A NATURAL LONG-TERM TRACK ANNEALING Experiment for Apatite1981
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2. METHODS AND GEOLOGICAL SETTING
Minerals were dated using essentially the methods described by Gleadow and Lovering (1978) and Gleadow and Brooks (1979). All ages were measured by the external detector method 169
Nucl. Tracks, Vol. 5, Nos. 1/2, pp. 169--174, 1981. Printed in Great Britain
0191-278X/81/010169-06502.00/0 © 1981 Pergamon Press Ltd.
A NATURAL LONG-TERM TRACK ANNEALING EXPERIMENT FOR APATITE
and between 9 and 30 grains were counted for each mineral sample. To compare the ages of sphenes and apatites from outcrop samples a combined age was calculated as if all the individual mineral grains came from a single age source. For these combined ages errors were calculated by the approximate method of Johnson et al. (1979) and quoted as two standard deviations. Measurements of track length were made on fully-etched confined tracks (TINTS and TINCLES, Lal et al., 1969) and only horizontal tracks were used so as to avoid having to correct for the dip of each track. For each apatite 50 length measurements were made and averaged. The Otway Basin was formed during the early stages of continental rifting between Australia and Antarctica and marks the site of their later separation during the Eocene. The initial phase of sedimentation during the early Cretaceous produced the Otway Group, a 3 km thick sequence of non-marine sediments. The Otway Group now outcrops in two broad areas of the basin where it has clearly never been deeply buried. Elsewhere, such as in the area of the drill-holes, it is overlain by an essentially flat-lying cover of late
A. J. W. GLEADOWand I. R. DUDDY Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia
(Received 19 January 1981)
Abstract--Fission track ages of apatites from volcaniclastic sandstones in several deep drill-holes in southern Victoria, Australia, decrease from 120 Myr near the surface to zero near the bottom of the deepest holes. Track fading occurs between 60 and 1250C, a narrower temperature interval than predicted from laboratory annealing studies, but the 50% track-loss temperature (98°C) is very close to earlier predictions for the estimated heating time in the drill-holes of 10 to 40 Myr. Average track lengths, measured on confined tracks (TINTS, etc.), also decrease with increasing down-hole temperature. Track-length reduction relative to fresh, induced tracks was found in apatites from all depths and even outcrop samples which show no reduction in their fission track ages. 1. INTRODUCTION FISSION track dating of deep drill-hole samples (Naeser and Forbes, 1976; Naeser, 1979) has given important new information on the geological annealing of fission tracks in apatite. Such studies provide a vital test of previously predicted geological annealing temperatures based on extrapolated laboratory annealing data and show that apatite ages decrease to zero at depths of a few kilometres in the Earth's crust. In a 3000m hole drilled in a Mesozoic metamorphic complex near the Eielson Air Force Base in Alaska Naeser found a steady decrease in apatite age with depth from about 100 Myr near the surface to about 14Myr at the bottom. The bottom-hole temperature in the hole was 96°C and the apatite fission track ages extrapolate to zero at a temperature of 105°C. Slow uplift in this area since metamorphism means that apatites at the bottom of the hole have been at a temperature above about 100°C for more than 100 Myr. A similar decrease in apatite age with depth was found in a geothermal test hole drilled by Los Alamos Scientific Laboratory in Precambrian crystalline rocks on the flank of the Pleistocene Valles Caldera in New Mexico. The young age of the volcanic activity in this area means that the duration of heating in this hole was much shorter than at Eielson, about 1 Myr, and the bottom hole temperature is much higher (197°C at 2900 m). In this hole the apatite age decreased to zero at a higher temperature of 135°C as would be expected for a shorter heating time. It was not possible from either of these holes, however, to define the temperature at which track fading in apatite begins. We have now studied the variation in apatite ages with depth in several deep drill holes from sedimentary basins in southern Victoria, Australia, which show both the beginning and ending of track fading. The most important of these are three exploration wells in the Otway Basin, Flaxmans-1, Eumeralla-1 and Port Campbell-4, which are between 2.5 and 3.5 km deep. The present geothermal gradient in these holes is about 33°Ckm -~ giving maximum bottom-hole temperatures up to 125°C. A fourth hole, Wellington Park-l, from the nearby Gippsland Basin reaches a maximum temperature of 157°C at a depth of 3642 m. Additional information about fission track annealing can be obtained from measurements of track size. Therefore we have also measured the progressive reduction in fission track lengths in apatite with increasing temperature in the various deep holes.
170
Байду номын сангаас
A . J . W . G L E A D O W and I. R. DUDDY Cretaceous and Tertiary marine deposits. Very similar early Cretaceous deposits occur in the Gippsland Basin to the east. The Otway Group consists dominantly of fluviatile lithic sandstones with interbedded mud. stones and minor coal seams. Most of the detritus in the sandstones is volcanogenic with abundanl rounded clasts of glassy volcanic rock fragments~ altered glass and fresh phenocrysts. The clasts were derived from volcanics of mildly alkaline or high-K Calc-aikaline intermediate composition containing apatite, sphene and occasional zircon crystals as accessories. The rocks show a depthzoned sequence of zeolite minerals up to laumontite grade. 3. RESULTS AND DISCUSSION
Minerals were dated using essentially the methods described by Gleadow and Lovering (1978) and Gleadow and Brooks (1979). All ages were measured by the external detector method 169
Nucl. Tracks, Vol. 5, Nos. 1/2, pp. 169--174, 1981. Printed in Great Britain
0191-278X/81/010169-06502.00/0 © 1981 Pergamon Press Ltd.
