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Reply to the comment made by Aubourg et al.
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Journal of Volcanology and Geothermal Research 122 (2003) 145^148 www.elsevier.com/locate/jvolgeores
Reply to the comment made by Aubourg et al. Emilio Herrero-Bervera a; , E. Can‹on-Tapia b , G.P.L. Walker c a
Paleomagnetics and Petrofabrics Laboratory, Hawaii Institute of Geophysics and Planetology (HIGP), University of Hawai’i at Manoa, 1680 East West Road, Honolulu, HI 96822, USA b Division de Ciencias de la Tierra, CICESE, P.O. Box 434843, San Diego, CA 92143, USA c Geology Department, University of Bristol, Bristol BS8 1JR, UK Received 4 March 2002; accepted 25 September 2002
We welcome the comment made by Aubourg et al. (2003) as it gives us the opportunity to make some corrections to our paper on the dikes from Skye (Herrero-Bervera et al., 2001). Due to unfortunate circumstances, some of the sites included in that paper were reported with a £ow direction inverted 180‡ relative to that originally interpreted. The three intrusives shown in ¢gure 2 of that paper belong to this group, as correctly pointed out by Aubourg et al. (2003). All of the intrusives that were reported with an inverted sense of £ow in Herrero-Bervera et al. (2001) are listed in Table 1 with the correct azimuth. Consequently, ¢gures 4a,b of Herrero-Bervera et al. (2001) also have to be modi¢ed. The corrected diagrams are shown in Fig. 1 of this reply. After the corrections have been made, we would like to note, ¢rst, that as the inversion of the £ow direction of some dikes was apparently the result of an aleatory error, after the correction is made neither dikes nor conesheets display a de¢nite £ow pattern. This can be observed on the corrected ¢gures (Fig. 1), which therefore leaves the main conclusions of our paper unchanged. Second, we would like to use this opportunity
* Corresponding author. E-mail address: [email protected] (E. Herrero-Bervera).
to make some comments about the utility of AMS as a magma £ow indicator and the di¡erent methods that can be used to this end. In order to infer £ow direction from our data, Aubourg et al. (2003) rely on the use of a geometric approach that does not seem to be di¡erent from that of the opposed imbrication of the maximum principal axis (Kmax ) laid out in the seminal paper by Knight and Walker (1988). Unfortunately, the details of their method are unknown to us because it has not been published. In any case, the most important aspect of the imbrication method is Table 1 Corrected data of the intrusives Intrusive no. Originally reported azimuth Corrected azimuth 01 04 09 12A 14 17A 21 22 24 25 25A 32 32A 34 40 47 51
220 121 196 199 144 026 355 276 216 290 076 279 309 146 230 269 079
0377-0273 / 03 / $ ^ see front matter C 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-0273(02)00468-7
VOLGEO 2554 10-2-03
040 301 016 019 324 206 175 096 036 110 246 099 129 326 050 089 259
146
E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 122 (2003) 145^148
Fig. 1. (a) Inferred £ow directions in dikes from the Cuillin Hills center. The arrows represent the inferred absolute magma-£ow directions based on paired groupings of the Kmax axes for dikes. The inset represents also the inferred absolute magma-£ow directions of the sites sampled in three di¡erent locations of the regional dike swarms. (b) Conesheet and sill complex inferred £ow directions from the Cuillin Hills and Blaven conesheet and sill complex. The arrows represent the inferred absolute magma-£ow directions based on paired groupings of Kmax axes for the intrusive conesheets and sills. Dashed arrows correspond to sills.
