HR: 08:45h
AN: V51G-04 [Abstracts]
TI: How does recycling of sediment components in arc magmatism really work?
AU: * Kelemen, P
EM: peterk@ldeo.columbia.edu
AF: Columbia, LDEO, Palisades, NY 10964, United States
AU: Hacker, B
EM: hacker@geol.ucsb.edu
AF: UCSB, ICS, Santa Barbara, CA 93106, United States
AU: Austin, N
EM: naustin@mit.edu
AF: MIT, EAPS, Cambridge, MA 02139, United States
AB:
Past work indicates substantial recycling of a sediment component rich in LILE, Th, Sr, Pb and LREE in arcs. For
example, in the relatively well-constrained case of Central America, Plank et al (Geology 02) estimate that 80% of
subducted, sedimentary Th is recycled in arc magmas. To understand how such a component is transferred from
subducted sediment to arc lava, we examined trace-element variation in (a) mid-crustal (0.4 GPa) contact
metamorphic rocks (Austin & Kelemen, Fall 06 AGU) and (b) ultrahigh-pressure (UHP, > 3 GPa)
metasediments. Most UHP samples were metamorphosed along subduction-zone geotherms (Hacker, Int Geol
Rev 06), but some record substantially higher T (e.g., Erzgebirge & Kokchetav, Massone EPSL 03).
Unmelted,
mid-crustal metapelites are indistinguishable from pelitic sediments for the entire suite of elements analyzed by
ICP-MS at WSU. Melt extraction from the mid-crustal metapelites led to systematic depletion of incompatible
elements in high-grade hornfels. Depletion increases with decreasing distance to the contact with a mafic pluton,
most clearly at peak T > 750°C.
In contrast, although many UHP metapelites record PT above the
aqueous fluid-saturated solidus, and have fluid inclusions and/or hydrous phases, compared to pelites they
show no detectable depletion of "fluid-mobile" elements such as LILE
(Cs, Rb, Ba, U, K), Sr and Pb, no depletion of "fluid-immobile,
incompatible" elements such as Th and LREE, and no systematic change in key
soluble/insoluble ratios such as Ba/Th or K/Zr up to ~1000 C. Mobility of incompatible elements is evident
for T > 1000 C, well above PT for subduction-zone geotherms. Presumably, trace phases rich in LILE, Th and
LREE persist to ~1050 C in metapelites at UHP conditions.
How can our observations be reconciled with
the recycled sediment component in arc lavas? Our preferred hypothesis is that low-density metasediments rise
into the mantle wedge when heating yields viscosities low enough for density-driven instabilities (Ringwood
JGSL 74; Marsh AJS 76; Gerya & Yuen EPSL 03; Kelemen et al, Treatise on Geochem 03). In the wedge,
metasedimentary diapirs heat as they rise, and undergo large degrees of super-adiabatic partial melting which
exhaust trace phases, releasing the sediment component observed in arcs.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3654 Ultra-high pressure metamorphism
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting