Geoscience Reference
In-Depth Information
or an undegassed reservoir. They also cannot be
treated the same as other quantities in mixing
calculations and in statistics. Means of ratios can
be misleading. The robust measures of central
tendency for ratios are the
geometric mean
and
the
median
, these are invariant to inversion of the
ratio.
Arithmetic means
of ratios are widely used,
even in this topic, but they can be misleading.
Smaller-scale averages tend to have more out-
liers and these have given rise to ideas about
different reservoirs. The
convecting mantle
is geo-
chemical jargon for the
upper mantle
,the
MORB-
reservoir
or for the
source
of homogenous basalts.
A homogenous product does not imply a homoge-
nous source.
Plume
This is a catch-all word in mantle studies,
although it has a precise fluid dynamic meaning.
In tomographic studies it means
red
,aregionthat
has relatively
slow seismic velocities
. In isotope geo-
chemistry it means any composition other than
MORB, as in
the plume component
. In marine geo-
physics it is any
anomalously shallow region
.Ingeol-
ogy it often means any region except ridges and
arcs with extensive magmatism. In geophysics
it means a hot buoyant upwelling from a deep
unstable thermal boundary layer, heated from
below. [
do plumes exist
]
Convection vs. stirring
Stirring is one way to homogenize a fluid. Gravity
and centrifuging are good ways to de-homogenize
a fluid. Thermal convection is not necessarily
a good homogenizer. The mantle is stirred by
plate motions and subduction but these also
are not efficient homogenizers. Homogeneity of
basalts is more likely to be a result of the central
limit theorem; the averaging out of heterogeneity
by the sampling process. Large-scale averages of
the properties of the upper mantle tend toward
Gaussian
distributions
with
small
variance.
