Geology Reference
In-Depth Information
100
OIB, EM 1
OIB, EM 2
CHILE RIDGE,
Segm. 3
PRIM. MANTLE
OIB, HIMU
MORB -
CC MIXTURES
OIB, other
HIMU -
CC MIXTURES
MORB
10
CONT. CRUST
-5
0
5
10
Av e
(Nd)
Figure 10.7. Average Nb/U versus Nd isotopic composition for suites of mantle-
derived oceanic rocks.
ε
(Nd) is the deviation, in parts per 10 000, from the primitive
mantle
143
Nd/
144
Nd. From Hofmann [176]. Reprinted from
Nature
with permis-
sion. Copyright Macmillan Magazines Ltd.
10.4 Mantle heterogeneity
The mantle is chemically heterogeneous on all scales, from mineral grains through
hand specimens and melting zones to ocean basins. This ranks as a primary obser-
vation, and it is central to the interpretation that follows. Although this hetero-
geneity has become more widely recognised [60], its implications have not been
fully explored. Furthermore, the heterogeneity is just as evident and important for
MORBs as for OIBs, though OIBs span a larger range of values, typically about
double the range of MORBs, depending on the isotopic system.
Heterogeneity of MORBs at the largest scale is evident in Figure 10.2, where
differences can be seen particularly between the Indian Ocean and the Pacific
Ocean, with the Atlantic Ocean tending to be intermediate. These differences
are even more clearly illustrated in colour by Hofmann [60]. Heterogeneity at
intermediate scales, down to a few degrees of latitude, are evident in Figure 10.8.
It is evident that there are variations at the scale of a few degrees comparable to
the variation along the entire ridge, for example near 14
◦
N. Large variations have
also been found within single hand specimens [60], suggesting that such variations
are present in the source, though they will tend to be homogenised within magma
chambers during extraction.
The heterogeneity is evident not just in trace elements but also in major elements.
This is inferred from variations in rocks derived by melting the mantle [192-194]. It
is also inferred from occasional outcrops of mantle material [195]. Mantle rocks are
commonly observed to comprise two main rock types, the dominant peridotite and
secondary, but common, eclogite [196]. Some care is required in attributing these
observations, because some of the detail in those rocks may reflect the particularities
