- Email: [email protected]
The slot technique for rock magnetic sampling
Physics of the Earth and Planetary Interiors, 78 (1993) 51—56 Elsevier Science Publishers B.V., Amsterdam
51
The slot technique for rock magnetic sampling Brooks B. Eliwood
*,a William D. MacDonald b John A. Wolff a Department of Geology, UTArlington, P.O. Box 19049, Arlington, 7X 76019, USA Department of Geological Sciences, State University of New York, Binghamton, NY 13902-6000, USA a
b
(Received 20 July 1992; revision accepted 4 January 1993)
ABSTRACT
We have developed a new technique for paleomagnetic sampling that allows one to sample extremely friable, poorly to moderately lithified sedimentary and volcanic materials. The method uses a portable electric drill and a dual blade masonry wheel (or two sharp steel cutting blades) with a 2 cm separation, which cuts two parallel slots in the material being sampled. A second set of cuts at right angles to the first produces a fixed, cubic pedestal around which a plastic sample box can be inserted, oriented and extracted. The method has allowed us to take samples at localities where we were not able to sample using other methods. To test the method we have sampled the Bandelier Tuff in northeast New Mexico and have measured the anisotropy of magnetic susceptibility (AMS) on boxes taken using the new method and on cylinders drilled at the same sites using standard techniques. Results show better AMS within-site precision for box samples taken using the new method. We conclude that the method is convenient, reliable and may be preferred in certain types of paleomagnetic studies.
1. Introduction: sampling procedures in paleomagnetism Several sampling methods are employed in paleomagnetic studies. The standard method now used involves sampling cores (approximately 2.5 cm diameter) which are drilled using portable gasoline-powered, modified chain saw motors and oriented using either a sun or Brunton compass. In the laboratory these cores are sliced into specimens approximately 2.2 cm long to conform to an optimum length-to-diameter ratio range of 0.82— 0.90 (Banerjee and Stacey, 1967; Noltimier, 1971). Hand samples are often collected where an orientation mark is placed on the sample before it is extracted from the outcrop (Girdler, 1967). This has been shown useful in rocks like shales which readily break along multiple bedding planes (Schieber and Ellwood, 1991). Hand samples are later sub-sampled in the laboratory using a fixed *
Corresponding author.
0031-9201/93/$06.00 © 1993
—
drill press. In moist, fine, unconsolidated sediments, samples are most often recovered using plastic sample boxes of various sizes and dimensions. These boxes are usually pushed into the sediments, but forceful sampling may deform the sample and adversely alter its magnetic properties (Gravenor et al., 1984). Friable or poorly lithified samples are often very difficult to sample and no standard method is employed, although a number of methods have been suggested. For example, it is possible to impregnate the rock before sampling (Bell, 1939; Galehouse, 1968), but this is often difficult or unsuccessful and is generally quite time consuming. Forcefully driving cylinders into the rock tends to disrupt the very small magnetic grains responsible for the magnetic properties in these samples (Gravenor et al., 1984; Lerbekmo, 1990). Alternatively, a cubic pedestal can be carved out of the sediment by hand and a plastic sample box inserted over the pedestal, oriented, extracted and capped.
Elsevier Science Publishers B.V. All rights reserved
52
2. The new sampling method When sampling friable, poorly to moderately lithified sediments and pyroclastic deposits, we have developed a method which uses either a dual blade masonry wheel (here cutting Austin Chalk samples; Fig. 1), or two sharp steel blades used to cut more friable materials such as the
B B. ELLWOOD ET AL
baked underclays in ancient hearths. Blades are separated by 2 cm and produce two parallel slots in the material being sampled. The first Cuts, typically normal to the rock face, are made along horizontal lines, so that the top edge of the box is horizontal. A second set of cuts at right angles to the first (Fig. 1) produces a fixed pillar of square cross-section around which a plastic sample box
Fig. 1. Masonry blades cutting Austin Chalk limestone samples, with slots 2 cm apart.
