes: XY(Si,Al)2O6

Common types of peridotite
  • Dunite: mostly olivine
  • Wehrlite: mostly olivine plus clinopyroxene.
  • Harzburgite: mostly composed of olivine plus orthopyroxene, and relatively low proportions of basaltic ingredients (because garnet and clinopyroxene are minor).
  • Lherzolite: mostly composed of olivine, orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene). Partial fusion of lherzolite and extraction of the melt fraction can leave a solid residue of harzburgite.

The hand sample and thin sections below shows peridotite xenoliths in a basaltic groundmass.

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.Figure 3: Hand sample of peridotite xenolith.
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Figure 4: Peridotite crystals in basaltic groundmass.

Description: Olivine rich crystals of peridotite in a basaltic groundmass. Olivine crystals are green to yellow green in massive or granular form. Cubic and octahedral dark crystals of spinel are in the olivine crystal mass. Texture is porphyritic, groundmass is basalt. Estimated proportion is 35% peridotite cystals and 65% basaltic groundmass, Pyroxene and olivine are difficult to distinguish.

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Figure 5: Thin section: KH97-7. XPL/PPL

Description: XPL: Large fragments of olivine ranging from 2nd to 3rd order colors with rims showing a wide range of birefringence. Smaller grains of pyroxenes most obviously distinguished by defined cleavage patterns and low first order colors, brown, grey, black, OPX the largest pyroxene component. OPX 90 degree cleavage patterns evident in PPL and XPL. Olivine can be distinguished from higher order pyroxenes, CPX by fracture patterns opposed from cleavage. Vesicles obvious in PPL and XPL. Mineral proportions: 40% Pyroxene 60% Olivine


Xenolith Classification/ Interpretation
UMR.GIF
Figure 6: Peridotite naming triangle

Through mineral percentages the naming triangle can be used to classify the rock type of the peridotite crystals. According to the triangle and the thin section analysis, yielding an estimated 60% olivine and 40% pyroxene composition, the peridotite is most likely a lherzolite. Peridotites of this nature are commonly harzburgites, but the apparent higher composition of OPX is closer to the mineral composition of lherzolite.