Showing papers on "Phenocryst published in 1981"

Gradients in silicic magma chambers: Implications for lithospheric magmatism

Abstract: Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.

On the crystallinity, probability of occurrence, and rheology of lava and magma

Abstract: Given a set of comagmatic lavas of similar composition but varying crystallinity, a diagram can be constructed using only the modes of the phenocrysts that quantitively shows the sequence of crystallization This is done by plotting the amount of each phenocryst against the total crystallinity or percentage of melt of the lava itself A histogram of the total phenocryst content measures the probability of the magma to be erupted as lava This eruption probability (P E ) is the product of the probability of finding the magma at any state of crystallinity (thermal probability, P T ) and the rheological probability (P R ) of the magma being physically able to erupt (ie P E =P T P R ) It is shown that P E is given by dX/dT, where X is the crystallinity of the magma as a function of temperature (T) Because crystal production is generally nonlinear—in most rocks it is step-like—P E is a bellshaped curve stradling the temperature at which the magma is one half crystallized Near the liquidus it is most favorable rheologically for the magma to erupt But the probability is small of sampling a magma near its liquidus, because it cools quickly there It is maximum when there are high rates of crystal production, because it then cools slowly As the crystallinity increases, it reaches a critical point of maximum packing (ie lowest porosity) around 50–60% crystals where it becomes rheologically impossible to erupt The magma looses its potential to become a lava and it becomes a pluton From a histogram of crystallinity and P T ,P R can be found This technique, as well as the construction of the mode-crystallization (M-C) diagram, is illustrated using a set of Aleutian lavas These lavas also show that the point of critical crystallinity decreases with increasing silica content of the lava Because this critical crystallinity is much lower for granitic magmas, they are much more probable than basaltic magmas to become plutons Beyond this point, granitic magmas can only erupt as ash flows This correlation of critical crystallinity and silica content is used to show a method by which the viscosity of the magma can be estimated as a function of crystallinity This variation is found to compare favorably with Roscoe's equation of the dependence of viscosity on the concentration of suspended solids These results show that differentiation probably can not normally take place beyond this critical crystallinity The extraction of melt beyond this critical point by filter pressing is unlikely because the assemblage dilates upon stressing Only if the phenocrysts deform viscously can additional melt be extracted, and this can probably only occur with large (−30km) bodies

Oxygen isotope thermometry of basic lavas and mantle nodules

Abstract: Measurements have been made of the oxygen isotope and chemical composition of glass and phenocrysts in lavas and coexisting minerals in mantle nodules. Temperatures of formation of these assemblages have been estimated from various chemical thermometers and range from 855° to 1,300° C. The permil fractionations between coexisting orthopyroxene and clinopyroxene in the lavas and nodules are all near zero. The fractionations between pyroxene and olivine vary from +1.2 to −1.4 and are a smooth function of temperature over the entire range. This function is given by T(° C)=1151-173Δ (px-d)-68Δ2(px-d) and has an uncertainty of ±60° (2σ). At temperatures above 1,150° C, olivine in the nodules becomes more18O-rich than coexisting clinopyroxene, orthopyroxene, and plagioclase. In combination with the experimental work of Muehlenbachs and Kushiro (1974), the olivine-pyroxene fractionations indicate that olivine also becomes substantially more18O-rich than basaltic liquids above 1,200° C. Geothermometers based on the oxygen isotope equilibration of basaltic liquid with olivine, pyroxene, and plagioclase are presented.

Chemical evolution of a pleistocene rhyolitic center: Sierra La Primavera, Jalisco, México

Abstract: The late Pleistocene caldera complex of the Sierra La Primavera, Jalisco, Mexico, contains well-exposed lava flows and domes, ash-flow tuff, air-fall pumice, and caldera-lake sediments. All eruptive units are high-silica rhyolites, but systematic chemical differences correlate with age and eruptive mode. The caldera-producing unit, the 45-km3 Tala Tuff, is zoned from a mildly peralkaline first-erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Hf, Ta, Nb, Sb, HREE, Pb, Th, and U to a metaluminous last-erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. The earliest post-caldera lava, the south-central dome, is nearly identical to the last-erupted portion of the Tala Tuff, whereas the slightly younger north-central dome is chemically transitional from the south-central dome to later, moremafic, ring domes. This sequence of ash-flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Since that time, the lavas that erupted 75,000, 60,000, and 30,000 years ago have become decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U. They represent successive eruption of the uppermost magma in the post-95,000-year magma chamber. Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine ≧quartz >ferrohedenbergite >fayalite>ilmenite±titanomagnetite. Whereas major-element compositions of sanidine, clinopyroxene, and fayalite phenocrysts changed only slightly between eruptive groups, concentrations of many trace elements changed by factors of 5 to 10, resulting in crystal/glass partition coefficients that differ by factors of up to 20 between successively erupted units. The extreme variations in partitioning behavior are attributed to small changes in bulk composition of the melt because major-element compositions of the phenocrysts and temperature, pressure, and oxygen fugacity of the magma all remained essentially constant. Crystal settling and incremental partial melting by themselves appear incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post-caldera lavas. Transport of trace metals as volatile complexes within the thermal and gravitational gradient in volatilerich but water-undersaturated magma is considered the dominant process responsible for compositional zonation in the Tala Tuff. The evolution of the post-caldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced trace-metal-site availability in the silicate melt.

