Page 1 Geochemical Journal Vol. 19, Pp. 193 to 198, 1985 The
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GeochemicalJournalV ol.19,pp.193 to 198,1985 Theidentification ofthefall-unitsofthe Tokyo pumice bed by chemicalcom position offerrom agnetic minerals R Y UJI A O YA GI MiyazakiJunior High School,Miyam ae-ku,Kawasaki 213,Japan (R ece ived O ctob er 2, 1984 .- Accepted M arch 26,1985) Sam ples of the T okyo pum ice bed w ere collected from 13 Iocalities. Ferrom agnetic m inerals contained in the sam ples w ere allid entified astitanom agnetite by X -ray diffraction m etho d. Ti0 2 a nd V 203 c om ents o f titan o m agnetite provide in form ation useful foride nt ify in g e ach f al l-unit of th e T oky o pum ice bed . The bed at th e ref e re nc e outc rop w as d ivided i n 10 f all-un its on t he b asi s of visua l obser vation o f st rati fied str uctur e a nd chem ical com pos itio n of titan om agn etite. The pres ence oftw o a ddi tional fall- units w asal so co nfi r m ed by c om par ativ e stu dy of s am ple s f rom d ifferent loc al itie s. T he area co vere d b y a give n fall-uni t w a s bounded by using Ti0 2 cont ent o fti tan om agneti t e separ at ed from each sam ple. firm ed the source and the distribution area of IN TR O DUC TION the bed. H akam ada (1972) divided itinto 8 fall- H akone volcano, a representative one in the units on the basis of the field study at Endo-hara central Japan, w as investigated from petro- of O oiso Hill, K anagaw a Prefecture. H e also exam ined Si02 content and heavy m ineral as- graphical and volcanological points of view by K uno (1950, 1952). T he activity ofthe volcano sem blage of these fall-units, concluding that is considered to data back to the m iddle each fall-unit has different petrographic charac- Pleistocene. The volcano erupted lava and teristics. H arada (1976), how ever, claim ed from scoria, w hich form ed the body ofthe m ountain, the outcrop observ ation at m any localities that and m ore than 100 pum ice beds extend to the the bed consists of atleast 5 fall-units. T he present author investigated chem ical east. It continues to em it volcanic gases from its centralcone. com position of ferrom agnetic m inerals con- The Tokyo pum ice bed, one of air fall tained in m arker-tephra, w hich ar e distributed pum ice beds, w as recognized as a single cycle of over the Japanese Islands (A oyagi, 1970, 1983, eruptions (N akam ura et al,, 1963), the age of 1985). This study isintended m ainly to identify w hich w as determ ined to be 49,000 i 5,000Y BP the fall-units of the T okyo pum ice bed on the by the fission=track m ethod (M achida and basis of chem ical com position of ferror)agnetic Suzuki, 1971). The erupted volum e of the bed m inerals as w ell as field observ ation. is estim ated to be 4.8 km 3 by M achida and M oriyam a (1968). This is the largest air fall E XPERIM EN TAL P R OCEDU RES pum ice bed derived from Hakone V olcano. T he thickness of the bed distributed in the M ethod of sam pling The sam ples w ere collected from 13 Iocali- vicinity of Tokyo, about 80km aw ay from the source, varies from 5 to 20cm . It has been ties in the southern K anto district (Fig. I). T he noticed for a long tim e as the m ostim portant grain size of pum ice decreases usually upw ard m arker-tephra of K anto loam and studied geo- within each fall-unit. The num ber ofsam ples taken from each fall- graphically and petrographically by m any w orkers. M achida and M oriyam a (1968) con- unit depended on its thickness: one sam ple 193 194 R. A oyagi from a fall-unitthinner than 5cm ,tw o from that sam ples were sieved out in running w ater to 5 to 15cm thick,each from the upper and low er elim inate clayey substances finer than purn ice parts, and three from that thicker than 15cm , grarns. Glassy m aterial in unweathered pum ice w as each from upper,m iddle and low er parts. dissolved in hot 20% N aO H solution. W eathered Separation offerrom agnetic m inerals pum ice w as crushed with a w ooden ham m er, The pum ice sam ples dried