HR: 1340h
AN: V43D-1630    [Abstracts]
TI: Imprints of an "Arc" Signature onto Subduction Zone Eclogites from Central Guatemala
AU: * Simons, K K
EM: ksimons@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Sorensen, S S
EM: Sorensen@si.edu
AF: Smithsonian Institution, Department of Mineral Sciences, 10th and Constitution Avenue, Washington, DC 37012, United States
AU: Harlow, G E
EM: gharlow@amnh.org
AF: American Museum of Natural History, Department of Earth and Planetary Sciences, 79th and Central Park West, New York, NY 10024, United States
AU: Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: Queens College (CUNY), School of Earth and Environmental Sciences, 65-30 Kessina Blvd, Flushing, NY 11367, United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Hemming, N G
EM: hemming@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Hemming, N G
EM: hemming@ldeo.columbia.edu
AF: Queens College (CUNY), School of Earth and Environmental Sciences, 65-30 Kessina Blvd, Flushing, NY 11367, United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, Department of Earth and Planetary Sciences, 20 Oxford St, Cambridge, MA 02138, United States
AB: High-pressure, low-temperature (HP-LT) rocks associated with the Motagua fault zone in central Guatemala occur as tectonic blocks in serpentinite mélange. Dismembered jadeitite and albitite veins within the melange are crystallization products of subduction fluids at <400° C and 0.4-1.4 GPa. Lawsonite eclogites represent the deepest, coldest rocks, with peak metamorphic conditions of approx. 2.6 GPa and 480°C. They contain a subduction fluid overprint acquired during retrogression to blue- and green-schist-facies conditions, seen mostly as hydrous phases (e.g. phengite, glaucophane) in veins and overgrowths. The low temperatures recorded in these rocks indicate they have only seen an aqueous fluid, not a melt, and therefore, could provide a window into the acquisition of an arc signature at a cold margin. Trace-element patterns for both eclogite and jadeitite resemble arc lavas, with large enrichments in the most fluid mobile elements (e.g. Cs, Tl, Ba, Pb), moderate enrichments in U, Th, Be and LREE and generally little to no enrichment in HFSE and HREE, although enriched Nb in jadeitite indicates some HFSE mobility. Trace-element patterns also have similarities to average subducting sediment (GLOSS), with enrichments in Th, Be, Ba and Li that suggest a sediment contribution. Nd versus Sr isotopes lie to the right of the mantle array, indicating a hydrous fluid contribution from altered ocean crust or sediment. Overall, Guatemalan eclogites resemble counterparts from the Franciscan Complex (CA) and the Dominican Republic. Guatemalan and Franciscan eclogites are interpreted to have had a MORB protolith despite the arc trace element signature because of: 1) similarities in major elements to MORB; 2) HREE and HFSE abundances similar to MORB; and 3) high ¹⁴³Nd/¹⁴⁴Nd that overlap MORB values. The modifications that transformed these eclogites from a MORB trace element pattern to an arc one can be attributed to an aqueous subduction fluid at moderate depths (<75km). This transformation may be due to the increased solubilities of some minerals (e.g., jadeite, albite, clays, sulfates) at high pressure, high water/rock ratios from dehydration reactions, and an abundance of alkali-aluminosilicate components in subduction fluids. Together these may act to dissolve and transport trace elements (including elements considered insoluble like Nb) out of the slab and into the mantle wedge. The Guatemala data thus indicate that the arc geochemical fingerprint may be achieved at cold margins without the need for melting.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting