Volatiles under High Pressure - Physics and Chemistry of the Deep Earth - page 5

Libowitzky, 2006). These observations show that
the upper mantle is by no means completely dry
(Bell & Rossman, 1992). However, estimating
mantle abundances of water and other volatiles
from such data is difficult, because samples often
have lost water on their way to the surface; in
some cases, this water loss is evident in diffusion
profiles that may be used to constrain ascent
rates (Demouchy et al ., 2006; Peslier & Luhr,
2006; Peslier et al ., 2008). Moreover, many of
these xenoliths come from alkali basalts or
kimberlites. The source region of these magmas
may

quite well-constrained Ce and Nb contents of the
mantle to estimate the water and carbon dioxide
content in the MORB and OIB sources. Using this
method, Saal et al . (2002) estimated the volatile
contents of the MORB-source upper mantle to
be 142 ± 85 ppm H 2 O (by weight), 72 ± 19 ppm

CO 2 , 146 50

ppm F. In general, estimates of the water content
in the depleted MORB source using similar meth-
ods yield values of 100-250 ppm by weight for
H 2 O. In particular, the work by Michael (1995)
suggests some regional variability of the MORB
source water content. Much higher volatile con-
centrations with up to about 1000 ppm of H 2 O
have been obtained for the OIB source region (e.g.
Dixon et al ., 1997; Hauri, 2002). The CO 2 content
in the OIB source may range from 120 to 1830 ppm
CO 2 (Hirschmann & Dasgupta, 2009). If one as-
sumes that the MORB source is representative
for most of the mantle and the OIB source con-
tributes a maximum of 40% to the total mantle,
these numbers would translate to a total mantle
carbon budget in the order of (1-12) . 10 23 gofC
(Dasgupta & Hirschmann, 2010). A similar cal-
culation assuming 142 ppm H 2 OintheMORB
source and 1000 ppm H 2 OintheOIBsource
would give a bulk water reservoir in the man-
tle of 2 . 10 24 g, i.e. about 1.4 ocean masses. The
uncertainty in this estimate is, however, quite
significant and the number given is likely to be
only an upper limit of the actual water content.
Water has a strong effect on the physical prop-
erties, particularly density, seismic velocities and
electrical conductivity of mantle minerals (Jacob-
sen, 2006; Karato, 2006). In addition, water may
change the depth and the width of seismic dis-
continuities (e.g. Frost & Dolejs, 2007), because it
stabilizes phases that can incorporate significant
amounts of water as OH point defects in their
structure. These effects may be used for a re-
mote sensing of the water content in parts of the
mantle that are not accessible to direct sampling.
The dissolution of water as OH point defects
in minerals generally reduces their density and
both P and S wave seismic velocities (Jacobsen,
2006). This is mostly due to the formation of
cation vacancies that usually - but not always

± 35 ppm S, 1 ± 0.5 ppm Cl and 250 ±

be more enriched in volatiles than the

normal mantle.
Mid-ocean ridge basalts (MORB) tap a volatile-
depleted reservoir that is believed to represent
most of the upper mantle. Ocean island basalts
(OIB) appear to come from a less depleted, likely
deeper source. Probably the best constraints on
volatile abundances in the mantle come from
MORB and OIB samples that have been quenched
to a glass by contact with sea water at the bottom
of the ocean (e.g. Saal et al ., 2002; Dixon et al .,
2002); the fast quenching rate and the confining
pressure probably suppressed volatile loss. In prin-
ciple, one can calculate from observed volatile
concentrations in quenched glasses the volatile
content in the source, if the degree of melting
and the mineral/melt partition coefficients of the
volatiles are known. Such calculations, however,
are subject to considerable uncertainties. A much
more reliable and widely used method is based
on the ratio of volatiles to certain incompatible
trace elements, such as H 2 O / Ce and CO 2 / Nb
(Saal et al ., 2002). These ratios are nearly con-
stant in MORB glasses over a large range of
H 2 OandCO 2 contents that represent different
degrees of melting and crystal fractionation. This
means that the bulk mineral/melt partition co-
efficient of H 2 O is similar to that of Ce and the
bulk mineral/melt partition coefficient of CO 2
is similar to Nb. For equal bulk partition coeffi-
cients, the H 2 O / Ce ratio and the CO 2 / Nb ratio
must be the same in the mantle source and in
the basalt, independent of the degree of melting.
Therefore, measured H 2 O / Ce ratios and CO 2 / Nb
ratios of the basalts can be used together with the