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

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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.
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