Interpreting water-rock interaction in ancient carbonates using multi-method microanalysis
- Univ. of Michigan, Ann Arbor (USA)
Modern mass spectrometers reliably measure very small samples, (i.e., 1 {mu}mol of CO{sub 2} and < 100 nanograms of Sr). It is now possible to analyze Sr, O, and C isotope ratios and Ca, Mg, Fe, Mn and Sr concentrations using a total sample of 1.0 mg. Such analyses can provide fine-scale geochemical trends that would be obscured by bulk-rock analyses. As a case study, diagenetic phases from a Late Devonian pinnacle reef from the Alberta basin have been microanalyzed to quantify water-rock interaction (marine cements, caliche, pendant, vadose cements, and sparry calcite cements). Unaltered marine cements have the lowest {sup 87}Sr/{sup 86}Sr ratios and the highest {delta}{sup 18}O values of all carbonate phases (0.70805 {plus minus} 2, {minus}4.8{per thousand} {plus minus}0.5) as well as the highest concentrations of (Mg) and (Sr) (approximately 8,500 and 350 ppm, respectively). These are assumed to represent average, initial rock compositions. In {sup 87}Sr/{sup 86}Sr-{delta}{sup 18}O crossplots, altered marine cement data diverge from this value toward higher {sup 87}Sr/{sup 86}Sr ratios and lower {delta}{sup 18}O values (to 0.70842 and {minus}8.0 {per thousand}). Vadose cements, caliche, and meteoric phreatic spars have {sup 87}Sr/{sup 86}Sr ratios and {delta}{sup 18}O values identical to altered marine cement data, with some spars having the same compositions as unaltered marine cements. These trends can be explained by two-component mixing models that use low W/R ratios, reasonable Sr concentrations, and distribution coefficients (0.05). However, a source of radiogenic strontium for diagenetic fluids is required to change initial {sup 87}Sr/{sup 86}Sr ratios to observed values.
- OSTI ID:
- 6796800
- Report Number(s):
- CONF-900605--
- Journal Information:
- AAPG Bulletin (American Association of Petroleum Geologists); (USA), Journal Name: AAPG Bulletin (American Association of Petroleum Geologists); (USA) Vol. 74:5; ISSN AABUD; ISSN 0149-1423
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
020200* -- Petroleum-- Reserves
Geology
& Exploration
58 GEOSCIENCES
580000 -- Geosciences
ALKALINE EARTH ISOTOPES
ALKALINE EARTH METALS
BETA DECAY RADIOISOTOPES
CALCIUM
CARBON ISOTOPES
CARBONATE ROCKS
CHEMISTRY
DIAGENESIS
ELECTRON CAPTURE RADIOISOTOPES
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOCHEMISTRY
HOURS LIVING RADIOISOTOPES
INTERMEDIATE MASS NUCLEI
IRON
ISOMERIC TRANSITION ISOTOPES
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
MAGNESIUM
MANGANESE
METALS
NUCLEI
OXYGEN 18
OXYGEN ISOTOPES
RADIOISOTOPES
ROCK-FLUID INTERACTIONS
ROCKS
SEDIMENTARY ROCKS
STABLE ISOTOPES
STRONTIUM
STRONTIUM 86
STRONTIUM 87
STRONTIUM ISOTOPES
TRACE AMOUNTS
TRANSITION ELEMENTS