Fig. 2. Sun compass being used to orient a box sitting on a hearth sample pedestal (2 cm~).
53
THE SLOT TECHNIQUE FOR ROCK MAGNETIC SAMPLING
can be inserted, oriented and broken off for extraction (Fig. 2). The azimuth of an arrow along the top edge of the box and the dip angle of the top front face of the box are sufficient to orient the sample. The broken end can then be shaved flush with the end of the box, and the cap inserted on the end, resulting in a nearly cubic sample. The blades are driven using a portable electric drill. Relatively light-weight, rechargeable batteries allow large numbers of sites to be sampled daily.
3. Comparative test of the method We were confronted with the problem of extensively sampling the Bandelier Tuff near Los Alamos, New Mexico; however, but many of the sites were friable and extremely difficult to sample using standard methods. We developed the new sampling method to overcome the problem and designed an empirical experiment to test the method. The Bandelier Tuff was sampled at two sites separated by approximately 0.5 km. At each site we drilled cylindrical samples using a portable gasoline-powered, converted chain saw and sliced box pedestals with masonry blades using an electric drill motor. All samples were oriented using a Bruton compass and/or a Sun compass and returned to the laboratory for anisotropy of magnetic susceptibility (AMS) analysis.
4. The AMS method and analyses of tuffs AMS analyses of tuffs to indicate flow direction, mode of emplacement and source has been an area of active research during the last decade (e.g. Ellwood, 1982; Knight et al., 1986; Wolff et al., 1989; MacDonald and Palmer, 1990). The AMS for rock specimens is expressed as a series of coefficients (x~1)which linearly relate an inducing magnetic field (H1) to an induced magnetization (Mi), taking the form M
H
=
i
Xi3
‘
/
and therefore is a second-rank tensor (Nye, 1969). It is commonly expressed as a triaxial ellip-
soid with orthogonal principal K1 (maximum), K2 (intermediate) and K3 (minimum) axes, representing the corresponding susceptibility directions and magnitudes, respectively. In general, these axes reflect the physical and crystallographic orientation of magnetic elements in a single sample. In rock samples, the AMS ellipsoid has been shown to reflect the petrofabric orientation (e.g. Khan, 1962; Bhathal, 1971; Hrouda, 1982; and many others), which results from any process physically orienting elements within a sample, such as flow, gravitational forces present during deposition, compaction, crystal growth, diagenesis and tectonic stress. Basically, the AMS ellipsoid shape expresses either the preferred alignment of included magnetic grains, or the overall crystallographic mineral orientation in the sample. Common usage of AMS parameters as well as standardization and calibration of the method has been discussed elsewhere (Ellwood et al., 1988). The AMS of Bandelier Tuff samples was measured using two automated, low-field (less than 5 mT), torsion-fiber magnetometers modified after Stone (1963). (See King and Rees (1962), and Stone (1963, 1967), for an in-depth description of the ‘torque meter’ method.) Basically, the sample is suspended from a torsion fiber in the presence of an alternating magnetic field generated by a coil surrounding the sample, and it is free to rotate in the horizontal plane. Rotation is damped using an oil damping pan located below the sample. A mirror is fixed to the sample and rotations of the sample are recorded by a laser beam reflecting from the mirror to a recorder. Calibration of the instruments was accomplished using a machined, seamless copper ring (Noltimier, 1964), thus eliminating many of the errors associated with variations in several parameters including torsion-fiber constants, sample volume, applied magnetic fields, and light path length (King, 1967). Measurements were made in three sample orientations by rotating the magnetic field-generating coil around the sample at 22.5°increments where deflections of the laser beam are recorded. A Fourier analysis of the deflection data allows identification of the important Fourier terms and isolation of the sin 29 term (second harmonic) for
54
B.B. ELLWOOD ET AL
each sample position. The second harmonic term is then used in calculating the AMS, thus elimi-
and A~are the eigenvalues of the mean AMS ellipsoid. C. is somewhat analogous to an a95
nating other Fourier terms shown to be sources of error (King, 1967). Initial susceptibility was measured using a bridge calibrated with standard salts,
(Fisher, 1953) for vectors, but rather gives an estimate of angular dispersion for AMS axial clusters. S1 is relatively insensitive to the number of samples used in calculating the mean direction and ranges from 1.0, for a perfect cluster, to 0.33, a random distribution of axes.
