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Journal of Volcanologyand Geothermal Research, 59 (1994) 279-293

Compositional variation of olivine-chromian spinel in Mg-rich magmas as a guide to their residual spinel peridotites Shoji Arai Department of Earth Sciences, Faculty of Science, Kanazawa University, Kakuma, Kanazawa 920-11, Japan

Abstract

Peridotite restites or last equilibrated peridotites for main magma groups are estimated on the basis of compositional relations between olivine (Fo content ) and chromian spinel (Cr~ = C r / ( C r + Al ) atomic ratio ). A particular magma or magma group develops a particular Fo-Cr~ variation trend ( = Fo-Cr~ fractionation line) during magmatic differentiation. A restite for the magma can be assessed by extrapolating the Fo-Cr~ fractionation line back to the olivine-spinel mantle array ( = a Fo-Cr~ trend for residual mantle peridotites). Cr~'s of chromian spinel in peridotite restites for magma types are estimated as follows: 0.2-0.6 (mostly 0.4-0.6) for MORB, 0.20.5 for intraplate alkali basalts, ca. 0.7 for intraplate tholeiites, ca. 0.8 for oceanic plateau basalts, 0.3-0.5 (mostly 0.4-0.5 ) for back-arc basin basalts, > 0.9 for boninites, ca. 0.9 for high-Mg arc tholeiites and high-Mg andesites, 0.1-0.9 (mostly 0.1-0.7) for Quaternary arc basalts of the Northeast Japan arc, and 0.7-0.8 for magmas which produced Precambrian layered intrusions. Arc magmas from a single-arc system can produce various peridotite restites within the upper mantle, which is due to a wide range of melting conditions (depth and degree of melting, hydrous to anhydrous melting) and of source peridotites (fertile to refractory ).

1. Introduction

Mantle processes related to primary magma genesis are subjects of debate. For example source mantle peridotites for a particular magma can not be specified unless the degree of partial fusion is given. We can estimate, however, the peridotite restite(s) for a given magma group to some extent on the basis of F o ( o l i v i n e ) - C r ~ ( = C r / (Cr + Al ) atomic ratio of chromian spinel) relationships of the Mg-rich magmas, combined with the Fo-Cr# relationships of the known mantle peridotite restites (the olivine-spinel mantle array) (Arai, 1987, 1990b). This procedure may make an assessment of genetic links between vol-

canic rocks and mantle restites much easier. The results may also place some constraints on origins of mantle materials we can obtain. The lava-restite genetic links and the origin of alpine-type peridotites were discussed on the basis of Cr~ of chromian spinel by Dick and Bullen ( 1984); The method used in the present paper allows estimation of Cr~ of spinel in primary magmas in equilibrium with mantle restite by extrapolating the observed Fo-Cr~ relationships to those in mantle peridotites. As a case, island-arc mantle restites will be estimated by a systematic evaluation of the Fo-C~ relationships of Quaternary Mg-rich arc magma frotn the Northeast Japan arc, one of the typical island arcs on the Earth.

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S. A rai / Journal of Volcanology and Geothermal Research 59 (1994) 2 79-293

2. Fo-Cr# relationships during magmatic differentiation Olivine and chromian spinel are often co-precipitated from primary or nearly primary magmas: they are commonly found in Mg-rich early cumulates as well as in Mg-rich volcanics. Behaviors of Cr in chromian spinel and in magmas are complex, depending on pressure (e.g., Sigurdsson and Schilling, 1976; Dick and Bullen, 1984), oxygen fugacity (Hill and Roeder, 1974; Fisk and Bence, 1980; Barnes, 1986; Murck and Campbell, 1986 ), temperature (Fisk and Bence, 1980; Murck and Campbell, 1986) and magma chemistry (Dick and Bullen, 1984; Allan et al., 1988). I empirically determine the compositional changes of chrornian spinel during differentiation of magmas by monitoring the change of Fo content of coexisting olivine. Chromian spinel in volcanics occurs as (micro)- phenocrysts or minute euhedral inclusions in olivine phenocrysts. In general, TiO2 and Fe203 contents of chromian spinel almost monotonously increase, although considerably scattered in some cases, with a decrease of Fo content of olivine (e.g., Arai and Takahashi, 1987; Arai, 1990b, 1992). Some examples of the Fo-Cr~ relationships during differentiation of magmas are shown in Fig. 1 and are named here the Fo-Cr~ fractionation lines (Fig. 1 ). For volcanic rocks, the FoCr~ fractionation lines can often be drawn by data even from a single thin section. For example, a Shodo-shima basalt associated with the Setouchi high-Mg andesite (Tatsumi and Ishizaka, 1981 ) or a Neogene arc-tholeiite from Ryozen, the Northeast Japan arc (Yoshida et al., 1985) have a wide range of Fo (more than l0 mole units) for phenocryst olivine which has spinel inclusions (Fig. 1 ). The Cr~ of spinel does not always decrease, but also sometimes increases or is unaffected, with a decrease of the Fo of coexisting olivine (Fig. 1 ). Fig. l shows that the FoCr~ fractionation lines which start at high Crg and at low Cr~ tend to have negative slopes and positive slopes, respectively. The Fo-CI~ fractionation lines are nearly straight and they or their extensions cross the olivine-spinel mantle array at high angles (Fig. l ). Note that, as used

