Iron–titanium oxide minerals in block-and-ash-flow deposits: implications for lava dome oxidation processes

Journal of Volcanology and Geothermal Research 138 (2004) 283 – 294 www.elsevier.com/locate/jvolgeores

Iron–titanium oxide minerals in block-and-ash-flow deposits: implications for lava dome oxidation processes Takeshi Saito*, Naoto Ishikawa, Hiroki Kamata Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihon-Matsu, Sakyo-ku, Kyoto 606-8501, Japan Received 7 October 2003; accepted 28 July 2004

Abstract The Ikeshiro pyroclastic-flow deposit at Yufu volcano, Kyushu in Japan, consists of typical block-and-ash flows generated by collapse of the Ikeshiro lava dome erupted ca. 2000 years ago. Lava clasts in the Ikeshiro pyroclastic-flow deposit were previously found to consist to two types of rock sample with different magnetic mineral assemblages established by rock magnetic experiments [Saito et al., 2003, J. Volcanol. Geotherm. Res. 126, 127–142]. We examined iron–titanium oxides in samples from the Ikeshiro pyroclastic-flow deposit with an optical microscope and with an electron microprobe analyzer. As a result, samples were classified into two types with different iron–titanium oxide mineral assemblages. Type A oxides are characterized by homogeneous titanomagnetite and titanohematite. Type B oxides are exsolved and composed of two or three phases: Ti-poor titanomagnetite, titanohematite, pseudobrookite and rutile. The reconstituted compositions of type B oxides show the same Fe/Ti ratio as type A oxides. This indicates that type B oxides are produced by oxidation of type A oxides. Type A oxides yield an equilibrium temperature of about 800–850 8C at an oxygen fugacity of NNO+2 using a two-oxide geothermobarometer. This indicates that deuteric oxidation in the lava dome separated samples of each type. Type A rocks originated in oxygen-poor parts of the lava dome with temperatures of about 800–850 8C, while type B rocks originated in oxygen-rich parts of the lava dome. Type A oxides remained unoxidized, while type B oxides oxidized and were transformed into complex grains by intense deuteric oxidation. The collapse of the lava dome generated the Ikeshiro pyroclastic flow. All oxides were quenched from about 800–850 8C and preserved their compositions by rapid cooling. D 2004 Elsevier B.V. All rights reserved. Keywords: lava dome; deuteric oxidation; block-and-ash flow; titanomagnetite; titanohematite; pseudobrookite

1. Introduction

* Corresponding author. Tel.: +81 75 753 6723; fax: +81 75 753 6872. E-mail address: [email protected] (T. Saito). 0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.07.006

The understanding of lava dome eruptions has advanced remarkably within recent decades. Dome growth and generation of block-and-ash flows were observed and studied precisely at Mt. Pele´e, West

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Indies (La Croix, 1904), Merapi volcano, Indonesia (Voight et al., 2000), Santiaguito volcano, Guatemala (Stoiber and Rose, 1969), and Mount St. Helens, United States (Swanson and Holcomb, 1990). Particularly, in Japan, the 1990–1995 eruption of Unzen volcano has been studied from various perspectives. The growth and collapse of lava domes were observed in detail and some models of the generation of block-and-ash flows were proposed (Sato et al., 1992; Nakada and Fujii, 1993; Ui et al., 1999). These previous studies were discussed on the basis of visual observations of the lava dome just before generating a block-and-ash flow. Changes of the conditions in the lava dome between eruption and collapsing were still not revealed, although they control the generation of block-and-ash flows. This is not only because steep topography of lava domes prevents us from surveying and sampling at will but also because silicate minerals, which most petrologists use, do not record the conditions in the dome. Iron–titanium oxide minerals are very useful for studying lava domes. They are oxidized in such circumstances as lava domes and transformed into composite multiphase grains whose phases have distinct chemical compositions. In addition, two solid solution series, titanomagnetite (Fe3 x Tix O4) and titanohematite (Fe2 y Tiy O3), acquire thermoremanent magnetization during cooling from above the Curie temperature (Tc) or Ne´el temperature, although the pseudobrookite series (Fe2 z Ti1+z O5) are all paramagnetic. Their magnetic properties depend on compositions, grain sizes and volume of oxide minerals. Therefore, if we identify iron–titanium oxides and determine their properties, we can estimate the oxidation process of iron–titanium oxides during cooling in the lava dome. However, few studies from such a point of view have been carried out. Numerous studies have adopted oxides as geothermobarometers in order to estimate the equilibrium state achieved in the magma reservoir or conduit. Although some researchers have used them as an oxidation index for lava flows or midocean ridge basalts (e.g. Watkins and Haggerty, 1967; Gromme´ et al., 1969; Anderson and Wright, 1972; Haggerty, 1976), none has tried to estimate the conditions in a lava dome by studying the oxidation state of iron–titanium oxides.