5. Statistical procedures Statistical procedures used in calculating mean ellipsoid azimuths and magnitudes are those reported by Schmidt et al. (1988) which were in turn based on the method of Mark (1973). The method involves summing all individual eigenvectors and eigenvalues. Within-site scatter is represented by the eigenvalues of the mean AMS ellipsoid. The parameter C1 is used here to characterize the angular departure, and C1
=
cos~(S1)
where S1 = A~/(A~+ “2
+
‘~‘3)
Cylinders K1D = 305.3
~
K~CI 367
__
6. Results of AMS measurement AMS data are conventionally presented using equal area plots like those for Bandelier cylinder and box samples presented in Fig. 3. Site DR 15 is an example of devitrified Bandelier Tuff, whereas site DR 3 represents typical vitric, nonwelded material. Comparisons of AMS data from cylindrical samples (top in Fig. 3) versus box
(2)
samples (bottom in Fig. 3) show that K1 axial clusters are significantly better for box samples
(3)
(K1C1 Fig. 3).sites As DR expected, similar in between 3 and K1 DR azimuths 15 (owingare to a close site proximity). There is also a close
)
K1D = 201.6 KiI,4.7 K~Ci 40.1
~I..
DR 15 - Devitritied
DR 3- Vitric Non-welded
Boxes
C
)ic~
DR 15- Devitrlfied
K1D309.0 K11 = 19.9 0.9
~
)K1CI
K1D=298.7(29 K11 22.5(19.6) = 16.9 (36
DR 3- VitrIc Non-welded
Fig. 3. Bandelier Tuff cylinders and boxes for which between-site and within-site comparisons have been made. K1D the mean AMS long axis orientation in the sample; K1C1 is the mean axial dispersion for the site.
and
K1!
are
55
THE SLOT TECHNIQUE FOR ROCK MAGNETIC SAMPLING
correspondence in directions between the cylinders and boxes from the devitrified site, DR 15. The large deviation observed for cylindrical sampies from DR 3 appears to result from sampling errors. Drilling such friable materials tends to abrade them, producing smaller diameter cores, thus making orientations more difficult. In these very inhomogeneous materials, hard grains (lithics, quartz and sanidine phenocrysts) tend to break loose and circulate inside the drill, ‘reaming’ the core and often breaking the core prematurely in a wedging effect. In the saw technique, such particles are swept immediately from the cut, do not circulate, and do not abrade the specimen unduly or break it prematurely. The technique presented here has many advantages and a few disadvantages. This technique is not appropriate for harder materials, but only for relatively soft and friable materials. It has been tested on and works well in ashflow tuffs, chalk, and baked soil or ‘underclay’; it is probably also well suited for soft sandstone, siltstone, till, mudstone, some shales, mans, coals and other materials of similar friability and consolidation, In such materials, the new method is superior to conventional methods (e.g. drilling or hand-carving) because it is faster, produces specimens of better shape, allows more precision in the orientation, and is accompanied by less waste (broken specimens). Moreover, serial sampling, i.e. sideby-side in horizontal or inclined series, is made both easier and more accurate. The equipment required (battery-powered motor, arbor, masonry blade or equivalent, recharger) is lightweight, readily available, compact and inexpensive. This eases the sampling of more remote sites and expands the scientific scope. As there is no need for water or gasoline, the scarcity (also weight and bulk) of such commodities is of less consequence. These advantages are only partially offset by attendant negatives. The batteries require recharging, and this requires typically several batteries and nightly access to electricity and an a.c. charger or d.c. equivalent. (Our experience has been that three batteries are sufficient for a long day’s drilling.) Although no water is required, in some materials a great deal of dust is created. -
.
