PROSIDING TAHUNAN AHLI GEOFISIKA INDONESIA Pertemuan Ilmiah Tahunan ke-29, Yogyakarta 5-7 Oktober 2004 THE SUITABILITY OF ANDESITIC ROCKS FROM YOGYAKARTA FOR PALEOMAGNETIC STUDY La Ode Ngkoimani1,2), Satria Bijaksana2), The Houw Liong2) 1) Department of Physics, Haluoleo University, Kendari 2) Department of Physics, Institut Teknologi Bandung e-mail :
[email protected] Abstrak Kajian paleomagnetik sangat bergantung pada kualitas remanen magnetik yang tersimpan pada batuan. Kualitas remanen magnetik yang baik dicirikan dengan stabilitas remanen, baik intensitas maupun arah, terhadap demagnetisasi. Dalam batuan beku, remanen seperti ini biasanya dihasilkan oleh bulir-bulir halus magnetit. Sebagai bagian dari rekonstruksi paleomagnetik Pulau Jawa, kami telah menguji kesesuaian batuan andesit dari 14 situs yang diambil dari Daerah Istimewa Yogyakarta. Batuan-batuan ini sebagian besar berumur Paleogen-Neogen. Hanya satu situs yang berumur Kapur Akhir. Pengujian dilakukan dengan melakukan pengukuran paleomagnetik yang diikuti dengan pengukuran anisotropi magnetik berupa AMS (Anisotropy of Magnetic Susceptibility) dan AAS (Anisotropy of Anhysteretic Susceptibility). Untuk mengenali mineral magnetik yang dominan pada batuan tersebut, kami juga melakukan pengukuran IRM (Isothermal Remanent Magnetization). Hasil demagnetisasi AF (Alternating Field) menunjukkan bahwa NRM (Natural Remanent Magnetization) pada sebagian besar conto meluruh secara perlahan dan mempertahankan arahnya secara stabil. Pada sebagian kecil contoh, terdapat komponen sekunder yang dapat dihilangkan dengan mudah pada medan rendah. Arah karakteristik pada setiap situs yang ditentukan dengan perangkat lunak PMGSC ternyata cukup meyakinkan dengan nilai alpha-95 yang bervariasi antara 2.4 sampai 8.8 derajat dan nilai k berkisar antara 40 samapi dengan 249. Pengukuran IRM menunjukkan bahwa mineral magnetik yang dominan adalah magnetit sebagaimana ditunjukkan oleh medan saturasi yang rendah. Pengukuran anisotropi magnetik menunjukkan bahwa conto cukup isotropik sebagaimana ditunjukkan oleh nilai prosentase anisotropi yang rendah. Dengan memplot suseptibilitas DC dan suseptibilitas anhisteretik, kami berhasil menunjukkan bahwa bulir-bulir magnetit mempunyai kisaran ukuran antara 0.1 sampai dengan 200 mikrometer. Secara keseluruhan, hasil-hasil di atas menunjukkan bahwa batuan andesit dari Yogyakarta dapat digunakan secara meyakinkan dalam kajian paleomagnetik Kata kunci : Paleomagnetik, andesit, kelayakan, Yogyakarta Abstract Paleomagnetic study relies on the quality of magnetic remanence preserved in rocks. Good quality magnetic remanence is inferred from its stability both in intensity and in direction against demagnetization. In igneous rocks , this kind of remanence is generally carried by fine-grained magnetite. As a part of paleomagnetic reconstruction of Java, we have tested the suitability of andesitic rocks taken from 14 sites in Yogyakarta Special Territory. These rocks are mostly Paleogene-Neogene in age. One sites is, however, Late Cretaceous in age. The paleomagnetic suitability was carried out by carrying out a regular paleomagnetic measurement followed by measurements of magnetic anisotropy in the form of AMS (Anisotropy of Magnetic Susceptibility) and AAS (Anisotropy of Anhysteretic Susceptibility). To identify the predominant magnetic mineral in the rocks, we have also performed a measurement of IRM (Isothermal Remanent Magnetization). The result of AF (Alternating Field) demagnetization show that the NRM (Natural Remanent Magnetization) in most samples decayed slowly retaining their stable direction. In a few samples, we have identify a secondary component that can be erased easily in low demagnetization fields. The characteistic direction of magnetic remanence for each site determined using the PMGSC software is found to be reliable with alpha-95 varies from 3.1 to 8.8 degree and k-value from 40 to 317. IRM measurement shows that the predominant magnetic mineral is indeed magnetite as indicated by low saturation field. Measurement of magnetic anisotropy showed that the samples are quite isotropic as indicated by the low value of percent anisotropy. By ploting DC susceptibility and anhysteretic susceptibility, we are also able to show that the magnetite grains vary in size ranging from 0.1 to 200 micrometer. The above result indicate that the andesitic rocks from Yogyakarta can be used reliably for paleomagnetic study. Key words: Paleomagnetic, andesitic, suitability, Yogyakarta
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I.
