Authigenesis

Authigenesisis the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described asauthigenic.Authigenic sedimentarymineralsform during or aftersedimentationbyprecipitationorrecrystallizationas opposed todetritalminerals, which are weathered by water or wind and transported to the depositional location. Authigenic sediments are the main constituents of deep sea sedimentation, compared to shallow waters or land where detrital sediments are more common.

Authigenic minerals inmarine sediment

Authigenesis process

Basic steps of saturation and precipitation

The authigenesis process is driven by particles of sediment that are not inthermodynamic equilibriumwith the conditions surrounding it. This often occurs in the ocean where changing conditions due to biological processes is common. In water, changing oxygen content or changing saturation causesprecipitationof minerals into the sediments, changing the state of the sediments. Precipitation is the main process of authigenesis on the surface of the ocean floor, with carbonates being the primary precipitates. For any mineral to be precipitated, the water must besupersaturatedwith respect to that mineral. For example, the area of deposition must be above thecarbonate compensation depthor the pore waters must be sufficiently saturated due to dissolution of other grains forcalciteto precipitate. The highporosityof deposited sediments allow for precipitation of minerals in the porous features, as the pore water can often have different saturation than the surrounding waters, thus diminishing the porosity and creating authigenic minerals.

Over time, sediments become more buried bydepositionandprecipitation.This causescompactionandcementationto occur and decreases porosity farther, changing the control on the authigenesis process from fluid composition to temperature. Increased temperatures around sediments leads to increased reactions in order for the sediment to reach thermodynamic equilibrium with the surroundings. This often is done throughdehydration reactionsbecause these reactions have a high entropy release.

Metamorphism

At larger depths, the line between authigenesis andmetamorphismis blurred. In metamorphicpetrologyan authigenic mineral is one formedin situduringmetamorphism,again by precipitation from fluids orrecrystallization.However, minerals created by temperatures above 250 degrees Celsius are generally agreed upon to not be authigenic minerals, but rather metamorphic minerals.

Common authigenic minerals in sedimentary rocks

Authigenic glauconite fromDorset,England

• calcium carbonate
• apatite
• clays
• hematite
• limonite
• chamosite
• siderite
• silica
• glauconite
• pyrite

See also

• Neomorphism

External links

• Authigenic Sediments