Serpentinization

In geosciences, serpentinization is the transformation of rocks containing olivine into serpentinites , or the formation of serpentine minerals from olivines. Both processes are characterized by different geochemical processes.

Serpentinization of peridotites

Serpentinization in the conversion of peridotites to serpentinites occurs at temperatures between 300 and 500 ° C and increased pressure in the lithospheric mantle . This process is described in detail in the article serpentinite .

Serpentinization on the ocean floor

Serpentinization is also the name given to those changes that occur through the decomposition of olivines through hydration (i.e. through absorption of water) at temperatures of up to 260 ° C during metasomatosis . This process takes place, for example, near mid-ocean ridges , where olivine-rich mantle rocks can meet with seawater. The conversion of iron-containing olivine under these conditions to serpentine and iron-rich brucite is described by the following reaction equation:

${\displaystyle \mathrm {2\;Mg_{1,8}Fe_{0,2}[SiO_{4}]+3\;H_{2}O\longrightarrow Mg_{2,85}Fe_{0,15}[(OH)_{4}|Si_{2}O_{5}]+Mg_{0,75}Fe_{0,25}(OH)_{2}} }$

As a further variant, water effects in low metamorphic proportions of the green slate facies are also noteworthy, which leads to the formation of serpentine from olivines.

Serpentinization as a source of hydrogen and methane

On the ocean floor, black smokers are found on their axis of expansion in thermal springs of up to 400 ° C and somewhat off the axis of expansion in cooler (up to about 90 ° C), alkaline thermal springs of the Lost City hydrothermal springs type with the thermal water hydrogen (H ₂ ) and Methane (CH ₄ ) promoted. The formation of hydrogen is seen as a consequence of serpentinization. The iron-rich brucite formed during serpentinization (see reaction equation above) reacts with silicon dioxide (SiO ₂ ) dissolved in the water to form serpentine, magnetite (Fe ₃ O ₄ ), water and hydrogen:

${\displaystyle \mathrm {57\;Mg_{0,75}Fe_{0,25}(OH)_{2}+30\;SiO_{2}\longrightarrow 15\;Mg_{2,85}Fe_{0,15}[(OH)_{4}|Si_{2}O_{5}]+23\;H_{2}O+4\;Fe_{3}O_{4}+4\;H_{2}} }$

The following reaction equation summarizes the serpentinization of olivine with the formation of magnetite and hydrogen:

${\displaystyle \mathrm {6\;Mg_{1,5}Fe_{0,5}[SiO_{4}]+7\;H_{2}O\longrightarrow 3\;Mg_{3}[(OH)_{4}|Si_{2}O_{5}]+Fe_{3}O_{4}+H_{2}} }$

It is assumed that methane is produced by a Fischer-Tropsch synthesis from the carbon dioxide (CO ₂ ) also contained in the thermal water and the hydrogen formed during serpentinization . A general reaction equation for the formation of alkanes from carbon dioxide and hydrogen in the Fischer-Tropsch synthesis is:

${\displaystyle \mathrm {CO_{2}+\left(2+{\frac {m}{2n}}\right)H_{2}\longrightarrow {\frac {1}{n}}C_{n}H_{m}+2\;H_{2}O} }$

The occurrence of hydrogen and methane in alkaline thermal waters on the ocean floor is discussed in connection with the formation of life: Such an environment is seen as a favorable prerequisite for this. The formation of methane proceeds according to the reaction equation

${\displaystyle \mathrm {CO_{2}+4\;H_{2}\longrightarrow CH_{4}+2\;H_{2}O} }$

It is also assumed that these chemical reactions are one of the possible causes of the methane detected in the Martian atmosphere .

Individual evidence

1. ↑ ^{a b} Wolfgang Bach, Holger Paulick, Carlos J. Garrido, Benoit Ildefonse, William P. Meurer, Susan E. Humphris: Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15 ° N (ODP Leg 209, Site 1274) . In: Geophysical Research Letters . Vol. 33, 2006, L13306. doi : 10.1029 / 2006GL025681
2. ↑ ^{a b c} Giora Proskurowski, Martin D. Lilley, Jeffery S. Seewald, Gretchen L. Früh-Green, Eric J. Olson, John E. Lupton, Shean P. Sylva, Deborah S. Kelley: Abiogenic hydrocarbon production at Lost City Hydrothermal Field. In: Science . Vol. 319, 2008, pp. 604-607.
3. ↑ ^{a b} William Martin: Everything has a beginning, including evolution: hydrothermal vents and the origin of life (PDF). ( Memento of the original from January 31, 2012 in the Internet Archive ) Info: The  archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Biol. Unserer Zeit, 3/2009 (39), pp. 166-173. doi : 10.1002 / biuz.200910391 .
4. ↑ Ulrich Knittel: Does weathering and not life create methane?  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. astronews.com, June 15, 2005. Retrieved December 28, 2008  

literature

• Roland Vix: Rock determination in the field . Munich 2005, p. 78 ISBN 3-8274-1513-6
• Wolfhard Wimmenauer : Petrography of igneous and metamorphic rocks . Stuttgart (Enke) 1985, pp. 286-289 ISBN 3-432-94671-6