The Colima volcanic complex, Mexico: Part II. Late-quaternary cinder cones

Abstract: Since the late Pleistocene, eleven cinder and lava cones have erupted on the floor of the southern Colima graben, NE and NW of the large, active, andesitic volcano Colima. Scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites (9), leucite-basanites (3), and minettes (15), the commonest variety of mica lamprophyre. These samples represent primitive, high temperature magmas with 47.6–50.3% SiO2, 7.4–15.3% MgO, 2.5–4.4% K2O, and 2.2–9.9% normative nepheline. All members of this basic alkalic suite contain Mg-olivine (Fo75–94), chromite, augite, and late plagioclase and titanomagnetite. The petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile-bearing phases phlogopite, apatite, and analcime during late stages of crystallization. As these phases increase in abundance, presumably reflecting a rise in magmatic volatile content, there are corresponding increases in the whole rock concentrations of 16 incompatible elements. Although these incompatible elements are relatively abundant even in the basanites, many are highly concentrated in the minettes: Ba≦ 4,200 ppm, Sr≦3,100 ppm, Zr≦ 550 ppm, Ce≦190 ppm, Hf ≦18 ppm. Among the incompatible elements, the degrees of enrichment in the minettes relative to the basanites decrease in the order: H, Th, Ce, La, Nd, Zr, Hf, U, Ba, Sm, Eu, Pb, P, Nb, Sr, Ti. These enrichments may reflect the increasing importance of minor, incompatible element rich mantle phases during partial melting. The concentrations of alkali metals K, Na, Rb, and Cs do not correlate with these other elemental enrichments. The leucite-basanties have similar incompatible element contents to many minettes, differing from them only in the presence of leucite rather than analcime, and Ti-F-rich groundmass phlogopite rather than hydrous phlogopite phenocrysts; thus the leucite-basanites represent relatively dry equivalents of minettes. Two of the eleven cinder cones are calc-alkaline in nature and do not belong to the basanite-minette group; the easternmost cone is constructed of high-Al basalt, and the northernmost of basaltic andesite. The high-Al basalt (49.5% SiO2, 9.3% MgO, 221 ppm Ni) closely approximates a parental magma to the post-caldera andesitic suite of V. Colima (56.5–61.6% SiO2). The basaltic-andesite is relatively enriched in incompatible elements compared to the high-Al basalt — V. Colima trend. The ne-normative basanite-minette samples are highly enriched in incompatible elements, while the contemporaneous hy — qz-normative calc-alkaline suite, encompassing the high-Al basalt and V. Colima's andesites, is characterized by relatively low abundances of these elements. No likely mineral assemblage can relate the alkaline and calc-alkaline suites through crystal fractionation; they probably represent fundamentally different melting events. During the Quaternary, the main focus of andesitic volcanism in the southern Colima graben has migrated southward with time. Volcan Colima marks its present position, 5 km south of the Pleistocene volcano Nevado de Colima, and another 15 km from the still older Volcan Cantaro. The eruptions of basic alkalic magma probably occurred during the late stages of Nevado's life and through the life of V. Colima. They generally migrated from west to east with time, towards V. Cantaro. The most recent cone, V. Apaxtepec, is the only one east of the andesitic Colima-Cantaro axis. The oldest and the two youngest cones produced basanites, while minettes dominated at cones of intermediate ages. The cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of V. Colima. The southern end of the Colima graben can be viewed, then, as the volcanic analog of many classical, post-plutonic, hypabyssal lamprophyre localities.

