We present an inventory of B, Cl and Li concentrations in (a) key
minerals from a set of ultramafic samples featuring the main
evolutionary stages encountered by the subducted oceanic mantle, and in
(b) fluid inclusions produced during high-pressure breakdown of
antigorite serpentinite. Samples correspond to (i) nonsubducted
serpentinites (Northern Apennine and Alpine ophiolites), (ii)
high-pressure olivine-bearing antigorite serpentinites (Western Alps
and Betic Cordillera), (iii) high-pressure olivine-orthopyroxene rocks
recording the subduction breakdown of antigorite serpentinites (Betic
Cordillera). Two main dehydration episodes are recorded by the sample
suite: partial serpentinite dewatering during formation of metamorphic
olivine. followed by full breakdown of antigorite serpentine to olivine
+ orthopyroxene + fluid. Ion probe and laser ablation ICP-MS (LA
ICP-MS) analyses of Cl, B and Li in the rock-forming minerals indicate
that the hydrous mantle is an important carrier of light elements. The
estimated bulk-rock B and Cl concentrations progressively decrease from
oceanic serpentinites (46.7 ppm B and 729 ppm Cl) to antigorite
serpentinites (20 ppm B and 221 ppm Cl) to olivine-orthopyroxene rocks
(9.4 ppm B and 45 ppm Cl). This suggests release of oceanic Cl and B in
subduction fluids, apparently without inputs from external sources.
Lithium is less abundant in oceanic serpentinites (1.3 ppm) and the
initial concentrations are still preserved in high-pressure antigorite
serpentinites. Higher Li contents in olivine, Ti-clinohumite of the
olivine-orthopyroxene rocks (4.9 ppm bulk rock Li), as well as in the
coexisting fluid inclusions, suggest that their budget may not be
uniquely related to recycling of oceanic Li, but may require input from
external sources.
Laser ablation ICP-MS analyses of fluid inclusions in the
olivine-orthopyroxene rocks enabled an estimate of the Li and B
concentrations in the antigorite breakdown fluid. The inclusion
compositions were quantified using a range of salinity ovalues (0.4-2
wt.% NaClequiv) as internal standards, yielding maximum average
D-fluid/rock(B) similar to 5 and D-fluid/rock(Li) similar to 3.5. We
also performed model calculations to estimate the B and Cl loss during
the two dehydration episodes of serpentinite subduction. The first
event is characterized by high fluid/rock partition coefficients for Cl
(similar to 100) and B (similar to 60) and by formation of a fluid with
salinity of 4-8 wt.% NaClequiv. The antigorite breakdown produces less
saline fluids (0.4-2 wt.% NaClequip) and is characterized by lower
partition coefficients for Cl (25-60) and B (12-30). Our calculations
indicate that the salinity of the subduction fluids decreases with
increasing depths. D-fluid/rock(B)/(fluid/rock) D-Cl < 1 (similar to
0.5) indicates that Cl preferentially partitions into the evolved
fluids relative to B and that the B/Cl of fluids progressively
increases with increasing depths and temperatures.
Despite light element release in fluids, appreciable B, Cl and Li are
still retained in chlorite, olivine and Ti-clinohumite beyond the
antigorite stability field. This permits a bulk storage of about 10 ppm
B, 45 ppm Cl and 5 ppm Li, i.e., concentrations much higher than in
mantle reservoirs. Chlorite is the Cl repository and its stability
controls the Cl and H2O budget beyond the antigorite stability; B and
Li are bound in olivine and clinohumite. The subducted oceanic mantle
thus retains light elements beyond the depths of arc magma sources,
potentially introducing anomalies in the upper mantle. (C) 2004
Elsevier B.V. All rights reserved.