Showing papers on "Phenocryst published in 1986"
TL;DR: In the Western Ghats between latitudes 18° 20′ N and 19° 15′ N, 7000 km2 of Deccan basalt has been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts.
Abstract: In the Western Ghats between latitudes 18° 20′ N and 19° 15′ N, 7000 km2 of Deccan Basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts. Using over 70 measured vertical sections, major and trace element analyses of nearly 1200 samples, and rare-earth and87Sr/86Sr determinations for over 60 samples, we divide the basalt into three subgroups and ten formations. In this paper we describe the seven principal formations in the area and the most prominent individual flows. The Kalsubai Subgroup is formed by the lower five formations, the Jawhar, Igatpuri, Neral, Thakurvadi, and Bhimashankar formations, from botton to top. In these formations amygdaloidal compound flows predominate and have a typically high MgO content, including picrite basalt (> 10% MgO) and picrite (> 18% MgO) with phenocrysts of olivine and clinopyroxene. These flows are separated by others which contain giant plagioclase phenocrysts and have more evolved chamical compositions. The Lonavala Subgroup overlies the Kalsubai and is composed of two formations, the Khandala and the Bushe. Both are readily recognized in the field and by their chemical compositions. The Wai Subgroup includes the upper three formations, the Poladpur, the Ambenali, and the Mahabaleshwar. The whole subgroup is composed of simple flows with well-developed flow tops, small phenocrysts of plagioclase, pyroxene and olivine, and relatively evolved bulk compositions. Distribution and variation in thickness of the straitigraphic units within the Western Ghats provide a first comprehensive view of the development of the Deccan volcanic edifice. The persistent southerly dip and gentle southerly plunging anticlinal form of the flows, the lensoid shape of many of the formations, and nearly randomly oriented feeder-dike system are together interpreted as evidence of a central volcanic edifice formed as the Indian plate drifted northward over a mantle plume or hot spot.
342 citations
TL;DR: K-feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts as mentioned in this paper, which can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave.
222 citations
TL;DR: The major and trace element and isotopic compositions of the SW Mull basal lavas show that they assimilated substantial amounts of Moine metasediments progressively as they fractionated within the upper crust as mentioned in this paper.
Abstract: Some of the magmas that were extruded to form the 59 Ma, early, plateau-forming, basalt-dominated floods in the British Tertiary Volcanic Province mixed with small amounts of sialic melt during their uprise through the continental crust, while others underwent concomitant fractional crystallization and crustal assimilation. Magma batches of the first group originated in the uppermost asthenosphere and mostly ponded at the Moho, where they crystallized fractionally to form a basalt-benmoreite suite. The major element compositions of members of this series show that they underwent negligible further fractional crystallization between the Moho and the surface. Nevertheless, they paused long enough during their uprise to dissolve their high-pressure phenocrysts and, in many cases, to mix with small amounts of acid melt from lower crustal, granulite-facies, Archaean Lewisian leucogneisses. Basalts and tholeiitic andesites of the second group outcrop locally at the base of the lava piles in Mull and Skye. Their major element compositions show that they equilibrated within the upper third of the crust, under conditions approximating to anhydrous 1-atm cotectic equilibria. Most of these lavas are in SW Mull, where Proterozoic Moine metasediments form the uppermost crust. The major and trace element and isotopic compositions of the SW Mull basal lavas show that they assimilated substantial amounts of Moine metasediments progressively as they fractionated within the upper crust. Pb isotope data reveal that, before their final upper crust evolution, some of the SW Mull basal lavas had already mixed with small amounts of lower crust acid melts.
102 citations
TL;DR: In this paper, two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and fine-grained basaltic inclusions.
Abstract: Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.
85 citations
TL;DR: In this article, the authors divide porphyritic and aphyric boninites into two major types: (A) porphyric type (phenocryst mode exceeds 1%) and (B) anaphric mode is less than 1%).
81 citations
TL;DR: In this article, fine-grained rims in the inclusions indicate that these inclusions are a quenched mafic magma incorporated into their rhyolite host.
