Showing papers on "Phenocryst published in 1983"

Fractionation of pyroxene-phyric MORB at low pressure: An experimental study

Abstract: One-atmosphere melting experiments are used to assess the role of clinopyroxene in producing the compositional variations observed in mid-ocean-ridge basalts (MORBs) from the North Atlantic. Analog models of natural glasses and associated phenocrysts show that several possible parental magmas may undergo low pressure fractional crystallization involving olivine and spinel, followed by plagioclase, and then by augite. The phenocryst phase assemblages in natural deep-sea basalts are closely correlated with the major element compositions of their associated quenched glasses, and the projections of these glasses on the Oliv-Cpx-Qtz pseudoternary correspond to the 1-atmosphere phase boundaries and reaction points defined by laboratory experiments. Comparison of natural phenocryst's with experimental phases indicates that the augites preserved in moderately fractionated MORB from the FAMOUS area may have formed at or near the ocean floor and need not be relics of high pressure processes.

Trace element abundances and mineral/melt distribution coefficients in phonolites from the Laacher See volcano (Germany)

Abstract: Twenty six whole rocks, seven matrix and fifty three mineral separates from the compositionally zoned late Quaternary Laacher See tephra sequence (East Eifel, W Germany) were analyzed by instrumental neutron activation. These data document the chemical variation within the Laacher See magma chamber prior to eruption with a highly fractionated phonolite at the top and a more mafic phonolite at its base as derived from other data. Incompatible elements such as Zn, Zr, Nb, Hf, U, light and heavy rare earths are extremely enriched towards the top whereas compatible elements (e.g. Sr, Sc, Co, Eu) are strongly depleted. Semicompatible elements (Ta and some middle REE) are depleted at intermediate levels. This chemical variation is shown by whole rock and matrix data indicating the phonolite liquid was compositionally zoned regardless of phenocryst content. Hybrid rocks (phonolite-basanite) show the largest concentrations for compatible elements. All elements (except Rb) display continuous compositional variations with regard to the stratigraphic position of pumice samples. From these data we are able to distinguish three main units: An early erupted highly fractionated magma, the main volume of evolved phonolite and a mafic phonolite as the final products.

Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes

Abstract: Re-examination of petrochemical data for high-silica ash-flow tuffs suggests that crystal-liquid fractionation plays a major role in the differentiation of high-silica magmas. For the high-silica Bishop Tuff, the enrichment and depletion of 27 major and trace elements correlates with the bulk crystal-liquid partition coefficients (calculated from the data of Hildreth) in ways that are consistent with crystal fractionation. Comparison of the Bishop Tuff with other high-silica tuffs and rhyolites with different compositional variations and different phenocryst assemblages shows the correspondence of chemical gradients with mineralogy. Crystal zoning studies in the Cordillera Paine granite in southern Chile suggest that the evolution of differentiated liquids similar to those in the Bishop Tuff results from crystal fractionation. The physical mechanisms of separation of liquid and crystals for the Paine granite and Bishop Tuff were quite different, however. Differentiation of the high-silica ash flows might result from advection of a less dense liquid from a crystallizing boundary into the top of the magma chamber.

Geochemical studies of abyssal lavas recovered by DSRV Alvin from Eastern Galapagos Rift, Inca Transform, and Ecuador Rift: 2. Phase chemistry and crystallization history