Pleistocene high‐silica rhyolites of the Coso Volcanic Field, Inyo County, California

Abstract: The high-silica rhyolite domes and lava flows of the bimodal Pleistocene part of the Coso volcanic field provide an example of the early stages of evolution of a silicic magmatic system of substantial size and longevity. The rhyolites are sparsely porphyritic to virtually aphyric, containing qz + pl + san + bi + hb + mt ± allanite ± opx ± cpx ± fa ± il ± ap ± zircon phenocrysts. Major and trace element compositions of all 38 rhyolite extrusions are consistent with derivation from somewhat less silicic parental material by liquid state differentiation processes in compositionally and thermally zoned magmatic systems. Seven chemically homogeneous eruptive groups emplaced approximately 1.0, 0.6, 0.24, 0.17, 0.16(?), 0.09, and 0.06 m.y. ago can be distinguished on the basis of trace element and K-Ar data. The oldest two groups are volumetrically minor and geochemically distinct from the younger groups, all five of which appear to have evolved from the same magmatic system. Erupted volume-time relations suggest that small amounts of magma were bled from the top of a silicic reservoir at a nearly constant long-term rate over the last 0.24 m.y. The interval of repose between eruptions appears to be proportional to the volume of the preceding eruptive group. This relationship suggests that eruptions take place when some parameter which increases at a constant rate reaches a critical value; this parameter may be extensional strain accumulated in roof rocks. Extension of the lithosphere favors intrusion of basalt into the crust, attendant partial melting, and maintenance of a long-lived silicic magmatic system. Consideration of the mass of magma that must have been intruded into the crust in order to explain the anomalously high heat flow near the center of the rhyolite field and comparison of age, volume, mineralogic, and compositional characteristics of the rhyolites with those of caldera-forming systems suggest that the Coso silicic system may contain a few hundred cubic kilometers of magma. The Coso magmatic system may eventually have the potential for producing voluminous pyroclastic eruptions if the safety valve provided by rapid crustal extension becomes inadequate to (1) defuse the system through episodic removal of volatile-rich magma from its top and (2) prohibit migration of the reservoir to a shallow crustal level.

Three S-type volcanic suites from the Lachlan Fold Belt, southeast Australia

Abstract: The concept that granitoids of the Lachlan Fold Belt in southeast Australia, are derived from either igneous or sedimentary source rocks (I- or S-type), can be extended to volcanic rocks of the same age. Three suites of late Silurian S-type volcanics are described, two of which can be matched quite closely with plutonic equivalents. A large part of the Paleozoic continental margin volcanic activity in southeast Australia consisted of the magmatic recycling of old metasedimentary crust, probably of late Proterozoic age. The three volcanic suites are moderately to strongly peraluminous, with the corresponding presence of Al-rich minerals. Variation within the volcanic suites is ascribed chiefly to progressive removal of restite, or source material residual from partial melting. The most mafic suite, the Hawkins Suite, contains plagioclase, cordierite, orthopyroxene, biotite and quartz as restite components, with less abundant almandine. These rocks are chemically equivalent to mafic biotite-rich and cordierite-bearing granitoids from parts of the Berridale and Murrumbidgee Batholiths. Garnet is absent from the Goobarragandra Suite volcanics, which are a little more felsic and close in compositon to granitoids of the Young and Maragle Batholiths. The S-type character of the Laidlaw Suite is less pronounced although a sedimentary source seems to be established. Such a source would be less mature than those for the other two volcanic suites. The Laidlaw Suite resembles, but cannot be closely identified with, felsic S-type granitoids of the Murrumbidgee and Berridale Batholiths. Low-grade regional metamorphism in most of the volcanic rocks has resulted in much mineralogical alteration and mobility of alkali and alkaline earth elements. Despite this alteration, unaltered rocks are present in all three suites. Compositional data for the phenocryst phases within the unaltered rocks has allowed estimation of some intensive parameters. Hawkins Suite volcanics were extruded directly from their source region and preserve phenocryst equilibria established at 5–6 kbar at 800°C. The Laidlaw and Goobarragandra Suites reequilibrated at lower pressures than their source before extrusion. The Laidlaw Suite reequilibrated at estimated temperatures of 725°–730°C, ƒO2 of 10−15.5 to 10−14 bars, and ƒ(H2O) of 2 to 2.5 kbar. Differing mineral compositions in the three suites are related to differing source rock compositons and oxygen fugacities. Relative biotite and orthopyroxene mg is possibly pressure dependent.