here, the differentiation of magma may include various processes, including crystallization differentiation, magma mixing and assimilation. The magma mixing and assimilation may make the Fo-Cr~ lines more diffuse than in simple crystallization differentiation (cf. Allan et al., 1988). The Fo-Cr~ relationships are also affected by the above-solidus cooling rate of magmas because Mg-Fe diffusivity in olivine is different from, i.e. higher than, Cr-AI diffusivity in spinel (e.g., Scowen et al., 1990). In cumulates the Fo-Cr~ relations could be different from those in volcanics because of much lower cooling rate and possible interaction with intercumulus melts. In spite of these possible complexities the slopes of the Fo-Cr~ fractionation lines for volcanic rocks are apparently controlled by the presence or absence of plagioclase as a co-precipitate with olivine and spinel (Figs. 2 and 3; Arai, 1990b). Cr~'s of spinel coexisting only with olivine_pyroxenes decrease or are kept almost constant with decreasing Fo of olivine (Figs. 13 ); they increase with decreasing Fo of olivine when spinel coexists with olivine and plagioclase (Figs. 2 and 3). For example Kajishiyama and Misaki basalts, southwest Japan, for which FoCr~ fractionation lines have negative slopes (Fig. 2 ), do not have or rarely have plagioclase phenocrysts coexisting with olivine and spinel (Fig. 3 ). On the contrary Takakusayama and Rishiri hasalts, for which Fo-Cr~ fractionation lines have positive slopes (Fig. 2), have large amounts of plagioclase phenocrysts coexisting with olivine and spinel (Fig. 3). Co-precipitation of large amounts of plagioclase can make spinel depleted in A1 relative to Cr. Primary magmas formed by lower-degree partial melting may tend to be saturated with plagioclase, co-precipitating with olivine and chromian spinel, in earlier stage of solidification than those formed by higher-degree partial melting (cf. Ishiwatari, 1985), and are expected to have such Fo-Cr~ trends. The Vshaped Fo-Crg lines are possible if olivine +chromian spinel+ plagioclase crystallize after olivine+chromian spinel. However, they are found neither in volcanics nor in cumulates in this study. The Fe 3÷ content of chromian spinel also af-

S. Arai / Journal of Folcanology and Geothermal Research 59 (1994) 279-293

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Fig. 1. Relationships between Fo of olivine and Cr~ of chromian spinel ( =Fo-Cr~ fractionation lines) in some volcanic rocks. Manam=volcanics from the Manam volcano, Papua-New Guinea (Johnson et al., 1985). Setogawa=a Miocene intraplate-type picrite basalt from the Setogawa belt, central Japan. Shodo-shima=a Miocene island-arc olivine basalt associated with the Setouchi high-Mg andesites from Shodo-shima, western Japan (Tatsumi and Ishizaka, 1981 ). Ryozen=a Miocene island-arc olivine tholeiite from Ryozen, the Northwest Japan arc (Yoshida et al., 1985 ). Takakusayama=a Miocene intraplate-type alkali olivine basalt from Takakusayama, central Japan (Arai, 1990b), which is considered to be fragment of accreted seamount (Tonouchi and Kobayashi, 1982). OSMA = olivine-spinelmantle array (Arai, 1987; 1990a).

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Fig. 2. Fo-Cr~ fractionation lines with positive or negative slope. Kajishiyama, an intraplate-type nepheline basanite of 5 Ma from Tsuyama district, southwest Japan (Arai, 1990b; Iwamori, 1991 ). Misaki, an intraplate-type olivine basalt of 0.55 Ma from Oki-Dogo island in the Sea of Japan (Uchimizu, 1966). Rishiri, an island-arc alkali olivine basalt from the Rishiri volcano in the Sea of Japan off Hokkaido (Arai and Takahashi, 1987 ). For Takakusayama see Fig. 1.

3. Fo-Cr# relationships of main magma groups: Observations 3.1. M O R B MORB, mostly of N-type, lie within or near the olivine-spinel mantle array (Arai, 1987, 1990a), indicating their relatively undifferentiated nature (Fig. 5A). A possible Fo-Cr~ fractionation line is drawn. Cr~ of spinel is concentrated within a relatively narrow range, 0.4-0.6 (Fig. 5A).

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Fig. 4. Ratios of trivalent cations of chromian spinel for two types of basalts (See Figs. 2 and 3 ). Note the Kajishiyama and the Misaki spinels only weakly increase their Cr~'s with a decrease of Fo of olivine (arrow) whereas the Takakusayama and the Rishiri spinels rapidly increase their Cr~'s with a decrease of Fo of olivine (arrow).

3.2. Intraplate alkaline basalts Alkali basalts, which often have xenoliths of deep origin, are mainly available from intraplate regions. Most of the data of Fig. 5B are from the Tertiary to Quaternary mantle xenolith-bearing alkali basalts from the Southwest Japan arc (Arai, 1990b), but data from other provinces are mostly included in the compositional range of the Japanese Cenozoic alkali basalts (Fig. 5B). For example a possible Fo-Cr~ fractionation line for alkali olivine basalts from Diamond Craters, Oregon (Russel and Nicholls, 1987), may have a similar slope to the Takakusayama one (Fig. 5B). For the alkali basalts from the Hawaii-Emperor Seamounts may plot in the same region of Fig. 5B, that is Cr#