Yufu volcano is one of the most active Quaternary stratovolcanoes in the Hohi volcanic zone in central Kyushu, Japan (Kobayashi, 1984; Hoshizumi et al., 1988). It was recently listed as one of the Japanese active volcanoes assigned by the Japan Meteorological Agency and it is attracting not only volcanologists’ but popular attention. Its last activity emplaced lava domes and their collapses generated many blockand-ash flows ca. 2000 years ago. One of them, the Ikeshiro pyroclastic-flow deposit, was found to have two types of rock sample with different magnetic mineral assemblages as demonstrated by rock magnetic experiments (Saito et al., 2003). It was suggested that deuteric oxidation, which occurred in the lava dome, caused the separation into two types. In this paper, we examine iron–titanium oxides in samples from the Ikeshiro pyroclastic-flow deposit with an optical microscope in reflected light and determine the chemical compositions of the oxides with an electron microprobe analyzer. Based upon these results, we discuss oxidation states of rock samples and deuteric oxidation processes occurring in the lava dome.

2. Geology, sampling and classification of rocks Yufu volcano in Kyushu, Japan, has produced many pyroclastic flows and lava flows over the last 35,000 years (Kobayashi, 1984; Hoshizumi et al., 1988). It is blanketed with one summit lava and eight lateral volcano lavas, one of which, located on the northwestern slope, is named the Ikeshiro lava dome. The last activity emplaced the Ikeshiro lava dome and the following summit lava about 2000 years ago. They collapsed repeatedly and generated many blockand-ash flows. One of the flows emplaced on the northwestern foot is named the Ikeshiro pyroclasticflow deposit (Kobayashi, 1984; Fig. 1). The Ikeshiro pyroclastic-flow deposit is a nonwelded block-and-ash-flow deposit. It is composed of dense hornblende-andesite juvenile clasts and a matrix of the same lithology. Some of the clasts include vesicular autoliths with diktytaxitic structure. The Ikeshiro pyroclastic-flow deposit is more than 10 m thick and consists of at least 3 flow units. Each unit is separated by fine ash layers. The base of the top unit is fines-depleted as well as rich in clasts; it is similar to

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Fig. 1. Simplified geologic map of the north side of Yufu volcano after Saito et al. (2003). The location of samples (Sl) is also shown. Triangles are Quaternary volcanoes in Kyushu. A, Aso; K, Kuju; S, Sakurajima; T, Tsurumi; U, Unzen.

the lithofacies of a ground layer (Walker et al., 1981). The top of the central unit contains segregation pipes filled with lithic fragments (e.g. Francis, 1993). The clasts are mostly poorly vesiculated and commonly oxidized to varying degrees. Unoxidized clasts are mostly grayish or light grayish in color, similar to a fresh lava. Oxidized clasts usually have reticular cracks with reddish tints. Samples were collected at one outcrop from the central unit of the Ikeshiro pyroclastic-flow deposit (Fig. 1). We sampled 14 lava clasts oxidized to various degrees. We classified our samples into two types after Saito et al. (2003). Unoxidized samples consisting of grayish groundmass and fresh phenocrysts are classed as type A. Strongly oxidized samples consisting of reddish groundmass and weathered phenocrysts, especially broken down hornblendes with opacite rims, are classed as type

B. Eight of the 14 samples were type A and the rest were type B. By using rock magnetic methods, Saito et al. (2003) revealed magnetic mineral assemblages and properties of each type as follows: Type A contains titanomagnetite (Tc=480–485 8C), whose grain size is mainly multidomain. Type B contains Ti-poor titanomagnetite (Tc=500–580 8C), hematite and titanohematite (Tc=215–220 8C). The grain size of titanomagnetite in type B is smaller than that in type A.

