Boron isotopic discrimination for subduction-related serpentinites

Abstract

The Guatemala Suture Zone (GSZ), Guatemala, is a region that contains two distinct suture-related serpentinite mélanges straddling the Motagua fault and an ophiolitic complex paired with the northern mélange. The serpentinite matrix of the mélanges formed by subduction-fluid hydration of peridotite from the deep mantle wedge. The occurrence of serpentinite from both exhumed subduction channel mélange and ophiolite is not uncommon in paleo–suture zones, but distinguishing them and their tectonic origin can be difficult. A new method of discrimination, based on boron isotopes in serpentine from both mélanges and ophiolite, as well as on mica and pyroxene from the metamorphic and vein-rock blocks embedded within the mélanges, has been developed. The metamorphic and vein samples have mainly negative δ11B, ranging from –15.3‰ to +4.3‰, in the same range as the serpentine from the mélanges (–14.4‰ to +9.7‰). In addition to being the most negative δ11B values ever measured in serpentinite, comparable values from vein minerals indicate that the same fluid serpentinized the overlying mantle. In contrast, serpentine samples from the ophiolite have positive δ11B, in the range 0‰ to +18.0‰, consistent with hydration by seawater-derived fluids. As the GSZ displays two mélanges whose serpentinite originated from two different deep subductions and mantle hydration, we hypothesize that the negative signature of exhumed mélange serpentine is the norm and that the B isotopic signature can be a useful tool to discriminate the tectonic origin of serpentinization in paleo–suture zones.

Origin of Boron and Brine Evolution in Saline Springs in the Nangqen Basin, Southern Tibetan Plateau

“…The large relative mass difference between the two isotopes ( 11 B and 10 B) and the different properties of the two dominant aqueous species of boron (i.e., B(OH) 3 and B(OH) 4 − ) lead to a significant isotopic fractionation of B [10,27]. Due to their soluble and incompatible character, the unique geochemical characteristics of B isotopes are widely used to assess numerous key geological processes, such as dehydration and metamorphism during slab subduction, ancient oceanic pH levels, the evolution of the formation of continental crust [28][29][30][31][32][33], magmatism and the formation of hydrothermal ore deposits [25,26,34,35], the causes of anthropogenic contamination, wastewater recharge [36][37][38], the origin of salt lakes and groundwater [27,39,40], sedimentary environment, and water-rock interactions [41,42].…”

Boron isotope variations in Tonga‐Kermadec‐New Zealand arc lavas: Implications for the origin of subduction components and mantle influences

“…can be high in some cases (10–40‰) [ Martin et al . [] due to interaction with seawater, they are typically lowered by dehydration processes, making it difficult to know a priori the isotopic composition of material being added to arc magma sources. Boron isotopic variations in arc lavas provide the most direct constraints on this question.…”

“…Thus, it is expected that B systematics of ascending magmas are relatively insensitive to modification by wall rock interactions. On the other hand, because B is enriched in altered oceanic crust [Thompson and Melson, 1970;Seyfried et al, 1983;Spivack and Edmond, 1987;Leeman, 1996;Staudigel et al, 1996], most marine sediments [Ishikawa and Nakamura, 1992;Leeman and Sisson, 1996], and serpentinized upper oceanic mantle [Bonatti et al, 1984;Benton et al, 2001;Hattori and Guillot, 2003;Tenthorey and Hermann, 2004;Scambelluri et al, 2004;Savov et al, 2005;Vils et al, 2008Vils et al, , 2009Tonarini et al, 2011;Pabst et al, 2012;Scambelluri and Tonarini, 2012;Deschamps et al, 2013;Harvey et al, 2014;Ryan and Chauvel, 2014;Martin et al, 2016], subduction of such materials can lead to anomalous enrichment of B in subarc mantle domains [Ryan and Langmuir, 1993;Leeman, 1996;Savov et al, 2005]. Relative to other incompatible elements, B tends to be selectively enriched in most arc lavas-with maximum enrichments (at any given subduction zone) observed in frontal rather than back-arc regions [Leeman et al, 1994[Leeman et al, , 2004Ishikawa and Nakamura, 1994;Ryan et al, 1995;Tonarini et al, 2001].…”

“…Although Geochemistry, Geophysics, Geosystems 10.1002/2016GC006523 initial d 11 B values in slab components (abyssal peridotites, oceanic crustal rocks, etc.) can be high in some cases (10-40&) [Martin et al [2016] due to interaction with seawater, they are typically lowered by dehydration processes, making it difficult to know a priori the isotopic composition of material being added to arc magma sources. Boron isotopic variations in arc lavas provide the most direct constraints on this question.…”

Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox

“…Heavy δ 7 Li for the PREMA end‐member (+5.1 ‰) is consistent with previous results (Elliott et al, ; Krienitz et al, ; Nishio et al, ), who showed that HIMU (PREMA)‐type lavas are characterized by δ 7 Li up to +7.4‰, which they interpreted as being due to recycling of dehydrated, less‐altered oceanic crust. The DampEM C‐O‐H‐Cl fluid end‐member has δ 11 B of −6.6 ‰, consistent with the boron isotopic composition of dehydrated sediments (−1 to −8 ‰) (Ishikawa & Nakamura, ) or melange serpentinites representing mantle wedge hydration at significant depths (30 to > 70 km) (Martin et al, ).…”