Sub-arc xenolith Fe-Li-Pb isotopes and textures tell tales of their journey through the mantle wedge and crust

Year:

2018

Type of Publication:

Article

Authors:

Schaefer, Bruce; Adam, John; Gerdes, Mitchell; Piazolo, Sandra; Turner, Simon; Williams, Helen; Blichert-Toft, Janne; Liu, Xiao-Ming; Maury, Rene

Journal:

Geology

Volume:

46

Number:

11

Pages:

947-950

Month:

September

ISSN:

0091-7613

BibTex:

@article{Schaefer2018, author = "Bruce Schaefer and John Adam and Mitchell Gerdes and Sandra Piazolo and Simon Turner and Helen Williams and Janne Blichert-Toft and Xiao-Ming Liu and Rene Maury", abstract = "Island arcs provide insights into the origin and recycling of continental crust, but questions remain concerning source metasomatism, the depth of differentiation, the potential role of amphibole fractionation, and the time scales involved. Basaltic andesites on Batan Island (Philippines) contain {\&}gt;=150 Ma peridotite fragments encased in hornblendite and gabbroic rinds produced via melt-rock reaction. The peridotites have some of the lightest d7Li and d57Fe values yet measured in mantle rocks. These values are too fractionated to have been created solely by equilibrium partial melting and thus require a combination of melt depletion and slab fluid addition and may be derived from diffusion-modified melt channel wall rocks. Stable isotope signals are easily modified by diffusive equilibration between peridotite and host magma, so the preservation of light d7Li and d57Fe here suggests magma ascent rates of {\textasciitilde}10 km yr-1. We show that melt-wall rock reactions at {\textasciitilde}25-30 km depth led to the crystallization of amphibole ({\textpm} plagioclase) followed by gabbroic fractionation at {\textasciitilde}7 km depth. The former provides a location and mechanism for the {\&}quot;cryptic{\&}quot; amphibole fractionation observed in these and perhaps many other arc lavas and may obviate the requirement for delamination of cumulates.", day = "27", doi = "10.1130/G45359.1", issn = "0091-7613", journal = "Geology", month = "Sep", number = "11", pages = "947-950", title = "{S}ub-arc xenolith {F}e-{L}i-{P}b isotopes and textures tell tales of their journey through the mantle wedge and crust", url = "https://dx.doi.org/10.1130/G45359.1", volume = "46", year = "2018", }

Abstract:

Island arcs provide insights into the origin and recycling of continental crust, but questions remain concerning source metasomatism, the depth of differentiation, the potential role of amphibole fractionation, and the time scales involved. Basaltic andesites on Batan Island (Philippines) contain >=150 Ma peridotite fragments encased in hornblendite and gabbroic rinds produced via melt-rock reaction. The peridotites have some of the lightest d7Li and d57Fe values yet measured in mantle rocks. These values are too fractionated to have been created solely by equilibrium partial melting and thus require a combination of melt depletion and slab fluid addition and may be derived from diffusion-modified melt channel wall rocks. Stable isotope signals are easily modified by diffusive equilibration between peridotite and host magma, so the preservation of light d7Li and d57Fe here suggests magma ascent rates of {\textasciitilde}10 km yr-1. We show that melt-wall rock reactions at {\textasciitilde}25-30 km depth led to the crystallization of amphibole ({\textpm} plagioclase) followed by gabbroic fractionation at {\textasciitilde}7 km depth. The former provides a location and mechanism for the "cryptic" amphibole fractionation observed in these and perhaps many other arc lavas and may obviate the requirement for delamination of cumulates.