Abstract: A diverse suite of lavas recovered by DSRV Alvin from the eastern Galapagos rift and Inca transform includes mid-ocean ridge tholeiitic basalts (MORB), iron- and titanium-enriched basalts (FeTi basalts), and abyssal andesites. Rock types transitional in character (ferrobasalts and basaltic andesites) were also recovered. The most mafic glassy basalts contain plagioclase, augite, and olivine as near-liquidus phases, whereas in more fractionated basalts, pigeonite replaces olivine and iron-titanium oxides crystallize. Plagioclase crystallizes after pyroxenes and iron-titanium oxides in andesites, possibly due to increased water contents or cooling rates. Apatite phenocrysts are present in some andesitic glasses. Ovoid sulfide globules are also common in many lavas. Compositional variations of phenocrysts in glassy lavas reflect changes in magma chemistry, temperature of crystallization, and cooling rate. The overall chemical variations parallel the chemical evolution of the lava suite and are similar to those in other fractionated tholeiitic complexes. Elemental partitioning between plagioclase-, pyroxene-, and olivine-glass pairs suggests that equilibration occurred at low pressure in a rather restricted temperature range. Various geothermometers indicate that the most primitive MORB began to crystallize between 1150° and 1200°C with fo2 < 10−7 atm. Coexisting iron-titanium oxides in more evolved lavas yield temperatures ∼1025°C to as low as 910°C withfo2 from 10−8 to 10−12 atm. PH 2 o could have been as high as 1 kbar during andesite crystallization. Compositions of the lavas from the Galapagos rift follow the experimentally determined (1 atm-QFM) liquid line of descent. Least squares calculations for the major elements indicate that the entire suite of lavas can be produced by fractional crystallization of successive residual liquids from a MORB parent magma. FeTi basalts represent 30–65 cumulative weight percent crystallization of plagioclase, augite, and olivine. An additional 30–50% fractionation of pyroxenes, plagioclase, titanomagnetite, and possible apatite is required to generate andesite from FeTi basalt liquids. The presence of partially resorbed mafic xenocrysts in some andesites, FeTi basalt inclusions in these xenocrysts, high-silica glass inclusions in basaltic andesites, and the transitional chemistry of basaltic andesites are evidence that some magma mixing occurred during crystal fractionation. The diversity of lava types recovered at single dive sites suggests that low-pressure fractional crystallization is a very efficient process beneath the eastern Galapagos rift and that isolated magma bodies must be present at shallow levels beneath the accretionary locus. Voluminous FeTi basalts erupted at the rift-transform intersection are genetically related to the rift lavas, but their restricted chemistry reflects different thermal and tectonic controls on their petrogenesis.

Melting kinetics of a plagioclase feldspar

Abstract: Partial melting experiments on plagioclase feldspar have been carried out to investigate textures and kinetics of the melting. A labradorite single crystal was heated at one atmosphere pressure and temperatures within its melting interval as a function of time. So called honeycomb, fingerprint, or sieve textures were produced except for the runs just below the liquidus. The melting was initiated by heterogeneous nucleation of melt at the surface and/or interior (cracks and possively dislocations) of the crystal. The pattern of the melt is dendritic with a few μm arm spacing. After the melt develops throughout the crystal, the volumes of melt and residual crystal become larger and smaller, respectively, without changing the arm spacings. The melt is homogeneous and has the approximate temperature dependent liquidus composition irrespective of the time. There are compositional gradients in the residual crystal after short periods of melting. The An content of the crystals increases with increasing time until it finally reaches equilibrium with the melt after several thousands minutes of heating. It is concluded that the enlargement of the melt, the main process of the melting, is controlled by diffusion in the crystal. The fact that partial melts have the composition of the equilibrium liquidus even from the first several minutes strongly suggests that the local equilibrium at the crystal-liquid interface is satisfied during the melting. Some of the honeycomb, fingerprint, and sieve textures found in xenoliths and phenocrysts of sodic plagioclase in volcanic rocks would be caused by heating events (such as magma mixing) during which temperatures of magmas were temporarily higher than the solidus of some of the minerals.

Systematics of modal phenocryst assemblages in submarine basalts: Petrologic implications

Abstract: Phenocryst assemblages in ocean-ridge basalts generally show an increasing proportion of plagioclase as the total amount of phenocrysts increases. The variations in phase assemblages, as well as most crystal-liquid Kd's, are similar to variations (equimodal trends) predicted by low-pressure laboratory experiments, suggesting that many of these basalts have experienced varying degrees of low-pressure cyrstallization prior to quenching, with little sorting of crystals and liquid. Important exceptions include moderately to highly phyric basalts enriched either in plagioclase or olivine which lie well off the experimental trends. In these basalts, megacrysts and xenocrysts usually cited as evidence for magma mixing commonly represent a small proportion of the total crystalline phase assemblage. However, phase proportions for many of these basalts lie well outside the range that could be produced by simple mixing; selective gravitative sorting either prior or subsequent to mixing appears to be the likely explanation for these phyric basalts. A relation between spreading rate and phase proportions is neither supported nor refuted by the data, which as yet do not adequately represent fast-spreading ridges.

