In subduction zones, the accretionary prism is formed as sediments and other materials accumulate and are subjected to increased pressure and temperature due to tectonic processes. The mineral assemblages that develop in the accretionary prism are influenced by the conditions of temperature, pressure, and depth prevailing in this unique geological environment.
Mineral Assemblages in Accretionary Prisms:
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Clay Minerals:
- Formation: Clays like smectite can form from the alteration of volcanic ash and other sediments at relatively low temperatures and pressures. As sediment is subducted, the clay can undergo hydration and diagenesis.
- Temperature & Pressure: Focused on lower temperatures (generally below 200 °C) and relatively low pressure (around 2-3 kbar).
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Illite and Chlorite:
- Formation: With increasing depth and temperature, these clay minerals can transform into illite and eventually chlorite, indicating a transition to diagenesis and low-grade metamorphism.
- Temperature & Pressure: Occurs at temperatures between approximately 200-400 °C and pressures increasing to around 3-6 kbar.
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Mica and Feldspars:
- Formation: Further metamorphism can lead to the formation of micas (like biotite) or feldspar under moderate to high-grade conditions.
- Temperature & Pressure: Typically develops at higher temperatures of around 400-600 °C and pressures of 6-12 kbar.
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Metamorphic Rocks:
- Formation: More significant subduction depths and higher temperatures can result in the formation of schists and gneisses, with minerals like garnet and staurolite potentially appearing.
- Temperature & Pressure: Occurs at pressures above 12 kbar and temperatures exceeding 600 °C.
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High-pressure, Low-temperature (HP/LT) Minerals:
- Formation: In the depths of the subduction zone, specific high-pressure minerals like lawsonite and glaucophane can form. These minerals are indicative of the unique conditions found in subduction-related metamorphism.
- Temperature & Pressure: Generally develop under conditions of high pressure (>12 kbar) and lower temperatures (200-400 °C), which are typical of subduction zones where oceanic crust is being forced into the mantle.
Why Do These Minerals Form Here?
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Depth-Dependent Pressure and Temperature: As sediments are subducted, they experience increasing pressure and temperature with depth. The pressure increases due to the weight of the overlying materials, while temperature can increase due to geothermal gradients and the heat generated by tectonic processes.
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Fluid Influx: Subduction zones also typically involve fluids (from the hydration of minerals and water trapped in sediments). This fluid activity significantly contributes to the alteration processes that lead to the formation of these specific mineral assemblages.
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Chemical Environment: The minerals that form are also dependent on chemical reactions under varying conditions of temperature, pressure, and the presence of fluids. These reactions often produce stable minerals under the given physical conditions.
In summary, mineral assemblages in the accretionary prism of subduction zones are heavily influenced by the interplay of temperature, pressure, and depth, leading to a variety of low-grade to high-grade metamorphic minerals evolving in response to the geological processes occurring in these dynamic environments.