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2 Geothermal Resources Council TRANSACTIONS, Vol. 14, Part II, August 1990 ESTlMATION OF MAGMA TEMPERATURE USING ZlKCON MORPHOLOGY Masao " * Geothermal Kesearch Center, Kyushu University 36 Hakozaki, Fukuoka 812, JAPAN ABSTRACT The temperature of magma reservoirs can be estimated according to the morphology of zircon crystals contained in igneous rocks. The prism index (PI) of a crystal is defined as the ratio of the sum of the four { 100 } widths to the total length of the prism perimeter. Within a temperature range from 600" C to 900" C, the magma temperature is given by T(" C) = 300xPI f 600. This geothermometxr is particularly useful for zircons from felsic rocks whose magma temperatures are riot high. Practical applications to volcanic rocks from the Otake- Hatchobaru area in Japan suggest that low temperature magma reservoirs are more effectjve heat source for geothermal systems than high temperature ones. the oxygen equilibration among the assemblage plagioclase-pyroxene-magnetite in mafic igneous rocks (Anderson et til., 1971). One of the weak points in the above t,hree geothermonieters is that they require two or three coexisting minerals, which cannot always be found in a rock sample. In this study, the morphology of zircon crystals is used for the estimation of magma temperature. Although there is quite a wide variety of zircon crystal forms, they can be quan~i~tively described using four indexes: flatness, elongation, prism and pyramid (, 1990). Of these, the prism index is closely related to the Formation temperature of zircon (Pupin, 1980). The zircon geothermometer is a product of the combination of the above two studies. INTRODUCTION For the evaluation of igneous-related geothermal systems in the U. S. A., Smith and Shaw (1979) developed a rationale according to which the age of volcanism and the volume of magma reservoirs arc two major factors. In addition, they suggested that the magma t~niperature also affects the' lifetime of the systems: a slab of high temperature magma reservoirs will cool down more rapidly than a cube of low temperature ones. There are two kinds of geothermometers: one based on the chemical compositions of coexisting minerals, and the other on the isotopic partition among coexisting minerals. The former is slightly affected by pressure, whereas the latter is a function of temperature only. In the system of MgzSiOt- CaMgSir?OG, the amount of Ca in coexisting clinopyroxene and orthopyroxene becomes large as the temperature increases. This two-pyroxene thermometer (Lindsley and Anderson, 1983) is effective at temperatures above 1000" C. Similarly, the two-feldspar geothermon~eter f Fu hrman and Lindsley, 2988) relies on the phase relationships in the ternary-system of CaA1~Si20~-NaA1Si30~-KA1SisO~, and it is effective for temperatures ranging from 600" C to 1000" C. The isotopic geothermometer uses EXPERIM E KTA L Zircon crystals were separated from rock samples of 1 kg each through crushing, panning, magnetic separation, heavy liquid separation, and hand picking. The ~Iorp~~olo~y of 15 crystals for each sample was described as follows: First, a crystal form is classified into Type 100 or Type 110, depending on the projected plane, either the i or the i. 110 f prisni (Fig. 1). 111 the figure, the vertical distance is called "height,", the horizontal distance "width", and the depth distance "thickness". Next, the following vertiral and horizontal distances were measured to calculate the terms required: the crystal height H=be, which is constant for each crystal, regardless of the projected plane: the crystal width W=(actfd)/B; the middle prisiii width W,,,=(gitlj)/Z; the prism height Hp F=(2hk+2glt2ij+aftcd)/8, the pyramid heighl HF-, =fh-hpf f/2, and the crystal thickness '~=ZW(~~-hk~ /(ZH-af-cd). The subscripts 100 and 110 for T and W in the following equations -refer to Type 100 and Type 110 respectively. The flatness index is the ratio of' the crystal thickness to the crystal width, or, when the ratio is larger than 1, its reciprocal, 1445

