The solid Earth - Geochemistry: An Introduction - page 228

0.5135

Depleted
quadrant

Mid-ocean ridge
basalts (MORB)

Andesites

0.5130

Seawater

BSE

0.5125

B

Continental crust

Ocean island
basalts (OIB)

0.5120

Enriched
quadrant

A

0.5115

0.702 0.704 0.706 0.708 0.710 0.712

87 Sr / 86 Sr

Relationship between 87 Sr

144 Nd in the sources of oceanic basalts and andesites.
The more incompatible character of Rb (compared with Sr) and of Nd (compared with Sm) during
melting of mantle rocks explains the negative correlation of these ratios. The MORB originate
from a source that is more depleted in incompatible elements by successive melting of the
mantle than the OIB source. This depletion is ancient (several billion years), as indicated by the
composition of slowly accumulating radiogenic elements. Andesites derive from either
hybridization of the mantle by the continental crust (A), or melting of the mantle contaminated
by dehydration fluids rich in seawater (B).

86 Sr and 143 Nd

Figure 11.9 / /

respect to their mantle source is visibly much greater for the highly incompatible elements
(La, Ce) than for the less incompatible elements (Yb, Lu). The inescapable conclusion is
that MORB depletion in highly incompatible elements, such as La, Ce, but also Th and Ba,
compared with more compatible elements such as Sr, Yb, and Ti (see Fig. 2.7) , implies that
their source is depleted in these highly incompatible elements to an even greater extent.
The range of radiogenic isotope compositions show that depletion of the mantle under
the mid-ocean ridges in incompatible elements is a very ancient phenomenon. The highly
incompatible radioactive isotope 87 Rb is greatly diminished compared with the more com-
patible radiogenic isotope 87 Sr, and in the course of geological time a relatively low
87 Sr/ 86 Sr ratio of the asthenospheric mantle results (0.7025). By contrast, the parent iso-
tope 147 Sm is more compatible than the daughter isotope 143 Nd. It is therefore enriched
in residual mantle by comparison with the melt. In the course of time, the asthenospheric
mantle acquires a relatively high 143 Nd/ 144 Nd ratio (see (4.33 ) ) of the order of 0.5131. The
187 Os/ 188 Os ratio evolves like 143 Nd/ 144 Nd, but in a more extreme way.
The values of the isotopic ratios of present-day magmatic rocks are usually plotted in
a series of binary diagrams, the most popular being undoubtedly the plot of 143 Nd/ 144 Nd
versus 87 Sr/ 86 Sr ( Fig. 11.9 ) . The geochemical jargon “depleted” simply refers to a deficit