INTRODUCTION
The State of
Crust-Mantle Recycling Science
Subduction zones are the modern sites of continental crust
formation and destruction. Continental growth occurs today by
accretion of island arcs and magmatic additions to the crust at
arcs. Crustal destruction occurs by subduction of crustal
material (seawater components, marine sediments, and basaltic
crust) at oceanic trenches. Thus, the geochemical and physical
evolution of the Earth's crust and mantle depends in large part
on the fate of subducted material at convergent margins
(Armstrong, 1968; Karig and Kay, 1981). The crustal material on
the downgoing plate is recycled to various levels in the
subduction zone. Some of it returns to the shallow crust during
forearc accretion and dewatering, some returns to the arc crust
via volcanism, some is mixed back into the deep mantle, and some
may even re-emerge in mantle plumes. Despite strong evidence for
these different types of crustal recycling (from seismic imaging,
drilling, and isotopic tracers), and despite the important
ramifications for mantle evolution, continent formation, and
geochemical cycles, few studies have focused on quantifying
crustal fluxes through subduction zones.
Von Huene and Scholl (1991) calculated a large global flux of subducted sediment‹as high as modern crustal growth rates. Their calculations, however, represent an upper limit on sediment fluxes into the mantle because some material is cycled back to the upper plate. It is a common misconception that the sediment contribution to the volcanic arc is trivial (~1%), based on isotopic mixing arguments, which constrain only the proportion of sediment to the mantle and not the proportion of the total subducting budget that contributes to the arc. To calculate the latter, estimates of input fluxes (sediment) and output fluxes (volcanic) are required. Earlier flux balances by Karig and Kay (1981) for the Marianas suggested that 10% of the sedimentary section contributes elements to the arc, whereas more recent calculations (Plank and Langmuir, 1993; Zheng et al., 1994) give values of 30%-50% globally.
These estimates, however, have large uncertainties because none of them take into account all of the crustal outputs. Plank and Langmuir (1993) do not consider underplating or erosion at the forearc; von Huene and Scholl (1991) do not consider recycling to the arc; and neither study considers the mobile components dissolved in fluids that are lost to the forearc. It is entirely possible that the 50%-70% recycling efficiency to the deep mantle suggested by Plank and Langmuir (1993) could be reduced to 0% for many important element tracers, given the other shallower outputs that have yet to be taken into account. Clearly, the difference between 70% and 0% recycling would lead to vastly different outcomes for mantle evolution and structure.
The Role
of Drilling
The recycling equation involves many variables-aging of the
oceanic lithosphere, flow of material through accretionary
prisms, and fluid circulation at active margins-that are linked
across a convergent margin and can be explored in combination
through a drilling transect across a margin (Fig. 1; Scholl et al.,
1996). The incoming section of sediment and altered oceanic crust
can be drilled near trenches. The extent of sediment accretion,
underplating, erosion, and subduction can be determined by
combining forearc drilling with seismic reflection images and
material balance considerations. The fluids lost from the
downgoing plate can be sampled by drilling into fault zones and
serpentine seamounts. Output to the arc can be determined from
the chemical composition of the volcanics and from arc growth
rates. The flux of crustal material that is eventually recycled
to the mantle is then the input minus the output. Because the
bulk sediment is not conserved through the entire subduction
process, chemical tracers must be used to track the sediment and
deduce the recycling processes. Thus, the problem is impossible
to solve by remote means and is completely dependent on drilling
to recover material for chemical analysis.
Determination of crustal fluxes into the mantle is not yet possible largely because the various parts of the problem are being investigated at different margins. Although this is a good way to understand individual processes, it is not a good way to determine the behavior of the entire system. The approach that we emphasize here is to try to solve the recycling equation at a few margins where significant progress can be made by a focused drilling effort.
Recent studies have focused on tracers of the subduction process (fluids and chemical components from the subducted sediment and basalt). Subducted material has been successfully identified in forearc fluids and serpentinite mud volcanoes (Mottl, 1992; Fryer, 1992), in arc volcanoes (Tera et al., 1986; Plank and Langmuir, 1993), and in backarc volcanism (Stolper and Newman, 1994). Laboratory experiments have helped in understanding the dehydration and/or melting processes in the downgoing slab that send material back to the overlying crust. What is now needed is to move beyond the tracer approach to begin to try to mass balance the recycling process. How much subducted material returns to the arc lithosphere vs. how much is mixed back into the mantle? How is the subducted material chemically modified during this process? These are the questions behind Crustal Mass Balances, a theme that appears in the Ocean Drilling Program (ODP) Long Range Plan, the MARGINs Initiative Science Plan, and a JOI/USSAC Workshop on Crustal Recycling (Scholl et al., 1996).