Geoscience Reference
In-Depth Information
Chapter 14
Magmas: windows into the mantle
If our eye could penetrate the Earth
and see its interior from pole to
pole, from where we stand to the
antipodes, we would glimpse with
horror a mass terrifying riddled with
fissures and caverns
Tellus Theoria Sacra (1694), Thomas Burnet
if one mixes together all the materials known to
enter or leave the mantle, one can obtain cosmic
ratios of the refractory elements except for Mg/Si
and other elements likely to enter dense refrac-
tory residues of mantle differentiation. Likewise,
this mix is deficient in the siderophile elements,
which are plausibly in the core, and the volatile
elements, which were probably excluded from
the beginning.
Magmas are an important source of infor-
mation about conditions and composition of
the Earth's interior. The bulk composition, trace-
element chemistry, isotope geochemistry and
volatile content of magmas all contain infor-
mation about the source region and the pro-
cesses that have affected the magmas before
their eruption. Mantle fragments, or xenoliths,
found in these magmas tell us about the mate-
rial through which the magmas have passed on
the way to the surface. Representative composi-
tions of various magmas are given in Table 14.1.
Three principal magma series are recognized:
tholeiite, calc-alkaline and alkali. The various
rock types in each series may be related by vary-
ing degrees of partial melting or crystal sepa-
ration. The dominant rock type is tholeiite, a
fine-grained dark basalt containing little or no
olivine. Tholeiites are found in both oceanic
and intraplate settings. Those formed at mido-
cean ridges are low-potassium and LIL-depleted
and relatively high in Al, while those found on
oceanic islands and continents are generally LIL-
enriched. Ridge tholeiites differ from continental
and island tholeiites by their higher contents of
Al and Cr, low contents of large-ion lithophile
Various kinds of magmas, ranging from mido-
cean-ridge basalts (MORB) to kimberlite (KIMB)
emerge from the mantle. Material is also recycled
into the mantle; sediments, oceanic crust, delam-
inated continental crust, water and peridotite.
Parts of the mantle are inaccessible to direct
sampling and we can only infer their composi-
tions by subtracting off the sampled components
from what is thought to be the original composi-
tion. There is no assurance that we are currently
receiving samples from all parts of the Earth,
although this is the hope of many geochemists.
There is reason to believe that the chemical strat-
ification of the mantle is irreversible, and that
there exist hidden 'reservoirs' (a bad term for a
permanent repository). Nevertheless, the accre-
tional stratification of the Earth may have placed
most of the low-melting point lithophile ele-
ments within reach. In fact, one can construct a
plausible compositional model for the mantle by
isolating the dense refractory depleted products
of differentiation in the deep mantle. This would
make the Mg, Si and Fe contents of the mantle
uncertain but these can be constrained by geo-
physics, mineral physics and cosmology. In fact,