.
Eye protection and a respirator filter are essential, especially in non-welded tuffs. The electric motors are not so loud as gasoline drill motors; nevertheless, hearing protection is recommended for the operator. One disadvantage is the difficulty in performing thermal demagnetization on samples recovered using this technique. It is necessary first to remove the sample from the plastic box before heating. For very friable samples, this can be a difficult problem, and after heating samples may swell and no longer fit back into the box.
7. Conclusions This work presents a new method for obtaining paleomagnetic samples. It is clear that when friable materials are being sampled, box samples obtained using this method tend to provide resuits which give greater within-site (intrasample) precision than do drilled cylinders. A wide variety of materials can be sampled in this way, and sample disruption appears to be quite minor.
Acknowledgments Funding was provided by NSF Grant EAR 9105981 to Wolff and Ellwood. We would like to thank Suzanne Ellwood and Marty Horn for their help in sampling the Bandelier Tuff.
References Baneijee, 5K. and Stacey, F.D., 1967. The high field torquemeter method of measuring magnetic anisotropy in rocks. In: D.W. Collinson, K.M. Creer and S.K. Runcorn (Editors), Methods in Paleomagnetism. Elsevier, Amsterdam, Bell J.F., 1939. Notes on the use of methyl methacrylate ‘lucite’ in a geological laboratory. Econ. Geol., 34: 804—811. Bhathal, R.S., 1971. Magnetic anisotropy in rocks. Earth-Sci. Rev., 7: 227—253. Ellwood, B.B., 1982. Estimates of flow direction for calc-alkaline welded tuffs and paleomagnetic reliability from anisotropy of magnetic susceptibility measurements: central San Juan Mountains, southwest Colorado. Earth Planet. Sci. Lett., 59: 303—314.
56 Ellwood, B.B., Hrouda, F. and Wagner, J.-J., 1988. Symposia on magnetic fabrics: introductory comments. Phys. Earth Planet. Inter., 51: 249—252. Fisher, R.A., 1953. Dispersion on a sphere. Proc. R. Soc. London, Ser. A, 217: 295—305. Galehouse, J.A., 1968. Anisotropy of magnetic susceptibility as a paleocurrent indicator: a test of the method. Geol. Soc. Am. Bull., 79: 387—390. Girdler, R.W., 1967. The collection and orientation of rock samples. In: D.W. Collinson, KM. Creer and S.K. Runcorn (Editors), Methods in Paleomagnetism, Elsevier, Amsterdam, pp. 8—10. Gravenor, C.P., Symons, D.T.A. and Coyle, D.A., 1984. Errors in the anisotropy of magnetic susceptibility and magnetic remanence of unconsolidated sediments produced by sampling methods. Geophys. Res. Lett., 11: 836—839. Hrouda, F., 1982. Magnetic anisotropy of rocks and its application in geology and geophysics, Geophys. Surv., 5: 37—82. Khan, MA., 1962. The anisotropy of magnetic susceptibility of some igneous and metamorphic rocks. J. Geophys. Res., 67: 2873—2885. King, R.F., 1967. Errors in anisotropy measurements with the torsion balance. In: D.W. Collinson, K.M. Creer and S.K. Runcorn (Editors), Methods in Paleomagnetism, Elsevier, Amsterdam, pp. 387—398. King, R.F. and Rees, Al., 1962. The measurement of the anisotropy of magnetic susceptibility of rocks by the torque method, J. Geophys. Res., 67: 1565—1572. Knight, M.D., Walker, G.P.L., Ellwood, B.B. and Diehl, J.F., 1986. Stratigraphy, paleomagnetism and magnetic fabric of the Toba Tuffs: constraints on the source and eruptive styles, J. Geophys. Res., 91: 10355—10382. Lerbekmo, J.F., 1990. A tool for obtaining oriented samples of weakly to moderately indurated sedimentary rocks for paleomagnetic measurements. Sediment. Geol., 66: 295— 299.
