 |
| Titel |
Using stable Mg isotopes to distinguish dolomite formation mechanisms: A case study from the Peru Margin |
| VerfasserIn |
Vasileios Mavromatis, Patrick Meister, Eric H. Oelkers |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250088941
|
| Publikation (Nr.) |
 EGU/EGU2014-3122.pdf |
|
|
|
|
|
| Zusammenfassung |
| Early diagenetic processes in marine sediments lead to formation of authigenic carbonates
with distinctive chemical and isotopic compositions. Such environments represent one of the
few geological settings where contemporaneous dolomite formation can be observed. In this
study we examined diagenetic dolomites and their adjacent pore fluids in a 250 m thick
sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg
201 (Site 1230) using stable Mg isotopes. Previous studies revealed the presence of two
types of dolomite: type (I) dolomite forms at ~6 m below seafloor (mbsf) due to an
increase in alkalinity associated with anaerobic methane oxidation, and type (II)
forms at focused sites below 230 mbsf due to episodic inflow of deep-sourced fluids
into an intense methanogenesis zone. The pore fluid δ26Mg values progressively
increase with depth from values similar to seawater (i.e. –0.8 ) in the top few meters
below seafloor (mbsf) to 0.8±0.2 owithin the sediments located below 100 mbsf.
Type I dolomites have a δ26Mg of -3.5 oand exhibit apparent dolomite-pore fluid
fractionation factors of about -2.6 oconsistent with previous studies of dolomite
precipitation from seawater. In contrast, type II dolomites have δ26Mg values ranging
from -2.5 to -3.0 oand exhibit apparent dolomite-fluid fractionation factors of
-3.1 to -3.6oṪhe larger apparent fractionation factor of the type II dolomite could
stem from 1) Rayleigh fractionation in a semi-closed system or 2) changes in fluid
composition following dolomite precipitation. The difference in the Mg isotope
composition of the studied dolomites demonstrates that Mg isotopes can aid to evaluate
whether diagenetic phases formed under open or closed-system conditions in marine
sediments. Hence they provide an additional tool to reconstruct geochemical conditions
conducive to early diagenetic processes that can now be applied to the geological record. |
|
|
|
|
|