3. Oxide mineralogy Iron–titanium oxides in samples of each type were characterized via optical microscope and electron microprobe analyses. Polished thin sections were examined by optical microscope in reflected

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Fig. 2. Backscattered electron images of representative iron–titanium oxide minerals in Ikeshiro samples. (a) is in a type A sample, the other images are in type B samples. Refer to bar in lower center for scale. (a) Homogeneous titanomagnetite with titanohematite (dark). (b) Oxidized titanomagnetite with many titanohematite laths. (c) Oxidized titanohematite with complex lamellas. The host titanohematite is pseudomorphed by pseudobrookite (grey) and titanohematite with small rutile blebs (black). In the upper left-hand region, many rutile lamellas (dark grey) are exsolved in titanohematite. The grain in the lower center is oxidized titanomagnetite, similar to (b). (d) Oxidized titanohematite with rutile (dark grey) and pseudobrookite (grey). (e) Completely pseudomorphed crystals after primary titanomagnetite (left) and titanohematite (right). Titanohematite is replaced by pseudobrookite (grey) and rutile (dark grey). White matrix is titanohematite. Titanomagnetite is pseudomorphed by pseudobrookite along {111} relic planes in a host of titanohematite. (f) Homogeneous titanomagnetite and associated titanohematite with pseudobrookite lamellas. This grain is an inclusion in hornblende.

Type A

Type B

Titanomagnetite 1

2

Titanohematite Titanomagnetite

3

4

5

6

7

8

10

11

12

13

2.00 2.81 0.02 0.00 1.27 0.06 0.96 85.96 93.08

1.23 2.53 0.02 0.44 5.10 0.07 0.66 83.18 93.23

5.90 6.29 2.60 0.36 2.86 0.00 0.78 74.79 93.58

1.02 1.39 0.05 0.05 14.27 0.02 0.22 74.73 91.75

1.21 2.98 1.24 0.00 1.79 2.15 1.64 1.19 0.99 0.48 0.88 0.62 0.18 0.74 1.32 0.92 0.00 0.00 0.00 0.28 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.07 0.00 0.32 15.75 37.01 31.30 0.00 25.71 17.85 8.87 5.06 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.36 0.69 0.47 0.00 0.88 0.39 0.42 0.65 74.05 57.93 61.09 88.80 68.09 72.84 79.71 81.63 92.34 99.09 95.00 89.70 96.66 94.04 91.96 89.79

14

15

16

17

18

19

20

21

22

1.37 1.27 2.02 1.87 2.00 2.40 0.01 0.00 0.42 0.14 0.00 0.13 5.85 5.58 9.00 0.01 0.00 0.31 0.53 0.65 0.63 84.17 86.35 80.50 93.94 95.85 95.41

1.95 0.54 0.14 0.00 39.91 0.00 0.40 51.21 94.15

4.50 1.25 0.00 0.18 47.71 0.00 0.35 41.82 95.81

Fe2O3 FeO Total

55.90 57.66 48.81 43.19 46.97 26.04 27.44 65.46 63.83 55.99 52.43 71.07 69.34 34.07 39.60 98.32 54.52 67.91 83.45 89.18 55.20 33.86 34.47 36.58 40.68 39.55 31.69 31.01 28.08 28.52 32.80 27.61 10.79 11.65 27.27 25.46 0.33 19.04 11.74 4.62 1.39 1.54 99.54 101.63 100.30 99.71 100.12 101.46 98.98 100.98 99.47 98.84 98.83 98.87 99.29 102.50 98.97 99.55 102.12 100.84 100.32 98.72 99.68

43.57 2.61 100.17

0.16

0.27 0.37

0.30

0.75

0.73

2.37 3.17 0.00 0.00 0.91 0.00 1.00 86.98 94.42

9

MgO Al2O3 SiO2 CaO TiO2 Cr2O3 MnO FeO Total

Mole fraction 0.17 of Ti-rich comp

1.34 0.58 2.26 1.43 1.14 1.78 0.43 0.28 0.25 0.34 0.00 0.02 0.00 0.00 0.00 0.22 12.63 10.11 40.42 38.13 0.00 0.00 0.00 0.00 0.47 0.79 0.62 0.46 79.55 81.81 55.12 55.70 95.38 95.41 98.85 96.24