B.B. ELLWOOD ET AL.
MacDonald, W.D. and Palmer, H.C., 1990. Flow directions in ash-flow tuffs: a comparison of geological and magnetic susceptibility measurements, Tshirege member (upper Bandelier Tuff), Valles caldera, New Mexico, USA. Bull. Volcanol., 53: 45—59. Mark, D. 1973. Analysis of axial orientation data including till fabrics. Geol. Soc. Am. Bull., 84: 1369—1374. Noltimier, H.C., 1964. Calibration of a spinner magnetometer with a wire loop. J Sci. Instrum., 41: 55. Noltimier, H.C., 1971. A model for grain dispersion and magnetic anisotropy in sedimentary rocks. J. Geophys. Res., 76: 3990—4002. Nye, J.F., 1969. Physical Properties of Crystals. Oxford University Press, London, 322 pp. Schieber, J. and Ellwood, B.B., 1991, Application of Anisotropy of magnetic susceptibility (AMS) to paleocurrent studies in several shale basins. EOS (Am. Geophys. Union) 137: (abstract). Schmidt, V.A., Ellwood, B.B., Nagata, T. and Noltimier, H.C., 1988. The measurement of anisotropy of magnetic susceptibility using a cryogenic (SQUID) magnetometer and a comparison with results obtained from a torsion-fiber magnetometer. Phys. Earth Planet. Inter., 51: 365—378. Stone, D.B., 1963. Anisotropic magnetic susceptibility measurements on a phonolite and on a folded metamorphic rock. Geophys. J. R. Astron. Soc., 7: 375—390. Stone, D.B., 1967. Torsion-balance method of measuring anisotropic susceptibility. In: D.W. Collinson, K.M. Creer and S.K. Runcorn (Editors), Methods in Paleomagnetism. Elsevier, Amsterdam, pp. 381—386. Wolff, iA., Ellwood, B.B. and Sachs, S.D., 1989. Anisotropy of magnetic susceptibility in welded tufts: application to a welded-tuft dike in the Tertiary Trans-Pecos Texas volcanic province, USA. Bull. Volcanol., 51: 299—310.
51
The slot technique for rock magnetic sampling Brooks B. Eliwood
*,a William D. MacDonald b John A. Wolff a Department of Geology, UTArlington, P.O. Box 19049, Arlington, 7X 76019, USA Department of Geological Sciences, State University of New York, Binghamton, NY 13902-6000, USA a
b
(Received 20 July 1992; revision accepted 4 January 1993)
ABSTRACT
We have developed a new technique for paleomagnetic sampling that allows one to sample extremely friable, poorly to moderately lithified sedimentary and volcanic materials. The method uses a portable electric drill and a dual blade masonry wheel (or two sharp steel cutting blades) with a 2 cm separation, which cuts two parallel slots in the material being sampled. A second set of cuts at right angles to the first produces a fixed, cubic pedestal around which a plastic sample box can be inserted, oriented and extracted. The method has allowed us to take samples at localities where we were not able to sample using other methods. To test the method we have sampled the Bandelier Tuff in northeast New Mexico and have measured the anisotropy of magnetic susceptibility (AMS) on boxes taken using the new method and on cylinders drilled at the same sites using standard techniques. Results show better AMS within-site precision for box samples taken using the new method. We conclude that the method is convenient, reliable and may be preferred in certain types of paleomagnetic studies.
1. Introduction: sampling procedures in paleomagnetism Several sampling methods are employed in paleomagnetic studies. The standard method now used involves sampling cores (approximately 2.5 cm diameter) which are drilled using portable gasoline-powered, modified chain saw motors and oriented using either a sun or Brunton compass. In the laboratory these cores are sliced into specimens approximately 2.2 cm long to conform to an optimum length-to-diameter ratio range of 0.82— 0.90 (Banerjee and Stacey, 1967; Noltimier, 1971). Hand samples are often collected where an orientation mark is placed on the sample before it is extracted from the outcrop (Girdler, 1967). This has been shown useful in rocks like shales which readily break along multiple bedding planes (Schieber and Ellwood, 1991). Hand samples are later sub-sampled in the laboratory using a fixed *
Corresponding author.