79 citations
TL;DR: Plagioclase and biotite phenocrysts in ignimbrites erupted from the Toba caldera, Sumatra, show microstructures and textures indicative of shock stress levels higher than 10 GPa as discussed by the authors.
Abstract: Plagioclase and biotite phenocrysts in ignimbrites erupted from the Toba caldera, Sumatra, show microstructures and textures indicative of shock stress levels higher than 10 GPa. Strong dynamic deformation has resulted in intense kinking in biotite and, with increasing shock intensity, the development of plagioclase of planar features, shock mosaicism, incipient recrystallization, and possible partial melting. Microstructures in quartz indicative of strong shock deformation are rare, however, and many shock lamellae, if formed, may have healed during post-shock residence in the hot ignimbrite; they might be preserved in ash falls. Peak shock stresses from explosive silicic volcanism and other endogenous processes may be high and if so would obviate the need for extraterrestrial impacts to produce all dynamically deformed structures, possibly including shock features observed near the Cretaceous/Tertiary boundary. 38 references, 3 figures.
74 citations
TL;DR: The post-caldera Kameni islands of the Santorini volcanic complex, Aegean Sea, Greece are entirely volcanic and were formed by eleven eruptions between 197 B.C. and 1950 as discussed by the authors.
Abstract: The post-caldera Kameni islands of the Santorini volcanic complex, Aegean Sea, Greece are entirely volcanic and were formed by eleven eruptions between 197 B.C. and 1950. Petrographic, mineral chemical and whole-rock major and trace element data are presented for samples of lava collected from the products of seven eruptive cycles which span the entire period of activity. The main phenocryst phases are plagioclase, clinopyroxene, orthopyroxene and titaniferous magnetite, which are weakly zoned (e.g. plagioclase — An55 to An42). The lavas are typical calc-alkaline dacites and show a restricted range of composition (from 64.1 to 68.4 wt. % SiO2). The phenocrysts were in equilibrium with the melts at temperatures of 960–1012 °C, pressures of 800–1500 bars and oxygen fugacities of 10−9.6-10−9.9 bars. The pre-eruptive water content of the magmas was 3–4 wt. % but since the lavas contain only 0.1–0.4 wt. % H2O, a considerable amount (about 0.01–0.015 km3) of water was lost prior to or during eruption. This indicates that the magmas rose to the surface gradually allowing the (largely) non-explosive loss of volatiles. The lavas were probably extruded initially from more or less cylindrical conduits which developed into fissures as the eruptions proceeded. The post-caldera lavas evolved from more mafic parental magmas (basalt-andesite) via fractional crystallization. The small range of compositional variation shown by these lavas can be explained in terms of near-equilibrium crystallization. Analyses of samples of lavas belonging to single eruption cycles and to individual flows indicate that the underlying magma chamber is compositionally zoned. The average composition of erupted magma has remained approximately constant since 1570 A.D. but that fact that the 197 B.C. magma was sligthly richer in SiO2 provides additional evidence that the magma chamber is compositionally zoned. Crystal settling has not affected the composition of the magma over a 2,200 year period of time which indicates that the melts do not behave as Newtonian fluids. Zonation was thus probably established prior to the 197 B.C. eruption though it is possible that it is developed and maintained by crystal-liquid differentiation processes other than crystal settling (e.g. boundary layer crystallization). The data indicate that there has been no significant cooling during 2,200 years; the maximum amount of cooling is <50 °C and is probably less than ∼30 °C. Two hypotheses are considered to explain the thermal and chemical buffering of the post-caldera magma chamber: (i) The magma chamber is large and heat losses due to conduction are largely compensated by latent heat supplied by thick, partially crystalline cumulate sequences. (ii) Periodic influx of hot mafic magma, which does not mix with the dacitic magma, inhibits cooling. The second alternative is favored because the post-caldera lavas differ geochemically from the pre-caldera lavas which signifies that a new batch of magma was formed and/or emplaced after the catastrophic eruption of 1390 B.C., and hence that mafic magmas may still be reaching upper crustal levels.
69 citations
TL;DR: The chemical zoning of augite phenocrysts in alkaline basalts from the Middle Basalt Series lavas of Gough Island was studied in detail for the major and trace elements using an ion microprobe as mentioned in this paper.
64 citations
TL;DR: In this article, the Lachlan Fold Belt of southeastern Australia is divided into two types depending on whether or not the primary magma contains restite crystals, i.e., mafic granitic and volcanic rocks containing restite phenocrysts.
Abstract: Comagmatic granitic and volcanic rocks are divided into two types depending on whether or not the primary magma contains restite crystals. Examples of both of these types are discussed from the Lachlan Fold Belt of southeastern Australia.Volcanic rocks containing restite phenocrysts are chemically identical to the associated plutonic rocks containing the same amount of restite. The more mafic granitic rocks correspond in composition to the most phenocryst-rich volcanics (up to 60% phenocrysts), and thus cannot be cumulate rocks produced by fractional crystallization, but must represent true magma compositions. These restite-bearing magmas result from partial melting in a source region up to the rheological critical melt percentage, which we estimate to be about 40% in the S-type Hawkins Suite of volcanics.Melts which escape their restite at the source, before the critical melt percentage is reached, are able to undergo fractional crystallization in high level magma chambers by heterogeneous crystallization on chamber walls. In this case volcanic products from the top of the chamber are more felsic than the plutonic products, the plutonics are crystal cumulates and the volcanics are composed of the complementary fractionated liquid. Those phenocrysts present in the volcanics were probably eroded from the chamber walls and are less abundant (< 20%) than in the restite-retentive volcanic products.
64 citations
TL;DR: In this article, 18 flows from a basal stratigraphic sequence on the Aleutian Island of Atka were analyzed for major elements, trace elements and initial 87Sr/86Sr ratios Petrographically, these lavas contain abundant plagioclase (24-45%) and lesser amounts of olivine (<7%), magnetite and clinopyroxene phenocrysts.
Abstract: Eighteen flows from a basal stratigraphic sequence on the Aleutian Island of Atka were analyzed for major elements, trace elements and initial 87Sr/86Sr ratios Petrographically, these lavas contain abundant plagioclase (24–45%) and lesser amounts of olivine (<7%), magnetite and clinopyroxene phenocrysts Compositionally, the lavas are high-alumina (∼20wt%) basalts (48–51 wt% SiO2) with low TiO2 (<1%) and MgO (<5%) Within the section, compositional variations for all major elements are quite small While MgO content correlates with olivine phenocryst contents, no such relationship exists between the other oxides and phenocryst content These lavas are characterized by 8–10 ppm Rb, high Sr (610–669 ppm), 308–348 ppm Ba and very constant Zr (23–29 ppm) and Sc (23–29 ppm) abundances Ni and Cr display extremely large compositional ranges, 12–118 ppm and 12–213 ppm, respectively No correlation exists between trace element concentrations and phenocryst contents Strontium isotopic ratios show a small but significant range (070314–070345) and are slightly elevated with respect to typical MORB No systematic correlation between stratigraphic position and petrography or geochemistry is evident REE abundances measured on six samples are LREE enriched ((La/ Yb)N = 220–281) and display similar chondrite normalized patterns One sample has a slight positive Eu anomaly but the other lavas do not Compared to other Aleutian basalts of similar silica content, these lavas are less LREE enriched and have lower overall abundances The geochemical characteristics of these basalts suggest they represent true liquid compositions despite their highly porphyritic nature Published phase relations indicate fractionation of a more MgO-rich magma could not have produced these lavas The high Al2O3 and low MgO and compatible element abundances suggest a predominantly oceanic crustal source for parental high-alumina basalts
TL;DR: In this article, an electron microprobe was used to analyze the glass inclusions in the North Tonga arc of the Tongan forearc during the 1984 Natsushima cruise.
TL;DR: In this paper, a complete spectrum of trends from one where all REE increase with differentiation (peralkaline series of the Chinati Mountains, Texas) to one when all except Ce decrease with fractionation (Batopilas, Mexico).
TL;DR: In this article, the effects of post-eruption alteration on the δ18O of these lavas were taken into account by analyzing phenocrysts or by using the extrapolation procedure developed for the nearby Alban Hills center.