Pseudobrookite

0.03 0.04 0.15 0.17 0.28 0.31

0.68 0.60 0.01

0.48

0.34

0.17 0.10 0.18

0.35

Fe2O3 and FeO calculated on the basis of oxide stoichiometry. Mole fraction of Ti-rich component (Fe2TiO4 for titanomagnetite, FeTiO3 for titanohematite and FeTi2O5 for pseudobrookite) is also shown. Analyses: 1, 2, 3, large, homogeneous titanomagnetite; 4, 5, small homogeneous titanomagnetite; 6, 7, large homogeneous titanohematite; 8, 9, titanomagnetite separated by many titanohematite laths; 10, homogeneous titanomagnetite inclusion in plagioclase; 11, small discrete titanomagnetite inclusion in hornblende; 12, 13, titanohematite lamellas in titanomagnetite; 14, titanohematite inclusion in hornblende; 15, titanohematite inclusion in plagioclase; 16, small discrete hematite; 17, 18, titanohematite with pseudobrookite and rutile; 19, 20, titanohematite with pseudobrookite pseudomorphed along {111} relic planes; 21, 22, pseudobrookite with rutile and titanohematite.

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Table 1 Representative electron microprobe analyses (wt. %) of iron-titanium oxide minerals in the Ikeshiro samples

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light and by backscattered electron images using JEOL JXA-8800K and JXA-8900R electron microprobe analyzers to identify occurrences, assemblages and textures of iron–titanium oxide minerals. Images of typical oxides are shown in Fig. 2. Based upon these observations, electron microprobe analyses of representative iron–titanium oxides were performed (Table 1). Analyses of iron–titanium oxides were recalculated on the basis of oxide stoichiometry to determine Fe2O3 and FeO from total Fe. The mole fraction of the titanium-rich component (x: Fe2TiO4 for titanomagnetite, y: FeTiO3 for titanohematite, z: FeTi2O5 for pseudobrookite) was also determined. Results of microprobe analyses are plotted on TiO2– FeO–1/2Fe2O3 ternary diagrams (Fig. 3).

3.1. Type A Oxides in type A samples are characterized by homogeneous titanomagnetite and titanohematite (Fig. 2a). There are no exsolution lamellas. Some titanomagnetite grains mantle or attach to titanohematite. Chemical compositions of titanomagnetite are within the range 0.14bxb0.36, while titanohematite is within 0.71byb0.80 (Fig. 3a). Smaller titanomagnetite grains are more titaniferrous with 0.30bxb0.39. 3.2. Type B Oxides in type B samples are oxyexsolved and show complex textures and compositions. Most

Fig. 3. TiO2–FeO–1/2Fe2O3 ternary diagrams showing compositions of iron–titanium oxide minerals in samples of type A (a) and type B (b). P P P Minor components allocated as follows: FeO= R2+=Fe2++Mg+Mn+Ca, Fe2O3=1/2 R3+=1/2(Fe3++Al+Cr) and TiO2= R4+=Ti+Si.

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titanomagnetite in type B samples is separated by abundant trellis titanohematite lamellas (Fig. 2b). Lamellas occur along {111} planes of the host titanomagnetite and are distributed uniformly. The widths of lamellas range from a few Am to 20 Am, with a distribution peaking around 10 Am. Titanohematite lamellas are homogeneous and there is no finer lamella within the lamellas. The composition of the titanomagnetite is very low (x below 0.08; shown as diamonds in Fig. 3b). It is nearly pure magnetite. Trellis lamellas are Ti-poor titanohematite with 0.16byb0.37 (shown as diamonds in Fig. 3b). Titanohematite frequently occurs with pseudobrookite (Fig. 2c,d,e). This titanohematite is more titaniferrous (0.33byb0.51) than that of trellis lamellas (shown as circles in Fig. 3b). Pseudobrookite is in the range 0.04bzb0.34 (shown as bars in Fig. 3b). Sometimes small rutile blebs occur within the pseudobrookite region. Some pseudobrookite is exsolved along {111} relic planes in a host of titanohematite (Fig. 2e). This titanohematite is less titaniferrous (0.07byb0.17) than that of trellis lamellas (shown as triangles in Fig. 3b). In addition, hematite occurs as discrete small grains. Its composition is yb0.02 (shown as stars in Fig. 3b). Some oxides occur as inclusions in hornblende or plagioclase (Fig. 2f). They are not so exsolved as discrete oxide minerals. Their chemical compositions are more titaniferrous (0.14bxb0.19, 0.60byb0.73) than discrete oxides and almost in the range of type A samples (shown as crosses in Fig. 3b).

