Shallow forearc mantle dynamics and geochemistry; new insights from IODP Expedition 366

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doi: 10.1016/j.lithos.2018.10.038
Author(s): Debret, B.; Albers, E.; Walter, B.; Price, R.; Barnes, J. D.; Beunon, H.; Facq, S.; Gillikin, D. P.; Mattielli, N.; Williams, H.
Author Affiliation(s): Primary:
Université Libre de Bruxelles, Département Geosciences, Environnement et Société, Brussels, Belgium
University of Bremen, Germany
University of Lorraine, France
University of Texas at Austin, United States
University of Cambridge, United Kingdom
Stony Brook University, United States
University of Texas, United States
Union College, United States
Volume Title: Lithos (Oslo)
Source: Lithos (Oslo), Vol.326-327, p.230-245. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0024-4937 CODEN: LITHAN
Note: In English. 104 refs.; illus., incl. 1 table, sketch map
Summary: The Mariana forearc is a unique setting on Earth where serpentinite mud volcanoes exhume clasts originating from depths of 15 km and more from the forearc mantle. These peridotite clasts are variably serpentinized by interaction with slab derived fluid, and provide a record of forearc mantle dynamics and changes in geochemistry with depth. During International Oceanic Discovery Program (IODP) Expedition 366, we recovered serpentinized ultramafic clasts contained within serpentinite muds of three different mud volcanoes located at increasing distance from the Mariana trench and at increasing depth to the slab/mantle interface: Yinazao (distance to the trench: 55 km/depth to the slab/mantle interface: 13 km), Fantangisna (62 km/14 km) and Asut Tesoru (72 km/18 km). Four different types of ultramafic clasts were recovered: blue serpentinites, lizardite-serpentinites, antigorite/lizardite- and antigorite-serpentinites. Lizardite-serpentinites are primarily composed of orange serpentine, forming mesh and bastite textures. Raman and microprobe analyses revealed that these textures contain a mixture of Fe-rich brucite (XMg ∼0.84) and lizardite/chrysotile. Antigorite/lizardite- and antigorite-serpentinites record the progressive recrystallization of mesh and bastite textures to antigorite, magnetite and pure Fe-poor brucite (XMg ∼0.92). Oxygen isotope compositions of clasts and pore fluids showed that the transition from lizardite to antigorite is due to the increase in temperature from 200°C to about 400°C within the forearc area above the slab/mantle interface. Lizardite-, antigorite/lizardite- and antigorite-serpentinites displayed U-shaped chondrite normalized Rare Earth Element (REE) patterns and are characterized by high fluid mobile element concentrations (Cs, Li, Sr, As, Sb, B, Li) relative to abyssal peridotites and/or primitive mantle. The recrystallization of lizardite to antigorite is accompanied by a decrease in Cs, Li and Sr, and an increase in As and Sb concentrations in the bulk clasts, whereas B concentrations are relatively constant. Some clasts are overprinted by blue serpentine, often in association with sulfides. Most of these blue serpentinites were recovered at Yinazao and the uppermost units of Fantangisna and Asut Tesoru suggesting alteration in the shallower portions of the forearc, possibly during exhumation of the clasts. This episode of alteration resulted in a flattening of REE spectra and an increase of Zn concentrations in serpentinites. Otherwise, no systematic changes of ultramafic clasts chemistry or mineralogy were observed with increasing depth to the slab. The samples document previously undescribed prograde metamorphic events in the shallow portions of the Mariana subduction zone, consistent with a continuous burial of the serpentinized forearc mantle during subduction. Similar processes, induced by the interaction with fluids released from the downgoing slab, likely occur in subduction zones worldwide. At greater depth, breakdown of brucite and antigorite will result in the massive transfer of fluids and fluid mobile elements, such as As, Sb and B, to the source of arc magmas.
Year of Publication: 2019
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 18 Geophysics, Solid-Earth; Antigorite; Basins; Brucite; Chrysotile; Electron probe data; Expedition 366; Fluid phase; Fore-arc basins; Geodynamics; IODP Site U1491; IODP Site U1492; IODP Site U1493; IODP Site U1494; IODP Site U1495; IODP Site U1496; IODP Site U1497; Igneous rocks; International Ocean Discovery Program; Isotope ratios; Isotopes; Izu-Bonin-Mariana Arc; Leg 195; Lizardite; Mantle; Mariana Trench; Metaigneous rocks; Metals; Metamorphic rocks; Metasomatic rocks; Migration of elements; North Pacific; Northwest Pacific; O-18/O-16; ODP Site 1200; Ocean Drilling Program; Oxides; Oxygen; Pacific Ocean; Peridotites; Phase transitions; Plate convergence; Plutonic rocks; Raman spectra; Rare earths; Serpentine group; Serpentinite; Sheet silicates; Silicates; Spectra; Stable isotopes; Subduction zones; Trace elements; Ultramafics; West Pacific
Coordinates: N134000 N200000 E1490000 E1450000
Record ID: 2019046601
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands