Pargasites in Lherzolite and Websterite Inclusions from Itinome-Gata, Japan

J. Japan. Assoc. Min. Petr. Econ. Geol. 68, 303-310, 1973

PARGASITES IN LHERZOLITE AND WEBSTERITE INCLUSIONS FROM ITINOME-GATA, JAPAN

KEN-ICHIRO AOKI and IKUKO SHIBA

Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Sendai, Japan

Pargasite occupies up to 4 volume per cent of lherzolite inclusions and up to 8 per cent of websterite inclusions. It occurs as discrete primary crystals and as interstitial grains among other minerals averaging less than 2 volume per cent. New analyses are presented for six representative pargasites. They are characterized by high Cr2O2 and wide variation of K2O and Mg/(Mg+Fe) ratio. The following origin of the pargasites is hypothsized from the mineralogy and chemist ry of the host rocks. Original garnet lherzolite and websterite were located at a depth of up to 50 to 75km. During upward transport, to 25km deep in the uppermost part of the mantle, the garnet was finally transformed to olivine+plagioclase+spinel and both orthopyroxene and clinopyroxene were recrystallized toward a relatively rich Tschermak's molecule. Jadeite and Tschermak's components separated from pyroxenes and silica liberated from the breakdown of garnet produced plagioclase, pargasite and orthopyroxene. However, minor newly-formed minerals were not always in equilibrium with major consti tuents. It is demonstrated that the lherzolite mantle just below the M-discontinuity contains only up to 0.04 per cent water.

INTRODUCTION (Kushiro, 1970). Because the former has been found only in kimberlite diatremes at Oxburgh (1964) suggested that am Buell Park, Arizona (Aoki, et al., 1972), it phibole would be stable in an upper mantle of peridotite composition, playing a very is thought to be a rare occurrence in the important role in basaltic magma genera mantle. The latter, which is sometimes tion. Since then, many investigators have found in lherzolite and websterite inclusions carried out studies on the stability relations in alkali basalts and also in mylonitized of various types of amphiboles under high peridotite intrusions on St. Paul and St. pressure and temperature conditions. As Peter of the Mid-Atlantic Ridge (Melson et a result of experimental work, it is known al., 1967), is considered to be the most now that the richterite50-tremoliteso solid common type of amphibole in the upper solution has the highest field stability for mantle. Serprisingly little attention has pressure among the amphibole group, up to been drawn to the mineralogical and 40 kb (corresponding to 130km in depth) chemical characteristics of pargasite in these (Hariya and Terada, 1973). Pargasite is natural rocks in spite of its importance. with stability up to 30 kb (100km deep) Most of the lherzolite and websterite

(Manuscript received, July, 13. 1973) 304 K. Aoki and I. Shiba inclusions from Itinome-gata, Japan, especially the latter, have wide range of contain negligible to small amounts of modal variations of essential constitutents pargasitic amphiboles (Kuno, 1967; Kuno and of chemical compositions. and Aoki, 1970; Aoki and Shiba, 1973). Pargasite constitutes from nil to 4 per To obtain more detailed information about cent by volume of the lherzolites and from ultramafic rock fragments directly derived nil to 8 per cent of the websterites. It has from the top of upper mantle, chemical a tendency toward high concentrations in

compositions of pargasites have been deter garnet-bearing series. However, the aver mined by an electron microprobe X-ray age percentages of pargasites in these in analyzer and wet chemical conventional clusions would be less than 2 per cent. Its methods. mode of occurrence is rather complicated.

In the lherzolite series, pargasites occur BRIEF PETROGRAPHY either as short prismatic anhedral grains

The ultramafic inclusions at Itinome which are not poikilitic in form and are less gata, northeastern Japan are rare con than 1.4mm in length along the c crystal stituents in air-fall lapilli of alkali basalt lographic axis or as interstitial grains among erupted in Holocene time (10,000 years ago). the other minerals such as olivine, ortho

These inclusions which appear to be directly pyroxene, clinopyroxene, chromian spinel derived from the upper mantle can be classi and/or garnet (now completely brokendown fied into two groups: lherzolite series and through pyroxene-green spinel symplectite websterite series. Both series are further to olivine-plagioclase-green spinel aggregate).

