Hydrogen isotopes as a proxy for the [sup 18]O content of water in carbonates
- NASA, Houston, TX (United States). Johnson Space Center
Water resides within carbonates as trapped fluid inclusions, adsorbed water, and as water bound within the crystal structure. Analysis by the CO[sub 2] equilibration method or micro-analysis of water by conversion of water directly to CO[sub 2] using guanidine hydrochloride requires hundreds of milligrams of carbonate for analysis. A viable alternative to these methods is the thermal decrepitation of carbonate for hydrogen isotopic analysis. Using this technique, water is generated by heating milligram-sized samples in vacuo at 625 C for 2 hours. Water is trapped cryogenically in evacuated glass tubing containing Zn and reacted according to established protocol for hydrogen isotopic analysis. As a test of this technique, delta D and delta O-18 of synthetic nesquehonite (MgCO[sub 3] [center dot] 3H[sub 2]O) were determined. [delta] D values from thermally extracted water and the precipitating solution are identical within error for both normal and deuterium-spiked solutions at 9 and 23 C. [delta] O-18 values for solution water are identical to the measured delta O-18 of solution water. In addition the delta O-18 of nesquehonite, measured by standard H[sub 3]PO[sub 4] acid digestion is within 2[per thousand] of the predicted value using the paleotemperature equation for calcite and the delta O-18 and temperature of the precipitating solution. Conversely, measured delta O-18 values for water from the thermal decrepitation of large samples (> 500 mg), determined by the CO[sub 2] equilibration method, are enriched in O-18 compared with actual solution values while CO[sub 2] liberated during heating is depleted in O-18 compared with values calculated to be in equilibrium with solution. This suggests that isotopic exchange between H[sub 2]O and CO[sub 2] occurs at elevated temperature before these two oxygen-bearing components can be separated cryogenically.
- OSTI ID:
- 6000425
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CARBONATE ROCKS
ISOTOPE RATIO
CARBON DIOXIDE
COMPARATIVE EVALUATIONS
DATA ANALYSIS
DEUTERIUM
INCLUSIONS
MAGNESIUM CARBONATES
MEASURING METHODS
OXYGEN 18
SAMPLE PREPARATION
WATER
ALKALINE EARTH METAL COMPOUNDS
CARBON COMPOUNDS
CARBON OXIDES
CARBONATES
CHALCOGENIDES
EVALUATION
EVEN-EVEN NUCLEI
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
ISOTOPES
LIGHT NUCLEI
MAGNESIUM COMPOUNDS
NUCLEI
ODD-ODD NUCLEI
OXIDES
OXYGEN COMPOUNDS
OXYGEN ISOTOPES
ROCKS
SEDIMENTARY ROCKS
STABLE ISOTOPES
580000* - Geosciences