『Abstract
Diverse carbonate dissolution methods have been applied to bulk
carbonate rocks in order to target least altered components for
strontium isotope stratigraphy (SIS). This is especially important
for Precambrian and Cambrian studies for which no suitable skeletal
material is available. On proven method for bulk limestones involves
the removal of up to a third of the powdered sample using an acid
pre-leach before partial dissolution of the rest of the sample
using a weak acid or acid buffer solution. We applied a similar
technique to dissolve a range of lithologies (limestone, dolostone
and phosphorite) and compared the strontium isotopic composition
of various leaches to contemporaneous seawater. Our results vindicate
this approach and allow us to conclude that some dolomitic and
phosphatic rock components may retain a near-primary seawater
87Sr/86Sr composition once contaminant strontium
from secondary calcite and other phases has been removed. Commonly
applied trace element (e.g. Mn/Sr, Mg/Ca) and isotopic (C,O) screening
cut-offs were also examined in the light of these results, and
proved unreliable particularly in the case of dolostones and partially
dolomitized limestones. We recommend that rigid sample selection
and sequential leaching procedures be applied to all SIS studies
bulk materials and propose a general protocol for strontium isotope
studies on marine authigenic rocks which may also be applicable
to rare earth element and other geochemical studies of marine
authigenic minerals in the future.
Keywords: Strontium isotope stratigraphy; Limestone; Dolostone;
Phosphorite; Dissolution methods』
1. Introduction
2. Geological setting and stratigraphy
3. Methods
3.1. Sample selection and extraction
3.2. Dissolution of samples and mass spectrometric analysis
4. Results
4.1. Elemental results
4.2. Sr isotope results
5. Discussion
5.1. Selection criteria
5.1.1. Petrographic criteria
5.1.2. Geochemical selection
5.2. Physical extraction
5.3. Chemical pre-treatment
5.4. Sample dissolution
6. Discussions
Acknowledgments
References