Abstract: 18O/16O,87Sr/86Sr and chemical analyses were made on 39 lavas and ignimbrites from M. Vulsini, the most northerly district of the K-rich Quaternary Roman Province of Italy. These rocks belong mainly to the undersaturated, leucite-bearing (High-K) series, but also included are samples from the less abundant, SiO2-saturated, hypersthene-(quartz)-normative (Low-K) series. The effects of post-eruption alteration on theδ18O of these lavas were taken into account by analyzing phenocrysts or by using the extrapolation procedure developed for the nearby Alban Hills center. Because of the high Sr contents (500–2400 ppm), the87Sr/86Sr ratios of these rocks were little affected by such alteration processes. The M. Vulsini volcanics have Sr- and O-isotopic ratios much less uniform, and on the average much higher, than at any of the other volcanic centers of the province:87Sr/86Sr=0.7097 to 0.7168;δ18O=6.5 to 13.8. This is attributable to the fact that M. Vulsini is one of the sites of greatest crustal assimilation and hybridism between K-rich Roman magmas and SiO2-rich Tuscan anatectic magmas. The High-K series parent magmas at M. Vulsini had a very high and uniform87Sr/86Sr=0.7102 to 0.7104, and a somewhat more variableδ18O=+5.5 to +7.5; they must have come from an upper mantle source region previously metasomatically enriched in87Sr and LIL elements. These18O/16O and87Sr/86Sr ratios are identical to the parent magma at the Alban Hills, 120 km to the south, where Low-K lavas are absent. Low-K series magmas at M. Vulsini originated from a lower-87Sr source region than the High-K series (<0.7097); a similar relationship is observed in all of the other localities in Italy where the two magma series coexist.
TL;DR: Oxygen isotope compositions were measured on 129 quartz, feldspar, and biotite phenocrysts from ash-flow tuffs and lava domes erupted from the Oligocene central Nevada and central San Juan caldera complexes as discussed by the authors.
Abstract: Oxygen isotope compositions were measured on 129 quartz, feldspar, and biotite phenocrysts from ash-flow tuffs and lava domes erupted from the Oligocene central Nevada and central San Juan caldera complexes. Most of the ash-flow tuffs are compositionally zoned with low-phenocryst rhyolite bases and high-phenocryst quartz-latite tops, but both within individual units and throughout each of the eruptive sequences at each locality, the δ18O values are remarkably constant. δ18O values of the central Nevada magmas range from +9.1 to +9.8 per mil: These values are high and indicate the involvement of high-18O geosynclinal sediments in the melting process. Magmatic δ18O values decrease by only about 0.4 per mil from the initial eruption sequence to the middle eruptive, the giant Monotony Tuff (3000 km3). The initial higher δ18O values are reestablished in the late eruptive sequence, but decrease again by about 0.4 per mil in the latest ring-fracture eruptions. δ18O values in the central San Juan magmas range from +6.8 to +7.5: These values are relatively low and indicate involvement of lower cratonal crust and upper mantle in the melting process. Magmatic δ18O values decrease by about 0.4 per mil from the early sequence (Fish Canyon, Carpenter Ridge, and Mammoth Mountain Tuffs) to the late sequence (Wason Park, Nelson Mountain, and Snowshoe Mountain Tuffs). 18O/16O fractionations among phenocrysts in both Nevada and Colorado are much smaller than among corresponding minerals in plutonic granitic rocks. These fractionations also decrease from stratigraphically lower to higher samples in each cooling unit, so the 18O/16O data agree with other evidence that these represent quenched equilibrium at magmatic temperatures, and that prior to eruption the tops of the magma chambers were cooler than the deeper portions. In striking contrast to what is observed in Iceland and in the late-Tertiary to Quaternary southwest Nevada and Yellowstone caldera complexes, we have found no evidence for any low-18O rhyolitic magmas. Thus, low-18O rhyolitic magmas must be less common than heretofor believed, and their origin must be a result of special circumstances involving the timing, depth, and intensity of meteoric-hydrothermal activity. We tentatively suggest that extensional tectonics and regional rifting may be one of the prerequisites for their development.
