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= Mariana Mud Volcanoes = Mud volcanoes in the Mariana fore-arc are a hydrothermal geologic landform which erupt slurries of mud, water, and gas. There are at least 10 mud volcanoes in the Mariana fore-arc which are actively erupting. This includes the recently studied Yinazao, Fantagisna (informally known as Celestial seamount), Asut Tesoro (formerly Big Blue), and South Chamorro serpentine seamounts. These mud volcanoes erupt a unique, serpentine rich, mud composition which is related to the geologic setting in which they have formed. Serpentine mud is the product of mantle metasomatism due to subduction zone metamorphism and slab dehydration. As a result, the serpentine mud erupted from these mud volcanoes often contains pieces of mantle peridotite material which hasn't fully altered during the serpentinization process. Serpentine mud volcanoes in the Mariana fore-arc are often located above faults in the fore-arc crust. These faults act as conduits for the hydrated mantle material to ascend towards the surface.

Geologic Setting
The Marina fore-arc is located at the convergent margin between the Pacific and Philippine tectonic plates. The Mariana arc is part of the combined Izu-Bonin-Mariana subduction system which extends form Japan to the north to Palau in the south. The fore-arc is located to the east of the Mariana volcanic arc, both of which have formed due to the subduction of the Pacific plate beneath the Philippine plate. Subduction of the Pacific plate beneath the Philippine plate began between 52-50 Ma during a period of tectonic plate reorganization in the western Pacific.

Formation of Serpentine Mud
As the Pacific plate sinks into the mantle the pelagic sediments and hydrated basalts begin to dehydrate, releasing water and volatiles into the mantle beneath the Mariana arc. This mixture of water and volatiles interact with the mantle peridotite material beneath the overriding Philippine plate. The peridotite beneath the Mariana fore-arc has a harzburgite composition, thus it contains mostly olivine and orthopyroxene (enstatite). This means that serpentine in the fore-arc mantle forms via the hydration of olivine (Mg2SiO4) and enstatite (MgSiO4). This reaction results in the formation of Mg-rich serpentine (Mg3Si2O5(OH)4) and brucite (Mg(OH)2). Since serpentine is a mineral group, there are multiple serpentine minerals which form during this chemical process. This includes antigorite, lizardite, and chrysotile, all of which share the same chemical composition but vary crystallographically.

2Mg2SiO4 + 3H2O = Mg3Si2O5(OH)4 + Mg(OH)2

6MgSiO3 + 3H2O = Mg3Si2O5(OH)4 + Mg3Si4O10(OH)2

Mg2SiO4 + MgSiO3 + 2H2O = Mg3Si2O5(OH)4