Geoscience Reference
In-Depth Information
convention, are considered to be hotspot mag-
mas. Although island-arc magmas and oceanic-
island magmas differ in their petrography and
inferred pre-eruption crystal fractionation, they
overlap almost completely in the trace elements
and isotopic signatures that are characteristic of
the source region. It is almost certain that island-
arc magmas originate above the descending slab,
although it is uncertain as to the relative con-
tribution of the slab and the intervening man-
tle wedge. Water from the descending slab may
be important, but most of the material is pre-
sumed to be from the overlying 'normal' mantle.
Whether the subducted oceanic crust also melts
below island arcs is controversial. It is curious
that the essentially identical range of geochemi-
cal signatures found in oceanic-island and conti-
nental flood basalts have generated a completely
different set of hypotheses, generally involv-
ing a 'primitive' lower-mantle plume source. If
island-arc magmas and boninites originate in the
shallowest mantle, it is likely that island and
continental basalts do as well. Subduction, and
bottom side erosion and delamination, rather
than plumes, are probably the primary causes of
shallow mantle enrichment.
The main diagnostic difference between some
islands and most other magmas is the higher
maximum values and the greater spread of
the
3
He/
4
He ratios of the former. High ratios
are referred to as 'primitive,' meaning that the
helium-3 is assumed to be left over from the
accretion of the Earth and that the mantle is not
fully outgassed. The word 'primitive' has intro-
duced semantic problems since high
3
He/
4
He
ratios are assumed to mean that the part of
the mantle sampled has not experienced partial
melting. This assumes that (1) helium is strongly
concentrated into a melt (relative to U and Th), (2)
the melt is able to efficiently outgas, (3) mantle
that has experienced partial melting contains no
helium and (4) helium is not retained in or recy-
cled back into the mantle once it has been in a
melt. As a matter of fact it is difficult to outgas
amelt.
Magmas must rise to relatively shallow depths
before they vesiculate, and even under these
circumstances gases are trapped in rapidly
quenched glass or olivine cumulates. Although
most gases and other volatiles are probably con-
centrated into the upper mantle by magmatic
processes, only a fraction manages to get close
enough to the surface to outgas and enter the
atmosphere. High helium-3 contents relative to
helium-4, however, require that the gases evolved
in a relatively low uranium--thorium reservoir for
most of the age of the Earth. Shallow depleted
reservoirs such as olivine cumulates may be the
traps for helium.
Komatiites
Komatiites are ultrabasic melts that occur mainly
in Archean rocks. The peridotitic variety appar-
ently require temperatures of the order of
1450--1500
◦
C and degrees of partial melting
greater than 60--70% in order to form by melting
of dry peridotite. At one time such high temper-
atures and degrees of melting were thought to
be impossible. Even today, similar degrees of par-
tial melting of eclogite, at much lower tempera-
tures, are considered unlikely. At high pressure it
is possible to generate MgO-rich melts from peri-
dotite with smaller degrees of partial melting. It
is even possible that olivine is replaced as the liq-
uidus phase by the high-pressure majorite phase
of orthopyroxene, again giving high-MgO melts.
Komatiites may represent large degrees of melt-
ing of a shallow olivine-rich parent, small degrees
of melting of a deep peridotite source, melting
of a rock under conditions such that olivine is
not the major residual phase, or melting of wet
peridotite at much lower temperatures. At depths
greater than about 200 km, the initial melts of a
peridotite may be denser than the residual crys-
tals. This may imply that large degrees of partial
melting are possible and, in fact, are required
before the melts, or the source region, become
buoyant enough to rise. The ratios of some trace-
elements in some komatiites suggest that garnet
has been left behind in the source region or that
high-pressure garnet fractionation occurred prior
to eruption. The existence of komatiites appears
to refute the common assumption that melt-
crystal separation must occur at relatively small
degrees of partial melting. The rarity of komati-
ites since Precambrian times could mean that the
mantle has cooled, but it could also mean that
a suitable peridotite parent no longer exists at
