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Chemical and Mineralogical Properties of Tephras and Mobility of Chemical Elements in Tephra-Derived Soils

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Chemical and Mineralogical Properties of Tephras and Mobility of Chemical Elements in Tephra-Derived Soils Chemical and Mineralogical Properties of Tephras and Mobility of Chemical Elements in Tephra-Derived Soils Ichiro YAMADA Department of Agro-Environmental Management, Kyushu National Agricultural Experiment Station (Nishigoshi, Kumamoto, 861-11 Japan) Abstract The SiO2 content of tephras was closely correlated with the contents of other major elements except for the K2O content in Japan tephras, which varied considerably with the volcanic zones where the volcanoes were located. Volcanic glass was abundant in rhyolitic and basaltic tephras and the contents of heavy minerals and plagioclase were high in dacitic, andesitic and basaltic andesitic tephras. Although the heavy mineral assemblage was correlated with the rock types of tephras and volcanic zones in the Japan tephras, these correlations were not clear in the New Zealand tephras. The composition of volcanic glass in andesitic, dacitic and rhyolitic tephras was felsic and non-colored glass with refractive indices about 1.50 was observed. In the mafic tephras, the composition of glass was intermediate and mafic and the colored glass with indices greater than 1.52 was observed. The stability sequence of the primary minerals in tephra-derived soils under temperate and humid climate conditions was graded as follows: volcanic glass < plagioclase = olivine < augite < hyperthene common hornblende ferromagnetic minerals. The mobility sequence of the major elements in tephra-derived soils under these climate conditions was considered to be as follows: Cao, Na2O > SiO2 > MgO, K2O > Al2O3, Fe2O3. Discipline: Soil, fertilizers and plant nutrition Additional key words: volcanic glass, primary minerals, volcanic ash, Andosols 82 JARQ 31 (2) 1997 provinces with a significant reflection on the chemi­ alumina basalt and alkali basalt contents, while the cal and mineralogical properties of volcanic products tholeliite petrographic province in Japan is lacking are different between Japan and New Zealand t>. In in New Zealand 1>. It is important to investigate the Japan, there are petrographic provinces with high properties of tephras in these counlries because 20 14 19- 12 18 - OD 17 . 10 ~ ~ ~ 16 . § 8 "' 0 15 · ON"' 6 Q) -"' 14 u. < 13 ~ -~ 4- ~~i 12 : 2 · 11 . i_ 10+.-, -,.,,..,....,..,,..,.,..,.,.....,...,.,..,..,....,,...,...-.--,..,..., 0 45 so 55 60 65 70 75 80 45 50 55 60 65 70 75 80 10 12 9 · • 8 ~ D 10 oa 7 ~ ....... 8 § 6 • • 0 ~ ~ 5 ..: 0 6 ~ 4 - <..>"' 3 · 4 - .. \~ 2 ..: 1 ..: 2 .irnf 0 JI.: -,., .,.,.., -r-,r--~-,-,,-~~~ - 0 • 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80 4 . 6 . 3.5 • 5 0 D 3 ....... 2.5 · 4 · ~ ~ 0 2 ·JI~~. ON 3 :,,,:;"" jl D 1.5 :z"' 2- D 1 ..: 0.5 1 -: 0 -t-.--'-1.,.,.,.-r,-'TT"!-r,--r--,-,..,...,......., 0 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80 1.4 0.2 1.2 D D~ O 0 0. 15 ~ § 0.8 ~ 0 o 0.1 ""0.6 ...: c:: "~C\:Jo £10D\ I- :s 6 0.4 0.05 A b 0.2 - 0-1---.--.--,.,-...,..,..~~....---. 0 45 so 55 60 65 70 75 80 45 50 55 60 65 70 75 80 SiO (%) SiO (%) 2 2 D Japan e N.Z. A USA Fig. l. Contents of major clements in relation to silica in tephras I. Yamada : Chemical and Mineralogical Characterisrics of Tephras and Volcanic Soils 83 tephras easily undergo a weathering process and of Japan tephras including the 26 samples reported release chemical elements affecting the fertility of by Shoji et al. 8>, 9 New Zealand samples and 2 USA tephra-derived soils. samples 10>. Correlation equations between Si02 Shoji et al. 3> classified tephras into 5 rock types content (x) and contents of other elements (y) of on the basis of silica content, as follows; rhyolite tephra samples of Japan and New Zealand are as (70 to I OOOJo Si02), dacite (62 to 700Jo Si02), andesite follows: (58 to 620Jo Si02), basaltic andesite (53 .5 to 580/o Japan tephras (n = 33): AbOJOJo y = - 0.250x Si02) and basalt (45 to 53.50Jo Si02). + 31.52 r = - 0.882***, Fe20 30Jo y = - 0.385x + Fig. I shows the analytical data of 33 samples 30.34 r = - 0.924***, MgOOJo y = - 0.247x + 17.78 100 wt.% K?muinupuri-c Towada-a 80 (rhyolite) (rhyolite) 60 VG VG 40 HM lo"- 20 /0-............ ---0 -o ---o- Pl o\ o o Pl /0 • 0 o-- , -., .......-H~ 100 ·-·-·-· 80 Towada-Cu Tarumae-a (dacite) (andesite) 60 VG VG 40 20 80 Zao Hoei (basaltic andesite) (basalt) 60 VG VG 40 20 o--o--- / 0 0-0- 0 Pl , _ ,............._ , , - • --- HM -----..0,::-:.,.. s 10 20 so 100 200 500 1000 S 10 20 so 100 200 500 1000 Fig. 2. Primary mineral composilion of frac1ion of each size VG : Volcanic glass. Pl : Plagioclase, HM: Heavy minerals. 