First‐order major element variation in basalt glasses from the Mid‐Atlantic Ridge: 29°N to 73°N

Abstract: Basaltic glasses from 29°N to 73°N on the Mid-Atlantic Ridge define two geographic and compositional groups that are characterized by different major element concentrations, phase assemblages and partition coefficients. Group A glasses occur on the ridge segment north of Gibbs Fracture Zone, from 54°N, along the Reykjanes Ridge, the western volcanic zone of Iceland and Kolbeinsey Ridge, up to 70°N. Similar glasses occur also on the ridge from 29°N to 34°N. Group A is characterized by pronounced alumina depletion and iron-enrichment trends, lower silica and alkalis and higher iron and high CaO/Al2O3. Clinopyroxene is only present in the more evolved group A glasses (Mg-value < 58). Olivine-glass partition coefficients for Mg and Fe are systematically lower in group A than group B (Schilling and Sigurdsson, 1979), reflecting effects of melt composition on structure of the liquids. Group B glasses define the ridge segment north of 35°N over the Azores platform and up to Gibbs F. Z., at 53°N. They are also present on the Mohns Ridge north of the Jan Mayen F. Z. at 71°N and at least as far north as 73°. Group B glasses are characterized by higher silica and alkalis, low iron and lack of alumina depletion and iron-enrichment trends. They contain calcic clinopyroxene (Cpx) throughout the compositional range, whereas the sub-calcic augite is absent. The major-element variation within each group can be modeled quantitatively by fractional crystallization involving the phenocryst phases. In group A, the model involves only olivine (Ol) and plagioclase (Pl) extraction in the early stages, but a total of 60% crystallization of the most primitive magma is required, yielding a final solid extract of Ol-Pl-Cpx in the proportions 1:3.3:1.5. Group B fractionation models require clinopyroxene extraction throughout the compositional range, and 45% crystallization of the primitive magma is required to account for the most evolved glasses giving a total solid extract of Ol-Pl-Cpx in proportions 1:5:3.9. The range of glass compositions in the two groups forms two distinct but parallel cotectic trends within the basalt tetrahedron, which are believed to define the quaternary univariant line Ol-Pl-Cpx-Liq along which the magmas evolved during fractionation at low to intermediate pressure. It is proposed that the shift in phase boundaries and univariant lines reflects differences in melt and source compositions of the two groups. The distribution of the two groups is thus taken to indicate two major petrographic provinces of the Mid-Atlantic Ridge.

A summary of the geology and petrology of the Sierra La Primavera, Jalisco, Mexico

Abstract: The Sierra La Primavera, near Guadalajara, Mexico, is a Late Pleistocene rhyolitic center consisting of lava flows and domes, ash flow tuff, air fall pumice, and caldera lake sediments. All eruptive units are high-silica rhyolites, but systematic compositional differences correlate with age and eruptive mode. The earliest lavas erupted approximately 145,000 years ago and were followed approximately 95,000 years ago by the eruption of about 20 km3 of magma as ash flows that form the Tala Tuff. The Tala Tuff is zoned from a mildly peralkaline first-erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Nb, Sb, HREE, Hf, Ta, Pb, Th, and U to a metaluminous last-erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. Collapse of the roof zone of the magma chamber led to the formation of a shallow 11-km-diameter caldera in which lake sediments began to collect. The earliest postcaldera lava, the south-central dome, is nearly identical to the last-erupted portion of the Tala Tuff, whereas the slightly younger north-central dome is chemically transitional from the south-central dome to later, more mafic, ring domes. This sequence of ash flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Sedimentation continued and a period of volcanic quiescence was marked by the deposition of some 30 m of fine-grained ashy sediments. Approximately 75,000 years ago a new group of ring domes erupted at the southern margin of the lake. These domes are lapped by only 10–20 m of sediments as uplift resulting from renewed insurgence of magma brought an end to the lake. This uplift culminated in the eruption, beginning approximately 60,000 years ago, of aphyric lavas along a southern arc. The youngest of these lavas erupted approximately 30,000 years ago. The lavas that erupted 75,000, 60,000, and 30,000 years ago became decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U with time. They represent successive eruption of the uppermost magma in the postcaldera magma chamber. Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine ≥ quartz ≫ ferrohedenbergite > fayalite > ilmenite ± titanomagnetite. Major element compositions of sanidine, clinopyroxene, and fayalite phenocrysts vary only slightly between eruptive groups, but the concentrations of many trace elements change by factors of 5–10. This is reflected in phenocryst/glass partition coefficients that differ by factors of up to 20 between successively erupted units. Because the major element compositions of the phenocrysts and the pressure, temperature, and ƒO2 of the magmas were essentially constant, the large variations in partitioning behavior are thought to result from small changes in bulk composition of the melt. Crystal settling and incremental partial melting are by themselves incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post-95,000-year lavas. Rather, diffusional processes in the silicate liquid are thought to have been the dominant differentiation mechanisms. The zonation in the Tala Tuff is attributed to transport of trace metals as volatile complexes within a thermal and gravitational gradient in a volatile-rich but water-undersaturated magma. The evolution of the postcaldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced octahedral site availability in the silicate melt.