0031-9201/93/$06.00 © 1993
—
drill press. In moist, fine, unconsolidated sediments, samples are most often recovered using plastic sample boxes of various sizes and dimensions. These boxes are usually pushed into the sediments, but forceful sampling may deform the sample and adversely alter its magnetic properties (Gravenor et al., 1984). Friable or poorly lithified samples are often very difficult to sample and no standard method is employed, although a number of methods have been suggested. For example, it is possible to impregnate the rock before sampling (Bell, 1939; Galehouse, 1968), but this is often difficult or unsuccessful and is generally quite time consuming. Forcefully driving cylinders into the rock tends to disrupt the very small magnetic grains responsible for the magnetic properties in these samples (Gravenor et al., 1984; Lerbekmo, 1990). Alternatively, a cubic pedestal can be carved out of the sediment by hand and a plastic sample box inserted over the pedestal, oriented, extracted and capped.
Elsevier Science Publishers B.V. All rights reserved
52
2. The new sampling method When sampling friable, poorly to moderately lithified sediments and pyroclastic deposits, we have developed a method which uses either a dual blade masonry wheel (here cutting Austin Chalk samples; Fig. 1), or two sharp steel blades used to cut more friable materials such as the
B B. ELLWOOD ET AL
baked underclays in ancient hearths. Blades are separated by 2 cm and produce two parallel slots in the material being sampled. The first Cuts, typically normal to the rock face, are made along horizontal lines, so that the top edge of the box is horizontal. A second set of cuts at right angles to the first (Fig. 1) produces a fixed pillar of square cross-section around which a plastic sample box
Fig. 1. Masonry blades cutting Austin Chalk limestone samples, with slots 2 cm apart.
Fig. 2. Sun compass being used to orient a box sitting on a hearth sample pedestal (2 cm~).
53
THE SLOT TECHNIQUE FOR ROCK MAGNETIC SAMPLING
can be inserted, oriented and broken off for extraction (Fig. 2). The azimuth of an arrow along the top edge of the box and the dip angle of the top front face of the box are sufficient to orient the sample. The broken end can then be shaved flush with the end of the box, and the cap inserted on the end, resulting in a nearly cubic sample. The blades are driven using a portable electric drill. Relatively light-weight, rechargeable batteries allow large numbers of sites to be sampled daily.
3. Comparative test of the method We were confronted with the problem of extensively sampling the Bandelier Tuff near Los Alamos, New Mexico; however, but many of the sites were friable and extremely difficult to sample using standard methods. We developed the new sampling method to overcome the problem and designed an empirical experiment to test the method. The Bandelier Tuff was sampled at two sites separated by approximately 0.5 km. At each site we drilled cylindrical samples using a portable gasoline-powered, converted chain saw and sliced box pedestals with masonry blades using an electric drill motor. All samples were oriented using a Bruton compass and/or a Sun compass and returned to the laboratory for anisotropy of magnetic susceptibility (AMS) analysis.