more subdivided into garnet-bearing and The former pargasites would be formed as a

garnet-free types by their primary mineral primary phase and the latter were produced assemblages. Although the primary and secondarily during recrystallization of the secondary mineral assemblages of the two host rocks. Sometimes it is very difficult groups are the same, there is not a con to distinguish the primary phase. Even tinuous modal gradation between lherzolites in large grains no zonal structure is usually and websterites. Thus it is easy to draw a observed. Twinning on (100) is rare.

line between the two groups. Namely, Pargasites show very weak pleochroism;

the former contains more than 55 per cent X=colorless, Z=pale brown, c•ÈZ=19•K,

olivine and more orthopyroxene than clino dispersion r>v about Z axis, weak. pyroxene. The latter includes nearly zero In the websterite series, pargasites show to 30 per cent olivine. The population of rather similar occurrence with those of websterites is less than one tenth that of lherzolite series, but all of them appear to

lherzolites. The mode of occurrence and be produced during recrystallization. petrography of the inclusions have been They are usually irregular plates, with already described elsewhere (Kuno, 1967; maximum grain size attaining 1.6mm, or Kuno and Aoki, 1970; Aoki and Shiba, they fringe other silicates as a thin mantle.

1973), and only a brief summary of the Sometimes pargasites replace clinopyroxenes

important petrographic features of par along margins or cleavages. They have gasites in lherzolites and websterites need weak pleochroism, X=very light brown, be given here. Z=light brown, c•ÈZ=19•‹, dispersion

Both the lherzolites and websterites, v>r about X, weak, Pargasites in lherzolite and websterite inclusions, Itinom-gata 305

Table 1. Chemical analyses of pargasites in lherzolite and websterite inclusions from Itinome-gata, Japan

* Analyzed by electron microprobe X-ray analyzer. ** Aoki and Shiba (1973, unpublished). 1. from garnet lherzolite (6982313). 2. from garnet lherzolite (HK58012403), analyst; H. Haramura (Kuno, 1967). 3. from spinel lherzolite (6982314). 4. from garnet lherzolite (6982312), analyst; K. Aoki. 5. from spinel lherzolite (6982408), analyst; K. Aoki. 6. from garnet websterite (Pyroxenite E), analyst; K. Aoki. (Kuno and Aoki, 1970). 7. from garnet lherzolite (6982401). 8. from garnet websterite,(7181004), analyst; K, Aoki, 306 K. Aoki and I. Shiba