Green clinopyroxenes and associated phases in a potassium-rich lava from the Leucite Hills, Wyoming

Abstract: Green, salitic pyroxenes occur as megacrysts and as cores in diopsidic pyroxene phenocrysts and microphenocrysts in a wyomingite lava from Hatcher Mesa, Leucite Hills, Wyoming. Al-rich phlogopite (16–21% Al2O3), apatite, Fe-Ti-oxide, Mg-rich olivine (Fo93) and orthopyroxene (En61) also occur as megacrysts or as inclusions in diopside phenocrysts. All of these phases are found in ultramafic xenoliths in the host lava, and petrographic and chemical evidence is presented that the megacrysts originate by the disaggregation of the xenoliths. It is concluded that the latter are accidental fragments of the wall rocks traversed by the wyomingite magma and it is suggested that the clinopyroxene-rich xenoliths, from which the green pyroxenes are derived, formed in the upper mantle as a result of local metasomatism or by crystallization from magmas of unknown composition during an earlier igneous event. The precise role of the clinopyroxene-rich xenoliths (which also contain apatite, Fe-Ti-oxide and amphibole) in the genesis of the Leucite Hills magmas cannot be elucidated on the basis of the available data, but it is unlikely that they represent the source material from which these magmas are derived.

Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the ‘famous’ region

Abstract: Bulk rock major and trace element variations in selected basalts from the Famous area, in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. For a given Mg/Mg+Fe2+ atomic ratio of the mineral, the NiO content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts. The Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and, in addition, distinguishes the picritic basalts from the other basalt types. These differences indicate that the different magma groups could not have been processed through the same magma chamber, and preclude any direct inter-relationship via open or closed system fractional crystallization.

Trace element incorporation into growing augite phenocryst

Abstract: Since the introduction1 to geochemistry of crystallization theories developed in metallurgy, it has been thought that the effects of kinetic parmeters such as the rate of crystal growth and the rate of diffusive transport of elements in the melt could significantly affect the partition of trace elements between mineral and magma. This conclusion has not, however, been demonstrated unequivocally, mainly because the extent of variations of equilibrium partition coefficients with T, P and chemistry is not well known. Henderson and Williams2 found a correlation between morphology and apparent partition coefficient of uranium between olivine and basaltic melt. Data on diffusivity of elements (see ref. 3) and on crystal growth kinetics together with the development of secondary ion mass spectrometry (SIMS) techniques, enable crystal zoning to be studied with respect to trace elements and the kinetic effects to be evaluated quantitatively. Trace element zoning (or lack of it) of minerals is important, as many kinetic-based crystallization models predict trace element zoning in crystals. This letter presents trace element data on a sector-zoned augite phenocryst based on the ion-probe spot analyses. The fact that the slower-growing prism sector [100] is enriched in both compatible and incompatible elements relative to the faster-growing basal sector [111] strongly supports Dowty's4 model for crystal growth involving preferential adsorption of elements onto growing crystal faces in proportion to the charge/size ratio of the elements.

Crystallization paths of leucite-bearing lavas: Examples from Italy

Abstract: The salic phases found in leucite-basanites, -trachytes, and -phonolites may be used to portray crystallization in the system NaAlSiO_4-KAlSiO_4-CaAl_2Si_2O_8-SiO_2, the phonolite pentahedron. Only two lavas have been found that contain the assemblage leucite-nepheline-plagioclase-sanidine and liquid, a natural pseudo-invariant assemblage (at 900° C±100) equivalent to the isobaric invariant point of the four component system. The diversity of phases in this group of lavas illustrates the role of halogens in controlling their crystallization paths. Thus the presence of F in the leucite-basanites has stabilized magnesian biotite and suppressed sanidine, as has been found in other basanitic lavas (Brown and Carmichael 1969). The presence of Cl in these same lavas has induced the crystallization of sodalite, which takes the place of nepheline in the groundmass. However in the leucite-trachytes, biotite has suppressed olivine and coexists with sanidine and leucite. The presence of S may produce hauyne at the expense of nepheline, and in general sulphate minerals, which include apatite, have the role in lavas of low silica activity that pyrrhotite plays in liquids of high silica activity. Both pyroxenes and titaniferous magnetites in this suite of lavas are very aluminous. Groundmass crystals of pyroxene may have one-fifth of Si replaced by Al. Other phases which occur occasionally are melanite garnet and a potassium-rich hastingsite, but neither ilmenite nor a sulphide mineral has been found. Phenocryst equilibration temperatures, derived from olivine and Sr-rich plagioclase, are generally in the range from 1,050° C to 1,150° C. The high content of incompatible elements (e.g., K, Ba, Rb, F, Sr, P) in these lavas suggests that they represent a small liquid fraction from a mantle source which possibly contains phlogopite.

