Mantle chemical evolution - Mantle Convection for Geologists - page 179

Figure 10.17. La/Sm, normalised to primitive mantle, versus La for MORB from
three ocean basins. Lanthanum concentrations vary by about two orders of mag-
nitude; La/Sm varies by more than one order of magnitude. Data were extracted
from PetDB. From Hofmann [60]. Copyright Elsevier Science. Reprinted with
permission.
common composition, or in other words the peak or mode of the distribution. For
example, in Figure 10.16, NMORB strontium would be represented by the peak
around 0.7025, presumed symmetric. Higher Sr ratios that do not fit this peak are
then taken to represent other species, such as enriched MORB (EMORB).
However, there is no objective way to separate the NMORB and EMORB
populations, as has been emphasised also by Hofmann [60]. Figure 10.17 shows
La/Sm ratios for over 1000 MORB samples. This ratio is a measure of depletion or
enrichment, since it effectively measures the slope of the trace element distributions
in Figure 10.1. According to Hofmann, NMORB is often defined by La/Sm < 1,
but in this plot such a division is completely arbitrary.
Furthermore, because high La/Sm correlates with a high La concentration, the
exclusion of the high-La/Sm data will exclude much or most of the La content of
the MORB source. This is true in other systems too. For example, the samples with
high 87 Sr/ 86 Sr also tend to have higher Sr concentrations, so if the high- 87 Sr/ 86 Sr
samples are excluded, much of the Sr inventory may be missed.
10.6.4 Mean compositions, not end-members
If the heterogeneity evident in the data of Figures 10.2, 10.3 and 10.16 is a direct
reflection of the heterogeneity of the sources, as depicted in Figure 10.15(b), then