Origin of bimodal volcanism, southern Basin and Range province, west-central Arizona

Abstract: Miocene volcanic rocks in the Castaneda Hills area, west-central Arizona, are interbedded with continental clastic sedimentary rocks, minor limestone, gravity glide blocks of Precambrian(?) and Paleozoic(?) rocks, and monolithologic megabreccia. The sedimentary and volcanic units dip to the southwest and are offset by northwest-trending listric and high-angle normal faults. The listric faults coalesce at the Rawhide detachment fault, which overlies mylonitic gneiss. The volcanic suite is strongly bimodal; rocks with 55 to 71 wt % SiO 2 are rare. On the basis of age, geomorphic position, and petrography, five volcanic units can be distinguished: older basalts (18.7 and 16.5 m.y. old), quartz-bearing basalts (13.7 and 12.4 m.y. old), rhyolite lavas and tuffs (15.1 to 10.3 m.y. old), mesa-forming basalts (13.1 to 9.2 m.y. old), and megacryst-bearing basalts (8.6 to 6.8 m.y. old). Most of the basalts contain groundmass olivine and titanaugite phenocrysts and are alkali-olivine basalts. Many rhyolites contain more than 75 wt % SiO 2 . The initial whole-rock Sr isotopic composition of the basalts indicates that they are partial melts of an isotopically vertically heterogenous mantle. The chemical composition of some of the megacrysts in megacryst-bearing basalts with 87 Sr/ 86 Sr i equal to .7035 and .7038 supports a high-pressure mantle origin. The low (.7034) Sr ratio and lack of evidence for mixing with young rocks indicate that the quartz-bearing basalts were also derived from the mantle. Other basalts with 87 Sr/ 86 Sr i > 0.705 probably were derived from old, lithospheric mantle with a high Rb/Sr ratio and do not appear to be contaminated with old, upper-crustal material. The rhyolites have initial Sr isotopic ratios of 0.7093 and 0.7141. These ratios indicate that the rhyolites were not differentiated from the basalts. Partial melting of 1.3-b.y.-old lower-crustal material with Rb/Sr = 0.10 to 0.19 satisfactorily explains the isotopic ratios of the rhyolites. Granulite, which may constitute the lower crust in this part of Arizona, has Rb/Sr ratios similar to those required to produce the rhyolites. K substituted for Na during cooling and devitrification in some of the rhyolites. Partial melting of upper-mantle peridotite and old lower-crustal granulite from 19 to 7 m.y. ago in the Castaneda Hills area produced the bimodal volcanic suite. The nearly contemporaneous production of basaltic and rhyolitic magma from the Earth9s crust and mantle requires extremely heterogenous source regions. Asthenospheric upwelling associated with basin-range extensional tectonism probably produced the heating event that caused partial melting and basaltic magma generation at different levels in the mantle. Partial melting in the lower crust to produce rhyolitic magmas probably was caused by the intrusion of basalt magma. The basaltic and rhyolitic magmas formed in separate source regions, rose independently, and erupted at the same time and place.

Trace element geochemistry of high alumina basalt - Andesite - Dacite - Rhyodacite lavas of the Main Volcanic Series of Santorini Volcano, Greece

Abstract: Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.

Kerimasi: A Neglected Carbonatite Volcano

Abstract: The volcano Kerimasi lies on the Eastern Rift Zone of Africa and is built up of tuffs, agglomerates, and flows which range in composition from nephelinites to carbonatites. Many of the surface rocks are tuffs which contain lapilli with microphenocrysts of melilite and calcite. A coarse-grained apatite-sovite containing monticellite and periclase overgrowths on magnesioferrite outcrops within the crater. A flow near the crater rim contains calcite phenocrysts which are single crystals and show complex zoning with cathodoluminescence. These crystals are often aligned giving a trachyitic texture and are thought to represent primary magmatic crystallization of calcite.

