New Zealand Journal of Geology and Geophysics abstracts
Fluid inclusion evidence for geothermal structure beneath the
Southern Alps, New Zealand
D. CRAW
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
Abstract Fissure veins containing adularia, bladed calcite,
quartz, and chlorite occur in fractures in schist immediately west of the
mountain crest in the Southern Alps, an active collisional mountain range. The
vein minerals contain primary fluid inclusions which homogenise between 240 and
260deg.C. The fluids have low dissolved salt content (<2 wt% NaCl
equivalent) and low CO2 content (<1 wt%). Fluid inclusions in adularia
show physical (co-existing liquid and vapour) and chemical (variable CO2
contents) evidence for boiling during entrapment. The mineral assemblage is
similar to that seen in boiling zones of modern geothermal systems. Boiling
occurred at 500 +/- 150 m below topographic surface, or c. 1 km
above sea level, and fluid temperature was higher than rock temperature. In
contrast, fluids trapped in the same rock sequence at 300-350deg.C at
6-10 km under lithostatic and hydrostatic fluid pressure were
approximately the same temperature as host rock and define part of a conductive
thermal anomaly. The boiling zone developed due to topography-driven
two-dimensional circulation of meteoric water into the uplift-induced
conductive anomaly, followed by rapid buoyant rise of heated and partially
isotopically exchanged water to shallow levels under hydrostatic fluid
pressure.
Farther west, near the Alpine Fault, the conductive thermal anomaly has
resulted in fluid and rock temperatures of 300-350deg.C at <5-8 km
under lithostatic and hydrostatic fluid pressure. The fluid is mainly meteoric
in origin, but has partially exchanged isotopically with the host rock. Minor
buoyant rise of fluid has resulted in penetration of hot (200deg.C) fluid into
relatively cool rock at shallow levels (<2 km). Hot springs emanate
from the surface above this portion of the hydrothermal system, but these
springs are fed by topographically driven meteoric water, which penetrates to
only shallow levels in the crust and is isotopically distinct from the deeper
fluids.
Keywords fluid inclusions; boiling; immiscibility; fluid
pressure; heat flow; Southern Alps; tectonics; mountains
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 43-52
0028-8306/97/4001--0043 $2.50/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (1672K); (scanned from paper original: notes about this process)
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