Fluid-Mineral Interactions: A Tribute to H. P. Eugster © The Geochemical Society, Special Publication No.2, 1990 Editors: R. J. Spencer and I-Ming Chou Crystallization history of a pyroxenite xenolith in a granulite inferred from chemical and single-crystal X-ray data JrRI FRYDA Geological Survey, Malostranske namesti 19, 11821 Praha 1, Czechoslovakia and MILANRIEDER Institute of Geological Sciences, Charles University, Albertov 6, 12843 Praha 2, Czechoslovakia Abstract-Porphyroclasts of orthopyroxene and clinopyroxene in the pyroxenite have bulk com- positions corresponding to an equilibrium at about 1260°C and their content of Fe3+ indicates a relatively high oxygen fugacity during their crystallization. Later they were corroded during recrys- tallization by a reaction with olivine that yielded spinel grains and pyroxene neoblasts. This reaction proceeded in the subsolidus, under increasingly reducing conditions, and produced spinel of pro- gressively smaller grain size and increasing Al content. Finally, at about 910°C, the recrystallization ceased, at approximately the same time as the porphyroclasts underwent exsolution. Orthopyroxene porphyroclasts exsolved clinopyroxene and spinel lamellae, crystallographically oriented in the host. Clinopyroxene porphyroclasts probably exsolved enstatite first and then spinel, as a result of a strong reduction probably caused by the diffusion of hydrogen. Oxygen, thus freed, combined with hydrogen, yielding about 2.5 OR per seven R2+in the exsolved enstatite and converted it into an orthoamphibole- like phase that is perfectly crystallographically aligned in the host. It appears that the unmixing of spinel lamellae may be viewed as a redox reaction and that concomitantly forming hydrated phases may serve as a proof of the role of hydrogen during the evolution of mantle xenoliths. GEOLOGICAL SETTING PETROGRAPHY THEXENOLITHof olivine websterite studied comes The studied xenolith contains orthopyroxene from the abandoned Lichtenstein quarry located (OPX), clinopyroxene (CPX), olivine (OL), and on the SSW slope of Mount Kid, about 1.5 km spinel (SPN). The minerals vary in proportion, so from the village of Vysny near Cesky Krumlov, the volume percentages (OPX 45-65%, CPX 15- southwestern Czechoslovakia. The xenolith is about 25%,OL 15-30%, SPN < 3%)are only approximate. 70 em in diameter, embedded in a relatively ho- Nevertheless, according to Streckeisen's (1973) mogeneous, partly recrystallized kyanite-garnet fel- classification, the rock can be referred to as olivine sic granulite, which belongs to the Blansky les websterite. Mountains Massif, one of the largest granulite mas- Orthopyroxene is present in what appears to be sifs of the Moldanubian crystalline complex. While two generations. The first is large lamellar porphy- numerous ideas on the genesis of the granulites in roclasts (up to 2 em in size, Fig. 1), the second, the area have been put forth, it is generally agreed smaller lamellae-free neoblasts «2 mm). The por- that they formed at high pressures, near the base of phyroclasts are irregularly shaped because of cor- the Earth's crust (VRANA,1987;FIALAet al., 1987). rosion during recrystallization, but remain elon- Ultramafic rocks such as dunites, lherzolites, harz- gated parallel to [001]. They are intensely bent and burgites, and olivine websterites (most of them car- often kinked along deformation planes roughly rying pyrope garnet) represent about 5 vol.% of the perpendicular to [001], along which smaller frag- rocks of the Blansky les Mountains Massif, and so ments separated (Fig. 1). This deformation cannot far they have received relatively little attention. be younger than the exsolution of lamellae; if it Further details about the geology of the area can be were, we would not observe SPN grains to follow found in papers by FEDIUKOVA(1965, 1978), ROST deformation zones nor would the lamellae there be (1966), KODYM (1972), KODYM et at. (1978), by about an order of magnitude thicker than in VRANA(1979, 1987), SLABY(1983), or FIALAet at. unaffected parts of the porphyroclasts (Fig. 2). The (1987). The papers by VANBREEMENet at. (1982) density oflamellae is uniform throughout the grains and AFTALIONet at. (1989) deal with geochro- all the way to the border with other pyroxene grains. nology. However, along contacts with olivine, there may be 85 86 J. Fryda and M. Rieder FIG. I. a. Orthopyroxene porphyroclast, deformed and broken, surrounded by recrystallization products. Lamellae-free zones in the porphyroclast rim recrystallized olivine grains. Crossed polars, width of photograph = 4 mm. b. A plastically deformed OPX porphyroclast corroded during re- crystallization. Olivine in the lower left-hand corner exhibits wavy extinction. The lamellae in OPX do not seem to disappear along [100]opxas readily as along directions in the (lOO)opxplane. Crossed polars, width of photograph = 4 mm. c. Lamellae of spinel (SPN) in OPX porphyroclast seen in a section roughly perpendicular to [00l]opx. The SPN platelets lie parallel to (I OO)opxand apparently contribute to the easy parting along that plane. Plane-polarized light, width of photograph = 0.5 mm. a lamellae-free margin (0.5-1.0 mm thick). Con- (Fig. 3), but appear like COr or CO-containing fluid trary to porphyroclasts, neoblasts do not show de- inclusions known from mantle xenoliths (Fig. 3; formation, and occur jointly with recrystallized ol- KIRBY and GREEN, 1980, pI. 4-D; ANDERSON et ivine and large (vermicular) grains of spinel. al., 1984, Fig. l-E, F; BERGMAN and DUBESSY, Clinopyroxene porphyroclasts reach up to 7 mm 1984, Fig. 2a, b). The zones of inclusions seem to in size and contain abundant lamellae (Figs. 2 and have no crystallographic orientation and appear not 3) evenly distributed throughout except for lamel- to have a systematic relation to kink bands. The lae-free zones, about 0.5 mm wide, that follow the rock also contains small OL grains free from signs border with OL grains. Clinopyroxene porphyro- of deformation or inclusions which always accom- clasts do not show signs of plastic deformation, but pany OPX neoblasts and vermicular spinel. many are fragmented. Small lamellae-free fragments Spinel is red-brown, and its large (vermicular) surrounded by olivine cannot be distinguished from grains or aggregates, which represent the vast ma- possible CPX neoblasts. jority of spinel in the rock, may reach 5 mm across. The mean size ofOL grains is about 4 mm, larger They do not seem to associate preferentially with grains are elongate and often penetrated by kink any of the three primary minerals in the xenolith. bands. The larger grains also contain zones or veils Contacts of OPX and CPX porphyroclasts are of isometric inclusions that were not studied further punctuated with chains ofSPN grains (Fig. 3) rang- Crystallization history of pyroxenite xenolith in granulite 87 FIG. 2. Back-scattered electron images of pyroxene porphyroclasts. a, b. Section across a CPX porphyroclast, approximately perpendicular to [OOI]cpx.Most spinel lamellae (white) are parallel to (OIO)cpxand lamellae of the orthoamphibole phase (OA, black) are mostly parallel to (100)cpx, but there are some exceptions. The 'wrong' orientation of SPN lamellae seems to be associated with the OA phase. The (100)cpx is vertical in a. and horizontal in b. c. OPX porphyroclast, cut roughly perpendicular to [OlO]opx,showing a deformation zone with swollen CPX lamellae (light grey) and enriched in SPN grains (white). d. A section ofOPX porphyroclast, approximately parallel to [OOI]opx, illustrating the distribution of lamellae of SPN (white) and CPX (light grey). ing between 10-3 and 10-1 mm in size. Similar recorded on a DRON-2,0 diffractometer (Burevestnik, 0 grains of spinel « 10-2 mm) occur along defor- Leningrad), with scanning speed 0.5 (21.1)/minute,and processed by BURNHAM'S(1962) least squares program. mation zones in OPX porphyroclasts (Fig. 2) or The cell edge of spinel was based on film data extrapolated inside OL crystals. against the Nelson-Riley function (RIEDER,1971). In the nomenclature of MERCIER and NICOLAS An ARL-SEMQ microprobe was used to obtain chem- (1975), the texture of the Lichtenstein quarry xeno- ical compositions. The accelerating voltage was 15 kV, lith is porphyroclastic. These authors cite the inclu- current 50 nA, and counting time 20 seconds. Natural silicate minerals were used as standards for Ti, Cr, Mn, sion of spinel in olivine as evidence of a recrystal- Si, Fe, K, Na, Mg, Ca, and AI, and synthetic Ni olivine lization of olivine, following an intensive defor- was used for Ni. Analytical points were selected after ob- mation. However, such an effect is typical of the servation in back-scattered electron mode (accelerating genetically younger equigranular texture, and thus voltage 30-35 kV). Five to ten grains of each type of min- we may refer to the present texture as transitional eral (porphyroclast, neoblast, lamella) were analyzed. If homogeneous, the mineral was analyzed at three to six between porphyroclastic and equigranular. points, otherwise, up to 25 analyses were made. The Ca in olivine and Ni in OL, OPX, CPX, SP were determined EXPERIMENTAL with 100 nA current and 100 seconds counting time (used for Ca by ADAMSand BISHOP,1986). The bulk compo- Single-crystal X-ray work was done on a Dioptra sition ofOPX and CPX porphyroclasts was analyzed either precession camera (HANICet al., 1955). Powder data were using a defocussed electron beam (35 µm) or by allowing 88 J. Fryda and M. Rieder the beam to scan over rasters, 50 X 50 um, that were suits. After corrections for drift of the instrument and for combined so as to pave stretches several hundred µm in dead time of the detectors, the data were reduced according diameter. Both methods yielded practically identical re- to BENCEand ALBEE(1968). When assessing the bulk chemical compositions, one must be aware that the un- certainties may exceed the errors indicated because the assumption of a homogeneous distribution of each element in the X-ray generation range is not fulfilled.