Spinel lherzolite and harzburgite xenoliths hosted in an alkali
basalt dyke near Inver, Donegal,
Ireland show abundant evidence of interaction between xenolith minerals
and
the host melt. Of particular
interest are primary Cr-diopside and spinel with sieve-textured coronas.
Coronas on primary Cr-diopside are
up to 3 mm wide and are associated with veinlets of devitrified glass.
The
coronas comprise secondary Cr-diopside
with vermicular, interstitial alkali feldspar and chlorite grains up to
100 µm in size. The inclusion-free
Cr-diopside cores are Al- and Na-rich whereas the coronas are Al- and
Na-depleted and Ti-enriched.
Sieve-textured spinels have similar texture to the clinopyroxene grains
and
are also associated with veinlets
of infiltrated glass. However, the interstitial inclusions in the
sieve-textured region are chlorite and
nepheline. Inclusion-free spinel is part of a chromite–spinel
solid solution and is Ti-poor. Spinel in the coronas
has a greater chromite and ulvospinel component and falls close
to a mixing line with spinel in the host
alkali basalt. In addition to the sieve-textured grains, primary olivine
in contact with infiltrated glass has Fe-rich
rims, and orthopyroxene has broken down to form rims of olivine,
clinopyroxene and a K-rich phase
similar in composition to alkali-feldspar. Comparison of the compositions
of
the inclusion-free cores and
sieve-textured rims shows that the rims have chemical signatures consistent
with partial melting, that is, Al
and Na depletion for clinopyroxene and Cr-enrichment for spinel. The
textures of the coronas, particularly
those around spinel and the reaction margins on orthopyroxene are identical
to those produced during dissolution experiments.
We suggest that silicate liquid from the host magma infiltrated the
xenoliths during their ascent and since
it was not in equilibrium with the xenolith minerals caused reaction. The
occurrence of K-bearing interstitial
minerals in the sieve-textured grains and reacted orthopyroxenes indicate
that the coronas did not form by
simple melting since none of the minerals that underwent breakdown are
K-bearing. We suggest that the
sieve-textured grains formed initially by partial melting and reaction
associated with decompression and
infiltration of liquid from the host magma. The melts included in the
reacted phases were enriched in K by
diffusion from the Si-poor infiltrated melt into the more Si-rich melt
inclusions in the coronas.