4. Discussion 4.1. Correlation with magnetic properties Saito et al. (2003) reported the Curie temperature of titanomagnetite and titanohematite in samples of each type. The Curie temperature depends on the titanium component (Nagata, 1961). Therefore, we can estimate the chemical composition of titanomagnetite and titanohematite from Tc (Dunlop and ¨ zdemir, 1997, figs. 3.11 and 3.23). Estimated O composition of titanomagnetite in type A is xc0.2 and that in type B is xb0.1. Titanohematite in type B has yc0.5. In addition, the blocking temperature distribution indicates the existence of hematite in type B samples.

289

In this study, we identified all oxide minerals inferred from the rock magnetic analyses of Saito et al. (2003). Type A contains titanomagnetite with 0.14bxb0.39 and titanohematite with 0.71byb0.80. Because titanohematite with yN0.7 is not magnetic at and above room temperature, rock magnetic analyses cannot detect it. Titanomagnetite in type B samples shows a very low Ti content, nearly pure magnetite. These compositions are consistent with the estimated compositions described above. However, the results for titanohematite in type B samples are not completely consistent. Although rock magnetic analyses identified titanohematite with yc0.5 only, microprobe analyses identified titanohematite with 0.1byb0.5 besides hematite and titanohematite with yc0.5 (Fig. 3b). This may be because such titanohematite has a small saturation magnetization or the volume of this mineral is smaller than that of other minerals, although further rock magnetic analyses will be needed to identify this mineral. As for the grain size of titanomagnetite, type A contains abundant large homogeneous crystals, while titanomagnetite in type B is subdivided by thick titanohematite lamellas and its effective grain size becomes smaller. This is also consistent with an inference from the coercivity distribution (Saito et al., 2003). 4.2. Oxidation of iron–titanium oxide minerals Iron–titanium oxide minerals oxidize and exsolve quickly at high temperature (e.g. Haggerty, 1976, 1991). Haggerty (1991) defined seven high-temperature oxidation stages, C1 to C7 for titanomagnetite and R1 to R7 for titanohematite, and characterized each stage in detail by showing many photomicrographs. On the basis of oxide mineral assemblages, we now discuss oxidation state and process of our rock samples. Oxides in type A samples are characterized by homogeneous grains, while those in type B are exsolved and composed of two or three phases. According to the oxide classification by Haggerty (1991), type A oxides are classified as C1 and R1 stage. Oxides in type B samples are classified as more than C3 and R6 stage because titanomagnetite is separated by trellis lamellas and titanohematite occurs with pseudobrookite. This contrast in oxidation stages indicates that type B samples were oxidized and

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Fig. 4. Reconstituted compositions of type B oxides plotted on a ternary diagram. Original compositions before reconstitution are plotted as squares tied with solid lines and reconstituted compositions are indicated by circles. The composition range of the type A oxides is also shown as bars. Grey bands indicate Fe/Ti ratio of type A oxides.

transformed, while type A samples were not oxidized. Chemical compositions of oxide inclusions in silicates are the same as that of type A oxides (Fig. 3). This observation suggests that all oxide minerals in type B samples were type A before oxidation, oxide inclusions having been protected from oxidation by the surrounding silicates. In order to estimate the oxidation process, we reconstituted typical composite grains in type B

samples and determined their chemical compositions before oxidation. Using digital image-editing software, we simplified photo images of oxides down to GIF images using only two or three colors. We obtained the area ratio of each phase in the grain from the histogram of brightness. We made a rough estimate of composition of the homogenized grain by multiplying the area ratio by each chemical composition. The process is summarized in Table 2 and results are plotted on a ternary diagram (Fig. 4). Estimated oxidation processes are summarized in Fig. 5. Reconstituted compositions of oxides, which consisted of rutile, pseudobrookite and titanohematite (Table 2a), show almost the same Fe/Ti ratio as that of titanohematite in type A samples (Fig. 4). This result indicates that homogeneous titanohematite grains in type A samples transform to grains composed of rutile, pseudobrookite and titanohematite by oxidation. Reconstituted compositions of oxides showing trellis lamellas (Table 2b) have almost the same Fe/Ti ratio as titanomagnetite in type A samples. This result indicates that homogeneous titanomagnetite grains transform by oxidation to grains with titanohematite trellis lamellas. In addition, reconstituted compositions of oxides showing pseudobrookite lamellas along {111} relic

