Shallow sediments across large parts of Krishna-Godavari (K-G) basin offshore east central Indian contain abundant methane and gas hydrates. In this study, we carried out rock-magnetic and transmission electron microscope (TEM) analyses on the samples from a sediment core (MD161/Stn-8) to constrain the formation and preservation of greigite in shallow sediments and how this might link to reactions involving methane. Here, we report for the first time the occurrence of silicate-hosted iron sulfide (greigite) inclusions which is an important observation for understanding the preservation of magnetic minerals in gas hydrate systems. The magnetization of the greigite zone (17–23 mbsf) is carried by complex magnetic mineral assemblages of detrital iron oxides (titanomagnetite), diagenetic iron sulfide (greigite) occurring as nano-inclusions within larger silicate particles, and biogenic minerals. Elevated concentrations of dissolved pore water silica and alkalinity within the magnetically enhanced greigite zone suggest that silica diagenesis and silicate weathering triggered by paleo-methane seepage played a key role in crystallizing the diagenetically formed iron sulfide (greigite) into silicate matrix. The silicate-hosted magnetic inclusions protected the ferrimagnetic greigite from further diagenetic dissolution and prevented its conversion into stable pyrite. Three scenarios explaining the potential controls on the greigite preservation in gas hydrate marine sedimentary system have been proposed.

中文翻译：

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孟加拉湾 Krishna-Godavari 盆地天然气水合物系统中灰岩保存的控制

印度中东部近海 Krishna-Godavari (KG) 盆地大部分地区的浅层沉积物含有丰富的甲烷和天然气水合物。在这项研究中，我们对来自沉积岩芯 (MD161/Stn-8) 的样品进行了岩石磁性和透射电子显微镜 (TEM) 分析，以限制浅层沉积物中灰岩的形成和保存以及这可能与反应的联系涉及甲烷。在这里，我们首次报告了以硅酸盐为主体的硫化铁（灰岩）包裹体的出现，这是了解天然气水合物系统中磁性矿物保存的重要观察结果。灰岩带的磁化强度 (17–23 mbsf) 由碎屑氧化铁（钛磁铁矿）的复杂磁性矿物组合携带，成岩硫化铁（greigite）作为纳米包裹体出现在较大的硅酸盐颗粒和生物矿物中。磁增强的灰岩带内溶解孔隙水二氧化硅的浓度和碱度升高表明，由古甲烷渗流引发的二氧化硅成岩作用和硅酸盐风化在将成岩形成的硫化铁（灰岩）结晶成硅酸盐基质方面发挥了关键作用。以硅酸盐为主体的磁性包裹体保护亚铁磁性灰泥免受进一步成岩溶解并阻止其转化为稳定的黄铁矿。已经提出了三种方案来解释对天然气水合物海相沉积系统中灰岩保存的潜在控制。磁增强的灰岩带内溶解孔隙水二氧化硅的浓度和碱度升高表明，由古甲烷渗流引发的二氧化硅成岩作用和硅酸盐风化在将成岩形成的硫化铁（灰岩）结晶成硅酸盐基质方面发挥了关键作用。以硅酸盐为主体的磁性包裹体保护亚铁磁性灰泥免受进一步成岩溶解并阻止其转化为稳定的黄铁矿。已经提出了三种方案来解释对天然气水合物海相沉积系统中灰岩保存的潜在控制。磁增强的灰岩带内溶解孔隙水二氧化硅的浓度和碱度升高表明，由古甲烷渗流引发的二氧化硅成岩作用和硅酸盐风化在将成岩形成的硫化铁（灰岩）结晶成硅酸盐基质方面发挥了关键作用。以硅酸盐为主体的磁性包裹体保护亚铁磁性灰泥免受进一步成岩溶解并阻止其转化为稳定的黄铁矿。已经提出了三种方案来解释对天然气水合物海相沉积系统中灰岩保存的潜在控制。以硅酸盐为主体的磁性包裹体保护亚铁磁性灰泥免受进一步成岩溶解并阻止其转化为稳定的黄铁矿。已经提出了三种方案来解释对天然气水合物海相沉积系统中灰岩保存的潜在控制。以硅酸盐为主体的磁性包裹体保护亚铁磁性灰泥免受进一步成岩溶解并阻止其转化为稳定的黄铁矿。已经提出了三种方案来解释对天然气水合物海相沉积系统中灰岩保存的潜在控制。