Introduction Paleomagnetic is the study of past geomagnetic field within the framework of the geological time in intensity and direction. The study deals with measurement of magnetic records in rocks and its implication in tectonic reconstruction, magnetostratigraphy, and the bahavior of the Earth’s magnetic field. Magnetic recording in rocks, often referred to as magnetic remanence, is possible due to the presence of minute grains of magnetic minerals. Paleomagnetic study relies on the quality of magnetic remanence preserved in rocks. Good quality magnetic remanence is inferred from its stability both in intensity and in direction against demagnetization. In igneous rocks , this kind of remanence is generally carried by fine-grained magnetite. As a part of paleomagnetic reconstruction of Java, we have tested the suitability of andesitic rocks taken from 14 sites in Yogyakarta Special Territory. These rocks are mostly Paleogene-Neogene in age. One sites is, however, Late Cretaceous in age. II.
Methods Rock samples were collected at 14 locations (sites) all from sorrounding area of Yogyakarta Special Teritory. Paleomagnetic core samples were obtained using a MPPRD hand-drill (Magnetic Measurement Ltd., Lancashire, United Kingdom) with a water cooled diamond bit. The diameter of cores is 2.5 cm, and normally outcroping rocks are cored a depth varies from 5 – 15 cm. A Brunton compas is use for the direction orientation while Global Positioning System (GPS, Garmin etrex type) is use for geographic position of sites. The suitability of 134 standar core speciman was carried out by carrying out a regular paleomagnetic maesurement followed by, first, measurements of magnetic anisotropy in the form of AMS (Anisotropy of Magnetic Susceptibility) using a Bartington MS2 susceptibility meter with MS2B sensor (Bartington Instruments ltd., Oxford, United Kingdom). The AMS was measured by measuring in eight different direction and calculate the values of the principal susceptibily axes (χmax, χint, and χmin), by using MATLAB 5.2 software . The average suscptibility, χavg, and the percent degree of anisotropy susceptibility are defined as χavg = ( χ max + χ int + χ min ) / 3 and P = (( χ max / χ min ) − 1) ) x 100% recpectifully. Measurement of the natural remanent magnetization (NRM) of each sample using a Minispin magnetometer (Molspin Ltd., Newcastle upon Tyne, United Kingdom). The samples were then subjected to a stepwise alternating field demagnetization using a Molspin AF demagnetizer (Molspin Ltd., Newcastle upon Tyne, United Kingdom) with a peak field 100 mT. After each step of 2.5 mT, the remanence was measured again using Minispin. The process was repeated until the measured remanence was 10% of its oroginal NRM or less. The characteistic direction of magnetic remanence for each site was determined using the PMGSC software. To identify the predominant magnetic mineral in the rocks, we have also performed a measurement of IRM (Isothermal Remanent Magnetization). To invert the size distribution of magnetic grains, a number of samples were given ARM`(Anhysteretic of Remanent Magnetization) so that the anhysteretic susceptibility can be plotted as a function of DC susceptibility. The ARM was given in the presence of 0.1 mT direc (DC) field and the peak alternating (AC) field of 70 mT using a built-in system in the Molspin demagnetizer. III.
Results and Discussion The result of AF (Alternating Field) demagnetization show that the NRM (Natural Remanent Magnetization) in most samples decayed slowly retaining their stable direction. In a few samples, we have identify a secondary component that can be erased easily in low demagnetization fields. Figure 1 shows that the NRM decay curve and directional stability of typical sample (#SKP42).
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0.7 0.6
(a) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
0.5
(b)
0.4
NRM curve decay of #SKP42
0.3 0.2 0.1
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
0.0 -0.2
-0.1
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Horizont al Component Vert ical Component
Figure 1. (a) Intensity decay curves and (b) Zijderveld plot of progresive AF demagnetization for typical sample #SKP42. Scale in the orthogonal plots is in mA/m The characteristic direction of magnetic remanence for each site determined using the PMGSC software is found to be reliable with α-95 varies from 3.1 to 8.8 degree and k-value from 40 to 317. Figure 2 shows the stereoplot of paleopole position for typicaly samples (KSG and SKP) with low α-95 and high precision parameter, k.
(a)
N
(b)
N
α-95 = 4.2 k = 175
West
West
α-95 = 3.1 k = 317
Figure 2. Stereoplot of (a) #KSG site and (b) #SKP site
Figure 3 shows the results of IRM aquisition as a function of increasing magnetic field. As IRM saturates at relatively low field (about 0.1 T), the predominant of magnetic mineral in the samples is very likely to be magnetite (Fe3O4). The anhysteretic susceptibility versus DC susceptibility plot for a number of samples (Fig 4) shows that the magnetite grains in the samples are vary in size ranging from 0.1 to 200 micrometer.
