Geoscience Reference
In-Depth Information
about 300 km depth, or that the currently rela-
tively thick lithosphere prevents their ascent to
the surface. Diapirs ascending rapidly from about
300 km from an eclogite-rich source region would
be almost totally molten by the time they reach
shallow depths, and basaltic magmas would pre-
dominate over komatiitic magma.
Komatiites may be the result of particularly
deep melting. By cooling and crystal fractiona-
tion komatiitic melts can evolve to less dense
picritic and tholeiitic melts. Low-density melts,
of course, are more eruptable. It may be that
komatiitic melts exist at present, just as they did
in the Precambrian, but, because of the colder
shallow mantle they can and must cool and
fractionate more, prior to eruption. In any case
komatiites provide important information about
the physics and chemistry of the upper mantle.
The major-element chemistry of komatiites
and related magmas (komatiitic basalts) are more
similar to modern mafic subduction magmas
than to magmas thought to be produced by high
temperatures. Komatiites with high SiO
2
-contents
show similarities to modern subduction magmas
(boninites). SiO
2
contents of magmas generally
decrease as the pressure of melting increases
therefore high SiO
2
is difficult to explain in a
plume scenario. Boninites have similarly high
SiO
2
at high MgO contents because they are
high-degree melts produced at shallow depths. In
an anhydrous, high-temperature decompression
melting regime, high degrees of melting begin
at great depth; in the case of boninites, large
extents of melting can occur at shallow depth
because of high H
2
O contents (Crawford
et al.
,
1989; Parman
et al.
, 2001).
The chemical
similarities between
boninites, basaltic komatiites and
komatiites
suggests that komatiites were
produced by similar melting processes that
produce modern boninites, with the primary
difference being that the
mantle was about
100
◦
C hotter in the Archean
. Rather than
being products of deep mantle thermal plumes,
komatiites may be products of normal plate tec-
tonic processes.
Komatiitic rocks are depleted in both light
and heavy rare earth elements (LREE and HREE)
relative
relatively high TiO2 even in the most LREE-
depleted varieties. Uncontaminated komatiites
and picrites have similar isotopic ratios overlap-
ping MORB, indicating generation from a man-
tle source with a long-term depletion in LREE.
Geochemical characteristics of the komatiite--
picrite association, including REE and Nb/Y--Zr/Y
systematics, indicate chemical heterogeneities in
the source region. The high MgO contents of the
rocks has been used to support a mantle plume
model for their genesis but they do not have
the
enriched
or
primitive
isotopic
signatures
expected
for
plumes
and
significant
regional
uplift before volcanism is lacking.
Hot mantle
Hot mantle intersects the solidus at different
depths and it melts over a larger pressure interval
and to greater extents than cooler mantle. This is
why melting anomalies have been called
hotspots
,
and why they are attributed to hot upwelling
plumes. The melts produced at high-temperature
have higher MgO and FeO and lower Na
2
O, Al
2
O
3
and SiO
2
and lower LIL-contents than melts of
cooler mantle.
Archean volcanism produced high-MgO mag-
mas called
komatiites
that seemed to fit the crite-
ria for being the products of super-hot plumes.
These magmas, however, may be produced by
hydrous melting processes in the upper mantle,
possibly associated with subduction
(Parman
and Grove, 2001). [mantleplumes]
Some modern subduction related magmas,
boninites, show a large compositional overlap
with Archean basaltic komatiites. Komatiites
have a wide range of major and minor element
composition. High-SiO
2
komatiites resemble mod-
ern boninite magmas that are produced by
hydrous melting, while low-SiO
2
komatiites
resemble more closely modern basalts produced
by anhydrous decompression melting. If high
MgO magmas represent deep plume sources
then the isotopic and trace element patterns
should resemble ocean island basalts, but they do
not. The low-TiO
2
contents of komatiite magmas
requires their source to have been depleted (i.e.
to have had a melt extracted). The isotopic com-
positions of komatiites overlap those of depleted
to
middle
REE
(MREE)
and
possess
