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Etching and annealing studies of fission tracks in zircon
Abstracts-6th International FTD Workshop
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with the CAl contours of Harris (1979) which indicate higher temperatures (deeper burial) in eastern Pennsylvania. In the central part of the study area, Maryland, northeastern West Virginia and northwestern Virginia, apatite fission-track apparent ages appear to delineate three zones. In the western section (West Virginia), the apatite fission-track ages range from 178 ± 20 to 246 ± 37 Ma, which corresponds to a zone of CAl values ofless than 3.0. There is a central zone (West Virginia and Maryland) where the apatite fission-track apparent ages vary from 110 ± 28 to 152 ± 12 Ma. This decreases to 95 ± 18 Ma in northwestern Virginia (easternmost sample). These relationships are consistent with the CAl values which increase eastward from 3.5 to a maximum of 4.0. In southwestern Virginia, the mean apatite fissIOn-track apparent age IS 182 ± 6 Ma for five samples. At the boundary between the central and southern sections of the study area, there are two anomalous apatite fission-track apparent ages, 33 ± 2 Ma at Gap Mills, West Virginia and 48 ± 6 Ma at Allegheny, Virginia. The cause of these apparent ages of Tertiary time is under investigation. The mean confined track length for the Pennsylvania Tioga samples (1100 tracks) is 13.5 ± 1.5 microns. Confined fission-track length distributions for the sediment and ash bed samples in the southern Appalachian samples range from 12.3 ± 1.9 microns in the east (Hayfield, VA) to 13.1 ± 1.5 microns in the west (Durbin, West VA). These mean track lengths are shortened from an initial track length of 16 microns. Exposure to temperatures in the range of 20-125'C during the last 200 Ma can produce the observed track lengths.
CORRECT IONS FOR ANISOTROPIC FISSION TRACK AGES IN MINERALS A S. SANDHU, R. C. RAMOLA, SURINDER SINGH and H. S. VIRK Department of Physics, Guru Nanak Dev University, Amritsar-143005, India
EXPERIMENTS have been performed to study the effect of crystallographic structure on fiSSIOn track (FT) age determination in some crystalline minerals viz., apatite, zircon, quartz and garnet. Among these minerals, the effect of anisotropic track etching and annealing on FT age is found to be a maximum in the case of apatite. The FT age in apatite is a maximum (83.1 ± 3.8 Ma) in a plane parallel to the c-axis, and a minimum (63.0 ± 2.4 Ma) in the perpendicular plane. The annealing augments the anisotropy of track revelation in the crystals. The correction in FT age due to the anisotropic etching and annealing in apatite as well as in other minerals is reported
ETCHING AND ANNEALING STUDIES OF FISSION TRACKS IN ZIRCON A. S. SANDHU, SURINDER SINGH and H. S. VIRK Department of Physics. Guru Nanak Dev University, Amntsar-143005, India
THE ETCHING studies of fission tracks on various planes of zircon are carried out using two different sets of etchants' (al I I mixture of H 2 S0 4 :HF and (b) 4:15:6 mixture of NaOH:KOH:LiOH·H 20. It is observed that the track etching anisotropy IS more in the basic medium (b) than that in the acidic medium (a). The most rapid etching occurs along the z-axis. The value of bulk etch rate, Va' varies from 0.075/lmh- 1 for (100) plane to 0.108/lmh- 1 for (001) plane using HF:H 2 S04 mixture as the track etchant. The correspondmg vanation of track etch rate, VT , is observed to be 1.17-0.88 /lm h -I. This leads to the different values of etching effiCiency on various planes of zircon. The fossil tracks are etched more isotropIcally than mduced tracks. The degree of anisotropy increases with the annealing of both fossil and induced tracks.
CENOZOIC BASIN HISTORIES DETERMINED FROM BASEMENT ROCKS IN NEW ZEALAND DIANE SEWARD* InstItute of Nuclear SCiences, DSIR, PrIvate Bag, Lower Hutt, New Zealand
CENOZOIC sedimentary basins of the West Coast of South Island, New Zealand were formed on downwarping granitic basement. Some of this basement is composed of Cretaceous intrusives. Other *Present address: Instltut fuer Kristallographie und Petrographie, ETH-Zentrum, CH-8092, Zuerich, Switzerland.