4. The AMS method and analyses of tuffs AMS analyses of tuffs to indicate flow direction, mode of emplacement and source has been an area of active research during the last decade (e.g. Ellwood, 1982; Knight et al., 1986; Wolff et al., 1989; MacDonald and Palmer, 1990). The AMS for rock specimens is expressed as a series of coefficients (x~1)which linearly relate an inducing magnetic field (H1) to an induced magnetization (Mi), taking the form M
H
=
i
Xi3
‘
/
and therefore is a second-rank tensor (Nye, 1969). It is commonly expressed as a triaxial ellip-
soid with orthogonal principal K1 (maximum), K2 (intermediate) and K3 (minimum) axes, representing the corresponding susceptibility directions and magnitudes, respectively. In general, these axes reflect the physical and crystallographic orientation of magnetic elements in a single sample. In rock samples, the AMS ellipsoid has been shown to reflect the petrofabric orientation (e.g. Khan, 1962; Bhathal, 1971; Hrouda, 1982; and many others), which results from any process physically orienting elements within a sample, such as flow, gravitational forces present during deposition, compaction, crystal growth, diagenesis and tectonic stress. Basically, the AMS ellipsoid shape expresses either the preferred alignment of included magnetic grains, or the overall crystallographic mineral orientation in the sample. Common usage of AMS parameters as well as standardization and calibration of the method has been discussed elsewhere (Ellwood et al., 1988). The AMS of Bandelier Tuff samples was measured using two automated, low-field (less than 5 mT), torsion-fiber magnetometers modified after Stone (1963). (See King and Rees (1962), and Stone (1963, 1967), for an in-depth description of the ‘torque meter’ method.) Basically, the sample is suspended from a torsion fiber in the presence of an alternating magnetic field generated by a coil surrounding the sample, and it is free to rotate in the horizontal plane. Rotation is damped using an oil damping pan located below the sample. A mirror is fixed to the sample and rotations of the sample are recorded by a laser beam reflecting from the mirror to a recorder. Calibration of the instruments was accomplished using a machined, seamless copper ring (Noltimier, 1964), thus eliminating many of the errors associated with variations in several parameters including torsion-fiber constants, sample volume, applied magnetic fields, and light path length (King, 1967). Measurements were made in three sample orientations by rotating the magnetic field-generating coil around the sample at 22.5°increments where deflections of the laser beam are recorded. A Fourier analysis of the deflection data allows identification of the important Fourier terms and isolation of the sin 29 term (second harmonic) for
54
B.B. ELLWOOD ET AL
each sample position. The second harmonic term is then used in calculating the AMS, thus elimi-
and A~are the eigenvalues of the mean AMS ellipsoid. C. is somewhat analogous to an a95
nating other Fourier terms shown to be sources of error (King, 1967). Initial susceptibility was measured using a bridge calibrated with standard salts,
(Fisher, 1953) for vectors, but rather gives an estimate of angular dispersion for AMS axial clusters. S1 is relatively insensitive to the number of samples used in calculating the mean direction and ranges from 1.0, for a perfect cluster, to 0.33, a random distribution of axes.
5. Statistical procedures Statistical procedures used in calculating mean ellipsoid azimuths and magnitudes are those reported by Schmidt et al. (1988) which were in turn based on the method of Mark (1973). The method involves summing all individual eigenvectors and eigenvalues. Within-site scatter is represented by the eigenvalues of the mean AMS ellipsoid. The parameter C1 is used here to characterize the angular departure, and C1
=
cos~(S1)
where S1 = A~/(A~+ “2
+
‘~‘3)
Cylinders K1D = 305.3
~
K~CI 367
__
6. Results of AMS measurement AMS data are conventionally presented using equal area plots like those for Bandelier cylinder and box samples presented in Fig. 3. Site DR 15 is an example of devitrified Bandelier Tuff, whereas site DR 3 represents typical vitric, nonwelded material. Comparisons of AMS data from cylindrical samples (top in Fig. 3) versus box
(2)
samples (bottom in Fig. 3) show that K1 axial clusters are significantly better for box samples
(3)
(K1C1 Fig. 3).sites As DR expected, similar in between 3 and K1 DR azimuths 15 (owingare to a close site proximity). There is also a close
)
K1D = 201.6 KiI,4.7 K~Ci 40.1
~I..