None of the pargasites in the ultramafic Fen Alkaline Complex, Norway, Griffin, inclusions suffer during or after eruption-an 1973). Therefore, it is rather difficult to oxidation which is sometimes recognized in state the chemical variations and the amphiboles of some inclusions from other range of solid solutions among end members localities (cf. Aoki, 1963, 1970; Best, of amphibole groups for pargasites in 1970). lherzolites or websterites. However, no essential differences in compositions are CHEMISTRY found among them. Also, they are very Table 1 lists chemical analyses of similar to pargasites from high temperature, pargasites from lherzolite and websterite mantle-derived lherzolite intrusions (Green, inclusions arranged in order of decreasing 1964; Melson et al., 1967). 100 Mg/(Mg+Fe) ratio together with The relationships among complex solid chemical formulae calculated on the basis solutions in the calciferous amphiboles are of 23 oxygen atoms per formula unit. The practically expressed by AlIV-(AlVI+Fe+3 100 Mg/(Mg+Fe) ratio of the host rocks and +Ti) and AlIV-(Na+K) diagrams (Deer coexisting minerals are also given in Table 1 et al., 1962). The diagram of Fig. 1 show for comparison. that the pargasites from Itinome-gata are As is clear in the table, these am made up mainlyof pargasitemolecule, but phiboles have similar compositions to one they include small amounts of edenite and another and are characteristic of the tschermakite molecules. Fe+3 in pargasites pargasite series NaCa2(Mg, Fe), A12Si6O22 (OH)2, regardless of the different series or mineral assemblages of their host rocks. These pargasites are characterized by high A12O3,Cr2O3 and MgO and low total iron and K2O. The 100 Mg/(Mg+Fe) ratio shows a range from 88.6 to 72.8, but for most analyses it is higher than 82. The compositions of pargasites also resemble those of hornblendes from mafic inclusions such as amphibolite, hornblendite , horn blende gabbro and pyroxene gabbro which occur together with the ultramafic inclusions of Itinome-gata (Aoki, 1971). However, there is no overlap between them in 100 Mg/ (Mg+Fe) ratio; the ratios of the latter types range from 73.6 to 59.4. Chemical analyses of pargasites from lherzolite and websterite inclusions have only been Fig. 1. Plots of pargasites in AlIV-(AlVI+ Fe+3+Ti+Cr) and AlIV-(Na+K) reported from four localities throughout the diagrams (Deer et al., 1962). world (Itinome-gata, Japan, Kuno, 1967; Solid circle: from lherzolite and websterite, Kirsh volcano, South Yenen, Varne, 1970; Open circle: from hornblendite-hornblende gabbro and pyroxene gabbro (Aoki, 1971), Dish Hill, California, Wilshire et al., 1971; Cross: from amphibolite (Aoki, 1971), Pargasites in lheryolite and websterite inclusions, Itinom-gata 307 can not be determined by an electron micro kaersutites are rather gradational. These probe X-ray analyzer, so some analyses secondary pargasites formed at lower plotted outside of the pargasite-tschermakite pressure conditions, but the Mg-rich ones tie line in the AlIV-(A1VI+Fe+3+Ti+Cr) have very similar compositions to pargasites diagram. If it is assumed that such in lherzolite inclusions from other localities. pargasites also contain small amounts of Pargasites from Itinome-gata show Fe+3, like pargasites analyzed by wet some significant differences in chemical methods, they would be distributed between compositions when carefully compared with the pargasite-tschermakite line. those in other localities inclusions. The former pargasites have a wider variation in DISCUSSION K2O content usually ranging from trace to It should first be demonstrated that 0.25 per cent, one attaining 0.57 per cent pargasites in lherzolite and websterite (Column 5, Table 1). Such a high potassium inclusions of Itinome-gata are clearly dif pargasite has not been reported to date. ferent in chemical compositions from kaer Accordingly it seems likely that if potas sutites in wehrlite, clinopyroxenite, gabbro sium is selectively concentrated in pargasite and hornblendite inclusions belonging to among silicate minerals comprising upper the dunite-wehrlite-gabbro series (White, mantle peridotites, as is suggested by Ox 1966) and from megacrystic kaersutites burgh (1964), potassium and also sodium which sometimes occur with lherzolite in contents in basaltic magmas are controlled clusions in alkali basalts and related rocks by the volume of pargasites and the degree throughout the world. Kaersutites are of partial melting of peridotites or fractio considered to be of cognate origin. They nation of more primitive basalt magmas. precipitated from alkali basalt magmas at Indeed, the host rock of this pargasite has an early stage of fractionation under high the highest K2O content (0.09%) among pressure, high temperature, and wet condi the twenty one analyzed lherzolites from tions corresponding to few tens of kilometers