The Identification of the Fall-Units of the Tokyo Pumice Bed by Chemical Composition of Ferromagnetic Minerals

Geochemical Journal Vol. 19, pp. 193 to 198, 1985

The identification of the fall-units of the Tokyo pumice bed by chemical composition of ferromagnetic minerals

RYUJI AOYAGI

Miyazaki Junior High School, Miyamae-ku, Kawasaki 213, Japan

(Received October 2, 1984: Accepted March 26, 1985)

Samples of the Tokyo pumice bed were collected from 13 localities. Ferromagnetic minerals contained in the samples were all identified as titanomagnetite by X-ray diffraction method. Ti02 and V203 contents of titanomagnetite provide information useful for identifying each fall-unit of the Tokyo pumice bed. The bed at the reference outcrop was divided in 10 fall-units on the basis of visual observation of stratified structure and chemical composition of titanomagnetite. The presence of two additional fall-units was also confirmed by comparative study of samples from different localities. The area covered by a given fall-unit was bounded by using Ti02 content of titanomagnetite separated from each sample.

firmed the source and the distribution area of INTRODUCTION the bed. Hakamada (1972) divided it into 8 fall Hakone volcano, a representative one in the units on the basis of the field study at Endo-hara central Japan, was investigated from petro of Ooiso Hill, Kanagawa Prefecture. He also graphical and volcanological points of view by examined SiO2 content and heavy mineral as Kuno (1950, 1952). The activity of the volcano semblage of these fall-units, concluding that is considered to data back to the middle each fall-unit has different petrographic charac Pleistocene. The volcano erupted lava and teristics. Harada (1976), however, claimed from scoria, which formed the body of the mountain, the outcrop observation at many localities that and more than 100 pumice beds extend to the the bed consists of at least 5 fall-units. east. It continues to emit volcanic gases from The present author investigated chemical its central cone. composition of ferromagnetic minerals con The Tokyo pumice bed, one of air fall tained in marker-tephra, which are distributed pumice beds, was recognized as a single cycle of over the Japanese Islands (Aoyagi, 1970, 1983, eruptions (Nakamura et al., 1963), the age of 1985). This study is intended mainly to identify which was determined to be 49,000 ± 5,000YBP the fall-units of the Tokyo pumice bed on the by the fission-track method (Machida and basis of chemical composition of ferromagnetic Suzuki, 1971). The erupted volume of the bed minerals as well as field observation. is estimated to be 4.8 km3 by Machida and Moriyama (1968). This is the largest air fall EXPERIMENTALPROCEDURES pumice bed derived from Hakone Volcano. The thickness of the bed distributed in the Method of sampling vicinity of Tokyo, about 80km away from the The samples were collected from 13 locali source, varies from 5 to 20 cm. It has been ties in the southern Kanto district (Fig. 1). The noticed for a long time as the most important grain size of pumice decreases usually upward marker-tephra of Kanto loam and studied geo within each fall-unit. graphically and petrographically by many The number of samples taken from each fall workers. Machida and Moriyama (1968) con unit depended on its thickness: one sample

193 194 R. Aoyagi

from a fall-unit thinner than 5 cm, two from that samples were sieved out in running water to 5 to 15 cm thick, each from the upper and lower eliminate clayey substances finer than pumice parts, and three from that thicker than 15 cm, grains. each from upper, middle and lower parts. Glassy material in unweathered pumice was dissolved in hot 20% NaOH solution. Weathered Separation of ferromagnetic minerals pumice was crushed with a wooden hammer, The pumice samples dried at room tempera and its clayey fraction was washed away with ture were plunged into water, and floating water. Remaining crystalline fractions obtained pumice grains were picked up and dried again. from unweathered and weathered pumices were The pumice bed more than 100km apart put in water, and magnetic grains were separated from the source occurs sporadically as thin from non-magnetic grains with a hand magnet. lenticular forms, from which only small amounts Magnetic grains were powdered in a mortar, of samples were obtained. In this case, the and purified by repeating the above treatment.