Armalcolite-bearing Fe-Ti oxide assemblages in graphite-equilibrated salic volcanic rocks with native iron from Disko, central West Greenland

Abstract: Native iron-bearing strongly sediment-contaminated andesitic to dacitic lavas from the Maligât Formation on Disko contain an early phenocryst assemblage of plagioclase, low-Ca pyroxene(s) and ilmenite. The phenocrystic ilmenite has reacted to form complex Fe-Ti oxide-metal-sulphide aggregates, which contain one or more of the oxides ilmenite, armalcolite and rutile. The armalcolite is very similar to the lunar type 1 armalcolite of Haggerty (1973) and approximate compositionally the ternary system FeTi2O5-MgTi2O5-Ti3O5 (92 to 97 mol.%). When evidence from several salic rocks is combined the Fe-Ti oxide-metal-sulphide aggregates display reactions which may represent one isobaric invariant assemblage (ilmenite-armalcolite-rutile-iron) and the 3 boundary univariant reactions in the system Fe-Ti-O. The compositional and textural features of ferro-magnesian silicates, oxides and metals show that most rocks were affected by a rapidly declining fO2 during magma ascent and cooling, as displayed in the Fe-Ti oxide-metal-sulphide aggregates by the cross-cutting of one or several T—fO2 buffer curves in the system Fe-Ti-O. Prominent sulphidation reactions are observed in the oxide aggregates and are always of the type where FeO in oxide is replaced by FeS while the liberated oxygen is consumed in a reduction process. Carbon, bound as graphite or cohenite, occurs throughout the rocks and is mostly enclosed in phenocrysts and xenocrysts. When the salic magmas ascended from pre-eruption reservoirs at 1 to 1.5 kb the reduction was largely controlled by strongly pressure-dependent carbon-oxygen equilibria resulting in rapidly declining T—fO2 paths recorded by the oxide assemblages. In the simplified C-O gas system carbon-barometry (Sato 1979) applied to the selected rocks would indicate final equilibrium pressures of between 10 and 100 bars. The scarcity of preserved graphite in the lavas would suggest that the carbon-controlled reductions were terminated when available carbon was exhausted during the magma ascent and solidification.

The mineralogy and chemistry of the anorogenic tertiary silicic volcanics of S.E. Queensland and N.E. New South Wales, Australia

Abstract: The Late Oligocene-Early Miocene volcanism of this region is chemically strongly bimodal; the mafic lavas (volumetrically dominant) comprise basalts, hawaiites, and tholeiitic andesites, while the silicic eruptives are mainly comendites, potassic trachytes, and potassic, high-silica rhyolites. The comendites and rhyolites have distinctive trace element abundance patterns, notably the extreme depletions of Sr, Ba, Mg, Mn, P, Cr, V, and Eu, and the variable enrichment of such elements as Rb, Zr, Pb, Nb, Zn, U, and Th. The trachytes exhibit these characteristics to lesser degrees. The comendites are distinguished from the rhyolites by their overall relative enrichment of the more highly charged cations (e.g., LREE, Nb, Y, and especially Zr) and Zn. The phenocryst mineralogy of the trachytes and rhyolites comprises various combinations of the following phases: sodic plagioclase (albite-andesine), calcic anorthoclase, sanidine, quartz, ferroaugite-ferrohedenbergite, ferrohypersthene, fayalitic olivine, ilmenite, titanomagnetite, and rarely biotite (near annite) and Fe-hastingsitic amphibole. Accessories include apatite, zircon, chevkinite (ferrohedenbergite-bearing rhyolites only), and allanite (amphibole and botite rhyolites only). The comendites generally contain Ca-poor anorthoclase-sanidine, quartz, fluorarfvedsonite, aegirine and aegirine-augite (Zr-bearing), aenigmatite, and ± ilmenite. Coexisting Fe-Ti oxides are absent in the comendites and relatively uncommon in the rhyolites and trachytes. Where present, they indicate equilibration temperatures of 885°–980°C and fo2 between QFM and WM buffers. The magmas are thus interpreted to have been strongly water undersaturated during phenocryst equilibration, which is also consistent with the general paucity of pyroclastics, the rarity of hydrous mineral phases, and the extreme Fe-enriched ferromagnesian phenocryst compositions. The chemical and mineralogical data are interpreted to indicate the operation of extreme fractionation processes controlling the development of the silicic magmas, and the comendites, trachytes, and certain trachyte-rhyolite series are considered to have evolved from a mafic parentage. The available oxygen, Sr, and Pb isotopic data, however, point to some modification of the magnas through crustal equilibration processes. The remaining high-silica rhyolites are considered to most likely represent crustal partial melts but are again modified by extensive fractionation.