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1.2
Relative Intensity
1.0 0.8
BDA WTA
0.6
PLP
0.4
KLB
0.2 0.0 0
100
200
300
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Magnetic Field (mT)
Figure 3. IRM Plot of selected samples
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Anhysteretic Susceptibility
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60 50 40
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B DA GSR
PRA PL P
PW H KLB
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Figure 4. Plot of anhysteretic susceptibility vs DC susceptibility for selected sample The Result of paleomagnetic analysis for all samples are summarized in table 1. Teble 1 also list the results of AMS measurement. Average of bulk magnetic susceptibility varies from 16.42 x 10-2 SI to 72.23 x 10-2 SI indicate that all samples are igneous rocks. The sampels showed are quite isotropic as indicated by the low value of percent anisotropy vary from 1.20 to 6.11.
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Table 1. Summary of paleomagnetic and AMS measurement
Site ID Location
Paleomagnetic Measurement
AMS Measurement χavg P% x 10-2 SI 35.21 2.98
Rock type
D (°)
I (°)
α95
k
N
PLP/West Progo
2.1
-37.0
4.7
139.0
12
GSR/West Progo GIJ/West Progo KLB/West Progo BDA/West Progo
192.7 317.1 336.7 350.9
-2.7 -23.0 -63.7 -25.9
7.1 5.1 8.8 6.0
61.7 119.2 40.7 85.5
10 8 11 9
36.74 46.39 48.36 49.23
1.20 2.86 2.04 3.38
andesitic lava andesitic neck andesitic lava andesitic lava
GPW/West Progo
-49.0 -33.1 -25.1 -37.0 37.8 -21.4
3.3 4.9 3.1 4.1 4.2 3.6
249.4 127.9 317.3 181.3 175.0 242.1
9 8 8 10 9 8
39.84 51.63 37.66 23.83 53.36 16.42
2.2 6.11 2.61 3.81 3.27 3.46
andesitic dyke
GGB/West Progo SKP/West Progo PWH/West Progo KSG/West Progo WTA/Kalasan
337.2 353.8 347.2 338.3 135.8 345.8
andesitic lava andesitic dyke andesitic lava andesitic lava andesitic pillow lava
WDR/Kidul Hills PRA/Bantul WDO/Kidul Hills
60.3 337.4 349.6
-65.4 6.6 -26.4
3.9 6.5 8.5
199.0 74.3 43.4
11 11 10
30.73 72.23 26.74
3.20 5.93 2.89
andesitic lava lava/dyke andesit andesitic lava
andesitic lava
IV. Conclussion 1. The andesitic rocks from around Yogyakarta Special Teritory are magnetically quite isotropic with low <10%. 2. Most of samples have stable remanent as indicated both by intensity slowly decayed and by the consistency of declination and inclination. 3. The predominant magnetic mineral in andesitic rocks from Yogyakarta Special Teritory is magnetite (Fe3O4) and the remanence carried by vary grain size in ranging 0.1 to 200 micrometer. 4. Andesitic rocks from around Yogyakarta Special Teritory can be used and suitable for paleomagnetic study in the region. Acknowledgement We would like to thank Harman Amir, Arif Budiman, Bernandus, Sandra, W. Purnama, and M. Aziz for their assistance during the stage of this study. The measurement was carried out in Rocks Magnetic Laboratory Physics Department ITB. This study is supported financially by the Riset Unggulan Terpadu XI from Ministry Research and Technology of Indonesia. References Barradile, G.J., M. Stupavsky, 1995, Anisotropy of magnetic susceptibility: Measurement scheme, Geophysics Researh Letters, vol.22, no. 15, pp. 1957-1960 Bijaksana, S., Mufit, F., Sudarningsih, Kasmiati, S., Rusli, M., 2000b, Preliminary result of paleomagnetic study on Early Miocene diorite from Trenggalek, East Java, Proc. of 18th National Physics Symposium, Serpong, pp. 9-15 Butler, R. F., 1992, paleomagnetism, Magnetic Domains to Geologic terranes, Blacwell Scientific Publication, Massacusetts. Dunlop, D. J., O. Ozdemir, 1997, Rock Magnetism : Fundamental and frontiers, Cambridge University Press. Soeria-Atmadja, R., et al., J. of Southeast Asia Earth Sciences, 1994, vol.9, pp.112, Tauxe, L., Paleomagnetic principles and practice, Kluwer Academic Publishers, Dordecht, 1998, 299pp King, J., Banerjee, S.K., Marvin, J., dan Özdemir, Ö., 1982, A Compari-son of Different Magnetic Methods for Determining the Relative Grain Size of Magnetite in Natural Materials: Some Results from Lake Sediments. Earth and Planetary Science Letter, 59, 404-419,
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