TL;DR: In this paper, a 5.6m-thick porphyritic, alkaline dolerite dike in Rockport, Massachusetts, was measured and compared to the dike attitude to determine the degree of flow alignment across the Dike.
Abstract: Systematic variations in the volume percent, size, and orientation of plagioclase phenocrysts up to 12.0 cm long occur across a 5.6-m-thick porphyritic, alkaline dolerite dike in Rockport, Massachusetts. Field measurements indicate that phenocryst concentrations increase from nearly zero at the dike margin to 46.0 vol. % at its center. Average phenocryst size increases inward from 4.1 × 2.2 mm at the dike margin to 19.2 × 7.9 mm at the center. The increase in size and abundance of phenocrysts toward the center of the dike is interpreted as resulting from flow differentiation. The magma-flow direction is assumed to have been upward and parallel to the dike margins (N7°W stiike, 88°E dip). The strikes and dips of all elongate phenocrysts (viewed in cross section) within one traverse across the dike were measured and compared to the dike attitude to determine the degree of flow alignment across the dike. Average phenocryst strike deviations from dike strike increase inward 21.8° from the dike margin to its midpoint. Phenocryst dip-angle deviations from dike dip increase inward by 18.8°. This more pronounced Clow alignment of phenocrysts nearer the dike margins is interpreted as being a function of the more extreme velocity gradients (and resulting shear due to flow) within the marginal zones of the magma than within its interior. At least the outer few centimetres of phenocryst-free chilled dike margins may form primarily by rapid quenching of phenocryst-free magma rather than by flow differentiation. Whole-rock, major- and minor-element trends across this dike may be produced, in part, by flow differentiation. A1 2 O 3 , CaO, P 2 O 5 , and Na 2 O increase inward with plagioclase phenocryst concentrations, whereas SiO 2 , K 2 O, FeO, TiO 2 , MgO, and MnO decrease. This inward decrease in certain oxides, together with an inward (within-dike) decrease in the anorthite content of the cores of plagioclase phenocrysts, cannot be attributed to flow differentiation. The inward-decreasing chemical trends are believed to have resulted from one or a combination of several of the following processes: the concentration of silica and incompatible elements in glass (now devitrified) within chilled dike margins compared to interiors; plagioclase fractionation; continuous tapping of partial melts increasingly depleted of silica and incompatible elements; tapping of a zoned magma chamber; and greater crustal contamination of marginal liquids. Superposed on the chemical trends, the inward decrease in plagioclase anorthite content resulted from fractional crystallization.
TL;DR: A compositionally diverse suite of volcanic rocks, including tholeiites, phonolites, basanites and nephelinites, occurs as accidental blocks in the palagonitic tuff of Kaula Island as discussed by the authors.
Abstract: A compositionally diverse suite of volcanic rocks, including tholeiites, phonolites, basanites and nephelinites, occurs as accidental blocks in the palagonitic tuff of Kaula Island. The Kaula phonolites are the only documented phonolites from the Hawaiian Ridge. Among the accidental blocks, only the phonolites and a plagioclase basanite were amenable to K-Ar age dating. They yielded ages of 4.0–4.2 Ma and 1.8±0.2 Ma, respectively. Crystal fractionation modeling of major and trace element data indicates that the phonolites could be derived from a plagioclase basanite by subtraction of 27% clinopyroxene, 21% plagioclase, 16% anorthoclase, 14% olivine, 4% titanomagnetite and 1% apatite, leaving a 16% derivative liquid. The nephelinites contain the same phenocryst, xenocryst and xenolith assemblages as the tuff. Thus, they are probably comagmatic. The strong chemical similarity of the Kaula nephelinites and basanites to those from the post-erosional stage Honolulu Group on Oahu, the presence of garnet-bearing pyroxenites in the Kaula nephelinites (which previously, had only been reported in the Honolulu volcanic rocks) and the similar age of the Kaula basanite to post-erosional lavas from nearby volcanoes are compelling evidence that the Kaula basanites and nephelinites were formed during a “post-erosional” stage of volcanism.