Fig. 5. Estimated oxidation processes plotted on a ternary diagram. Primary unoxidized titanomagnetite and titanohematite (circles) are oxidized along the arrows and transformed into complex grains: squares are oxidized from primary titanohematite and triangles are oxidized from primary titanomagnetite. The composition range of the type A oxides is also shown as bars. Stars are reconstituted compositions after Table 2.

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Table 2 Reconstitution of typical iron-titanium oxides in type B samples Processed GIF image

Color

Composition

Area ratio (%)

Reconstituted composition

grey

titanohematite ( y=0.46) pseudobrookite (z=0.34) rutile

28

Fe1.58Ti1.08O4.35

dark grey black

grey black

grey black

planes (Table 2c) also have the same Fe/Ti ratio as titanomagnetite in type A samples. This result suggests that host titanomagnetite and titanohematite

69 3

titanohematite ( y=0.23) titanomagnetite (x=0.03)

63

titanohematite ( y=0.11) pseudobrookite (z=0.07)

90

Fe2.25Ti0.15O3.37

37

Fe1.89Ti0.21O3.2

10

trellis lamellas are oxidized to titanohematite and pseudobrookite, respectively, by further oxidation. We conclude that all the iron–titanium oxides in

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type B samples were produced by oxidation of type A oxides (Fig. 5). 4.3. Implications for lava dome oxidation processes Our results revealed that the two types of Ikeshiro samples show different oxidation states and that type B oxides were produced by oxidation of type A oxides. We propose that deuteric oxidation in the lava dome caused this separation. Next, we estimate the oxidation processes in the lava dome. We estimated the equilibrium state before eruption by a two-oxide geothermobarometer (Ghiorso and Sack, 1991). Results from type A samples are shown in Fig. 6 but we cannot apply the method to type B oxides. Titanohematite coexisting with titanomagnetite in type B oxides forms trellis lamellas (Fig. 2b) and has a Ti-poor composition (Fig. 3b). For such a chemical composition, the two-oxide geothermobarometer is not calibrated and not effective (see Ghiorso and Sack, 1991, Fig. 5). Oxides in type A samples yield wide ranges in temperatures (800–1070 8C) and oxygen fugacity (10 8–10 12, Fig. 6). Evans and Scaillet (1997) reported that the oxidation state of the Pinatubo dacite had been overestimated at an oxidation fugacity greater than NNO and calculated temperatures had been largely in excess. Oxygen fugacities are about two log units higher than NNO and our values of temperature are possibly overestimated. This could be one reason for the temperatures greater than 1000 8C that we got. Although more petrologic analyses will be needed for a more precise estimation of the equilibrium condition, the obtained values seem not to be unrea-

Fig. 6. Log oxygen fugacity vs. temperature relationship for type A oxides. The two-oxide geothermobarometer of Ghiorso and Sack (1991) was used.

