Evidence of hydrous differentiation and crystal accumulation in the low-MgO, high-Al2O3 Lake Basalt from Medicine Lake volcano, California
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Phenocryst
Porphyritic
Igneous differentiation
Incompatible element
Petrogenesis
Fractional crystallization (geology)
Anorthite
Porphyritic magnesian andesites (PMAs; SiO 2 ∼55 wt. %; MgO ∼7 wt.%) are found in the Miocene Setouchi volcanic belt, SW Japan. The PMAs are characterized by the presence of plagioclase phenocrysts, whereas the rather aphyric, mantle-derived high-Mg sanukitoid andesites (HMAs) found in the region do not contain such phenocrysts. The following petrographic observations suggest a role of mixing of magmas in producing the PMA magma: (1) reversely zoned pyroxene phenocrysts, both clino- and ortho-pyroxenes, are observed in PMAs; (2) normally zoned clinopyroxene may be in equilibrium with olivine but not with normally zoned orthopyroxene in terms of Fe-Mg partitioning; (3) plagioclase displays a wide compositional range (An 80-45 ) with a bimodal distribution; (4) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in HMA sanukitoids and the other identical to those in basalts, are recognized in terms of Ni-Mg and Cr-Al-Fe 3+ relations, respectively. The above petrographic characteristics may be best explained by the presence of three end-member magmas, namely, Mg-rich basalt and HMA magmas, both having olivine and clinopyroxene phenocrysts and a low-Mg, plagioclase-phyric andesite magma. Major, trace, and isotope compositions of these magmas may also support the magma-mixing origin for PMAs.
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Sr isotope compositions were determined for anorthite megacrysts, olivine phenocrysts and host basalts of Miyakejima volcano, Izu-Ogasawara arc, Japan. The anorthite megacrysts are of An97-94 in composition. The olivine phenocrysts have almost the same composition (Fo84-82) as olivine inclusions in the anorthite megacrysts. The 87Sr/86Sr ratios of anorthites range within a narrow variation of 0.70340-0.70352, while those of olivine phenocrysts and olivine inclusions show distinctively higher values (0.70357-0.70374) than those of anorthites. Host rock samples excluding megacrysts yield intermediate 87Sr/86Sr values between anorthite and olivine. This disequilibrium in 87Sr/86Sr ratios between phenocrysts and host rocks demonstrates that anorthites and olivines did not crystallize directly from the host magmas but were probably xenocrysts carried up by the magmas. The present data further suggest that the olivine phenocrysts and olivine inclusions in anorthite megacrysts have the same origin and that the crystallization of anorthite occurred posterior to olivine crystallization. The difference in 87Sr/86Sr ratios between olivine inclusions and host anorthite megacrysts provide an evidence for magma mixing between two types of magmas (both of basaltic in composition) with different 87Sr/86Sr ratios. The basaltic magmas which formed the Miyakejima volcano may also be explained by the mixing of above two end member magmas, at least in some stage (or stages) of magma evolution.
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SUMMARY Some thin basaltic intrusive sheets in south-eastern Iceland consist in cross-section of a porphyritic central zone sharply bounded by non-porphyritic margins. Within the porphyritic zone phenocrysts of plagioclase, augite and olivine are arranged in two main layers, an upper layer containing mostly plagioclase phenocrysts, and a lower layer containing concentrations of augite and olivine phenocrysts. The phenocrysts are considered to have been gravitationally sorted during the passage through the sheets of a highly fluid and strongly flowing porphyritic basalt magma.