A NATURAL LONG-TERM TRACK ANNEALING EXPERIMENT FOR APATITE
and between 9 and 30 grains were counted for each mineral sample. To compare the ages of sphenes and apatites from outcrop samples a combined age was calculated as if all the individual mineral grains came from a single age source. For these combined ages errors were calculated by the approximate method of Johnson et al. (1979) and quoted as two standard deviations. Measurements of track length were made on fully-etched confined tracks (TINTS and TINCLES, Lal et al., 1969) and only horizontal tracks were used so as to avoid having to correct for the dip of each track. For each apatite 50 length measurements were made and averaged. The Otway Basin was formed during the early stages of continental rifting between Australia and Antarctica and marks the site of their later separation during the Eocene. The initial phase of sedimentation during the early Cretaceous produced the Otway Group, a 3 km thick sequence of non-marine sediments. The Otway Group now outcrops in two broad areas of the basin where it has clearly never been deeply buried. Elsewhere, such as in the area of the drill-holes, it is overlain by an essentially flat-lying cover of late
A. J. W. GLEADOWand I. R. DUDDY Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia
(Received 19 January 1981)
Abstract--Fission track ages of apatites from volcaniclastic sandstones in several deep drill-holes in southern Victoria, Australia, decrease from 120 Myr near the surface to zero near the bottom of the deepest holes. Track fading occurs between 60 and 1250C, a narrower temperature interval than predicted from laboratory annealing studies, but the 50% track-loss temperature (98°C) is very close to earlier predictions for the estimated heating time in the drill-holes of 10 to 40 Myr. Average track lengths, measured on confined tracks (TINTS, etc.), also decrease with increasing down-hole temperature. Track-length reduction relative to fresh, induced tracks was found in apatites from all depths and even outcrop samples which show no reduction in their fission track ages. 1. INTRODUCTION FISSION track dating of deep drill-hole samples (Naeser and Forbes, 1976; Naeser, 1979) has given important new information on the geological annealing of fission tracks in apatite. Such studies provide a vital test of previously predicted geological annealing temperatures based on extrapolated laboratory annealing data and show that apatite ages decrease to zero at depths of a few kilometres in the Earth's crust. In a 3000m hole drilled in a Mesozoic metamorphic complex near the Eielson Air Force Base in Alaska Naeser found a steady decrease in apatite age with depth from about 100 Myr near the surface to about 14Myr at the bottom. The bottom-hole temperature in the hole was 96°C and the apatite fission track ages extrapolate to zero at a temperature of 105°C. Slow uplift in this area since metamorphism means that apatites at the bottom of the hole have been at a temperature above about 100°C for more than 100 Myr. A similar decrease in apatite age with depth was found in a geothermal test hole drilled by Los Alamos Scientific Laboratory in Precambrian crystalline rocks on the flank of the Pleistocene Valles Caldera in New Mexico. The young age of the volcanic activity in this area means that the duration of heating in this hole was much shorter than at Eielson, about 1 Myr, and the bottom hole temperature is much higher (197°C at 2900 m). In this hole the apatite age decreased to zero at a higher temperature of 135°C as would be expected for a shorter heating time. It was not possible from either of these holes, however, to define the temperature at which track fading in apatite begins. We have now studied the variation in apatite ages with depth in several deep drill holes from sedimentary basins in southern Victoria, Australia, which show both the beginning and ending of track fading. The most important of these are three exploration wells in the Otway Basin, Flaxmans-1, Eumeralla-1 and Port Campbell-4, which are between 2.5 and 3.5 km deep. The present geothermal gradient in these holes is about 33°Ckm -~ giving maximum bottom-hole temperatures up to 125°C. A fourth hole, Wellington Park-l, from the nearby Gippsland Basin reaches a maximum temperature of 157°C at a depth of 3642 m. Additional information about fission track annealing can be obtained from measurements of track size. Therefore we have also measured the progressive reduction in fission track lengths in apatite with increasing temperature in the various deep holes.
170
Байду номын сангаас
A . J . W . G L E A D O W and I. R. DUDDY Cretaceous and Tertiary marine deposits. Very similar early Cretaceous deposits occur in the Gippsland Basin to the east. The Otway Group consists dominantly of fluviatile lithic sandstones with interbedded mud. stones and minor coal seams. Most of the detritus in the sandstones is volcanogenic with abundanl rounded clasts of glassy volcanic rock fragments~ altered glass and fresh phenocrysts. The clasts were derived from volcanics of mildly alkaline or high-K Calc-aikaline intermediate composition containing apatite, sphene and occasional zircon crystals as accessories. The rocks show a depthzoned sequence of zeolite minerals up to laumontite grade. 3. RESULTS AND DISCUSSION