140°E

10 130 0 09 4 00 PJ0 O FUJI 0150 HILL 3 0 O HAKONE0 2 11 0 60

SAGAMI BAY

35°N 35°N N

0 10 20 30 km

1400E

Fig. 1. Location of sampling sites. 1: Fukawa, Odawara 2: Kissawa of Ooiso Hill, Hiratsuka, 3: Kanai, Totsuka-ku, Yokohama, 4: Negoya, Tsukui-machi, Kanagawa Pref., 5: Kamimizo, Sagamihara, 6: Hayashi, Yokosuka, 7: Ka jiyama, Tsurumi-ku, Yokohama, 8: Inukura, Miyame-ku, Kawasaki, 9: Yamada, Kohoku-ku, Yokohama, 10: Aka tsuka. Itabashi-ku, Tokyo, 11: Kisarazu 12: Nagareyama, 13: Nakadai, Yookaichiba. (• = unweathered pumice, o = weathered pumice. Fall-units of the Tokyo pumice bed 195

The sum of FeO, Fe203 and Ti02 contents of solution, as revealed by the absence of the these samples of powder exceeded 93.5 wt%, but strongest diffraction peak of the latter. This further purification was unsuccessful. The conclusion is also sustained by the fact that grains contained up to 4% of impurities (Aoyagi, lamellae texture formed by both minerals was 1985). scarcely observed under the microscope. Table 1 shows that Ti02 content of titano Identification of ferromagnetic minerals magnetite serves as diagnostic constituent for Isolated magnetic grains. were examined by differentiating a fall-unit from another, because X-ray diffractometry using Fe Ka radiation and Ti02 content remains constant within a given reflective microscopy. Titanomagnetite fall-unit, while it varies widely from one to (TiFe2O4-Fe304 solid solution) has diffraction another. From analytical error in Ti02 deter peaks at 20 38.0° (220) and 44.8° (311). mination, a variation in Ti02 content of within Ilmenite-hematite (TiFe03-Fe203) solid solution, ± 0.06 mol% is considered to be insignificant. commonly associated with titanomagnetite, has V203 is also available for the same purpose, a strong peak at 20 41.9° (104). but MnO and ZnO are not, because their con Lamellae texture, observed under the tents do not vary with the fall-unit. microscope, is known to be characteristic to the assemblage of both phases. Reference outcrop The Tokyo pumice bed at Kissawa of Ooiso Chemical analysis Hill (Locality No. 2), 29km distant from the Major constituents of ferromagnetic min source, is 120 cm thick and shows a fairly good erals, FeO, Fe203 and TiO2, were analyzed by grading. The pumice appears to be little the method described by Iwasaki et al. (1957). weathered. In the present study, this site is V203 was determined colorimetrically with N selected as the reference outcrop. In most of benzoyl-N-phenylhydroxylamine (Iwasaki et al., the fall-units observed here, the grain size of 1968). MnO and ZnO were determined by pumice decreases in the upward direction. It atomic absorption spectrometry after extraction is a normal grading. Some fall-units, however, of trivalent iron with isopropyl ether (Dadson show a reverse grading. The fall-units A, B and et al., 1936). C in Table 1 belong to this category. In this Major constituents and V203 were deter case, each fall-unit is defined by visual change mined in duplicate. Permissible analytical errors in grain size. Nine fall-units of A through I were were estimated to be ± 0.05 wt% for FeO and recognized by field observations, and a total Ti02, ± 0.25 wt% for Fe203, and ± 0.01 wt% for of 17 samples were collected from this site. V203. Those for MnO and ZnO were calculated As is seen in Table 1, each of the fall-units E to be ± 0.02 wt% and ± 0.01 wt%, respectively, through I has been proved to be homogeneous based on a presumed relative error of 5% in with respect to Ti02 content of titanomag atomic absorption measurements. netite. Chemical composition of ferromagnetic minerals is expressed in terms of ternary system Correlation of fall-units between different of FeO, Fe203 and Ti02, the contribution of localities impurities being ignored. Based on Ti02 content of titanomagnetite, fall-units observed at different localities were correlated with those at the reference outcrop. RESULTS AND DISCUSSION No fall-units corresponding to those at the Ferromagnetic mineral reference site were found at Fukawa (Locality Magnetic grains were all titanomagnetite. No. 1), 10km distant from the source. The This mineral is free from ilmenite-hematite solid pumice bed is 195 cm thick at this locality and 196 R. Aoyagi

Table 1. Description of the Tokyo pumice bed at the reference outcrop and chemical composition of titanomagnetites separated from each fall-unit