DR 15 - Devitritied
DR 3- Vitric Non-welded
Boxes
C
)ic~
DR 15- Devitrlfied
K1D309.0 K11 = 19.9 0.9
~
)K1CI
K1D=298.7(29 K11 22.5(19.6) = 16.9 (36
DR 3- VitrIc Non-welded
Fig. 3. Bandelier Tuff cylinders and boxes for which between-site and within-site comparisons have been made. K1D the mean AMS long axis orientation in the sample; K1C1 is the mean axial dispersion for the site.
and
K1!
are
55
THE SLOT TECHNIQUE FOR ROCK MAGNETIC SAMPLING
correspondence in directions between the cylinders and boxes from the devitrified site, DR 15. The large deviation observed for cylindrical sampies from DR 3 appears to result from sampling errors. Drilling such friable materials tends to abrade them, producing smaller diameter cores, thus making orientations more difficult. In these very inhomogeneous materials, hard grains (lithics, quartz and sanidine phenocrysts) tend to break loose and circulate inside the drill, ‘reaming’ the core and often breaking the core prematurely in a wedging effect. In the saw technique, such particles are swept immediately from the cut, do not circulate, and do not abrade the specimen unduly or break it prematurely. The technique presented here has many advantages and a few disadvantages. This technique is not appropriate for harder materials, but only for relatively soft and friable materials. It has been tested on and works well in ashflow tuffs, chalk, and baked soil or ‘underclay’; it is probably also well suited for soft sandstone, siltstone, till, mudstone, some shales, mans, coals and other materials of similar friability and consolidation, In such materials, the new method is superior to conventional methods (e.g. drilling or hand-carving) because it is faster, produces specimens of better shape, allows more precision in the orientation, and is accompanied by less waste (broken specimens). Moreover, serial sampling, i.e. sideby-side in horizontal or inclined series, is made both easier and more accurate. The equipment required (battery-powered motor, arbor, masonry blade or equivalent, recharger) is lightweight, readily available, compact and inexpensive. This eases the sampling of more remote sites and expands the scientific scope. As there is no need for water or gasoline, the scarcity (also weight and bulk) of such commodities is of less consequence. These advantages are only partially offset by attendant negatives. The batteries require recharging, and this requires typically several batteries and nightly access to electricity and an a.c. charger or d.c. equivalent. (Our experience has been that three batteries are sufficient for a long day’s drilling.) Although no water is required, in some materials a great deal of dust is created. -
.
.
Eye protection and a respirator filter are essential, especially in non-welded tuffs. The electric motors are not so loud as gasoline drill motors; nevertheless, hearing protection is recommended for the operator. One disadvantage is the difficulty in performing thermal demagnetization on samples recovered using this technique. It is necessary first to remove the sample from the plastic box before heating. For very friable samples, this can be a difficult problem, and after heating samples may swell and no longer fit back into the box.
7. Conclusions This work presents a new method for obtaining paleomagnetic samples. It is clear that when friable materials are being sampled, box samples obtained using this method tend to provide resuits which give greater within-site (intrasample) precision than do drilled cylinders. A wide variety of materials can be sampled in this way, and sample disruption appears to be quite minor.
Acknowledgments Funding was provided by NSF Grant EAR 9105981 to Wolff and Ellwood. We would like to thank Suzanne Ellwood and Marty Horn for their help in sampling the Bandelier Tuff.
References Baneijee, 5K. and Stacey, F.D., 1967. The high field torquemeter method of measuring magnetic anisotropy in rocks. In: D.W. Collinson, K.M. Creer and S.K. Runcorn (Editors), Methods in Paleomagnetism. Elsevier, Amsterdam, Bell J.F., 1939. Notes on the use of methyl methacrylate ‘lucite’ in a geological laboratory. Econ. Geol., 34: 804—811. Bhathal, R.S., 1971. Magnetic anisotropy in rocks. Earth-Sci. Rev., 7: 227—253. Ellwood, B.B., 1982. Estimates of flow direction for calc-alkaline welded tuffs and paleomagnetic reliability from anisotropy of magnetic susceptibility measurements: central San Juan Mountains, southwest Colorado. Earth Planet. Sci. Lett., 59: 303—314.
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