Geoscience Reference
In-Depth Information
0.150
Koolau
Hawaiian volcanoes
0.145
Mauna Loa
0.140
0.135
Mauna Kea
0.130
Upper mantle
0.125
0.120
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
18
O (per mil)
δ
Figure 11.12
Relationship between the
187
Os
18
OoflavasamplesfromthreeHawaiian
volcanoes. The variation in abundance in
18
O means that the source of these lavas contains a
constituent that was processed under low-temperature surface conditions. Isotopic fractionation
of Os requires the presence of a rock source that was once composed of ancient magmatic liquids
and cumulates. The combination of these two characteristics suggests that these basalts
remobilize ancient continental crust buried deep within the mantle. Other criteria preclude
188
Os ratio and
/
δ
from pelagic sediments (deep-sea clays), with high
176
Lu/
177
Hf ratios, through the basaltic
part of the weathered basalt crust, with high
18
O and
187
Re/
188
Os, to cumulate rocks
(gabbro) of the lower oceanic crust with their low
δ
is becoming ever clearer that the process forming most of the OIB source is recycling of
old oceanic crust, possibly after melting or devolatilization of fluids in the course of plate
subduction.
Overall, our current understanding of the isotope geochemistry of oceanic basalts can
be boiled down to three simple but strong statements: (i) it is created by two independent
groups of processes taking place, one at mid-ocean ridges and the other at subduction
zones; (ii) extraction of continental crust accounts for the overall depleted character of the
mantle; and (iii) the contrast between OIB and MORB cannot be created in a short time
interval, e.g. upon ascent and melting of a particular batch of mantle, but requires multiple
cycles of convection, upwelling, and subduction.
The interpretation of the isotope compositions of the inert gases in the various types of
basalt remains, however, not agreed upon. Typical ridge basalts have
3
He/
4
He ratios four
times lower than basalt of ocean islands such as Hawaii or Iceland. Let us refer back to
lution of the
3
He/
4
He ratios in the mantle source of ocean island and mid-ocean ridge
basalts by assuming constant parent/daughter ratios (
Fig. 11.13
). Helium is distinctly more
radiogenic in MORB than in OIB. The apparent
238
U/
3
He ratio integrated over the Earth's
history is therefore higher in the MORB source than in the OIB source. Given the much
more volatile character of He compared with U and Th, it is inferred that parts of the OIB
δ
