The effects of diagenesis and dolomitization on Ca and Mg isotopes in marine platform carbonates; implications for the geochemical cycles of Ca and Mg

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doi: 10.1016/j.gca.2014.07.025
Author(s): Fantle, Matthew S.; Higgins, John
Author Affiliation(s): Primary:
Pennsylvania State University, Department of Geosciences, University Park, PA, United States
Other:
Princeton University, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.142, p.458-481. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 113 refs.; illus., incl. 2 tables, sketch map
Summary: The Ca, Mg, O, and C isotopic and trace elemental compositions of marine limestones and dolostones from ODP Site 1196A, which range in depth (∼58 to 627 mbsf) and in depositional age (∼5 and 23 Ma), are presented. The objectives of the study are to explore the potential for non-traditional isotope systems to fingerprint diagenesis, to quantify the extent to which geochemical proxies are altered during diagenesis, and to investigate the importance of diagenesis within the global Ca and Mg geochemical cycles. The data suggest that Ca, which has a relatively high solid to fluid mass ratio, can be isotopically altered during diagenesis. In addition, the alteration of Ca correlates with the alteration of Mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks. Bulk carbonate δ44Ca values vary between 0.60 and 1.31 ppm (SRM-915a scale); the average limestone δ44Ca is 0.97 ± 0.24 ppm (1SD), identical within error to the average dolostone (1.03 ± 0.15 1SD ppm). Magnesium isotopic compositions (δ26Mg, DSM-3 scale) range between -2.59 ppm and -3.91 ppm, and limestones (-3.60 ± 0.25 ppm) and dolostones (-2.68 ± 0.07 ppm) are isotopically distinct. Carbon isotopic compositions (δ13C, PDB scale) vary between 0.86 ppm and 2.47 ppm, with average limestone (1.96 ± 0.31 ppm) marginally offset relative to average dolostone (1.68 ± 0.57 ppm). The oxygen isotopic compositions (δ18O, PDB scale) of limestones (-1.22 ± 0.94 ppm) are substantially lower than the dolostones measured (2.72 ± 1.07 ppm). The isotopic data from 1196A suggest distinct and coherent trends in isotopic and elemental compositions that are interpreted in terms of diagenetic trajectories. Numerical modeling supports the contention that such trends can be interpreted as diagenetic, and suggests that the appropriate distribution coefficient (K Mg) associated with limestone diagenesis is ∼1 to 5 × 10-3, distinctly lower than those values (>0.015) reported in laboratory studies. With respect to Mg isotopes, the modeling also suggest that diagenetic fractionation factors of ∼0.9955 (-4.5 ppm) and 0.9980 (-2 ppm) are appropriate for limestone diagenesis and dolomitization, respectively. Abstract Copyright (2014) Elsevier, B.V.
Year of Publication: 2014
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Alkaline earth metals; C-13/C-12; Ca-44/Ca-40; Calcium; Carbon; Carbonate platforms; Carbonate rocks; Carbonate sediments; Carbonatization; Coral Sea; Depositional environment; Diagenesis; Dolomitization; Equatorial Pacific; Geochemical cycle; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; Leg 130; Leg 194; Limestone; Magnesium; Marine sediments; Mathematical methods; Metals; Mg-26/Mg-24; North Pacific; Northwest Pacific; O-18/O-16; ODP Site 1196; ODP Site 807; Ocean Drilling Program; Ontong Java Plateau; Oxygen; Pacific Ocean; Sedimentary rocks; Sediments; South Pacific; Southwest Pacific; Stable isotopes; Statistical analysis; West Pacific
Coordinates: N033622 N033626 E1563730 E1563728
S210100 S210000 E1525200 E1525100
Record ID: 2014086883
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands