Geoscience Reference
In-Depth Information
These were the midocean-ridge basalt (MORB)
reservoir and an ocean-island basalt (OIB) and
continental flood basalt (CFB) reservoir. The
former was postulated to fill up the whole upper
mantle, and the latter was assumed to be a
primordial (chondritic) lower mantle. It is now
generally believed that the noble gases -- actu-
ally only He and Ne -- are the only reliable geo-
chemical indicators of lower mantle involvement
in surface volcanism. All other indicators have
been traced to the recycling of crust and sedi-
ments, or fluids therefrom, or to delamination.
High
3
He/
4
He has traditionally been assumed to
result from a high abundance of
3
He, and this
has been used to argue that the lower mantle
is undegassed in primordial volatiles. Plumes are
assumed to carry the high
3
He/
4
He signal from
the deep mantle to the surface. However, the
assumptions underlying this model require that
the deep mantle has a high absolute abundance
of He. In this model, the observed low abundance
of He in OIB is a paradox, one of the
helium
paradoxes
.
The MORB reservoir was originally thought
to be homogenous because some isotopic ratios
show less scatter in MORB than in OIB. The com-
mon explanation was that MORB were derived
from a well-stirred, convecting part of the man-
tle while OIB were derived from a different,
deeper reservoir. Alternatively, the homogeneity
of MORB can be explained as a consequence of
the sampling process, and the central limit theo-
rem. MORB is more likely to result from a
process
rather than from a
reservoir
;itisanaverage,not
a component.
The standard, two-reservoir model is rein-
forced by data selection and data filtering prac-
tices. Samples along ridges that are judged to
be contaminated by plumes -- magmas that are
assumed to be from a different reservoir -- are
often removed from the dataset prior to statis-
tical analysis. The definition of
plume inf luence
is arbitrary. For example, isotopic ratios which
exceed an arbitrary cutoff may be eliminated
from the dataset. In this way, the MORB dataset
is forced to appear more homogenous than it
really is. Despite this, various ridges still have
different means and variances in their isotopic
ratios, and these depend on spreading rate and
ridge maturity. The variance for many ridge seg-
ments increases as spreading rate decreases and,
by analogy, the observed high variance of various
OIB data-sets is consistent with slow spreading,
small sampled volume, or low degrees of melt-
ing or degassing.
Geochemical variations in a well-sampled sys-
tem such as a midocean ridge or an oceanic
island can be characterized by an average value,
or mean, and a measure of dispersion such as
the standard deviation or variance. When deal-
ing with isotopic ratios the appropriate measures
of central tendency are the
median
and the
geo-
metric mean
, since these are invariant to inver-
sion of the ratio. Likewise, when dealing with
ratios, the absolute concentrations must be taken
into account, in addition to the ratios. That is,
the ratios must be weighted appropriately before
being analysed or averaged.
Recycling of crustal materials and mantle
heterogeneity
Chemical and isotopic diversity within the man-
tle, and fertility variations, may be related to
recycling of oceanic crust, lithosphere and sed-
iments into the mantle by plate tectonic pro-
cesses. Delamination of the eclogitic portion of
over-thickened continental crust is an important
process for fertilizing the mantle. The partial
extraction of melts at ridges and volcanic islands
contributes, over time, to the isotopic diversity of
the mantle.
On the basis of radiogenic Pb, Nd and Sr iso-
topes, mainly in ocean-island basalts, five isotopi-
cally extreme mantle components or reservoirs
have been defined: (1) MORB (depleted or normal
MORB); (2) HIMU ('high-
μ
μ
238
U/
204
Pb);
' where
is
(3) EM1; (4) EM2; and (5) LOMU ('low-
') with low
time-integrated U/Pb. 'Enriched' refers to time-
integrated Rb/Sr, Sm/Nd or (U,Th)/Pb ratios higher
than primitive mantle (bulk silicate Earth). In
addition, new isotopic systems have been devel-
oped using isotopes of hafnium, osmium, oxy-
gen and neon. The end-member and reservoir
description of mantle variability, which includes
MORB, FOZO, HIMU, EM1 and EM2 was based
on isotopes of Sr, Nd and Pb. Osmium, oxygen
and helium isotope data have revealed the limi-
tations of this five-component classification. For
μ