TL;DR: In this article, the formation of the Batur caldera was signalled by a change in the composition of the erupting material from basaltic and andesitic to dacitic.
TL;DR: In this article, a package of lavas exposed in the Red River gorge of the Taos Plateau volcanic field, northern New Mexico, consists of hybrids produced by simple two-component magma mixing.
Abstract: The package of lavas exposed in the Red River gorge of the Taos Plateau volcanic field, northern New Mexico, consists of hybrids produced by simple two-component magma mixing. The sequence of flows, which is graded compositionally from basalt at the base of the section to dacite at the top, represents the outpouring of progressively more silicic hybrids during the mixing process. The end-members were basalt (50% SiO2) and dacite only slightly more silicic than the uppermost dacite in the gorge (61–62% SiO2). Hybrids (52–57% SiO2) are characterized by large skeletal olivine phenocrysts and exhibit linear chemical variations between end-member compositions for all major and trace elements. Major and trace element mass balance calculations concur on the proportions of end-members necessary to form each hybrid; chemical models limit the amount of subsequent olivine fractionation to less than 5%. The presence of groundmass olivine in andesitic hybrids implies that eruption occurred shortly after, or during, mixing. Estimated density and viscosity contrasts between the end-members would appear to inhibit mixing of basaltic and dacitic magmas, unless basalt were forcefully injected into a dacitic magma chamber. Chemical and petrographic similarities between other olivine andesites and two-pyroxene dacites of the Taos Plateau volcanic field and the Red River gorge hybrids suggest that magma mixing was an important process in the Taos Plateau magmatic system.
TL;DR: Petrographic, mineral chemical and whole-rock major oxide data are presented for the lavas of the Main Volcanic Series of Patmos, Dodecanesos, Greece.
Abstract: Petrographic, mineral chemical and whole-rock major oxide data are presented for the lavas of the Main Volcanic Series of Patmos, Dodecanesos, Greece. These lavas were erupted about 7 m.y. ago and range in composition from ne-trachybasalts through hy-trachybasalts and trachyandesites to Q-trachytes. To some extent, the ne-trachybasalts are intermediate in composition to the alkaline lavas found on oceanic islands and the calc-alkaline lavas of destructive plate margins. Major oxide variation is largely explicable in terms of fractional crystallization involving removal of the observed phenocryst and microphenocryst phases viz. olivine, plagioclase, clinopyroxene and Ti-magnetite in the mafic lavas, plagioclase, clinopyroxene, mica and Ti-magnetite in the evolved lavas. Apatite, which occurs as an inclusion in other phenocrysts or as microphenocrysts must also have been removed. However, mass balance calculations indicate that the chemistry of the hy-trachybasalts is inconsistent with an origin via fractional crystallization alone and the complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas show that they are hybrids formed by the mixing of 80-77% ne-trachybasalt with 20–23% trachyandesite. It is estimated that the mixing event preceded eruption by a period of 12 h-2 weeks suggesting that mixing triggered eruption. Combined fractionation and mixing cannot explain the relatively low MgO contents of the hy-trachybasalts and it is concluded that assimilation also occurred. Assimilation, and especially addition of volatiles to the magmas, may be responsible for the evolutionary trend from ne-normative to hy-normative magmas and was probably facilitated by intensified convection resulting from mixing. A model is presented whereby primitive magma undergoes fractionation in an intracrustal magma chamber to yield more evolved liquids. Influx of hot primitive magma into the base of the chamber facilitates assimilation, but eventually mixing yields the hy-trachybasalts and finally the ne-trachybasalts are erupted.
TL;DR: In this paper, the partitioning of Zr between clinopyroxenes and hydrous, intermediate magmatic liquids was investigated in a series of experiments at 9-10 kbar and 950°-1000°C.
TL;DR: In this paper, major element, Ni, Mn and Ca electron microprobe analyses of olivine phenocrysts in one of the most primitive basalts from the FAMOUS area of the Mid-Atlantic Ridge, 527-1-1, reveal two different olivines populations, distinguished by their zoning characteristics.
