Titanite Mineralization of Microbial Bioalteration Textures in Jurassic Volcanic Glass, Coast Range Ophiolite, California

Matthew R M Izawa,John W Shervais,Debarati Das,Karlis Muehlenbachs,Callum J Hetherington,Roberta L Flemming,Cynthia Schultz,Neil R Banerjee,Barry B Hanan

doi:10.3389/feart.2019.00315

Abstract

Volcanic glasses are rarely preserved in the rock record, and the quality of preservation generally declines with increasing age. Records preserved in ancient basaltic glasses therefore provide important links between processes operating in the distant past, and those that are active on the Earth today. Microbial colonization has been linked to the formation of characteristic structures in basaltic glass, including tubules and granule-filled tubules, which are thought to be produced by microbially mediated glass dissolution. Structures of similar occurrence and morphology but filled almost entirely with fine-grained titanite have been documented in some ancient metabasalts. It has been suggested that the ancient titanite-mineralized structures are mineralized equivalents of hollow tubules in modern glassy basaltic rocks, but a direct link has not been firmly established. We report the discovery of tubular bioalteration structures in fresh and minimally altered basaltic glasses of middle Jurassic (164 Ma) age from the Stonyford Volcanic Complex (SFVC), Coast Range Ophiolite, California. Tubular structures hosted in unaltered basaltic glass are typically hollow, whilst those in zones of zeolitic alteration are mineralized by titanite. Tubules are continuous across zeolite-glass interfaces, which mark an abrupt change from titanite-filled to hollow tubules, demonstrating that titanite growth occurs preferentially within pre-existing tubular structures. Titanite mineralization in the SFVC represent a link between tubular structures in modern basaltic glass and titanite-mineralized features of similar morphology and spatial distribution in ancient metabasalts. Our observations support a link between textures in modern glassy basaltic rocks and some of the oldest-known putative ichnofossils.

Highlights

The preservation of glass in ancient volcanic rocks is relatively rare, and occurs only where processes such as hydrothermal alteration, thermal metamorphism, and deformation were minimal or absent
(1) Tubules meeting the textural and geological context criteria for microbial ichnofossils occur throughout Stonyford Volcanic Complex (SFVC) basaltic glasses and are most consistent with the Tubulohyalichnus simplus ichnofossil taxon (McLoughlin et al, 2009)
(2) Titanite mineralization in candidate microbial ichnofossils is localized within zeolite-rich alteration and commonly located along fractures and cracks within basaltic glass

Summary

Introduction

The preservation of glass in ancient volcanic rocks is relatively rare, and occurs only where processes such as hydrothermal alteration, thermal metamorphism, and deformation were minimal or absent. Volcanic glass may be preserved in oceanic settings prior to subduction, and may be sampled by drilling (Furnes et al, 1996; Fisk and McLoughlin, 2013) Despite their rarity, ancient volcanic glasses provide important links between the past and processes that are active on the Earth today. Ancient volcanic glasses provide important links between the past and processes that are active on the Earth today They preserve compositional and isotopic data documenting evidence of their magmatic and tectonic origins that may be lost during thermal or hydrothermal metamorphism. The character of this activity is essentially identical to that of microbial alteration that is active today (Furnes et al, 1996; Fisk et al, 1998, 2003) and provides a link between currently observable/active processes and the past

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