Boron isotopes in boninites from the Izu-Bonin-Mariana arc system; insights into subduction initiation

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doi: 10.1180/minmag.2013.077.5.19
Author(s): Savov, Ivan P.; Agostini, Samuele
Author Affiliation(s): Primary:
University of Leeds, School of Earth and Environment, Leeds, United Kingdom
Other:
Istituto di Geoscienze e Georisorse, Italy
Volume Title: Goldschmidt abstracts 2013
Source: Mineralogical Magazine, 77(5), p.2144; Goldschmidt 2013, Florence, Italy, Aug. 25-30, 2013. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Note: In English. 4 refs.
Summary: The origin of boninites has long been debated and several mechanisms for their formation have been proposed. These rocks have both high MgO contents and somewhat LREE+LILE enrichments, thus their genesis should involve large percentage melting of highly melt-depleted but also highly fluid metasomatized mantle sources. One mechanism to explain boninite generation in the Izu-Bonin-Marianas (IBM) arc-basin system invokes sinking of cold Pacific oceanic slabs, trench retreat and intense backarc spreading- all resulting in anomalous mantle melting conditions and voluminous boninite eruptions close in time and space to the oldest mafic volcanic arc rocks in IBM at ∼ 51 Ma. Since the mantle is extremely depleted in B and has very negative δ11B ratios and the slab fluids have very positive δ11B signatures, B and B isotope ratios are able to trace the slab-fluid additions and the resulting boninite generation. We have selected a suite of representative boninites from the the Izu-Bonin (Bonin Ridge) and the Marianas (Guam Island) segments of the IBM. The boninites all have remarkably similar 87Sr/86Sr (0.7042- 0.7049) and 143Nd/144Nd (0.51283-0.51297) isotope signatures. Interestingly the boninites δ11B ranges from +0.9 to +5.1 permil (data always better than ±1 per mil), supporting models for early slab fluid liberation via dehydration of old foundering Pacific slabs. Such fluid additions are necessary for lowering the melting temperature of the depleted mantle wedge and for boninite generation. Based on the isotope (B, Nd, Sr) and trace element similarities we envision the boninite sources to be chemically similar to blueschist-containing serpentinite muds and serpentinized peridotites erupted today via mud volcanoes in the IBM forearc and successfully drilled during ODP Legs 195 and 125. Links between high Mg arc volcanic rocks and serpentinite-derived fluids are becoming increasingly common and we will discuss possible links between metamorphic petrology of the slab inventory, island arc magma generation and the usefulness of B isotopes as tracer.
Year of Publication: 2013
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Alkaline earth metals; Andesites; B-11/B-10; Bonin Ridge; Boninite; Boron; Genesis; Guam; Igneous rocks; Isotope ratios; Isotopes; Izu-Bonin Arc; Leg 125; Leg 195; Mariana Islands; Melting; Metals; Metamorphism; Micronesia; Nd-144/Nd-143; Neodymium; North Pacific; Northwest Pacific; Ocean Drilling Program; Oceania; Pacific Ocean; Plate tectonics; Rare earths; Slabs; Sr-87/Sr-86; Stable isotopes; Strontium; Subduction; Temperature; Volcanic rocks; West Pacific
Coordinates: N192955 N304929 E1464146 E1405509
N130000 N250000 E1470000 E1220000
Record ID: 2014025429
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland