Journal of Economic Geology (Dec 2024)
Seawater-originated fluids interactions with oceanic lithospheric mantle peridotites and formation of hornblendite dykes, as well as spadaite and dolomite veins in the Naein ophiolite (Isfahan Province, Iran)
Abstract
The Mesozoic ophiolitic mélange of Naein is located to the west of the Central-East Iranian Microcontinent (CEIM). In this ophiolite, the mantle peridotites cross cut by greenish, coarse-grained hornblendite dykes with up to 50 cm width. These dykes cross cut by carbonate veins with a few millimeters to a few centimeter width. Hornblendite dykes composed of Cr-spinel, magnesio-hornblende, chlorite, ilmenite, tremolite, calcite and dolomite. Hydrothermal spadaites (MgSiO2(OH)2·H2O) are formed in the late-stage phase. The chemical compositions of hornblendites indicate that hornblendes are magnesio-hornblende in composition (with a mean Mg# = 0.93) and chlorites are penninite and clinochlore, with a mean Mg# of 0.94. The Mg# and Cr# of Cr-spinels are 0.45 and 0.66, respectively. The presence of abundant hydrous minerals (hornblende and chlorite) and carbonate veins, as well as the chemical characteristics of hornblendes and Cr-spinels, indicates the non-magmatic origin of these dikes and veins, which were formed by the interactions of seawater-derived fluids with the uppermost mantle peridotites. The mineralogical and chemical characteristics of hornblendites demonstrate the mobility of elements such as Mg, Ca, Si, Al, Na, Cr, Fe, Ti and REE during the circulation of fluids derived from seawater within the uppermost mantle peridotites. This study suggests that the percolation of seawater ingression fluids in the uppermost mantle peridotites, resulted in the formation of hornblende dikes and, in the late-stage phase, the development of carbonate veins that contain calcite, dolomite and spadaite. Introduction Petrological and geochemical studies indicate that the influence of seawater affects the mineralogy and chemistry of the oceanic crust and uppermost mantle peridotites (Berger et al., 2005; Python et al., 2007; Akizawa et al., 2011; Akizawa and Arai, 2014; Torabi et al., 2017). Diopsidite, hornblendite and hydrothermal chromitite have formed as a result of reaction between mantle peridotites and penetrating hydrothermal fluids (Python et al., 2007; Torabi et al., 2017; Arai et al., 2020). In the Naein ophiolites mantle peridotites, fractures and cracks within the uppermost mantle peridotites (Harzburgite and dunite) (Fig. 3) have been filled with hornblendites (Torabi et al., 2017). In the last stage, CO2, Mg, Si and Ca-bearing hydrothermal fluids formed the carbonate veins, cross-cuting the peridotites and hornblendites (Fig. 4). In this research, the formation of the hornblendite dikes, carbonate veins and the rare mineral spadaite (MgO.SiO2.2H2O), which were formed by circulating fluids in mantle peridotites of the Nain ophiolite, will discuss. Materials and methods After the field studies, sampling and petrographic studies, polished thin sections of the selected fresh samples were used for point analyses by electron microprobe. Chemical analyses of mineral were performed at the Kanazawa University (Japan) using a wavelength-dispersive electron probe microanalyzer (EPMA) (JEOL JXA-8800R). The analyses were conducted at an accelerating voltage of 15 kV, a probe current of 15 nA (Table 1, 2 and 3) and counting time of 40 seconds. In addition to the microprobe, the minerals of the carbonate veins were investigated by scanning electron microscopy (SEM) (EDS-RONTEC) at an accelerating voltage of 20 kV in the Razi Metallurgical Research Center (RMRC) (Tehran) (Table 4). Discussion Hornblendite formation The petrographic, mineralogical and chemical specifications of the hornblendites indicate their non-magmatic origin (Torabi et al., 2017). These samples composed of primitive hydrous phases (such as Mg-hornblende and chlorite). Some of the primary Mg-hornblendes, have changed to tremolite due to retrograde metamorphism. These minerals indicate the penetration of hydrothermal fluids in the uppermost mantle section (Python et al., 2007; Torabi et al., 2017). The fluid composition is enriched in Cr, Mg, Fe, Si, Al, Ca, Na and HREE as a result of reacions with peridotites. The circulation of fluids through the fractures and veins of mantle peridotites has led to the formation of hornblendites (Torabi et al., 2017). In the hornblendites, the higher content of MgO contrasted to CaO reveals a considerable activity of Mg in circulation of hydrothermal fluids (Torabi et al., 2017). Carbonate veins formation After the formation of hornblendites in the upper mantle peridotites, carbonate veins were formed in the last stage. The presence of carbonate veins in peridotites reveals that these veins formed under the influence of circulating hydrothermal fluids at lower temperatures. These fluids are enriched in elements such as Mg, Ca, Si, CO2 and H2O. The carbonate veins are composed of calcite, dolomite, and spadaite. These carbonate veins cross-cut the hornblendites and peridotites. The presence of dolomite and calcite in carbonate veins, and hornblende (Ca-rich mineral) in hornblendite dykes, shows in the study area, the fluids have passed through Ca -rich rocks (limestone, gabbros) before reaching the uppermost mantle, resulting in the enrichment of the fluids in Ca and CO2. These mineralogical and chemical specifications possibly confirm seawater origin for the fluids. Spadaite Formation The occurrences of magnesium silicate spadaite (MgSiO2(OH)2·H2O), along with calcite and dolomite, developed under the influence of fluid–rock interaction, serpentinization of olivine and orthopyroxene, and subsequent dissolution of serpentine by CO2-bearing hydrothermal fluids. This hydrous magnesium silicate forms under basic conditions, at low temperatures and in the last stage. The Mg and Si-bearing hydrothermal fluids play an important role in the formation of spadaite. The formation of carbonate minerals (calcite and dolomite) in the uppermost mantle peridotites indicates a high fugacity of CO2 in hydrothermal fluids. The kind of new minerals seem to be influenced by ion activities in hydrothermal fluids (Birsoy, 2002), and as well as indirectly by pH. Mobility of Elements Seawater-derived fluids pass through the entire oceanic crust and extend to the uppermost mantle. The hornblendites in the Naein ophiolite were formed by a reaction between seawater ingression fluids and peridotites (harzburgite and dunite) at temperatures ranging from 700–850°C. The mineralogy and chemical characteristics of hornblendite dykes suggest that the circulation of hydrothermal fluids at high-temperatures helps the mobility of Cr, Mg, Ti, Fe, Ca, Si, Al, Na, and REEs (Torabi et al., 2017). The presence of hydrothermal chromite and ilmenite within the hornblendite dykes show mobility of Cr, Fe and Ti, in hydrothermal conditions during the circulation of high temperature silicate-rich fluids through mantle peridotites. The formation of hornblendites dykes (Torabi et al., 2017), diopsidites (Python et al., 2007; Akizawa et al., 2011; Akizawa and Arai, 2014) and hydrothermal chromitites (Arai et al., 2020), under The influence of metasomatic process, indicates that the activity of seawater ingression fluids alters the initial concentration of Ca, Mg, Cr and Si from the lower crust to the uppermost mantle section (Akizawa et al., 2011). Hydrothermal fluids change the chemical composition of minerals, lead to the decomposition of olivine and the formation of serpentine, modify the chemical composition of chromites and form chlorite and secondary chromites. The hydrothermal chromites of the hornblendites (Cr# 0.56 and Mg# 0.62) are chemically intermediate between to chromite found in the surrounding harzburgite (Cr# 0.56 and Mg# 0.62) and dunite (Cr# 0.79 and Mg# 0.41) (Fig. 6E and F), indicating dissolution of primitive chromite grains present in nearby peridotites and their reprecipitation in cracks and fractures during the formation of hornblendite dyke. Altered chromite grains in the hornblendites (Cr# 0.86 and Mg# 0.21) and peridotites (Cr# 0.91 and Mg# 0.17) suggest that hydrothermal fluids have leached Cr-spinel from the host rock and hornblendites (Fig. 6E and F). Conclusions The mineralogical and chemical properties of the Naein mantle hornblendites and their associated carbonate veins indicate a non-magmatic origin, suggesting that they have a hydrothermal nature. The circulation of seawater-derived fluids through the uppermost mantle peridotites will cause to the mobility of Cr, Ti, Fe, and REE. The hydrotermal spadaite formed by H2O, CO2, Mg, Ca and Si-bearing hydrothermal fluids, in the last stage phase that developed in a low-temperature environment under basic conditions. Calcite, dolomite and spadaite are minerals of the carbonate veins. Acknowledgments The authors thank the University of Isfahan and Kanazawa University for financial supports and laboratory equipments.
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