Saturday, December 11, 2010

What a Geologist Sees - Part 14 [Original Post Date 2/08/08]

As a follow-up to Part 13, the upper photo is of a broken "volcanic bomb" from Kilbourne's Hole maar volcano, showing the mantle xenolith within. The average mineral grain-size within the xenolith is approximately 2 mm in diameter. The rock peridotite is generally composed of olivine and a pyroxene, in the case of Kilbourne's Hole, I think the pyroxene is enstatite. The composition of the peridotite xenolith defines it as being "ultramafic". The basalt that encloses the xenolith is defined as "mafic". The ultramafic minerals are the first to solidify in a cooling magma, followed by the mafic minerals, which include olivine and pyroxene, as well as some amphiboles and calcium plagioclase feldspars. The slow rate of cooling is responsible for the visible grain sizes in these xenoliths contrasted with the more rapid rate of cooling of the enclosing basalt lava.

In the few places in the world where these types of xenoliths have been erupted, the xenoliths provide a little insight as to the mineral composition of the upper mantle. In both cases shown here, the matrix or host rock is a basalt.

One other mantle xenolith locality in the western United States is San Carlos, AZ, on the Apache reservation east of Phoenix. The photo below is of a sample of the San Carlos xenoliths.

In "What a Geologist Sees - Part 3", I described the concept of "Inclusions", which originated with James Hutton and Charles Lyell. Inclusions, such as these xenoliths, are older than the rock itself, i.e., the solidification of the xenoliths had already taken place when they were included in the rising magma body.

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