Carbonatite tuffs in the Laetolil Beds of Tanzania and the Kaiserstuhl in Germany

Abstract: Carbonatite lava and tephra are now well known. The only modern eruptive carbonatites, from Oldoinyo Lengai, Tanzania, are of alkali carbonatite, whereas all of the pre-modern examples are of calcite or dolomite. Chemical and stable isotope analyses were made of separate phases of Pliocene carbonatite tuffs of the Laetolil Beds in Tanzania and of Miocene carbonatite tuffs of the Kaiserstuhl in Germany in order to understand the reasons for this major difference. The Laetolil Beds contain numerous carbonatite and melilitite-carbonatite tuffs. It is proposed that the carbonatite ash was originally of alkali carbonate composition and that the alkali component was dissolved, leaving a residuum of calcium carbonate. The least recrystallized melilitite-carbonatite tuff contains early-deposited calcite cement and calcite pseudomorphs after nyerereite (?) that have contents of strontium and barium and δ 18O and δ 13C values suggestive of incomplete chemical and isotopic exchange during alteration and replacement of alkali carbonatite ash. Carbonatite tuffs of the Kaiserstuhl contain globules composed of calcite phenocrysts and microphenocrysts in a groundmass of calcite with a small amount of clay, apatite, and magnetite. The SrO contents of phenocrysts, microphenocrysts, and groundmass calcite average 0.90, 1.42, and 0.59 percent, respectively. The average δ 18O and δ 13C values of globules (+14.3 and −9.0, respectively) fall between those of coarse-grained intrusive Kaiserstuhl carbonatite (avg. +6.6, −5.8) and those of low-temperature calcite cement in the carbonatite tuffs (+21.8, −14.9). The phenocrysts and microphenocrysts are primary magmatic calcite, but several features indicate that the groundmass has been recrystallized and altered in contact with meteoric water, resulting in weathering of silicate to clay, leaching of strontium, and isotopic exchange. The weight of evidence favors an original high content of alkali carbonatite in the groundmass, with recrystallization following leaching of the alkalies.

Spinel minerals in transitional and alkali basaltic glasses from Iceland

Abstract: Crystallization of spinel minerals in transitional and alkali basalts from Iceland can be related to the FeO, MgO, TiO2 and Cr contents of the coexisting melt. Chromian spinel occurs in glasses in which TiO2 is less than 2.8 wt.% and the weight ratio FeO/MgO is less than 2.0, whereas titanomagnetite occurs when the same parameters are greater than 4 wt.% and 2.7, respectively. In addition, chromian spinel only occurs in basalts with Cr greater than 200 ppm. It is suggested that chromian spinel crystallizes, together with olivine, from liquids with olivine liquidus temperatures ranging from above 1,200° C to approximately 1,150° C. A discontinuity in spinel crystallization follows until below 1,100° C, where titanomagnetite starts to crystallize. Compositional variations in chromian spinel attached to, or included, in homogeneous olivine phenocrysts, however, cannot be related to equilibrium relations. Textural relations suggest homogeneous nucleation for titanomagnetite, whereas chromian spinel nucleates heterogeneously, dependent on growth of olivine phenocrysts. The composition of chromian spinels cannot in detail be related to physical and compositional parameters of the average melt, but may be related to local compositional relations in the melt adjacent to growing crystals. Such compositional variation around growing olivine crystals may be the prime reason for the non-equilibrium precipitation of included chromian spinels.

The volatile component of some pumice-forming alkaline magmas from the Azores and Canary Islands

Abstract: The abundances of pre-eruptive magmatic volatile species in the system H-O-S may be determined by application of thermodynamic methods to phenocryst assemblages commonly found in volcanic rocks, as demonstrated by Rutherford and Heming (1978). These methods are applied to alkaline pumice deposits, of airfall and ignimbrite type, from Tenerife (Canary Islands), Sao Miguel and Faial (Azores). It is argued that reliable temperature and fO2 buffering mechanisms found in rhyolitic magmas appear not to operate in more alkaline liquids. fH2O is estimated using biotite; the high values found are shown to be compatible with the violently explosive nature of the magmas concerned. fS2 is estimated from pyrrhotite composition. fH2, fH2S, fSO2, fSO3 are calculated from gas equilibria. Water fugacity may be very roughly estimated for non-biotite bearing samples from data on the sulphur species. Abundances of these species are similar in alkaline and calc-alkaline salic magmas. Volcanological implications, relating to the release of volatiles during explosive eruptions, are considered.

