Formation model of authigenic chimneys on the Quaker serpentinite mud volcano in the Mariana forearc

Authigenic chimneys, found at the top of serpentinite mud volcanos in the Mariana forearc, are induced by the seepage of low temperature and alkaline fluids. They are critical significant to trace the eruption of serpentinization fluids. However, few is known with regards to the formation mechanism of these chimneys. In this paper, detailed investigations are carried out on the petrology, mineralogy, and major elemental geochemistry of the chimneys collected from the Quaker serpentinite mud volcano in the Mariana forearc region so as to explore the formation model of these authigenic chimneys. Base on the mineral and elemental compositions, as well as neomorphic processes, three types of chimneys are identified. It is found that infancy chimneys are high in MgO (18.5%～37.5%) and low in CaO contents (12.2%～32.1%), and mineralogically composed of calcite (52.0%～77.6%) and magnesium-rich alkaline minerals, such as brucite, hydromagnesite, and hydrotalcite, while the mature chimneys are characterized by reduced MgO contents (1.5%～23.6%) and enhanced CaO contents (18.6%～53.3%), and mineralogically composed of calcite (59.8%), magnesium-rich minerals and aragonite (23.4%). Dead chimneys have the highest aragonite content (33.2%), but do not contain any magnesium-rich minerals. In addition, microscopic observation results have revealed the precursory magnesium-rich alkaline minerals replaced by aragonite. The variations of elemental and mineral compositions among different types of chimneys, and their petrological characteristics suggest that the fluid seepage induced calcite and brucite precipitation, while aragonite represents a replaced phase of brucite. Brucite occurrences indicate newly formed fabrics, while aragonite reflects an old precursory mineral. Micro-drilled samples from the same chimney cross section show successively decrease of MgO content and increase of CaO content from inner to outer, suggesting that the outer texture is older than the inner. The top of a chimney displays higher MgO and lower CaO contents than the bottom, indicating that the top is relatively younger.