Itinome-gata. Coexisting orthopyroxene and depth (cf. Aoki, 1970; Best, 1970; Binns clinopyroxene contain negligible amounts of et al., 1970). Pargasites have higher MgO K2O (Kuno and Aoki, 1970; Aoki and and Cr2O3 and lower total iron, TiO2 and Shiba, 1973). K2O than kaersutites. The ratio of 100 Mg/ One of the most important features of (Mg+Fe) in the former ranges usually pargasites from Itinome-gata is the wide from 91 to 82, while that of the latter ranges range of 100 Mg/(Mg+Fe) ratios from 88.6 from 79 to 57, and there is no overlap. to 84.2. Published pargasites in lherzolite However, Wilshire et al. (1971) have shown inclusions are limitted within the narrow that pargasites occurring interstitially in range from 91.0 to 87.9. By comparing peridotite inclusions in basanite from Dish this ratio with that of the coexisting min Hill, California were produced by the addi erals, a regular relationship is found among tion of enclosing magma. These crystals olivines, orthopyroxenes and clinopyroxenes. then reacted with the magma to form kaer With a decrease in 100 Mg/(Mg+Fe) ratio sutites during upward movement to the of olivines, which are the most abundant surface. In this case, observed composi constituents in lherzolites, the ratio of coex tional variations between pargasites and isting orthopyroxenes and cinopyroxenes 308 K. Aoki and I. Shiba decrease with the same rate as the olivines port to the uppermost part of the mantle (see Table 1). The distribution of Mg and at 25km deep, mineral assemblages chang Fe between olivine and orthopyroxene, oli ed from eclogite facies through granulite vine and clinopyroxene, and orthopyroxene facies to amphibolite facies probably accom and clinopyroxene, is closely related. The panied by a small amount of water addition. values of this ratio for coexisting mafic sili Most garnet were finally transformed to cates to pargasites from other localities also plagioclase, olivine and spinel, and some re tend to show similar relationships to those acted with pargasites to form secondary mentioned above. However, pargasites pargasite, spinel and plagioclase. All of the from Itinome-gata are fairly variable. The orthopyroxenes and clinopyroxenes were re most Mg-rich crystal coexists with the most crystallized to jadeite-free and Tschermak's Fe-rich olivine, orthopyroxene and clinopy molecule-rich pyroxenes. During this recry roxene, and the most Fe-rich pargasite is stallization process, separated jadeite and found with the most Mg-rich mafic silicates. Tschermak's components and silica (liberat This fact shows that these pargasites did ed by the breakdown of garnet) produced not always crystallize in equilibrium with plagioclase, pargasite and orthopyroxene. coexisting minerals, even in the case of However, these minor newly-formed miner primary phases. als were not always in equilibrium with Recently, Aoki and Shiba (1973) point major constituents. ed out that although Itinome-gata lherzo These characteristic reactions were ex lites are isochemical with other lherzolites, perimentally proposed by Green and Ring the compositions of orthopyroxenes and cli wood (1967) and Hill and Boettcher (1970) nopyroxenes clearly indicate recrystalliza and were also observed in the Hareidland tion. Furthermore, the garnets are comple eclogite, Norway (Mysen and Heier, 1972). tely transformed through pyroxene-spinel As already described in the previous symplectite to an aggregate of olivine, pla section, lherzolite inclusions contain averages gioclase and spinel. During these readjust of less than 2 per cent of pargasite as the ment under the conditions of falling pres sole hydrous mineral phase. Usually parga sure, most jadeite and some Tschermak's sites contain about 2 per cent of water. component are exsolved from the primary Accordingly, it is demonstrated that the pyroxenes. lherzolite mantle just below the M:discontin By combining these considerations, the uity in the Japan Sea Coast region of north following recrystallization process should be eastern Japan contains only up, to 0.04 per assumed regarding the formation of parga cent water, and the amount of water de sites. Original garnet lherzolites consisting creases with depth toward the interior, of olivine, orthopyroxene, clinopyroxene, except in the low-velocity zone. garnet and chromian spinel with negligible amounts of pargasite were located at depths ACKNOWLEDGMENTS of about 50 to 75km. At this point, parga The authors indebted to Dr. R. V. Fodor sites were in equilibrium with coexisting of the University of New Mexico for his minerals and had the same ratio of 100 kind help with analysis by an electron Mg/(Mg+Fe) as olivine, orthopyroxene and microprobe X-ray analyzer. Some part of chnopyroxene. During slow upward trans the costs of this study was defrayed by, the Pargasites in lherzolite and websterite inclusions, Itinom-gata 309