Abstract: Major element, Ni, Mn and Ca electron microprobe analyses of olivine phenocrysts in one of the most primitive basalts from the FAMOUS area of the Mid-Atlantic Ridge, 527-1-1, reveal two different olivine populations, distinguished by their zoning characteristics. The often skeletal Group I olivines have zoning profiles with high forsterite, high Ni, low Mn, and low Ca cores. These profiles can be explained by low pressure crystallization from the 527-1-1 magma on cooling. The equant, often megacrystic Group II olivines also have high forsterite, low Mn cores, but the cores have low Ni compared to rims. Thus they are normally zoned with respect to Mg, Fe, and Mn but reversely zoned with respect to Ni. For example, Ni ranges from 1,700 ppm at Fo90.5 in the core to 2,100 ppm at Fo89.5 at the rim. In view of the published whole rock and mineral data of le Roex et al. (1981), the most likely explanation for these data is that the Group II olivines are xenocrysts assimilated from solidified “plagioclase-pyroxene” basalts through which the 527-1-1 basalt ascended. The diffusion rate of Ni and size of the xenocrystic olivines are used to calculate the residence time of the assimilated olivines in the magma. An alternative hypothesis would be a high pressure origin for the Group II olivines. This would be possible if future experiments show that with increasing pressure the partition coefficient for Ni decreases relative to the partition coefficient for Mg for a given bulk composition. Current evidence suggests this is unlikely. The data from 527-1-1 and other samples from the FAMOUS area require magmas with distinct Ni-MgO-FeO characteristics. In general, MORB from different ridge segments fall on distinct trends on plots of MgOvs. Ni as well as MgOvs. FeO. Calculation of the MgO and Ni contents of primary magmas suggests a mechanism by which such distinct trends could come about.
TL;DR: Cerro Negro volcano in Nicaragua first erupted in 1850 and it has erupted at least ten more times, the last in February 1971 as mentioned in this paper, the last one being the most recent one.
Abstract: Cerro Negro volcano in Nicaragua first erupted in 1850. It has erupted at least ten more times, the last in February 1971. It is therefore a polygenetic, parasitic cinder cone that may eventually evolve into a composite cone. Lavas produced during the five important eruptions since 1923 have been sampled. These recent lavas from Cerro Negro are unusually phenocryst-rich basalts (29%–43% phenocrysts by volume) with phenocrysts of plagioclase (An 96−85 ), olivine (Fo 81−72 ), clinopyroxene (En 45 Wo 38 Fs 16 - En 41 Wo 45 Fs 14 ), and subordinate titanian magnetites (Usp 16–20 ). All of the basalts have relatively high Al 2 O 3 and low K 2 O and other incompatible element contents. There have been gradual but significant changes in lava composition with time. All multiply sampled flows show significant intra-lava inhomogeneity. The Fe-Mg contents of analyzed olivines indicate disequilibrium between the olivines and host lavas. All of the inter-lava compositional variation, most of the mineral-lava disequilibrium, and most of the intra-lava inhomogeneity are the result of differential olivine and clinopyroxene accumulation during magma ascent. The magma originated from an essentially homogeneous magma chamber. Phenocrysts in the basalts could have experienced much of their growth during ascent. Ascent velocities were probably on the order of 10 1 –10 −5 cm/sec. Increasing efficiency of crystal settling with time suggests that a narrow feeder, probably a north-south oriented dike, may be widening beneath the cone.
01 Jan 1986
TL;DR: The Coso volcanic field, from early basalt to polygenetic intermediate-composition volcanoes to a comparatively large-volume silicicic center, is thought to reflect systematic changes in the least principal stress (S/sub 3/) and the related tectonic extension rate as mentioned in this paper.
