Origins of rodingite-forming fluids from the seafloor to exhumed terranes: Insights from calcium, strontium, and oxygen isotopes

Abstract

Rodingites are metasomatic Ca-rich, Si-poor rocks typically formed on the seafloor during interaction of a gabbroic dike with seawater. However, rodingites are also common in exhumed metamorphic terranes, in which their formation and tectonic history are more complex. Here we measure the O, Ca, and Sr isotope compositions of seafloor and Alpine rodingites from various tectonic settings (obducted oceanic lithosphere, subducted oceanic lithosphere, fossil rifted margins). We focus on andradite-grossular garnet, one of the first phases to crystallize during rodingitization, and clinopyroxene to discern the origins of rodingite-forming fluids and thus the tectonic setting of rodingitization. δ^44/40Ca values of plagioclase (pl), clinopyroxene (cpx), and calculated whole rock (WR) from altered gabbro (i.e., rodingite protoliths) from the Mid-Atlantic Ridge span a range of ∼0.2 ‰ (1.24 to 1.40 ‰ for pl, n = 2; 1.30 ‰ for cpx, n = 1; 1.29 ‰ for WR, n = 1), whereas, garnet (grt), cpx, and WR from the Western and Central Alpine rodingites span ∼2.8 ‰ (0.01 to 2.52 ‰ for grt, n = 9; −0.32 to 1.58 ‰ for cpx, n = 9; −0.22 to 1.92 ‰ for WR, n = 9). ⁸⁷Sr/⁸⁶Sr ratios of mineral separates from the seafloor span from 0.702606 to 0.703700 (0.703516 to 0.703700 for pl, n = 2; 0.702606 for cpx, n = 1) and 0.70330 to 0.710559 for Alpines rodingites (0.704756 to 0.710559 for grt, n = 9; 0.70330 to 0.707080 cpx, n = 9). Lastly, δ¹⁸O values span ∼1.6 ‰ for the seafloor (5.7 to 6.4 ‰ for pl, n = 2; 4.8 ‰ for cpx. n = 2) and ∼ 6.4 ‰ for Alpine rodingites (−0.4 to 4.5 ‰ for grt, n = 9; 1.7 to 6.0 ‰ for cpx, n = 8). Isotope mass balance modeling shows that these data are overall consistent with rodingitization of a gabbroic dike via seawater on the seafloor and subsequent recrystallization with limited isotopic modification during subduction. A few Alpine rodingites with distinctly high or low δ^44/40Ca values compared to mantle values (e.g., δ^44/40Ca_WR = 1.92 ‰ at Erro-Tobbio and δ^44/40Ca_WR = −0.22 ‰ at Lago di Cigana) cannot be explained via metasomatism by seawater using the model presented here, requiring either evolution of the fluid source during rodingitization and associated serpentinization and/or Ca isotope disequilibrium.