VOLGEO 2554 10-2-03
E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 122 (2003) 145^148
the relative position of the samples within the intrusive and the systematic change in orientation that is observed when traversing the intrusive across its width. The better de¢ned these changes are in orientation, the more reliable will be the inferred £ow direction, even with the absence of a formal statistical evaluation. It is irrelevant that the Kmax axes alone or the plane de¢ned by the Kmax ^Kint axes (also known as the plane of magnetic foliation) are used to identify this change in orientation. The important aspect is that some of the axes of principal susecptibility display this systematic change while others do not. This last point is most important to clarify because there are some instances where the Kmax axes not only may not display a clear opposed imbrication, but may be perpendicular to the plane of intrusion (Rochette et al., 1991). Clearly, under those circumstances, the utility of the method described by Knight and Walker (1988), and probably that used by Aubourg et al. (2003), will be hampered. Alternative approaches to overcome this limitation include the identi¢cation of systematic changes in the orientation of the three axes of susceptibility, regardless of their type, as a function of the relative position of the sample relative to the boundaries of the cooling unit and, alternatively, collection of a rather large number of samples close to the margins of the intrusive. The former approach has been developed based on observations made on lava £ows (Can‹on-Tapia et al., 1996, 1997; Can‹on-Tapia and Pinkerton, 2000; Can‹on-Tapia and Coe, 2002; HerreroBervera et al., 2002) and re£ects the physical aspects of the £ow (e.g. variations of rate of shear, £ow regime, time of e¡ective solidi¢cation relative to movement of magma, etc.). The latter method relies on a statistical approach (Tauxe et al., 1998) that, on focusing on the rims of the intrusion, overlooks some of the physical complexities that occur during the £ow of magma, such as the possibility of a change in the direction of £ow with time, or the e¡ect that the relaxation of shear resulting from a pause in the £ow might have on the orientation of the principal susceptibility axes. Ideally, both approaches should be used in every case. Unfortunately, depending on other aspects, such as logistical problems, sometimes it is
147
necessary to adopt a sampling scheme that may not satisfy the requirements of both methods simultaneously. In our case, the sampling scheme adopted was amenable to using the physical approach rather than the statistical approach, which also explains why no reference was made originally (Herrero-Bervera et al., 2001) to the paper by Tauxe et al. (1998). Finally, another aspect mentioned by Aubourg et al. (2003) concerns the possibility of the occurrence of pyrrhotite as a mineral phase present in the intrusives from Skye, which they consider an uncommon mineral. According to Dunlop and Ozdemir (1997), pyrrhotite is a common magnetic mineral in gabbros, where it occasionally carries a larger fraction of the nuclear remanent magnetization than magnetite, and occurs not uncommonly in other intrusive rocks. Although the Curie temperature of pyrrhotite (Fe7 S8 ) is 320‡C, as correctly pointed out by Aubourg et al. (2003), another characteristic of this mineral is that it decomposes to magnetite at temperatures around 500‡C, and to hematite at higher temperatures, so its presence in a rock together with other magnetic components, such as titanomagnetite, might result in an irreversible thermomagnetic curve, such as those observed on some of the intrusives from Skye. Certainly, knowledge of the low-temperature properties of these rocks would help to perfectly characterize their magnetic mineralogy, but this identi¢cation will have little in£uence on the interpretation of our AMS results. In summary, we would like to end this comment by repeating that, despite the regrettable error in some of the £ow directions as originally published, the main results of that study remain valid, and its implications for the understanding of volcanic plumbing remain the same.
References Aubourg, C., Geo¡roy, L., Callot, J.P., 2003. Comment on paper: Magnetic fabric and inferred £ow directions of dikes, conesheets and sill swarms, Isle of Skye Scotland, J. Volcanol. Geotherm Res., by E. Herrero-Bervera, G.P.L. Walker, E. Can‹on-Tapia and M.O. Garcia, J. Volcanol. Geotherm Res., this issue. doi: 10.1016/S0377-0273(02)00467-5 Can‹on-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1996.