Reverse zoning in the resurgent intrusions of the Grizzly Peak cauldron, Sawatch Range, Colorado

Abstract: The deeply eroded Grizzly Peak cauldron, 17 by 23 km and source of the 34-m.y.-old Grizzly Peak Tuff, is located in the Sawatch Range in west-central Colorado. The Lincoln Gulch composite stock is exposed in the core of a resurgent dome in the northern part of the cauldron and consists of three units. Their forceful emplacement was a cause of cauldron resurgence. The first intrusion lifted its roof along a vertical fault that surrounds the stock. The second resurgent intrusion, a complex body with broad sill-like overhangs, was emplaced within the first intrusion by further piston uplift of the roof. The third intrusion, emplaced in the second, is a small funnel-shaped body. Normal faults in surrounding inter-layered intracaldera tuff and caldera-collapse breccia are radial to the intrusion-cored piston block. Most of the tuff dips away from the resurgent intrusions. Pyroclastic dikes cutting tuff that roofs the stock may have been emplaced by venting of the intrusions. The three resurgent intrusions are concentrically zoned, with the most differentiated material at the margins. From margin to core, compositions range from biotite granodiorite to hornblende-bearing biotite quartz monzodiorite. The intrusions are porphyroaphanitic throughout, but both phenocrysts and groundmass become coarser toward the cores. The compositional patterns are termed reverse zonations because most documented concentrically zoned intrusions become more silicic toward their cores. Crystal settling and marginal accretion of crystalline phases can be eliminated as means of forming reverse zoning because mafic-coreward gradients in element and mineral content are developed inward from contacts of all attitudes. Diffusion-based mechanisms of in situ differentiation are too slow, given the short solidification times for these sill-like bodies. We interpret reverse zonation to result from rearrangement of a vertically graded magma column during emplacement of the intrusions. Progressively more mafic magma from successively deeper levels in a subjacent chamber rose into the cores of the intrusions, displacing more silicic magma toward the margins. This emplacement pattern was preserved by rapid solidification, perhaps due to volatile loss on venting.

Origin of the trachyte–quartz trachyte–peralkalic rhyolite suite of the Oligocene Paisano volcano, Trans-Pecos Texas

Abstract: Volcanic rocks of the Paisano volcano include trachyte, quartz trachyte, and peralkalic rhyolite. Mafic rocks, hawaiite and mugearite, occur within units that stratigraphically underlie and overlie rocks of the volcano. Quartz trachyte, trachyte, and nephe-line trachyte occur as discordant plugs and dikes intruded into strata of the volcano. Central eruptions from dike swarms led to the formation of the shield complex of the volcano ∼ 35 m.y. ago. The central dike complex, a generalized eruptive sequence of rhyolite–quartz trachyte–trachyte within the eruptive products of the volcano, and the development of a 5-km-diameter caldera suggest the presence beneath the volcano of one or more shallow plutonic bodies in which differentiation may have occurred. Fractionation calculations, using whole-rock analyses to represent liquid compositions and electron-probe microanalyses of phenocryst minerals to represent compositions of fractionating phases, indicate plagioclase-plus-olivine control in the evolution of trachyte from mugearite, and anorthoclase control in the evolution of rhyolite from trachyte. The feldspar fractionation model is supported by strong enrichment of Rb and Zr, and strong depletion of Sr in the series, and by a striking Eu anomaly in REE plots. A compositional gap between mugearite and trachyte, analogous to the “Daly Gap” of oceanic islands, disappears when oxides of major elements are plotted versus Zr. Zr, however, cannot be used as a strict index of fractionation because Zr concentrations were buffered by the crystallization of zircon with feldspar in the crucial trachyte stage of magmatic evolution. In more advanced quartz trachyte and rhyolite stages, the melts greatly increased in peralkalinity and zircon did not crystallize. This effect produced Zr concentrations as high as 2,500 ppm in highly fractionated rhyolite. Glomeroporphyritic clusters of feldspar, zoned from andesine to calcic anorthoclase, along with augite, opaques, and, in some samples, olivine, are ubiquitous in mafic trachyte. These clusters have textures indicative of crystallization in intrusions. They suggest a genetic relationship between mugearite and trachyte where residual trachytic liquids are segregated by filter pressing from crystallizing magma of over-all mugearitic composition in subvolcanic chambers. Paisano igneous rocks closely resemble suites of volcanic rocks from Afro-Arabian central volcanoes associated with intracontinental rifting and suites from some oceanic islands. Alkalic rocks of the Paisano volcano may be related to mantle diapirism triggered by subduction processes.

