Authigenic Carbonate Rock
from Geologic Settings with and without Gas Hydrate from the Gulf of Mexico
Jung, Woodong1, Roger Sassen2
(1) Texas A&M University, College Station, TX (2) Texas A & M
University, College Station, TX
Authigenic carbonate rocks associated
with gas hydrate show a considerable difference in the mineralogy between
geochemical environments that favor gas-hydrate crystallization and
environments that retard gas-hydrate crystallization. This may be a useful tool
for gas-hydrate exploration. Authigenic carbonate
samples from the seafloor are recovered from the Atwater Valley 425,
Mississippi Canyon 118, and the The Elbow 711 in the Gulf of Mexico. We document the
carbonate isotopes (δ13C and δ18O) in the
geochemical and microbiologic context in which it is found. The δ13C
of authigenic carbonate rock rarely corresponds to
the δC of source carbon such as CH4.
We use petrographic thin-sections to photograph and
describe the texture, identify carbonate species and other minerals such as
barite, strontium, gypsum, and pyrite. We discover that the δ13C
of the carbonate is usually heavier than the source carbon (CH4).
This is because carbonate sometimes encloses substantial normal-marine skeletal
carbonate. For example, we have studied nodules formed of foraminifera (normal
marine carbonate) that are cemented with carbonate strongly depleted in 13C.
The mineralogy and trace-metal composition provides vital insight to the origin
of the fluids from which the carbonate forms. We further characterize the
unusual mineralogical components using an electron microprobe to detail map the
distribution of unusual elements in the thin-sections.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California