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Bangly Quarry is situated about two miles to the north-west of Haddington, and is well known for the fine porphyritic quartz-banakite that occurs there; and also for a so-called very fine porphyritic trachyte carrying large crystals of sanidine up to two inches in length. It is thus described in the Memoir of the Geological Survey for East Lothian, 1910, p. 79: ‶The finest example of a porphyritic trachyte in the Garleton area is met with in the Bangly or Silver Hill Quarry. . . . It belongs to the special group of quartz-banakites, and most of the normal phenocrysts are of plagioclase; but these are associated, in the large quarry, with sanidine crystals which are often two inches long, and in many cases conspicuously twinned according to the Carlsbad law. This rock is probably the finest example of a porphyritic trachyte in the British Isles; yet so local is the development of the late formed giant phenocrysts, that at the west end of the same quarry they have almost entirely disappeared.″ The face of Bangly Quarry is nearly 80 feet high, and looks towards the north. The rock is dark red-brown in colour, and the jointing is in smooth vertical planes, except at one part near the east end. At this place a vertical dyke (Plate XL., Fig. 1), about 12 feet wide, cuts through the whole thickness of the rock, in a nearly north-and-south direction. The jointing of the dyke is conspicuous and horizontal, and there is a chilled
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Sr isotope compositions were determined for anorthite megacrysts, olivine phenocrysts and host basalts of Miyakejima volcano, Izu-Ogasawara arc, Japan. The anorthite megacrysts are of An97-94 in composition. The olivine phenocrysts have almost the same composition (Fo84-82) as olivine inclusions in the anorthite megacrysts. The 87Sr/86Sr ratios of anorthites range within a narrow variation of 0.70340-0.70352, while those of olivine phenocrysts and olivine inclusions show distinctively higher values (0.70357-0.70374) than those of anorthites. Host rock samples excluding megacrysts yield intermediate 87Sr/86Sr values between anorthite and olivine. This disequilibrium in 87Sr/86Sr ratios between phenocrysts and host rocks demonstrates that anorthites and olivines did not crystallize directly from the host magmas but were probably xenocrysts carried up by the magmas. The present data further suggest that the olivine phenocrysts and olivine inclusions in anorthite megacrysts have the same origin and that the crystallization of anorthite occurred posterior to olivine crystallization. The difference in 87Sr/86Sr ratios between olivine inclusions and host anorthite megacrysts provide an evidence for magma mixing between two types of magmas (both of basaltic in composition) with different 87Sr/86Sr ratios. The basaltic magmas which formed the Miyakejima volcano may also be explained by the mixing of above two end member magmas, at least in some stage (or stages) of magma evolution.
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Abstract Phosphorus X‐ray maps of olivine phenocrysts in many type II (FeO‐rich) porphyritic chondrules in LL3.00 Semarkona and CO3.05 Y 81020 reveal multiple sets of thin dark/bright (P‐poor/P‐rich) layers that resemble oscillatory zoning. Such discrete layers are generally not evident in BSE images or in Fe, Cr, Ca, Al, Mg, or Mn X‐ray maps because rapid diffusion of these cations in olivine at high temperatures smoothed out their initial distributions, thereby mimicking normal igneous zoning. In contrast, the relatively slow diffusion of P in olivine preserves original dendritic or hopper morphologies of olivine crystals; these skeletal structures formed during quenching after initial chondrule melting. The skeletal olivine crystals were filled in with low‐P olivine during cooling after one or more subsequent heating events, mainly involving the melting of mesostasis. Crystallization of mafic silicates depleted the mesostasis in FeO and MgO and enriched it in silico‐feldspathic components. Sectioning of the olivine grains at particular orientations can produce apparent oscillatory zoning in P. Strong evidence of a secondary melting event is evident in Semarkona chondrule H5k. Phenocryst H5k‐2 in this chondrule has a relict core (with rhythmic P zoning layers) that was fractured and severed; it is overlain by a set of differently oriented subparallel P‐poor olivine layers. Chondrule C6f from Y 81020 contains a large multi‐lobed olivine phenocryst that still preserves hopper cavities, partially outlined by P‐poor/P‐rich olivine layers. The thin P‐rich rims surrounding many olivine phenocrysts could reflect a short period of rapid grain growth after a late‐stage chondrule reheating event.
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