This study deals with microtextures and fluid inclusions from veins and vesicles hosted in the Troodos Pillow Lavas that enable a conclusive model for vein formation during the post-magmatic stage of the Troodos supra-subduction zone. Three different types of veins from the Upper and Lower Pillow Lavas are distinguished and imply different modes of fracturing, fluid flow, and precipitation. (1) Syntaxial calcite-, quartz-, and zeolite-bearing veins are interpreted as mineralized extension fractures that were pervaded by seawater. This advective fluid flow in an open system changed later into a closed system characterized by geochemical self-organization. (2) Blocky and (3) antitaxial fibrous calcite veins are associated with host rock brecciation due to hydrofracturing and diffusion-crystallization processes, respectively. Based on aqueous fluid inclusion chemistry with seawater salinities in all studied vein types, the representative fluid isochores crossed with minimum hydrostatic pressure conditions yield vein mineral precipitation temperatures between 180 and 210 °C at 250 bar, independently of the Pillow Lava units. This points to a heat source for the circulating seawater and implies that vein and vesicle minerals precipitated shortly after pillow lava crystallization under dominant isobaric cooling conditions. Compared to previous suggestions derived from secondary mineral parageneses, significant higher temperatures of vein formation in the Troodos Pillow Lavas are proposed.

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.

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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...