Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow ...

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166 CHEN-HONG CHEN ET AL.TABLE 1 (Continued)Sample no.olcpxHP65KAP16 (114370/17)166KAP1 (114743/1)168KAPl(114S76/l)t170K.AP2 (114970/2)75KAP5 (115032/1)75KAP6 (115032/2)75KAP7 (115032/3)**75KAP8 (115032/4)75KAP9(U5032/5)t97989396949896979811434—211WeMite and olivine clinopyroxeniteXIX3"X4X5Af§**X5§X8X17X20X23Q§B4f§6511463 (114385/13)§**65115153 (114386/29)t |)65KAP17 (114370/18)66KAP2(114743/2)§66KAP3 (114743/3)68KAP1 (114876/l)tT*71264080606675618130404021577215332970601540332539177060557943288565213122222—p—21p2——P————* Except where indicated, percentages are basedon counts of at least 7000 points.t Composite xenolith.X Present, but
ULTRAMAFIC XENOLITHS FROM THE KAUPULEHU FLOW 167Schwarzmann, 1977). The most common texture is characterized by kink-banded orstrained olivine porphyroclasts (1-5 mm) in a matrix of finer-grained (0-2-0-4 mm) recrystallized,undeformed olivine grains, some of which have rounded to polygonal shapesand well-defined triple junctions. All samples except 75KAP6 contain small amounts ofclinopyroxene (augite and subcalcic augite, Table 1). It is pale green in thin section andtypically occurs as small (~ 30 /zm) interstitial grains. Clinopyroxene oikocrysts (up to1 mm across) with olivine inclusions occur in one sample (XI2), whereas clinopyroxenecrystals in samples AA, B4, and X4 are blocky and could possibly be cumulus (T. N. Irvine,pers. comm.). Opaque spinel grains of various shapes and sizes (20-400 /un) are ubiquitous.Some occur as small interstitial grains in the fine-grained matrix, some are euhedral orsubhedral grains included in olivine, and a few are enclosed in clinopyroxene. The euhedralhabit of spinel crystals in olivine indicates that they crystallized relatively early. Orthopyroxenewas found only in composite xenolith 68KAP1 and plagioclase was found in onedunite xenolith (75KAP7). Both minerals are interstitial. Inclusions of CO 2 and silicateglass are common along healed microfractures in both clinopyroxene and olivine (Roedder,1965; Kirby & Green, 1980).Wehrlite and olivine clinopyroxeniteThree types of clinopyroxene-rich xenoliths were identified by Jackson et al. (1981):(1) those that have deformed kink-banded olivine and clinopyroxene in a matrix ofrecrystallized olivine; (2) those that have kink-banded olivine but undeformed clinopyroxene;and (3) those with undeformed cumulus olivine and intercumulus clinopyroxene. InTable 1, samples XI, X3, X4, X5, X17, X20, X23, Q, B4, 65115153, and 68KAP1 are texturetype 1. Samples X5A, X8, and 66KAP2 are texture type 2. Cumulus grain shapes andpostcumulus poikilitic textures are common in types 2 and 3. Deformed olivine grains aregenerally smaller (0-5-1-0 mm) than the porphyroclastic olivine in the dunite xenoliths.Modal mineral proportions for wehrlite and clinopyroxenite xenoliths are given in Table 1.As in the dunite xenoliths, trains of CO 2 and glass inclusions are common in theclinopyroxene and deformed olivine. Rare small interstitial grains of orthopyroxene (only insample 68KAP1), spinel, and plagioclase also occur.MINERAL COMPOSITIONSOlivineOlivine compositions in the dunite xenoliths range from Fo 81 4 to Fo 894 (Fig. 2 andTable 2). There is no compositional difference between the large deformed and smallrecrystallized grains in the same sample. Olivine compositions in wehrlite and olivineclinopyroxenite xenoliths are generally more iron rich (Fo 81 _ 84 . 7 ) than those in the dunites,but the two compositional ranges overlap (Fig. 2 and Tables 2 and 3). NiO contents(0-05-0-28 wt.%) in olivines from wehrlites and clinopyroxenites also overlap those fromdunites (0-05-0-50 wt.%). Forsterite content and NiO content of olivine covary.SpinelExcept in samples X8, X9, X10, and Al, the chemical compositions of spinel grains withina single thin section are identical within analytical uncertainty (Tables 4 and 5). Despite thevariations of spinel composition in some samples, a reasonably good positive correlation
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Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow ...

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