VOLGEO 2554 10-2-03
148
E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 122 (2003) 145^148
The internal structure of lavas: insights from AMS measurements I: near vent ‘a’a. J. Volcanol. Geotherm. Res. 76, 19^ 46. Can‹on-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1997. The internal structure of lavas: insights from AMS measurements II: Hawaiian pahoehoe, toothpaste lava and ‘a’a. J. Volcanol. Geotherm. Res. 76, 19^46. Can‹on-Tapia, E., Pinkerton, H., 2000. The anisotropy of magnetic susceptibility of lava £ows: an experimental approach. J. Volcanol. Geotherm. Res. 98, 219^233. Can‹on-Tapia, E., Coe, R.S., 2002. Rock magnetic evidence of in£ation of a £ood basalt lava £ow. Bull. Volcanol. 64, 289^ 302. Dunlop, D., Ozdemir, O., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge University Press, Cambridge, 573 pp. Herrero-Bervera, E., Walker, G.P.L., Can‹on-Tapia, E., Garcia, M.O., 2001. Magnetic fabric and inferred £ow direction
of dikes, conesheets and sill swarms, Isle of Skye, Scotland. J. Volcanol. Geotherm. Res. 106, 195^210. Herrero-Bervera, E., Can‹on-Tapia, E., Walker, G.P.L., Tanaka, H., 2002. Magnetic fabric study and inferred £ow directions of lavas of the Old Pali Road, O’ahu Hawaii. J. Volcanol. Geotherm. Res. 118, 161^171. Knight, M.D., Walker, G.P.L., 1988. Magma £ow directions in dikes of the Koolau complex, Oahu determined from magnetic fabric studies. J. Geophys. Res. 93, 4301^4319. Rochette, P., Jenatton, L., Dupuy, C., Boudier, F., Reuber, J., 1991. Diabase dikes emplacement in the Oman Ophiolite: a magnetic fabric study with reference to geochemistry. In: Peters, T. (Ed.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere. Ministry of Petroleum and Minerals, Sultanate of Oman. Tauxe, L., Gee, J.S., Staudigel, H., 1998. Flow direction from anisotropy of magnetic susceptibility data: The bootstrap way. J. Geophys. Res. 103, 17775^17790.
VOLGEO 2554 10-2-03
Journal of Volcanology and Geothermal Research 122 (2003) 145^148 www.elsevier.com/locate/jvolgeores
Reply to the comment made by Aubourg et al. Emilio Herrero-Bervera a; , E. Can‹on-Tapia b , G.P.L. Walker c a
Paleomagnetics and Petrofabrics Laboratory, Hawaii Institute of Geophysics and Planetology (HIGP), University of Hawai’i at Manoa, 1680 East West Road, Honolulu, HI 96822, USA b Division de Ciencias de la Tierra, CICESE, P.O. Box 434843, San Diego, CA 92143, USA c Geology Department, University of Bristol, Bristol BS8 1JR, UK Received 4 March 2002; accepted 25 September 2002
We welcome the comment made by Aubourg et al. (2003) as it gives us the opportunity to make some corrections to our paper on the dikes from Skye (Herrero-Bervera et al., 2001). Due to unfortunate circumstances, some of the sites included in that paper were reported with a £ow direction inverted 180‡ relative to that originally interpreted. The three intrusives shown in ¢gure 2 of that paper belong to this group, as correctly pointed out by Aubourg et al. (2003). All of the intrusives that were reported with an inverted sense of £ow in Herrero-Bervera et al. (2001) are listed in Table 1 with the correct azimuth. Consequently, ¢gures 4a,b of Herrero-Bervera et al. (2001) also have to be modi¢ed. The corrected diagrams are shown in Fig. 1 of this reply. After the corrections have been made, we would like to note, ¢rst, that as the inversion of the £ow direction of some dikes was apparently the result of an aleatory error, after the correction is made neither dikes nor conesheets display a de¢nite £ow pattern. This can be observed on the corrected ¢gures (Fig. 1), which therefore leaves the main conclusions of our paper unchanged. Second, we would like to use this opportunity
* Corresponding author. E-mail address: [email protected] (E. Herrero-Bervera).
