Using stable Mg isotopes to distinguish dolomite formation mechanisms

Author(s)

Vasileios Mavromatis, Patrick Meister, Eric H. Oelkers

Abstract

The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~6m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~230mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid δ

²⁶Mg composition becomes progressively enriched in

²⁶Mg with depth from values similar to seawater (i.e. -0.8‰, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8±0.2‰ within the sediments located below 100mbsf. Type I dolomites have a δ

²⁶Mg of -3.5‰, and exhibit apparent dolomite-pore fluid fractionation factors of about -2.6‰ consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have δ

²⁶Mg values ranging from -2.5 to -3.0‰ and are up to -3.6‰ lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in

²⁶Mg and depletion in total Mg concentration below ~200mbsf is likely the result of Mg isotope fractionation during dolomite formation, The

²⁶Mg enrichment of pore fluids in the upper ~200mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.

Organisation(s)

Department of Geology

External organisation(s)

Université Toulouse III Paul Sabatier, Max-Planck-Institut für marine Mikrobiologie, University College London

Journal

Chemical Geology

Volume

385

Pages

84-91

No. of pages

8

ISSN

0009-2541

DOI

https://doi.org/10.1016/j.chemgeo.2014.07.019

Publication date

10-2014

Peer reviewed

Yes

Austrian Fields of Science 2012

105105 Geochemistry, 105121 Sedimentology

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology, Geology

Sustainable Development Goals

SDG 14 - Life Below Water

Portal url

https://ucrisportal.univie.ac.at/en/publications/1b19adca-86a9-4dbb-9244-774e3688730d