Max. grain Chemical composition of titanomagnetites Thickness Fall-unit Size of pumice FeO Fe203 Ti02 V203 MnO ZnO Ti02 (cm) (cm) (wt %) (mol %) 0.5 34.85 49.78 9.61 0.44 0.53 0.13 13.12 I 20 1.5 35.00 49.60 9.64 0.43 0.55 0.14 13.14 5.0 34.76 50.16 9.55 0.44 0.55 0.14 13.03 2.0 34.51 50.21 9.56 H 15 0.48 0.55 0.13 13.08 5.0 34.35 50.34 9.48 0.46 0.53 0.15 13.01 2.0 34.75 49.91 9.16 0.50 0.52 0.12 12.58 G 30 3.0 34.85 49.89 9.10 0.51 0.52 0.13 12.50 3.0 34.98 49.70 9.15 0.50 0.53 0.14 12.55 2.2 34.96 49.95 9.31 0.47 0.52 0.15 12.72 F 15 4.0 34.75 50.30 9.27 0.45 0.51 0.16 12.68 2.0 35.07 49.61 9.41 0.41 0.55 0.13 12.85 E 12 3.5 34.91 50.00 9.44 0.41 0.53 0.13 12.88 D' 5 3.0 35.01 50.12 8.99 0.50 0.52 0.15 12.31 D 6 4.0 35.40 48.91 9.74 0.44 0.62 0.15 13.24 C 9 1.7 35.45 48.32 10.72 0.43 0.61 0.19 14.43 B 5 1.0 35.80 49.11 10.56 0.34 0.62 0.15 14.19 A 2 0.4 34.81 48.86 10.72 0.36 0.66 0.17 14.51

TV 012

010 IH 130 V

5 40 8 0s o7

FUJI 0 A 1 2 30 11 6 HAKONE A

0 10 20 30 km i

Fig. 2. Distribution areas of some fall-units of Tokyo pumice bed. II, III, IV and Vdenote the distribution areas cor responding to the later early period, the early middle period, the later middle period to the early late period, and the latest period in the history of the cycle of eruption, respectively. • = unweathered pumice, 0 = weathered pumice. Fall-units of the Tokyo pumice bed 197

Table 2. Correlation between fall-units of the Tokyo pumice bed on the basis of TiO2 content of titanomagnetites

Locality No. 1* 2 3 4 5 6 7 8 9 10 11 12 13 Distance from 10 29 48 • 44 48 54 66 62 65 82 85 110 135 Hakone Vol. (km Fall-unit TiO2 (mol%) N2 12.60 12.52 12.56 I 13.10 13.10 13.06 13.02 13.04 13.04 12.99 13.05 H 13.05 12.98 13.06 12.97 G 12.54 12.51 12.61 12.65 12.60 12.54 F 12.70 12.83 12.80 12.73 12.80 12.77 12.79 E 12.87 12.80 12.82 D' 12.31 12.35 12.40 12.36 12.36 12.33 12.38 N1 12.80 12.76 12.77 12.72 12.77 D 13.24 13.18 13.18 C 14.3 B 14.19 14.22 14.22 14.10 A 14.51