84 JARQ 31(2) 1997 Table 1. Relationships between rock types and heavy mineral assemblage •> Basaltic Rhyolite Dacite Andesite Basalt Total andesite Olivine 7 10 17 (15%) Pyroxene 4 12 31 6 53 (470/o) Pyroxene· hornblende 17 II 28 (25%) Hornblende 13 14 (13%) Total 5 42 49 6 10 112 (4%) (380/o) (44%) (5%) (9%) (100%) a): Figure shows the number of samples. r = - 0.916***, Ca0% y = - 0.324x + 25.80 r = plagioclase were higher in intermediate tephras com­ - 0.926***, Na20% y = 0.107x - 3.139 r = 0.862***, posed of dacite, andesite and basaltic andcsite. K20% y=0.030x-0.701 r=0.340, Ti02% Compared with the contents of heavy minerals in y= - 0.018x+ 1.790 r= -0.579***, Mn0% fractions with different sizes, the contents were higher y = - 0.0029x + 0.315 r = - 0.536*** . in the 0.1 to 0.5 mm fraction and lower in the frac­ New Zealand tephras (n = 9): Al203 % y = tion finer than 0.05 mm. -0.248x+31.16 r=-0.923***, Pe203% Table I shows the relationship between heavy y = - 0.34Sx + 26. 79 r = - 0.976***, Mg0% mineral assemblage and rock types of tephras and 161 y = - 0.220x + 16.20 r = - 0.844***, Ca0% tephra-derived soils in Tohoku, Japan • The an­ 0 y = - 0.364x + 27.66 r = - 0.959* , Na20% desitic and dacitic samples accounted for the larger y = 0.049x + 0.330 r = 0.812***, K20% part of 112 samples while the samples of other rock y = 0.096x - 3.981 r = 0.926***, Ti02% types were not conspicuous. y = - 0.029x + 2. 325 r = - 0.996***, Mn0% Based on the heavy mineral composition, tephras y = - 0.0045x + 0.397 r = - 0.950***. and tephra-derived soils were divided into 4 types: The Si02 content of the tephras was closely cor­ olivine type, pyroxene type, pyroxene-hornblende type related with the contents of all the major elements and hornblende type16>. As shown in Table I, the except for K20 in the Japan tephras and with the pyroxene type accounted for almost half of all the contents of all the elements in the New Zealand samples. Olivine type was not observed in felsic tephras. The K20 content in the Japan tephras was samples. Hornblende type was not observed in an­ closely correlated with the petrographic provinces desitic to basaltic samples and pyroxene-hornblende (volcanic zone), being lowest in the Nasu volcanic type was not detected in mafic samples. These results zone (tholeliite petrographic province), intermediate indicated that the heavy mineral composition could in the Chokai volcanic zone (high alumina basalt be used to estimate the rock types, namely the charac­ petrographic province) and highest in the Daisen teristics of chemical elements in tephra-derived soils volcanic zone (alkali basalt petrographic province) 141, of Japan. However, the correlation between the whose volcanic zones are located in parallel to the heavy mineral composition and the rock types was Japan trench. The potassium contents in the tephras 11ot clear in the New Zealand tephras 10>. belonging to the tholeliite petrographic province were As for the correlation between the heavy mineral low, even though the chemical composition of the assemblage of tephras and volcanic zone of Japan, tephras was felsic. hornblende and biotite were detected in the tephras of the Daisen volcanic zone, but not in the tephras 2) Mineralogy of tephras of the Nasu volcanic zone. Hornblende was detect­ Fig. 2 shows the contents of volcanic glass, ed, but biotite was absent in the tephras of the Chokai plagioclase, and heavy minerals in 6 tephras with volcanic zone which is located between the Nasu vol­ 111 different rock types • The contents of light miner­ canic zone and Daisen volcanic zone. als which were dominated by volcanic glass were much higher than those of heavy minerals in all the 3) Volcanic glass in tephras samples. Especially, volcanic glass was overwhelm­ Since tephras were produced under the rapid cool­ ingly abundant in rhyolitic and basaltic tephras. On ing of magma, volcanic glass is the dominant primary the other hand, the contents of heavy minerals and mineral in tephras. Volcanic glass is also considered I. Yomodo: Chemical and Mi11eralogical Clwracrerisrics of Tephros and Vo/ca11k Soils 85 80 22 75 20 Al 20 3 70 • 18 65 16 0 60 14 • Si02 ss 12 so 10 45 so ss 60 65 70 75 45 so 55 60 65 70 75 10 5 9 4.5 8 Fe 203 4 MgO 7 3.5 6 3 s • • 2.5 0 • <.!) 4 2 D > 3 1.5 C. -..,0 2 1 C 1 • 0.5 Cl) • E 0 0 Cl) ~ 45 so ss 60 65 70 75 45 so ss 60 65 70 75 .c u Cl)"' 14 5 -0 12 . 4.5 D ~ _9 ~· • i....... CaO 4 .., 10 3.5 • • ...-,6;0 • "'C ..,Cl) 8 3 D o • C 2.5 i:.
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