3 TY Pe 00 A iio / 9 'r 1 Z i 2i i e 21 i e Fig. 1. Two types of zircon crystal morphology depending on the projected plane, Type 100 and Type 110. The crystal corners required for the morphological measurements are named alphabetically. The corners from a to f are on the outline of the crystal, and those from g to 1 are on the outline of the central prism. They are located at the following points: a=upper-left point between the prism and the pyramid, b=the highest point or edge, c=upper-right point between the prism and the pyramid, d=lower-right point between the prism and the pyramid, e=the lowest point or edge, f=lower-left point between the prism and the pyramid, g=upper-left point, h=the highest point or edge, i=upper-right point, j=lower-right point, k=the lowest point or edge, l=lower-left point. These definitions are valid for any crystal forms, including bipyramids. RESULTS AND DISCUSSION 'The prism index is the ratio of the sum of the four widths to the total length of the prism perimeter. The pyramid index is the ratio of the pyramid height of a crystal sample to that of an ideal crystal consistjng only of the { 301 j prism, namely, 1.36W I U,S for Type 100 and 1.44W for Type 110. A magma temperature is given by T(" C) = 3OOxP.I t 600 based on the relation between the relative development of prism faces and the estimated temperatures (Pupin, 1980). This zircon thermometer is valid only for a temperature range from 600" to 900" C, because the outside of this range occur only one kind of prism faces. The zircon morphology data for some volcanic rocks from the Otake-Hatchobaru area are plotted on Fig. 2 (prism-pyramid-elongation-flatness diagram). The zircon from the Fukakuragawa dacite (Fig. 2A) is { 110 } prism dominant and mostly prismatic with some long-prismatic crystals. The zircon from the Gotozan hornblende andesite (Fig. ZB), which is the heat source for the Hatchobaru geothermal system, is of intermediate type and short-prismatic. The PI of the Hohi pyroxene andesite (Fig. 2C) has a wide range from the intermediate type { 110 ). = { 100 } to { 100 } type; similarly the E1 ranges from short-prismatic to long-prismatic. The Yabakei welded tuff (Fig. 2D) is of typical { 100 } dominant type and prismatic. The pyramid index for all the samples has a similar range, from 0.3 to 0.6, indicating { 101 } dominant pyramids with small { 211 } faces. Table 1 is the summary of the zircon indexes for volcanic rocks and their estimated magma temperatures. Since zircon is generally considered to crystallize at later stages, the upper limit of the estimated temperature range may represent the minimum temperature of magma. For example, the highest estimated temperature for each sample is 1446

4 P - c 0- (A) Fukakuragawa daci te (1 IO) PRISM INDEX (1001 (110) PRISM INDEX (100) (short) ELONGAT ION INDEX ( 1 ong) , I I I I I I I I I , (110) PRISM INDEX (100) (1101 PRISM INDEX (100) Fig. 2. PPEF diagram for zircons of some volcanic rocks 'from the Otake-Hatchobaru geothermal area and its surroundings. The circles show the relation between prism and pyramid indexes, the crosses, that between flatness and elongation indexes. The straight lines link the two symbols for each crystal. The diameter of the circles is proportional to the cryst;al width W( p rn) and is given by D=O.llog W /log(looo). 1447

5 Table 1. Prism index and estimated temperature using zircon geothermometer for rocks from the Otake-Hatchobaru geothermal area and its surroundings, Japan. Sample Rock Associated Radiometric Prism Estimated number name system age (Ma) index temp. (" C) H-265 Hr-andesi te Kuju sulfur H-257 Hr-andesite Hatchobaru I H-261 Hr-andesite Otake 0.17-t H-256 Hr-andesite Well HT ' H-251 Hr-andesite Oguni H-268 Px-Hr-andesi te None H-269 Px-andesite None H-004 Welded tuff None F- 1 Hr-daci te None Kuju volcanic rocks (H-265=Hosshozan lava, H-Z57=GotOzan lava, H-261=Kuroiwayama lava, H-256=Ryoshidake lava, H-25t=Naitayama lava, H-268=Misokobushiyama lava), H-269=Hohi volcanic rocks, H-O04=Yabakei welded tuff, F-l=Fukakuragawa dacite. Radiometric age: H-004 (Uta & Suto, 1985), E'-1 (Watanabe &, 1986), Others (Watanabe et a1.,1987) much lower than the temperatures calculated using the two-pyroxene thermometer. The zircon lemperature, therefore, will be close to the temperahre at the time of intrusion, which is more useful for the evaluation of heat sources than the magma temperature at great depths. It is noteworthy that the magma temperatures of the heat sources for the Otirke and Hatchobaru geothermal systems are as low as " C. In conclusion, the zircon geothermometer proposed here is useful to estimate magma temperature for heat source rocks to which the two-pyroxene and the two-feldspar geothermometers can not be applied, because the required co-existing minerals are not present. RE FERE NC E S Anderson, A. T., Clayton, 11. N. and Maeda, T. K. (1971) Oxygen isotopic thermometry of mafic igneous rocks. J. Geol. 79, Fuhrman, M. L. and Lindsley, D. H. (1988) Ternaryfeldspar modeling and thermometry. Amer. Mineral., 73, , M. (1990) Zircon crystal morphology and its application to earth science. Jour. Geol. SOC. Japan, 96, Lindsley, D. H. and Anderson, D. J. (1983) A twopyroxene thermometer. Jour. Geophys. Res., 88, A A90 6. Pupin, J. P. (1980) Zircon and granite petrology. Contrib. Mineral. Petrol., 73, Smith, R. L. arid Shaw, H. R. (1979) Igneous-related geothermal systems. USGS Circular, 790, Uto, K. and Suto, S. (1985) K-Ar age determination of volcanic rocks from the Hohi geothermal area, Kyushu, Japan. Rept. Geol. Surv. Japan, 264, Watanabe, K.,,.M. and.f_ujjno, T. (1981) Fission track age of volcanoes in the Kuju volcanic region in relation to geothermal activity. J. Geoth. Res. SOC. Japan, 9, Wells, P. K. A. (1977) Pyroxene thermometry in simple, and complex systems. Contrib. Mineral. Petrol., 62,