Petrogenesis of Lavas from Cinder Cone Fields behind the Volcanic Front of Central America

Abstract: Small, monogenetic cinder cones and shield volcanoes occur in platform cone fields behind the volcanic front of Central America. Their lavas (BVF suite) are distinctly different from typical volcanic front lavas (VF suite). Most VF lavas are nearly aphyric basalts, containing only olivine and plagioclase phenocrysts. They have high $Al_{2}O_{3}, Na_{2}O, K_{2}O$, Ba, and Sr contents and moderate Ni, Cr, and Rb contents. In contrast to petrographic observations, major and trace element modeling and the decrease in the $CaO/Al_{2}O_{3}$ ratio with fractionation require substantial clinopyroxene fractionation and moderate degrees of fractionation for the BVF lavas. The contradiction between petrography and compositional trends can be resolved by the following model: (1) crystallization and separation of plagioclase, clinopyroxene, and olivine at approximately 5-10 kb; and (2) rapid magma ascent and surface eruption of nearly aphyric lavas. Near-surface crystallization involves only plagioclase and olivine du...

Geology and petrogenesis of the edgecumbe volcanic field, SE Alaska: The interaction of basalt and sialic crust

Abstract: The Edgecumbe volcanic field is a Holocene volcanic province located on Kruzof Island, SE Alaska. Exposed within the 260 km2 field are basalt, andesitic basalt, andesite, dacite and rhyodacite. The rhyodacites were erupted after the basalts and before the andesites. The volcanics, which are Al-rich (14–18 wt%) and lack an iron enrichment trend, range from tholeiites (47 wt% SiO2) through rhyodacites (72%), but a compositional gap of approximately 9 wt% separates the dacites and rhyodacites. Initial 87Sr/86Sr ranges from 0.70297 in the basalts to 0.70440 in a pyroxene andesite. δ 18O increases across the suite: 5.8‰ to 7.9‰. Plagioclase (An32–86) is the dominant phenocryst in all but one lava. Olivine (Fo58–86) occurs in the basic lavas (<53 wt% SiO2), but is replaced by orthopyroxene (En43–73) and clinopyroxene (En31Wo41-En48Wo40) in the more siliceous volcanics. In the basalts and rhyodacites, plagioclase is weakly zoned, but extreme zoning (<30 mole% An) is characteristic of phenocrysts in the intermediate lavas. Fractionation of the observed phenocryst assemblages could not have produced the more silicous volcanics. Instead they were generated by partial melting of intrusive basement (87Sr/86Sr=0.70487; δ 18O: 8.7–9.3) by basaltic magma and subsequent assimilation. Mass balance calculations show the rhyodacites are almost pure partial melt (<5% basaltic component) whereas the intermediate lavas contain between 30 and 60% partial melt.

The significance of groundmass ilmenite and megacryst ilmenite in kimberlites

Abstract: Criteria are suggested for distinguishing xenocrystic ilmenites from those indigenous to the host kimberlite. For instance, in contrast to groundmass grains, ilmenite xenocrysts usually are larger, have reaction rims of leucoxene and perovskite, exhibit strong magnesium enrichment outward, and sometimes have exsolution lamellae and deformation features. Most of the abundant ilmenite macrocrysts found in kimberlite appear to have been phenocrysts in a crystal mush unrelated to kimberlite. On the other hand, kimberlitic groundmass ilmenite is rare, but consistently more magnesian than the cores of macrocrysts. Strong Mg-enrichment patterns evident in the ilmenite macrocrysts probably developed during their attempt to equilibrate with the more magnesian, fractionating kimberlitic liquid. The hypothesis of extensive reaction of ilmenite with kimberlite melt/ fluid has implications with regard to the following: (1) the degree of differentiation of kimberlite melts; (2) the genesis of mantle megacrysts; (3) the reactivity of kimberlite; and (4) the usefulness of groundmass ilmenite as a petrogenetic indicator.

