Isotopic and Geochemical Study of Travertine and Hot Springs Occurring Along the Yumoto Fault at North Coast of the Oga Peninsula, Akita Prefecture

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1 J. Hot Spring Sci. /2 **2 * * * + 3 Isotopic and Geochemical Study of Travertine and Hot Springs Occurring Along the Yumoto Fault at North Coast of the Oga Peninsula Akita Prefecture Kyoko F URUHASHI Osamu N ISHIKAWA Osamu M ATSUBAYA * + + Tateo SHIRAISHI and Daizo ISHIYAMA Abstract In the Oga Hot Spring area in Akita Prefecture several hot springs are located along the Yumoto fault and travertine has been deposited there by past hot spring activities. To estimate the character of hot spring water at the time of travertine formation the carbon and oxygen isotopic ratios of travertine and the hydrogen and oxygen isotopic ratios of the hot spring water were examined. The carbon isotopic ratio of travertine is almost the same as that of precipitate from the present hot springs. This suggests that the source of dissolved carbonate in the hot spring water at the time of travertine formation is the same as the present one. The carbon isotopic ratio of dissolved carbonate in the present hot spring water is similar to that of carbonate rock occurring in the Nishikurosawa Formation around the Oga Hot Spring area suggesting that the dissolved carbonate in the hot spring water originates from carbonate rock in the Nishikurosawa Formation. The oxygen isotopic ratio of travertine is /. higher than that of precipitate from the present hot spring. From this it is inferred that the oxygen isotopic ratio of hot spring water at the time of travertine formation was /. higher than the present one. Based on the relationship of hydrogen and oxygen isotopic ratios the present hot spring water is estimated to originate from meteoric water reserved in marine formation. Therefore it is concluded that hot spring water at the time of travertine formation was also of meteoric water origin and that the di# erence of /. is caused by the fact that the degree of oxygen isotopic exchange between the carbonate rock and the hot spring water was more advanced at the time of travertine formation than at present. *+* *2/ + + * Faculty of Engineering and Resource Science Akita University + + Tegata Gakuenmachi Akitacity *+**2/ Japan. * Emeritus Professor

2 Key words : Oga Hot Spring Travertine Past activity of hot spring CarbonHydrogen Oxygen isotopic ratios. d O /. d O /. /. : + Fig. + +3/3 +30/ ; +31* +33 CaCO CaCO CO CaCO 4

3 /2 **2. ***.** m ** m + 2 m *** / +30* +323 /* NaCl Cl /. gl Fig. + **/ / 3 **1 2 Fig. + AF 0 / G +. ** ml CaCO CO **. CO CO Fig. + Map of the Oga Hot Spring area and localities of sample collection. 5

4 +** CO CaO CaCO CO CO CO CO CO H CO Varian MAT/* d Rsa dx + +* Rst + +0 dx d C d O dd R C C O O DH sa st PDB SMOW *+. + Table CaCO CO CaCO HCO CO CO H O CO CO CO Table + Fig. CO 0*. 2* //. CO CO /* Fig. CaCO CO 11. HCO CO /.. Friedman and O Neil +331 ) CaCOHCO. HCO HCO HCO CO CaCO CaCOHCO. CaCO CO 11. H CO HCO 6

5 /2 **2 Table + d C and d O of CaCO precipitate dissolved carbonate and degassing CO from the present hot springs. Hot springs Scale Dissolved Degassing CO carbonate * d C d O d C d C d O Oga Hotel * 141 Yuzankaku Oga onsen# + Yumoto Hotel +4. +* / /4.4* * Calculated values from the measured values in Fig.. Table Concentration d C and d O of CO in air including CO degassing from the hot springs and in the vicinity of the hot springs. Hot springs Concetration ppm d C d O Oga Hotel Yuzankaku Yumoto Hotel Yumoto spring air (Yunojiri) air (Yumoto) /1* 1+* ++.*./ * /4/ +4/.*4+.*43.+4 Fig. Carbon isotopic fractionation among CaCO precipitate dissolved carbonate and degassing CO at the present hot springs and the equilibrium fractionation at /*. 7

6 Fig. 3. // CaCO CaCO H CO HCO CO d C +*.. /. CO +. 0 CaCO CO CO CaCO. d C Table.. 1 * Table + d C 3. //... + CO CaCO d C d C Table d C and d O of travertine and carbonate rock in the Nishikurosawa Formation. Travertine Carbonate rock* Samples d C d O Samples d C d O Content ** A+ A A * * +4+ NK+ NK NK B+ 4 4* NK B *4+ +4 NK/ *40 /4+ +0 B C+ C D / *40 *4. *4 NK0 NK1 NK2 NK3 +4.4* * /40 / * D +4* +4+ NK+* D *40 *43 E+ * E +4/ +4+ E E. E/ F+ 4/ 4 F *42 4* G 43 4 * Carbonate minerals in the carbonate rocks are identified to dromite or protodolomite by Xray di# raction. ** Carbonate mineral contents in the carbonate rocks are shown in the carbonate rocks as weight percent of samples dried at ++* assuming CaMg (CO ) for chemi cal form of the mineral. 8

7 /2 **2 d C CO CO 1 1* km. + * / / *.. +32* d C / /+ CaCO CaCO H O O O ato O CaCO H O a t d d O d O +* lna CaCO H O d O HO CaCO d O H O d O d O Table + d O Table... 1 /. 2 /*..* Friedman and O Neil CaCO / d O d O d O /+ d O Table Table + /. /. + CaCO /. d O /. d O /. * d O 9

8 /. / dd d O Table. dd d O *. */. Cl... gl dd. d O */. Cl dd d O dd Fig. a b NaCl NaCl Cl */. gl dd d O */. Table. dd d O and chemical composition of the hot spring water and surface water. Hot springs Temp ph Isotopic ratios Chemical composition mg/ l d D d O Na K Mg Ca Cl SO. CO * Oga Hotel / * 3* +* 0 1* * ++* Yuzankaku / * 22 +* +3* 1*. 1** Oga onsen# /* 23 +*. 0 * + 10* Yumoto Hotel /* Yumoto spring / / +* /* Yumoto river +/ / Nomura river +14/ / / + Yunojiri (sea water) 43 24/ 0 *4 +**** 1* **.1/ 1** 0** * Dissolved carbonate shown as CO in unit of mg/ l. Fig. Relationships of dd vs. Cl (a) and dd vs. d O (b) of the present hot spring water and possible relationship of dd and d O of the hot spring water at the time of travertine formation (AC). 10

9 /2 ** ** + +** mgl d C *.* 2* d C Table / d O /. d O Fig. b A /* 1 Friedman and O Neil * 2 +** /. d O /. d O. d O */. 2 Cl * gl NaCl Cl +. gl d O NaK 1* +** +323 NaK NaK +** NaK +0 Table. NaK * // dd d O Fig. b d O /.. B d OC d O B C C dd d O 11

10 dd d O d O /. + d O 0 Table 0 + CaCO CO CaCO d C d C *0. 1. CaCO d C d C NaCl Cl dd dd d O */ CaCO CaCO /. CaCO d O /. d O /. 12

11 /2 **2 Friedman I. and O Neil J.R. ( +311) : Compilation of stable isotopic fractionation factors of geochemical interest Data of Geochemistry ChapterKK US Geol. Surv. Professional Paper..*KK. +3/3 : / : +.1 / :.1 // * : /* : / : * * : : Ca Na Cl./ /. +30* : 03. *** : / a ; ++*. **. : **0 3 1 /3 13

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