Government Fund allotted to the research eral. and Petrol., 38, 135-146. Hariya, Y. and Terada, S. (1973) Stability of for Japanese Geodynarnics Project, which is richterite50-tremolite50 solid solution at high greatly appreciated. pressures and possible presence of sodium cal cic amphibole in the upper mantle conditions. Earth Planet. Sci. Letters, 18, 72-76. REFERENCES Hill, R. E. T. and Boettcher, A. L. (1970) Water in

Aoki, K. (1963) The kaersutites and oxykaersutit the earth's mantle: melting curves of basalt es from alkalic rocks of Japan and surround water and basalt-water-carbon dioxide. Sci ing areas. Jour. Petrol., 4, 198-210. ence, 167, 980-981. Kuno, H. (1967) Mafic and ultramafic nodules •\ (1970) Petrology of kaersutite-bearing ultramafic and mafic inclusions in Iki Island, from Itinome-gata, Japan. In P. J. Wyllie, ed., Ultramafc and related rocks, 337-342. John Japan. Contr. Mineral. and Petrol., 25, 270- 283. Wiley and Sons, New York.

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rol., 30, 314-331. of basaltic magmas. Physics Earth Planet. •\, Fodor, R. V., Keil, K. and Dowty, E. Interiors, 3, 273-301. Kushiro, I. (1970) Stability of amphibole and (1972) Tremolite with high richterite-molecule content in kimberlite from Buell Park, Ari phlogopite in the upper mantle. Carnegie Inst. zona. Amer. Mineral., 57, 1889-1893. Wash. Year Book 68, 245-247. •\ and Shiba, I. (1973) Pyroxenes from Melson, W. G., Jarosewich, E., Bowen, V. T. and lherzolite inclusions of Itinome-gata, Japan. Thompson, G. (1967) St. Peter and St. Paul Lithos., 6, 41-51. rocks: a high temperature, mantle-derived in Best, M. G. (1970) Kaersutite-bearing inclusions trusion. Science, 155, 1532-1535. and kindred megacrysts in basanitic lavas, Mysen, B. O. and Heier, K. S. (1972) Petrogenesis Grand Canyon, Arizona. Contr. Mineral. and of eclogites in high grade metamorphic gneiss Petrol., 27, 25-44. es, exemplified by the Hareidland eclogite, Binns, R. A., Duggan, M. B. and Wilkinson,J. F. G. western Norway. Contr. Mineral. and Petrol., 36, 73-94. (1970) High pressure megacrysts in alkaline lavas from northeastern New South Wales. Oxburgh, E. R. (1964) Petrological evidence for Am. Jour. Sci., 269, 132-168. the presence of amphibole in the upper mantle Deer, W. A., Howie, R. A. and Zusman, J. (1962) and its petrogenetic and geophysical implica Rock-forming minerals, vol. 2 Chain silicates. tions. Geol. Mag., 101, 1-19. Longmans, Green and Co., Ltd., London. pp. Varne, R. (1970) Hornblende lherzolite and the upper mantle. Contr. Mineral. and Petrol., 263-314. 27, 45-51. Green, D. H. (1964) The petrogenesis of the high White, R. W. (1966) Ultramafic inclusions in basal temperature peridotite intrusion in the Lizard tic rocks from Hawaii. Contr. Mineral. and area, Cornwall. Jour. Petrol., 5, 134-188. Petrol., 12, 245-314. •\ and Ringwood, A. E. (1967) An experi mental investigation of the gabbro to eclogite Wilshire, H. G., Calk, L. C. and Schwarzman, E. C. transformation and its petrological applica (1971) Kaersutite-a product of reaction be tween pargasite and basanite at Dish Hill, tions. Geochim. Cosmochim. Acta, 31, 767-833. California. Earth Planet. Sci. Letters, 10, 281- Griffin, W. L. (1973) Lherzolite nodules from the 284. Fen alkaline complex, Norway. Contr. Min

一の目潟産レルゾライト・ウエプステライト中のパーガサイト

青木謙一郎・柴いく子

バ一ガサイ卜はレゾラの4vol.%ウエプステライhの8vol.%までを占め,初生鉱物として又は他の鉱物の間をうめて産する。その量は普通2vol.%以下である。新しく代表的な6つのバーガサイトについて分

析した。これらはCr2O3量が多いこと,K2OとMg/Mg+Feが大きく変化しているととが特微である。

母岩の鉱物学的及び化学的性質からパーガサイトの起源についての次のような仮説がなきれる。ざくろ石レルゾ 310 K. Aoki and I. Shiba

ライトとウエブステライトは初めは50～75kmの深さに存在した。その後マントルの最上部(25kmの深さ) に運ばれる間に,ざくろ石は最終的にかんらん石+斜長石+スピネル+両輝石に分解し,又斜方輝石,単斜輝石とも相対的にチェルマック分子に富む方向へ再結晶した。輝石から分かれたジェイダィト及びチェルマック成分とぎくろ石の分解で遊離したSiO2は斜長石,パーガサイト,斜方輝石を形成した。しかし新しく生成きれた少量の鉱物は主要鉱物とは必ずしも平衡ではなかった。 M面直下のレルゾライトから成るマントルは多くても0.04%の水しか含まないと考えられる。