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with the CAl contours of Harris (1979) which indicate higher temperatures (deeper burial) in eastern Pennsylvania. In the central part of the study area, Maryland, northeastern West Virginia and northwestern Virginia, apatite fission-track apparent ages appear to delineate three zones. In the western section (West Virginia), the apatite fission-track ages range from 178 ± 20 to 246 ± 37 Ma, which corresponds to a zone of CAl values ofless than 3.0. There is a central zone (West Virginia and Maryland) where the apatite fission-track apparent ages vary from 110 ± 28 to 152 ± 12 Ma. This decreases to 95 ± 18 Ma in northwestern Virginia (easternmost sample). These relationships are consistent with the CAl values which increase eastward from 3.5 to a maximum of 4.0. In southwestern Virginia, the mean apatite fissIOn-track apparent age IS 182 ± 6 Ma for five samples. At the boundary between the central and southern sections of the study area, there are two anomalous apatite fission-track apparent ages, 33 ± 2 Ma at Gap Mills, West Virginia and 48 ± 6 Ma at Allegheny, Virginia. The cause of these apparent ages of Tertiary time is under investigation. The mean confined track length for the Pennsylvania Tioga samples (1100 tracks) is 13.5 ± 1.5 microns. Confined fission-track length distributions for the sediment and ash bed samples in the southern Appalachian samples range from 12.3 ± 1.9 microns in the east (Hayfield, VA) to 13.1 ± 1.5 microns in the west (Durbin, West VA). These mean track lengths are shortened from an initial track length of 16 microns. Exposure to temperatures in the range of 20-125'C during the last 200 Ma can produce the observed track lengths.
CORRECT IONS FOR ANISOTROPIC FISSION TRACK AGES IN MINERALS A S. SANDHU, R. C. RAMOLA, SURINDER SINGH and H. S. VIRK Department of Physics, Guru Nanak Dev University, Amritsar-143005, India
EXPERIMENTS have been performed to study the effect of crystallographic structure on fiSSIOn track (FT) age determination in some crystalline minerals viz., apatite, zircon, quartz and garnet. Among these minerals, the effect of anisotropic track etching and annealing on FT age is found to be a maximum in the case of apatite. The FT age in apatite is a maximum (83.1 ± 3.8 Ma) in a plane parallel to the c-axis, and a minimum (63.0 ± 2.4 Ma) in the perpendicular plane. The annealing augments the anisotropy of track revelation in the crystals. The correction in FT age due to the anisotropic etching and annealing in apatite as well as in other minerals is reported
ETCHING AND ANNEALING STUDIES OF FISSION TRACKS IN ZIRCON A. S. SANDHU, SURINDER SINGH and H. S. VIRK Department of Physics. Guru Nanak Dev University, Amntsar-143005, India
THE ETCHING studies of fission tracks on various planes of zircon are carried out using two different sets of etchants' (al I I mixture of H 2 S0 4 :HF and (b) 4:15:6 mixture of NaOH:KOH:LiOH·H 20. It is observed that the track etching anisotropy IS more in the basic medium (b) than that in the acidic medium (a). The most rapid etching occurs along the z-axis. The value of bulk etch rate, Va' varies from 0.075/lmh- 1 for (100) plane to 0.108/lmh- 1 for (001) plane using HF:H 2 S04 mixture as the track etchant. The correspondmg vanation of track etch rate, VT , is observed to be 1.17-0.88 /lm h -I. This leads to the different values of etching effiCiency on various planes of zircon. The fossil tracks are etched more isotropIcally than mduced tracks. The degree of anisotropy increases with the annealing of both fossil and induced tracks.
CENOZOIC BASIN HISTORIES DETERMINED FROM BASEMENT ROCKS IN NEW ZEALAND DIANE SEWARD* InstItute of Nuclear SCiences, DSIR, PrIvate Bag, Lower Hutt, New Zealand
CENOZOIC sedimentary basins of the West Coast of South Island, New Zealand were formed on downwarping granitic basement. Some of this basement is composed of Cretaceous intrusives. Other *Present address: Instltut fuer Kristallographie und Petrographie, ETH-Zentrum, CH-8092, Zuerich, Switzerland.