Abstract
Volcanic arc basalts exhibit variable enrichments of B -
e.g., B/Zr ratios are generally 10-100 times higher than in
intraplate basalts. This characteristic can be attributed to
selective addition of B to arc magma sources via trasport in
slab-derived fluids. Work is in progress to develop a
systematic data base for arcs worldwide. To date, it can be
demonstrated that relative degree of B-enrichment (e.g., as
reflected in B/Zr interpolated at a reference B content) is
strongly correlated with indicators of slab thermal
structure (e.g., slab length; Figure 1).
B-enrichments in arc magmas are poorly correlated with
estimates of sediment subduction rate. It is inferred that
most B in arc magmas is derived from subducted altered
oceanic crustal rocks. This conclusion is supported by
measurements of B isotopic composition in arc lavas.
However, because such data are currently rather limited,
further work is underway to characterize the B systematics
in sediments outboard selected subduction zones (e.g.,
Aleutians, Tonga, South Sandwich) for which we have similar
data for the arc lavas.
Related studies have been completed or are in progress to
evaluate the concentration and/or isotopic composition of B
in related materials to evaluate the utility of these
parameters in diverse petrologic and geochemical
investigations (see references below).
Selected publications
Morris, J.D., Leeman, W.P., and Tera, F. (1990) Be
isotope and B/Be constraints on a subduction contribution to
volcanic arc magmas. Nature, 344, 31-36.Leeman, W.P., Carr,
M.J., and Morris, J.D. (1994) Boron geochemistry of the
Central American volcanic arc: constraints on the genesis of
subduction-related magmas. Geochim. Cosmochim. Acta, 58,
149-168.
Leeman, W.P., and Carr, M.J. (1995) Boron geochemistry of
the Central American volcanic arc: constraints on subduction
proceses. In: P. Mann (ed.), Geologic and Tectonic
Development of the Caribbean Plate Boundary in Southern
Central America: Boulder, CO, Geol. Soc. of America Spec.
Pap. 295, 57-73.
Ryan, J.G., Morris, J., Tera, F., Leeman, W.P., and
Tsvetkof, A. (1995) Cross-arc grochemical variations in the
Kurile arc as a function of slab depth. Science 270,
625-628.
Leeman, W.P. (1996) Boron and other fluid-mobile elements
in volcanic arc lavas: Implications for subduction
processes. In: Bebout, G.E., Scholl, D., Kirby, S., and
Platt, J.P. (eds.), Subduction Top to Bottom, AGU Monograph
96, 269-276.
Noll, P.D., Newsom, H., Leeman, W.P., and Ryan, J.G.
(1996) The role of hydrothermal fluids in the production of
subduction zone magmas: Evidence from siderophile and
chalcophile trace elements and boron. Geochim. Cosmochim.
Acta, 60, 587-611.
Ryan, J., Morris, J., Bebout, G., and Leeman, B. (1996)
Describing chemical fluxes in subduction zones: insights
from "depth-profiling" studies of arc and forearc rocks. In:
Bebout, G.E., Scholl, D., Kirby, S., and Platt, J.P. (eds.),
Subduction Top to Bottom, AGU Monograph 96, 263-268.
Leeman, W.P. (2000) Boron geochemical constraints on
subduction contributions: Relations to slab thermal
structure and other subduction parameters. Abstracts volume,
State of the Arc 2000 Conference, New Zealand.
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