sonable. Most oxides yield temperature of 800–850 8C at an oxygen fugacity of NNO+2. These are well within the fO2 range common for felsic rocks (Frost, 1991). Equilibration temperatures are appropriate for the crystal-rich andesite lava dome. In the 1990–1995 eruption of Unzen volcano, a temperature of 670 8C was measured from the surface of the lava dome (Taniguchi et al., 1996). These observations suggest that type A rocks originated as part of the lava dome. In the lava dome, intense deuteric oxidation occurred in some samples and type B rocks were produced. Type A rocks remained unoxidized and preserved a homogeneous composition. We suggest that type A rocks originated in oxygen-poor parts of the lava dome, for example the inner part of the dome (Fig. 7). On the other hand, type B rocks were strongly oxidized and changed their textures and compositions. Homogeneous titanomagnetite was transformed to Ti-poor titanomagnetite with trellis titanohematite lamellas (Fig. 5). Further oxidation transformed this assemblage into titanohematite with pseudobrookite lamellas. Homogeneous titanohematite was transformed to titanohematite and pseudobrookite with rutile blebs. These observations suggest that type B rocks come from oxygen-rich parts of the lava dome, for example the dome surface (Fig. 7). Then, collapse of the lava dome generated the Ikeshiro pyroclastic flow and lava blocks were quenched. Oxides in the lava blocks preserved their compositions by rapid cooling. The existence of titanohematite with intermediate composition supports this conclusion because it can be preserved only by rapid cooling from above 600 8C (Burton, 1991). Moreover, deuteric oxidation is not only controlled by oxygen fugacity but by temperature which is affected by the effusion and cooling rate of the lava. In lava dome eruptions, the effusion and cooling rate of the lava result in two different styles of dome growth (Nakada et al., 1995). The lava dome grew exogenously when the effusion rate was high and cooling rate was low, whereas the lava dome grew endogenously when the effusion rate was low and cooling rate was high. In the 1990–1995 eruption of Unzen volcano, Ui et al. (1999) reported that block-and-ash flows generated from the lava dome growing exogenously were derived from abundant fresh lava, whereas block-andash-flow deposits generated from the lava dome growing endogenously included lava blocks on the

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Fig. 7. Simplified model for the deuteric oxidation in the lava dome. The exogenous dome consists of fresh lava (type A), while the endogenous dome oxidizes and produces type B rocks in the surface.

surface which had suffered various degrees of high temperature oxidation. The exogenous dome generated much fresh lava due to successive supply of new lava, while the endogenous dome was exposed to the air for a long time at high temperature and oxidized because the hot lava intruded within the dome. These observations imply that type A rocks came from the lava dome growing exogenously or from the inner parts of the lava dome growing endogenously, while type B rocks came from the outer parts of the lava dome growing endogenously (Fig. 7).

5. Conclusions (1) Samples from the Ikeshiro pyroclastic-flow deposit are classified into two types with different iron–titanium oxide mineral assemblages. Type A oxides are characterized by homogeneous titanomagnetite and titanohematite. Type B oxides are exsolved and composed of two or three phases; Ti-poor titanomagnetite, titanohematite, pseudobrookite and rutile. (2) The identified titanomagnetite and titanohematite have compositions consistent with the rock magnetic properties described by Saito et al. (2003). (3) Type B oxides show a high oxidation state, while type A oxides are not oxidized. The reconstituted compositions of type B oxides show the same

Fe/Ti ratio as type A oxides. This indicates that type B oxides are produced by oxidation of type A oxides. (4) Type A oxides yield the equilibrium temperature of about 800–850 8C at an oxygen fugacity of NNO+2 using the two-oxide geothermobarometer. This indicates that deuteric oxidation in the lava dome separated samples of each type. Type A rocks originated in oxygen-poor parts of the lava dome at about 800–850 8C, whereas type B rocks originated in oxygen-rich parts of the lava dome. Type A oxides remained unoxidized, whereas type B oxides oxidized and were transformed into complex grains by intense deuteric oxidation. The collapse of the lava dome generated the Ikeshiro pyroclastic flow. All oxides were quenched from about 800–850 8C and preserved their compositions by rapid cooling. Acknowledgements ¨.O ¨ zdemir and B. We are grateful to D.J. Dunlop, O Scaillet for helpful reviews that improved the manuscript. The electron microprobe analyses were done at Beppu Geothermal Research Laboratory of Kyoto University, at Institute for Frontier Research on Earth Evolution of Japan Marine Science and Technology Center, and at Venture Business Laboratory of Kobe University. We would like to thank Y. Tatsumi and N. Tomioka for permission to use the microprobe in their laboratory. We also thank T. Kawamoto and H. Shukuno for instructing and helping with electron

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microprobe analyses. M. Funaki and M. Ozima are acknowledged for helping with identification of iron– titanium oxide minerals using the optical microscope. I. Iizawa helped with digital image processing. We also acknowledge comments and suggestions by M. Torii. This research was supported in part by a Grantin-Aid of the Fukada Geological Institute.

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