Boron isotopic characterization of ophiolitic serpentinites from Atlin terrane, Canadian Cordillera

Abstract

Atlin terrane ophiolites (Canadian Cordillera) exhibit non-Penrose pseudostratigraphy, with extensive serpentinites at the interface between upper crust and mantle. Petrography, Raman spectroscopy, electron probe microanalyzer (EPMA) and in-situ boron (δ¹¹B) isotopic analysis of serpentinites from Squanga Lake and Union Mt. ophiolites of Atlin terrane, identified four distinct phases of serpentine growth. Phase I (δ¹¹B = +1.6 ± 1 ‰ to +12.9 ± 1.3 ‰) and Ia (δ¹¹B = −5.7 ± 1.1 ‰ to +1.8 ± 1.1 ‰) serpentines have δ¹¹B signatures suggestive of seawater input, possibly along a detachment during oceanic core complex formation. These have up to 9 wt% FeO_tot concentrations, suggestive of primary mesh and bastite after ultramafic minerals. Phase I δ¹¹B values are typical of serpentinized ophiolite (−5.7 ‰ to +25 ‰). Phase II antigorite (δ¹¹B = −13.5 ± 0.8 ‰ to −6.3 ± 1.9 ‰; with propagated 1 S.E. error) and phase III fibrous chrysotile (δ¹¹B = −10.0 ± 2.5 ‰ to −2.7 ± 0.7 ‰) have strong negative δ¹¹B values distinct from phase I/Ia values. Phase II and III serpentines formed during listvenite alteration, coeval with Jurassic thrust imbrication of ophiolitic units and post-obduction plutonism and might be resulted from sediment-derived fluids. While δ¹¹B values of phase II antigorite align perfectly with those obtained from Dalayee Lake sediment (−14.6 ± 0.6 ‰ to −7.7 ± 2.1 ‰), the less negative δ¹¹B values of phase III chrysotile might indicate fluid evolution during exhumation. Phase IV antigorite flakes (δ¹¹B = −1.9 ± 1.4 ‰ to +1.5 ± 0.9 ‰) overprint phase II & III serpentines and might be related to post-obduction plutonism. These have lowest FeO_tot values (<2.3 wt%), compatible with multiple re-crystallisation pulses of Union Mt. antigorite.