to make some comments about the utility of AMS as a magma £ow indicator and the di¡erent methods that can be used to this end. In order to infer £ow direction from our data, Aubourg et al. (2003) rely on the use of a geometric approach that does not seem to be di¡erent from that of the opposed imbrication of the maximum principal axis (Kmax ) laid out in the seminal paper by Knight and Walker (1988). Unfortunately, the details of their method are unknown to us because it has not been published. In any case, the most important aspect of the imbrication method is Table 1 Corrected data of the intrusives Intrusive no. Originally reported azimuth Corrected azimuth 01 04 09 12A 14 17A 21 22 24 25 25A 32 32A 34 40 47 51
220 121 196 199 144 026 355 276 216 290 076 279 309 146 230 269 079
0377-0273 / 03 / $ ^ see front matter C 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-0273(02)00468-7
VOLGEO 2554 10-2-03
040 301 016 019 324 206 175 096 036 110 246 099 129 326 050 089 259
146
E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 122 (2003) 145^148
Fig. 1. (a) Inferred £ow directions in dikes from the Cuillin Hills center. The arrows represent the inferred absolute magma-£ow directions based on paired groupings of the Kmax axes for dikes. The inset represents also the inferred absolute magma-£ow directions of the sites sampled in three di¡erent locations of the regional dike swarms. (b) Conesheet and sill complex inferred £ow directions from the Cuillin Hills and Blaven conesheet and sill complex. The arrows represent the inferred absolute magma-£ow directions based on paired groupings of Kmax axes for the intrusive conesheets and sills. Dashed arrows correspond to sills.
VOLGEO 2554 10-2-03
E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 122 (2003) 145^148
the relative position of the samples within the intrusive and the systematic change in orientation that is observed when traversing the intrusive across its width. The better de¢ned these changes are in orientation, the more reliable will be the inferred £ow direction, even with the absence of a formal statistical evaluation. It is irrelevant that the Kmax axes alone or the plane de¢ned by the Kmax ^Kint axes (also known as the plane of magnetic foliation) are used to identify this change in orientation. The important aspect is that some of the axes of principal susecptibility display this systematic change while others do not. This last point is most important to clarify because there are some instances where the Kmax axes not only may not display a clear opposed imbrication, but may be perpendicular to the plane of intrusion (Rochette et al., 1991). Clearly, under those circumstances, the utility of the method described by Knight and Walker (1988), and probably that used by Aubourg et al. (2003), will be hampered. Alternative approaches to overcome this limitation include the identi¢cation of systematic changes in the orientation of the three axes of susceptibility, regardless of their type, as a function of the relative position of the sample relative to the boundaries of the cooling unit and, alternatively, collection of a rather large number of samples close to the margins of the intrusive. The former approach has been developed based on observations made on lava £ows (Can‹on-Tapia et al., 1996, 1997; Can‹on-Tapia and Pinkerton, 2000; Can‹on-Tapia and Coe, 2002; HerreroBervera et al., 2002) and re£ects the physical aspects of the £ow (e.g. variations of rate of shear, £ow regime, time of e¡ective solidi¢cation relative to movement of magma, etc.). The latter method relies on a statistical approach (Tauxe et al., 1998) that, on focusing on the rims of the intrusion, overlooks some of the physical complexities that occur during the £ow of magma, such as the possibility of a change in the direction of £ow with time, or the e¡ect that the relaxation of shear resulting from a pause in the £ow might have on the orientation of the principal susceptibility axes. Ideally, both approaches should be used in every case. Unfortunately, depending on other aspects, such as logistical problems, sometimes it is
147
necessary to adopt a sampling scheme that may not satisfy the requirements of both methods simultaneously. In our case, the sampling scheme adopted was amenable to using the physical approach rather than the statistical approach, which also explains why no reference was made originally (Herrero-Bervera et al., 2001) to the paper by Tauxe et al. (1998). Finally, another aspect mentioned by Aubourg et al. (2003) concerns the possibility of the occurrence of pyrrhotite as a mineral phase present in the intrusives from Skye, which they consider an uncommon mineral. According to Dunlop and Ozdemir (1997), pyrrhotite is a common magnetic mineral in gabbros, where it occasionally carries a larger fraction of the nuclear remanent magnetization than magnetite, and occurs not uncommonly in other intrusive rocks. Although the Curie temperature of pyrrhotite (Fe7 S8 ) is 320‡C, as correctly pointed out by Aubourg et al. (2003), another characteristic of this mineral is that it decomposes to magnetite at temperatures around 500‡C, and to hematite at higher temperatures, so its presence in a rock together with other magnetic components, such as titanomagnetite, might result in an irreversible thermomagnetic curve, such as those observed on some of the intrusives from Skye. Certainly, knowledge of the low-temperature properties of these rocks would help to perfectly characterize their magnetic mineralogy, but this identi¢cation will have little in£uence on the interpretation of our AMS results. In summary, we would like to end this comment by repeating that, despite the regrettable error in some of the £ow directions as originally published, the main results of that study remain valid, and its implications for the understanding of volcanic plumbing remain the same.
