Geoscience Reference
In-Depth Information
Enrichment and depletion processes
Vigorous stirring can homogenize a fluid by a
process known as
chaotic advection
. Dif-
fusive and thermodynamic processes, in the
absence of gravity, are homogenizers. Large-
scale melting can be a homogenizer. Plate tec-
tonic and other petrological processes, however,
create heterogeneities. Removal of small-degree
melts causes depletion of fertile regions (basalt
sources) -- or components -- and refractory regions
(depleted peridotites, cumulates). Small-degree
melts are enriched in LIL and become the crust
and the enriched components (EM) in the upper
mantle, such as kimberlite and carbonatites, and
in the sources of enriched magmas such as
EMORB and OIB. Large-degree melting occurs at
spreading centers and at thin spots of the litho-
sphere from upwelling mantle that has been
depleted (NMORB source) or enriched (EMORB
source) by the transfer of these small melt frac-
tions. Large-degree and large-volume melts blend
together large- and small-degree melts from a
large volume of the mantle and give fairly
uniform magmas with small variance in trace-
element and isotopic ratios, even if the shallow
mantle is heterogenous. This is called
melt aggre-
gation
or
blending
.
Melt extraction from partially molten rocks
or crystallizing cumulates is not 100% effi-
cient and residual melts help explain some of
the trace element and isotopic paradoxes of
mantle magmatism, such as apparent contradic-
tions between the elemental and isotopic com-
positions. Other sources of magmatic diversity
include recycling, delamination and melting of
diverse lithologies such as eclogite and peri-
dotite, which also have experienced various levels
of melt extraction and infusion. This heteroge-
nous upper mantle or
statistical upper
mantle assemblage
gives relatively homoge-
nous products when it experiences large degree
melting.
Magmas at ridges and thin spots
represent blends of melts from vari-
ous depths, lithologies
and extents of par-
tial melting. The compositions of ocean island
basalts, ocean ridge basalts, and residual mantle
reflect upper mantle processes of melt extraction,
migration and trapping -- as well as recycling
from the surface -- and the sampling/melting
compositions have been interpreted as trapped
melts, depleted residues and recycled and dela-
minated materials. On major element plots
(Chapter 15) the end-members are harzburgite
and MORB, or eclogite. Picrites, komatiites and
primitive mantle have intermediate composi-
tions. On LIL and REE plots, the extreme com-
positions are kimberlites and DMORB or abyssal
peridotite.
From an isotopic point of view, oceanic
basalts are also treated as multi-component sys-
tems involving mixtures of
DMM, EM1, EM2
and HIMU and C or FOZO
. These are short-
hand names for what are thought to be the
various enriched (EM) and depleted (DM) iso-
topic end-members of the mantle and there is
a large literature on each. There is no agree-
ment regarding the lithology or history that
goes with each component. Trends of OIB and
MORB isotopic compositions approach -- or con-
verge on -- a hypothetical component of the man-
tlereferredtoas
FOZO (focal zone) or C
(common)
. It has been assumed that this reflects
the composition of the lower mantle. It is not
clear why the most common component in
basalts should represent the deepest, rather than
the shallowest, mantle. Melts pond beneath, and
percolate through, the lithosphere, and inter-
act with it. A lithosphere or harzburgite com-
ponent may therefore be involved in most mag-
mas. If so, ultramafic rocks (UMR) may anchor
the ends of both major element and isotope
mixing arrays. UMRs are certainly the most
common or prevalent lithology of the shallow
mantle.
The average or
prevalent mantle com-
position
has been referred to as PREMA in the
isotope literature. In contrast to the end-member
components,
PREMA, C and FOZO
are interior
components -- on isotope diagrams -- and are
therefore either mixtures or sources. The one
extreme attribute of these average compositions
is that basalts falling near these compositions
tend to have higher variance in their
3
He/
4
He
ratios, and therefore contain some high
3
He/
4
He
samples. The most prevalent lithology of the
mantle -- peridotite -- may be implicated in this
component while EM and HIMU may reside in
the fertile or mafic components.
