Geoscience Reference
In-Depth Information
part
of
the
transition
region,
near
and
just
chemical heterogeneity at the tens of km scale,
the scales of recycled crust and lithosphere. The
hundreds of km scales are comparable to the
segmentation of ridges, trenches and fracture
zones, and the scales of delaminated crust along
island arcs. Chunks of slabs having dimensions
of tens by hundreds of km are inserted into the
mantle at trenches. They are of variable age,
and equilibrate and are sampled over various
time scales. The lateral dimensions of plates, and
the separation distances of trenches and aseis-
mic ridges are also likely to show up as scale
lengths in chemical and physical variations along
ridges.
The central limit theorem (CLT) is essential
in trying to understand the range and variabil-
ity of mantle products extracted from a het-
erogenous mantle. Depending on circumstances,
small domains -- tens to hundreds of km in
extent -- can also be isolated for long periods
of time until brought to a ridge or across the
melting zone. Mineralogy, diffusivity and solu-
bility are issues in determining the size of iso-
latable domains. When a multi-component man-
tle warms up to its solidus -- not necessarily the
same as the surrounding mantle -- the erupted
magmas can be variable or homogenous; this is
controlled by sampling theory, the statistics of
large numbers and the CLT. Even under a ridge
the melting zone is composed of regions of vari-
able melt content. The deeper portions of the
zone, and those regions on the wings, will expe-
rience small degrees of melting but these will be
blended with high-degree melts under the ridge,
prior to eruption. Magma cannot be considered
to be uniform degrees of melting from a chem-
ically uniform mantle. Blending of magmas is
an alternate to the point of view that convec-
tion is the main homogenizing agent of man-
tle basalts. There are also differences from place
to place and with depth, i.e. large-scale hetero-
geneities.
The isolation time of the upper mantle is
related to the time between visits of a trench or
a ridge. With current migration rates a domain
of the upper mantle can be isolated for as
long as 1 to 2 Gyr. These are typical mantle
isotopic
below 650 km. Plates that were young (
30 Myr)
at the time of subduction (e.g. Farallon slab
under western North America) and slabs sub-
ducted in the past 30 Myr may still be in the
upper mantle.
Old, thick slabs appear
to collect at 750
--
900 km
, the probable
base of the layer accessible to surface volca-
noes. The
quantitative and statistical
methods of determining the depth of
subduction
and comparison of
tomographic
and geodynamic models
are superior to the
visual analysis of selected color tomographic
cross-sections -- qualitative chromotomography.
A chemically stratified mantle will have some
deep high-velocity patches and some will appear
to correlate with shallower structures.
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Scale of mantle heterogeneity
In the plume model isotopic differences are
attributed to different large (400--2000 km in
extent) reservoirs at different depths. In the
marble cake and plum-pudding models the
characteristic dimensions of isotopic hetero-
geneities are centimeters to meters. Chemical
differences between ridge and nearby seamount
and island basalts may be due, in part, to the
nature of the sampling of a common heteroge-
nous region of the upper mantle. In order for
this to work there must be substantial chem-
ical differences over dimensions comparable to
the volume of mantle processed in order to fuel
the volcano in question, e.g. tens to hundreds
of km.
Chemical differences along ridges have char-
acteristic scales of 200--400 km. Inter-island dif-
ferences in volcanic chains, and seamount chem-
ical differences, occur over tens of km e.g. the
Loa and Kea trends in Hawaii
.Ifhetero-
geneities were entirely grain-sized or km-sized,
then both OIB and MORB would average out the
heterogeneity in the sampling process. If het-
erogeneities were always thousands of km in
extent and separation, then OIB and MORB sam-
pling differences could not erase this. There-
fore, there must be an important component of
ages
and
are
usually
attributed
to
a