Abstract: Major volcanic episodes occurred in the Coso Range at about 6 my, 4-25 my, and later than 1 my Petrographic features, such as quartz xenocrysts in basalt, sieved plagioclase phenocrysts, broad compositional ranges of phenocrysts within single samples, and occurrence of commingled bombs and lava flows, indicate that many of the intermediate-composition rocks formed by mixing of basaltic magma with silicic material A large volume, highly silicic center near Haiwee Ridge evolved 31-25 my ago This system erupted high-silica rhyolite air-fall and ash-flow tuff and in its later stages rhyodacite lava flows The evolution of the Coso volcanic field, from early basalt to polygenetic intermediate-composition volcanoes to a comparatively large-volume silicic center, is thought to reflect systematic changes in the least principal stress (S/sub 3/) and the related tectonic extension rate Beginning about 4 my ago, basalt was erupted onto a surface of little relief, ponding in a shallow north-trending basin Intermediate-composition magma was produced mainly between 35 and 33 my, at a time when S/sub 3/ may have decreased and the extension rate increased Thus, Pliocene volcanism progressed through a sequence of compositions and eruptive patterns that reflect the transition from tectonic stability to extension
TL;DR: In this article, the authors show that the "pyroxene paradox" is an illusion which arises in part because of the compositional dispersion of olivine fractionation trends from picritic parental magmas between the limiting cases of equilibrium and fractional crystallization.
Abstract: A persistent conclusion reached in studies of glass in the chilled margins of mid-ocean ridge basalts (MORB) is that occult clinopyroxene fractionation is required in order to explain the observed variation in glass compositions1. Successful crystal-fractipnation models commonly require crystallizing assemblages which are dominated by plagioclase and clinopyroxene rather than the plagioclase and olivine which dominate the phenocryst assemblages. The ‘pyroxene paradox’ arises from the observation that clinopyroxene phenocrysts are uncommon in lavas containing more than 8.5 wt% MgO (refs 2,3) and the fact that melting experiments on liquids with such compositions indicate that clinopyroxene appears only at 20–40°C below liquidus temperatures4–7. Based on a study8 of compositional relationships between glasses and chilled margins of pillow lavas from the Baffin Bay picrite suite, I suggest here that the ‘pyroxene paradox’ is an illusion which arises in part because of the compositional dispersion of olivine fractionation trends from picritic parental magmas between the limiting cases of equilibrium and fractional crystallization.
TL;DR: A recent reexamination of the literature provides little or no support for the hypothesis that large, embayed quartz grains (quartz eyes) that resemble phenocrysts and that occur in fine-grained, generally foliated aggregates are porphyroblasts as mentioned in this paper.
Abstract: Deformed rocks containing "quartz eyes" are common in mining areas and a correct interpretation of the origin of the quartz eyes is important for evaluating models for ore genesis, especially volcanogenic models. Reexamination of the literature provides little or no support for the hypothesis that large, embayed quartz grains (quartz eyes) that resemble phenocrysts and that occur in fine-grained, generally foliated aggregates are porphyroblasts. Quartz phenocrysts in deformed volcanic or volcaniclastic rocks can remain undeformed, even though the surrounding fine-grained matrix has undergone extensive deformation and recrystallization. Therefore, most, if not all, quartz eyes should be interpreted as residual phenocrysts.
TL;DR: The Aillik Bay lamprophyric dyke swarm comprises abundant sannaites, plus rarer olivine sannaite, aillikites, and carbonatites.
Abstract: The Aillik Bay lamprophyric dyke swarm comprises abundant sannaites, plus rarer olivine sannaites, aillikites and carbonatites Sannaites are characterized by phenocrysts of Ti–Al titansalite plus
TL;DR: In this paper, the authors show that the upper Cenozoic alkali basaltic rocks from Thailand contain megacrysts of clinopyroxene, olivine, garnet, ilmenite, spinel, magnetite, anorthoclase, plagioclase and nepheline in addition to typical spinel lherzolite xenoliths.
TL;DR: Spinifex veins and veins filled with swirling, tabular olivine grains, intrude the upper part of a 120 m thick lava lake in Munro Township, Ontario, Canada as discussed by the authors.
TL;DR: The South Pacific atoll of Mururoa is located in the southeastern part of the Tuamotu Archipelago as discussed by the authors, which is composed of submarine volcanic rocks resting on oceanic crust, then, above, subaerial volcanic rocks and finally a coral coating about 300 m thick.