GARNET
LHERZOLITE
One of the most incredibly rare rocks in my collection
is this garnet lherzolite from South Africa. This is a slice of a
xenolith nodule ("xeno" = foreign, and "lith" = rock) from
a kimberlite pipe in Kimberley, central South Africa. As magma rises in a
pipe during kimberlite formation, it plucks rock from the deep walls of the
pipe. Many kimberlite xenoliths turn out to be samples from the deep
crust, lithospheric mantle, and even possibly sub-lithospheric mantle.
This garnet lherzolite xenolith is such a sample of lithospheric or
sub-lithospheric mantle! Rocks such as garnet lherzolite and spinel
lherzolite have traditionally been inferred to be primitive mantle
materials.
This rock is a variety of peridotite, an ultramafic
intrusive igneous rock. It is dominated by two dark-greenish minerals: forsterite
olivine (having veinlets of serpentine) and enstatite orthopyroxene.
Notice that the rock also contains rounded, purplish-red crystals. These
are pyrope garnets. There are also widely scattered, bright
emerald green colored crystals of chromian diopside pyroxene.
Small amounts of accessory minerals are also present.
Garnet lherzolite xenolith nodule (unpolished slice, 9.4 cm
across). Dark green crystals = olivine & orthopyroxene; purplish-red
crystals = pyrope garnet; bright emerald green crystals = chromian diopside
pyroxene.
Garnet lherzolite (field of view ~1.6 cm across). Purplish red =
pyrope garnets; bright green = chromian diopside pyroxenes.
Age:
Precambrian, very likely Archean (probably late Mesoarchean or early
Neoarchean).