at room tem pera- and its clayey fraction w as w ashed aw ay w ith ture w ere plunged into w ater, and fl oating w ater. R em aining cry stalline fractions obtained pum ice grains w ere picked up and dried again. from unw eathered and w eathered pumices w ere The pum ice be d m ore than 100 km apart put in w ater, and m agnetic grains w ere separated from the source occurs sporadically as thin from non-m agnetic grains with a hand m agnet,. lenticular form s,from w hich only sm allam ounts M agnetic grains w ere pow dered in a m ortar, of sam ples w ere obtained. In this case, the and purified by repeating the above treatm ent. 140'E 12 10 13 0 Fig. 1. Location ofsam pling sites. I: Fukawa, Odawara 2: Kissawa of Ooiso Hill,Hiratsuka,3:K anai, Totsuka-ku, Yokoham a, 4: N ego y a , Tsu ku i-m achi, Kan ag aw a Pref, 5: Kam im izo, Sag ami hara, 6, H aya shi, Yoko suka, 7.･ Ka - 8 jiyama, Tsuru mi-ku, Yokohama, 8: Inukura,M iyam e-k u, Kawasaki, 9: Yam ada,Kohoku-ku, Yokohama,10.' A ka- 05 o ~ ~ ~~~ 4 tsuka.Itabashi-ku, Tokyo, 11.･ Kisarazu 12:N agareyama, 13:N akadai, Yookaichiba. (e= unweathered pumice,o= w eath ere d pumic e. FUJI OISO HILL 3 o ~:o~~~ o JL HAKONE ~ ~~ 2 ~( 11 o 6o SAGAMI BAY 35'N 35'N N o ,o 20 30km Fall -unit s oft he T ok yo p um ice be d 195 The sum of FeO, Fe203 and Ti02 contents of solution, as revealed by the absence of the these sam ples of pow der exceeded 93.5 w t%, but strongest diffraction peak of the latter. T his further purification w as unsuccessful. T he conclusion is also sustained by the fact that grains contained up to 4% ofim purities(A oyagi, lam ellae texture form ed by both m inerals was 1985). scarcely observed underthe m icroscope. Table I shows that Ti0 2 content of titano- ldentification of ferrom agnetic m inerals m agnetite serves as diagnostic constituent for differentiating a fall-unit from another, because Isolated m agnetic grains w ere exam ined by X-ray diffractom etry using Fe K a radiation and Ti0 2 content rem ains constant within a given reflective m icroscopy. Titanom agnetite fall-unit, w hile it varies w idely frorn one to (TiFe20 4-Fe30 4 Solid solution) has diffraction another. From analytical error in Ti0 2 deter- peaks at 2e 38.0' (220) and 44.8' (311). m ination, a variation in Ti02 content of within Ilm enite-hem atite (TiFe0 3-Fe20 3)solid solution, d: 0.06m ol% is considered to be insignificant. com m only associated with titanom agnetite, has V 20 3 is also available for the sam e purpose, a strong peak at 2e 41.9' (104). but M nO and ZnO are not, because their con- Lam ellae texture, observed under the tents do not vary with the fall-unit. m icroscope,is know n to be characteristic to the assem blage of both phases. R eference outcrop The T okyo pu mice bed at Kissaw a of O oiso Chem icalanalysis Hill (Locality N o. 2), 29 km distant from the M ajor constituents of ferrom agnetic m in- source, is 120 cm thick and show s a fairly good erals, FeO, Fe20 3 and Ti02, w ere analyzed by grading. The pum ice appears to be little the m ethod described by lw asaki et al. (1957). w eathered. In the present study, this site is V20 3 W as determ ined colorim etrically with N- selected as the reference outcrop. In m ost of benzoyl-N-phenylhydroxylam ine (Iw asaki et al., the fall-units observ ed here, the grain size of 1968). M nO and ZnO were determ ined by pum ice decreases in the upw ard direction. It atom ic absorption spectrom etry after extraction is a norm al grading. Som e fall-units, how ever, of trivalent iron with isopropyl ether (D adson show a reverse grading. The fall-units A, B and etal., 1936). C in T able I belong to this category. In this M ajor constituents and V 203 w ere deter- case, each fall-unit is defined by visual change m ined in duplicate. Perm issible analytical errors in grain size. Nine fall-units of A through I w ere were estim ated to be :!: 0.05w t% for FeO and recognized by field observ ations, and a total Ti02,:t 0.25 w t% for Fe203, and ~: 0.01 wt% for of 17 sam ples w ere collected from this site.