Geochemistry of Vein Calcites Hosted in the Troodos Pillow Lavas and Their Implications for the Timing and Physicochemical Environment of Fracturing, Fluid Circulation, and Vein Mineral Growth

Abstract

Calcite veins hosted in pillow lavas of the Late Cretaceous Troodos suprasubduction zone ophiolite provide insights into the timing and physicochemical environment of postmagmatic fracturing and fluid circulation through oceanic crust. This study presents rare earth element and yttrium (REE+Y) concentrations, δ 13 C, δ 18 O, 87 Sr/ 86 Sr, and clumped isotopic (Δ 47 ) compositions of vein calcites in order to investigate their fluid sources, formation temperatures, and precipitation ages. These geochemical data are combined with microtextural analyses. Intersections of 87 Sr/ 86 Sr ratios of vein calcites with the Sr isotope seawater curve suggest two distinct calcite veining phases. Major calcite veining within an interval of~10 Myr after crust formation is characterized by microtextures that point to extensional fracturing related to crack and sealing, host rock brecciation, and advective fluid flow. These vein calcites show REE+Y characteristics, 87 Sr/ 86 Sr ratios, and clumped isotopic compositions indicative of precipitation from seawater at <50°C. Extended fluid residence times intensified fluid-rock interactions and lowered Y/Ho ratios of some blocky vein calcites, whereas crack and sealing resulted in pristine seawater signatures. Low 87 Sr/ 86 Sr ratios of localized high-temperature blocky vein calcites point to the involvement of hydrothermal fluids. These calcites show Mn-controlled oscillatory growth zonations that probably developed in a closed system out of equilibrium. Later calcite veining (<75 Ma) may have coincided with rotation and/or uplift of the Troodos ophiolite. Microtextures of these vein calcites indicate fluid diffusion and fracture-independent crystallization pressure-driven veining. Their variably modified seawater signatures resulted from diffusion-related fluid interaction with hydrothermal sediments. Plain Language SummaryThe Troodos ophiolite (Cyprus) formed as oceanic crust 92 million years ago along a mid-ocean ridge above a young subduction zone before it was uplifted to its present position 2,000 m above sea-level. Therefore, the Troodos ophiolite constitutes a suitable research object to understand the formation and alteration of oceanic crust. Subsequent to its formation, the oceanic crust underwent structural and mineralogical changes. This investigation explores the age, chemical and physical conditions of these changes using an elemental and isotopic approach. Seawater entered the oceanic crust through fractures and in cases exchanged elements and isotopes with ambient rocks. Calcium carbonate (calcite) precipitated from these waters at temperatures mostly <50°C and filled fractures. These structures are termed veins. A few vein calcites formed at temperatures up to~220°C and show distinguishable zones that developed automatically without external input. The principal mineralization finished~10 million years after the onset of oceanic crust formation. Later calcite mineralization, less than 75 million years ago, is characterized by diffusive fluid flow and fibrous calcite...

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

“…The precipitation ages of FAB‐hosted vein calcites fall mainly within an interval of 25 Myr after crust formation (~52 Ma). This represents the typical temporal framework during which >80% of secondary mineralization is completed (e.g., Coogan & Gillis, , and references therein; Hart et al, ; Hart & Staudigel, ; Quandt, Micheuz, Kurz, Kluge, et al, ; Richardson et al, ; Staudigel & Hart, ; Staudigel et al, ). The precipitation ages also overlap with the onset of pelagic carbonate deposition within extensional graben and half‐graben basins along the Izu‐Bonin forearc (Robertson et al, ).…”

“…The volcanic units of the Izu-Bonin forearc and rear arc are substantially and similarly pervaded by veins that document extensive fluid flow through the oceanic crust and subsequent secondary mineral precipitation (Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, 2015;Christeson et al, 2016;Reagan et al, 2015). Numerous previous studies on calcite veins pervading the oceanic crust used δ 18 O, δ 13 C, and 87 Sr/ 86 Sr isotopic compositions, rare earth element and yttrium (REE+Y) concentrations, and fluid inclusions to investigate the geochemical composition of circulating fluids, and the age and temperature of secondary mineral precipitation (e.g., Alt & Teagle, 2003;Alt et al, 1998;Brandstätter et al, 2016;Brandstätter et al, 2018;Hart & Staudigel, 1978;Quandt et al, 2018;Quandt, Micheuz, Kurz, Kluge, et al, 2019;Schroeder et al, 2015). However, studies on calcite veins from the Izu-Bonin-Mariana trench in particular are rare.…”

“…REE+Y concentrations, as well as radiogenic strontium ( 87 Sr/ 86 Sr), stable oxygen (δ 18 O) and carbon (δ 13 C), and clumped (Δ 47 ) isotopic compositions record the geochemical compositions of circulating fluids and temperature of vein mineral precipitation. The new clumped isotope method (Ghosh et al, ) has been rarely applied on secondary calcites from the oceanic crust (Coogan et al, ; Quandt, Micheuz, Kurz, Kluge, et al, ). In contrast to δ 18 O thermometric calculations, clumped isotopes yield precipitation temperatures without requiring knowledge of the parental δ 18 O fluid composition, which in turn may be calculated (Ghosh et al, ).…”

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

“…Similarly, angular and non-rotated host rock fragments may be easily pieced together. Curved fibers, particularly in antitaxial veins from the Troodos ophiolite, complicate tracking of the displacement trajectory (Quandt et al 2019 ).…”

“…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.…”