Geochemistry and Microtextures of Vein Calcites Pervading the Izu‐Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351. (19th February 2020)

• Record Type: Journal Article
• Authors: Quandt, D.; Micheuz, P.; Kurz, W.; Bernasconi, S. M.; Hippler, D.; Krenn, K.; Hauzenberger, C. A.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 21:Number 2(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-02-19
• Subjects: Izu‐Bonin forearc and rear arc -- Calcite veins -- Geochemistry -- IODP 352 and 351 -- Microtextures; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2019GC008745
• Languages: English
• ISSNs: 1525-2027

Abstract

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 (δ 18 O, δ 13 C, 87 Sr/ 86 Sr, 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 87 Sr/ 86 Sr ratios intersect the 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr. In the rear arc, low‐T rock alteration lowered the circulating seawater in δ 18 O and 87 Sr/ 86 Sr. Thus, vein calcites precipitated from modified seawater with up to 20–30% basaltic 87 Sr/ 86 Sr 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. Plain Language Summary: Our knowledge of the oceanic crust relies on crustal pieces uplifted above sea level and sampling by dredging, diving, and drilling expeditions. Since uplift may involve the alteration and destruction of original geological features, International Ocean Discovery Program Expeditions 352 and 351 drilled into the Izu‐Bonin forearc and rear arc crust (Western Pacific), revealing rocks unaffected by these processes. The volcanic rocks in these drill cores are characterized by fractures through which seawater percolated from which calcium carbonate (calcite) precipitated (veining). By analyzing crystal shapes, elemental and isotopic compositions of vein calcites, this research project investigated the causes of fracturing, the geochemical composition of the percolating fluids, and the calcite formation temperatures. In the forearc calcites formed at temperatures <30 °C in fractures that opened due to high fluid pressures. These calcites precipitated from fluids ranging from unmodified seawater to seawater that was weakly modified by fluid‐rock interaction. Most seawater‐derived calcites formed between ~35 and ~33 million years. In the rear arc calcites precipitated at temperatures up to 74 °C from seawater that was modified by low‐temperature fluid‐rock interaction during which elements and isotopes were exchanged. The rear arc geology facilitated fluid diffusion and fracture‐independent veining. Key Points: In the Izu‐Bonin forearc vein calcites precipitated at <30 °C from pristine to weakly modified seawater in hydrofractures Calcite veining (max. 74 °C) in the rear arc is characterized by weak to moderate fluid‐rock interaction, diffusive and advective fluid flow A sedimentary cover and the lack of faults facilitated diffusion‐fed and crystallization pressure‐driven antitaxial veining in the rear arc