References Aubourg, C., Geo¡roy, L., Callot, J.P., 2003. Comment on paper: Magnetic fabric and inferred £ow directions of dikes, conesheets and sill swarms, Isle of Skye Scotland, J. Volcanol. Geotherm Res., by E. Herrero-Bervera, G.P.L. Walker, E. Can‹on-Tapia and M.O. Garcia, J. Volcanol. Geotherm Res., this issue. doi: 10.1016/S0377-0273(02)00467-5 Can‹on-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1996.
VOLGEO 2554 10-2-03
148
E. Herrero-Bervera et al. / Journal of Volcanology and Geothermal Research 122 (2003) 145^148
The internal structure of lavas: insights from AMS measurements I: near vent ‘a’a. J. Volcanol. Geotherm. Res. 76, 19^ 46. Can‹on-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1997. The internal structure of lavas: insights from AMS measurements II: Hawaiian pahoehoe, toothpaste lava and ‘a’a. J. Volcanol. Geotherm. Res. 76, 19^46. Can‹on-Tapia, E., Pinkerton, H., 2000. The anisotropy of magnetic susceptibility of lava £ows: an experimental approach. J. Volcanol. Geotherm. Res. 98, 219^233. Can‹on-Tapia, E., Coe, R.S., 2002. Rock magnetic evidence of in£ation of a £ood basalt lava £ow. Bull. Volcanol. 64, 289^ 302. Dunlop, D., Ozdemir, O., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge University Press, Cambridge, 573 pp. Herrero-Bervera, E., Walker, G.P.L., Can‹on-Tapia, E., Garcia, M.O., 2001. Magnetic fabric and inferred £ow direction
of dikes, conesheets and sill swarms, Isle of Skye, Scotland. J. Volcanol. Geotherm. Res. 106, 195^210. Herrero-Bervera, E., Can‹on-Tapia, E., Walker, G.P.L., Tanaka, H., 2002. Magnetic fabric study and inferred £ow directions of lavas of the Old Pali Road, O’ahu Hawaii. J. Volcanol. Geotherm. Res. 118, 161^171. Knight, M.D., Walker, G.P.L., 1988. Magma £ow directions in dikes of the Koolau complex, Oahu determined from magnetic fabric studies. J. Geophys. Res. 93, 4301^4319. Rochette, P., Jenatton, L., Dupuy, C., Boudier, F., Reuber, J., 1991. Diabase dikes emplacement in the Oman Ophiolite: a magnetic fabric study with reference to geochemistry. In: Peters, T. (Ed.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere. Ministry of Petroleum and Minerals, Sultanate of Oman. Tauxe, L., Gee, J.S., Staudigel, H., 1998. Flow direction from anisotropy of magnetic susceptibility data: The bootstrap way. J. Geophys. Res. 103, 17775^17790.
VOLGEO 2554 10-2-03