Petrology of historic rhyolite and dacite from usu volcano, north japan

Abstract: by A fte r several t housand years of dormancy, Usu Volcano re newed its act ivity in 1663, and seven erup­ tions occurred up to the present. The essentia l products are typica l of low-K rhyolit e a nd dacite, and varied with time from rh yolit e 10 dacite, decreasing in sili ca a nd in creasi ng in A 120 3, CaO, MgO, and FeO + Fe20J contents, with a s li ght variati on in trace-element abundances, but their Sr iSOlope ratios remains almost cons­ tant (0.70396 - 0.70399). In consistent wi th the varia tion in chemistry of the rock s, most of the plagioclase phenocrysts become more calcic and orthopyroxene more magnesian. The seq uence of hi stori c erupti o ns and the varia tion in composition of the products can be interpreted in terms of a compositio nally zoned magma chamber wh ich has formed before the first hi stori c eruption. The presence of disequilibrated phenocrysts in the hi storic felsic rocks, i.e. calcic plagioclase, magnesian orthopyroxene, cl inopyroxene, and pa rgasit ic hornblende, indicates a complex magma process. The process of incorporatio n of t hese crystals into the fel sic magma and the fo rmation of the zoned magma chamber are di sc ussed.

Submarine ‘crystal tuffs’: their origin using a Lower Devonian example from southeastern Australia

Abstract: Summary. The submarine crystal-rich volcaniclastics of the Lower Devonian Merrions Tuff contain concentrations of angular to euhedral volcanic quartz, plagioclase and orthoclase. Lithic fragments, largely altered vitriclasts, are minor components and the average matrix content is 37.6%. Associated dacite/andesite and rhyodacitic lavas have average groundmass contents of 64.2%. Rare shards in the graded, pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions. The sedimentology of the volcaniclastics suggests subsequent redeposition by cold-state mass-flow processes. The volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption.The high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed, so concentrating the crystal fraction. The crystal-rich character of the crystal-tuffs is not simply due to explosive eruption. Several primary and secondary factors/processes could have interacted to ultimately produce the crystal-rich character, these being: (i) eruption of highly crystallized magmas (≤ 65% phenocrysts), (ii) concentration of crystals in primary eruption columns, (iii) concentration of crystals in any resulting pyroclastic flows, fine vitric ashes being elutriated out and being carried away in accompanying, trailing ash clouds, (iv) concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean, and (v) concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine/subaerial settings.

Conditions of phlogopite crystallization in ultrapotassic volcanic rocks

Abstract: Phlogopite occurs as an early crystallizing mineral in many ultrapotassic lavas of basaltic affinities. Based on high-pressure experiments in lavas of these compositions, the early crystallization of phlogopite is controlled in large part by the bulk compositions of the liquids from which it crystallizes but also by the total pressure and by the aH20, with early phlogopite forming under a narrow range of aH20, less than that represented by H20-saturated conditions. Variations in fo~ do not appreciably affect phlogopite crystallization but high ac02 suppresses its crystallization. In ultrapotassic magmas, phlogopite will preferentially incorporate K20 , TiO2, MgO, and A1203 relative to the coexisting early silicate minerals, olivine and clinopyroxene, and thus, on fractionation of these minerals, phlogopite will be more effective in reducing these oxides in residual liquids. Phenocrysts and microphenocrysts of phlogopite in ultra- potassic lavas are directly related with respect to their K/Ti, K/A1, K/(K + Na), and Mg/(Mg + Fe) ratios. Tex- tural relations suggest phlogopite may form by reaction relationships involving liquid with olivine, and/or clino- pyroxene. Such relationships are supported by the experi- mental studies on ultrapotassic rock compositions.