* No fall-units corresponding to those observed at the reference site (Locality No. 2) were found at this locality. The Ti02 contents of titanomagnetites separated from 6 fall-units, observed here and named as a, b, c, d, e and f, in the upward direction, were: (a) 15.61, (b) 15.00, (c) 14.99, (d) 15.38, (e) 15.28 and (f) 15.13 mol% (see text for details). composed of 6 fall-units, provisionally named tents of titanomagnetite contained in them. a, b, c, d, e and f in the upward direction. They These are possibly correlated with the fall show a uniform grading within each fall-unit. units H or I, but the distinction between them The largest grain size of pumice in these fall is ambiguous, as they are similar to each other units is 12, 7, 10, 7. 4.5 and 5 cm, respectively. in both TiO2 and V2O3 contents. It can only TiO2 content of titanomagnetite of each fall be said that the eruptions related to the deposi unit is more than 15 mol%. No such high value tion of these fall-units were the most violent, is found elsewhere. As the Ti02 content de deduced from the large grain size of pumice creases from lower to upper fall-units, the found at the reference outcrop and the area pumice bed observed at Odawara may underlie covered with them. the Tokyo pumice bed. The uppermost layers at Negoya, Kamimizo History of a cycle of eruption depositing the and Yamada (Localities Nos. 4, 5 and 9) differ Tokyo pumice bed distinctly from any of fall-units at the reference From the areas covered with each fall-unit, outcrop in Ti02 content of titanomagnetite. as defined by Ti02 content of titanomagnetite These layers are characterized by the presence contained, the history of the cycle of eruption of light-blue pumice weathered. Concludingly depositing the Tokyo pumice bed can be de they are a new fall-unit overlying the fall-unit I, scribed as follows: and named N2. Another new one, having no 1) The early period: During this period, the correlation with those at the reference outcrop, eruption was calm. Pumice was not transported was found between the fall-units D and D'. This over a long distance and piled up near the fall-unit, named N1, was observed at Kanai, source. The fall-units observed at Fukawa were Hayashi, Kajiyama, Inukura and Yamada (Lo produced during this period. calities Nos. 3, 6, 7, 8 and 9). 2) The later early period: Pumice was de Pumice layers located at Akatsuka, Nagare posited in the eastern area of the source, blown yama and Nakadai (Localities Nos. 10, 12 and by westerly wind. The fall-units A, B and C cor 13) are thin and lenticular, being far from the respond to the products of this period. The fall source. The assignment of these layers was unit B is found even at Kisarazu (Locality No. made on the basis of their TiO2 and V203 con 11) situated beyond Tokyo Bay, where the fall 198 R. Aoyagi unit is 3.0 cm thick and contains pumice grains of 0.08 cm in diameter. The variation in grain REFERENCES size of pumice at the reference outcrop suggests that the eruption was becoming active during Aoyagi, R. (1970) The relationship between chemical this period. composition of titanomagnetite and colored minerals coexisting with it in Kanto and Shinshu loams. 3) The early middle period: The fall-units D, Chikyu Kagaku 4, 1-11 (Japanese). D' and E belong to this period. The eruption Aoyagi, R. (1983) A fundamental study of identifica related to the deposition of the fall-unit D' tion of marker-tephras by means of main chemical appears to be most violent during this period. composition of ferromagnetic minerals. Chikyu As compared with the later early period, these Kagaku 17, 109-119 (Japanese). are characterized by a sudden decrease in Ti02 Aoyagi, R. (1985) A method for the identification of marker-tephras by means of micro-component content of titanomagnetite. containing ferromagnetic minerals. submitted to Chi 4) The later middle period to the early late kyu Kagaku. period: These periods correspond to the fall Dadson, R. W., Forney, G. J. and Swift, E. H. (1936) units F, G, H and I. These are all more than 15 Extraction of ferric chloride from hydrochloricacid cm thick at the reference outcrop. During the solution by isopropyl ether. J. Am. Chem. Soc. 58, preceding periods, no fall-unit exceeded 15 cm 2573-2577. Hakamada, K. (1972) Tokyo pumice at the view in thickness. This fact reveals that the volcanic point of one-cycle eruption. Daiyonki Kenkyu 11, activity of Hakone attained to its highest stage. 44 (Japanese). The fall-unit H or I can be observed even at a Harada, M. (1976) The distribution of Tokyo pumice site 13 5 km distant from the source. layer and the sedimental mode. Kanto no Yonki 3, 5) The latest period: No deposit was formed at 51-55 (Japanese). the reference outcrop. The fall-units of this Iwasaki, I., Katsura, T., Yoshida, M. and Tarutani, T. (1957) Rapid analysis of magnetite and ilmenite. period can be found only at Negoya, Kamimizo Bunseki Kagaku 5, 211-215 (Japanese). and Yamada (Localities Nos. 4, 5 and 9). Iwasaki, I., Ozawa, T. and Yoshida, I. (1968) Photo metric determination of vanadium in magnetite, ilmenite, chromite and igneous rocks. Bunseki CONCLUSION Kagaku 17, 986-990 (Japanese). It was shown that Ti02 content of titano Kuno, H. (1950) Geology of Hakone Volcano and magnetite in the Tokyo pumice bed provides a adjacent areas Part 1. J. Fac. Sci., Univ. Tokyo, Sec. 11 7, 257-279. mean of identifying each fall-unit constituting Kuno, H. (1952) Explanetary text of the geological the pumice bed. The extention of each fall map of Japan: Atami. unit can be traced over a long distance by using Machida, H. and Moriyama, A. (1968) The develop Ti02 content of titanomagnetite as an indicator. ment of Mt. Fuji and Mt. Hakone Volcanoes analysed from tephro-chronological study in the Ooiso Hills. Acknowledgments-The author is indebted to Dr. M. Chirigaku Hyoron 41, 241-257 (Japanese). Harada, Ooshimizu High School, for his kind help in Machida, H. and Suzuki, M. (1971) A chronology of the field work and Mr. T. Furuta, Ocean Research the late Quaternary tephras as established by fis Institute, University of Tokyo, for his valuable com sion-track dating. Kagaku 41, 362-370 (Japanese). ments on the manuscript. Nakamura, K., Aramaki, S. and Murai, I. (1963) Vol canic eruption and nature of pyroclastic deposits. Daiyonki Kenkyu 3, 13-30 (Japanese).