Geochemistry and Microtextures of Vein Calcites Pervading the Izu‐Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

Abstract

International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu‐Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB‐like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion‐fed and crystallization pressure‐driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, and Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB‐hosted low‐T (<30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35–33 and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite‐hosted low‐T (<30 °C) vein calcites precipitated from seawater that was modified by fluid‐rock interactions. Mixing calculations yield a mixture of >95% seawater and <5% basaltic 87Sr/86Sr. In the rear arc, low‐T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20–30% basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates.

Post-magmatic fracturing, fluid flow, and vein mineralization in supra-subduction zones: a comparative study on vein calcites from the Troodos ophiolite and the Izu–Bonin forearc and rear arc

“…Blocky calcites in the Izu–Bonin forearc are in cases accompanied by argillaceous vein selvedges, phillipsite and/or harmotome (Fig. 3 ), and palagonite (Quandt et al 2020a ), which are interpreted as low-T (< 75 °C) precipitates and alteration products (Inoue 1995 ; Chipera and Apps 2001 ; Stroncik and Schmincke 2002 ). In all study areas, blocky calcites typically enclose host rock fragments and altered glass (palagonite) shards (Quandt et al 2018 , 2020a ).…”

“…They are characterized by a median line and growth competition between inwardly growing elongate-blocky crystals (Fig. 2 g, h, Table 3 ; Quandt et al 2018 , 2020a ). These microtextures are particularly well developed in the Troodos ophiolite and resulted from similar rates of mineral growth and fracture opening during which the nucleation of new crystals was suppressed (Fisher and Brantley 1992 ; Hilgers et al 2001 ; Bons et al 2012 ).…”

“…Syntaxial veins in the Izu–Bonin forearc are solely composed of calcite. In the Izu–Bonin rear arc, syntaxial veins are absent (Quandt et al 2020a ). CL microscopy reveals that elongate-blocky calcites from the Troodos ophiolite show a high, homogeneously distributed CL; whereas, elongate-blocky calcites from the Izu–Bonin forearc are characterized by non-luminescent cores and luminescent grain boundaries (Quandt et al 2018 , 2020a ).…”

“…For reasons of clarity, we strictly adopt the vein-type classification after Bons et al ( 2012 ) for this study considering only blocky, syntaxial, and antitaxial vein types. Hence, transitional veins marking the textural transition from syntaxial to antitaxial vein growth (Quandt et al 2020a ) are assigned to the antitaxial vein type. Similarly, vesicle calcites as well as late-stage calcites situated in cavities where vein sealing by early-stage quartz or analcime was incomplete (Quandt et al 2018 ) are classified as blocky veins.…”

“…Secondary mineralization and a sedimentary cover, however, decrease the crustal permeability and act as a thermal and hydraulic seal for the oceanic basement (e.g., Anderson and Hobart 1976 ; Spinelli et al 2004 ; Coogan and Gillis 2018 ). These post-magmatic processes are the subject of this paper in which we compare published data of mineralized veins from the Late Cretaceous Troodos ophiolite (Cyprus) and the Early Eocene Western Pacific Izu–Bonin forearc and rear arc (Alt et al 1998 ; Gillis et al 2015 ; Quandt et al 2018 , 2019 , 2020a ; Weinzierl et al 2018 ; Coogan et al 2019 ). This data set comprises (1) petrographic observations based on thin section and cathodoluminescence (CL) microscopy, (2) mineralogical determinations concluded from Raman spectroscopy, (3) mineral formation temperatures derived from δ 18 O, clumped isotope (Δ 47 ), and fluid inclusion thermometry, (4) parental fluid compositions inferred from rare earth element and yttrium (REE + Y) and isotope ( 87 Sr/ 86 Sr, δ 18 O, δ 13 C, Δ 47 ) geochemistry, (5) and relative ages of vein calcite precipitation deduced from 87 Sr/ 86 Sr stratigraphy.…”