Igneous scapolite-bearing associations in the Chaîne des Puys, Massif Central (France) and Atakor (Hoggar, Algeria)

Abstract: A scapolite+amphibole+clinopyroxene+Fe-Ti oxide+apatite association has been found as megacrysts in tephra from the Enval-Volvic volcanic line, east of the Chaine des Puys (Massif Central, France) and in the cinder cone of the Segueika volcano (Atakor, Algeria). In both kinds, lavahosts are basanites. Although never seen together in a single xenolith, a study of their inclusions indicates close genetic relationship between all 5 phases. This association must be considered as a paragenesis that crystallized within a narrow PT range. Volcanological, petrological and geochemical data suggest that these megacrysts are high-pressure phenocrysts rather than mechanically desintegrated fragments derived from coarse rocks. The composition of the amphibole suggests a pressure between 5 and 15 kb; Fe-Ti oxides imply a temperature close to 1,100° C.

Two‐Mica Granites of Northeastern Nevada

Abstract: The field settings are described and analytical data are presented for six two-mica granites from north-eastern Nevada. High δ18O and 87Sr/86Sr values indicate that all are S-type granites, derived from continental crust. The major element chemistry and accessory mineral contents of these rocks also are characteristic of S-type granites. Chemical, X ray, and other data are presented for the micas recovered from these granites. The muscovites are notably high in Fe2O3, FeO, and MgO. Except for one hydrobiotite, each of the biotites has an MgO content near 6.0 weight percent. Two different types of two-mica granites are recognized in the area of this study. One type is distinguished by the presence of many biotite euhedra within muscovite phenocrysts and by an unusual suite of accessory minerals completely devoid of opaque oxides. This type probably resulted from anatexis of late Precambrian argillites under conditions of relatively low oxygen fugacity, along a line that roughly coincides with the westward disappearance of continental basement. In the other textural type of two-mica granite the micas are equigranular and there is a greater variety of accessory minerals. The magmatic evolution of this type also appears to reflect the influence of late Precambrian argillites; there may be age differences between the two types of two-mica granites.

The ultramafic cumulate series, Gardiner complex, East Greenland

Abstract: The ultramafic cumulate series of the ultramafic, alkaline and carbonatite bearing Gardiner complex in East Greenland is divided in: 1) Contact zone of plagio-clase-bearing alkaline rocks chilled to the surrounding gneisses and alkaline lavas; 2) a banded sequence of dunites to mt-pyroxenites; 3) a zoned dunite — cpx-dunite ring and 4) in the centre of the complex ol-pyroxenites and mt-pyroxenites. The zones and bands are superimposed with gradational contacts and are increasingly younger towards the centre of the complex. Primocrysts and intercumulus phases, which are equivalent to phenocryst phases in magmatic liquids show that these rocks accumulated from nephelinitic to nepheline-hawaiitic magmas and the contact rocks from less alkaline basanitic magma types similar to the regional alkaline magmas. The cumulates apparently formed in a magma chamber of a nephelinitic volcano, resting on the regional basalts of the Kangerdlugssuaq area.

Amphibole-Bearing Inclusions from Boisa Island, Papua New Guinea: Evaluation of the Role of Fractional Crystallization in an Andesitic Volcano

Abstract: Boisa Island is the tip of a Quaternary island-arc volcano. It consists of an older cone made up of porphyritic, amphibole-free, mafic rocks, and two younger cumulodomes of hornblende-bearing high-silica andesite. One of the cumulodomes contains coarse-grained igneous inclusions consisting of amphibole, plagioclase, clinopyroxene, orthopyroxene, olivine, and spinel. Some of these minerals in the andesites themselves have both cognate and xenocrystal components. Both the anhydrous phenocryst assemblage in the basalts and the amphibple-bearing assemblage of the inclusions provide low residuals in least-squares linear-mixing calculations, suggesting that fractionation of either assemblage in the basalts could have given rise to the andesites. However, this is not supported by the results of Rayleigh-fractionation calculations for several incompatible elements. Four possible interpretations are: (1) the Rayleigh-fractionation results are inappropriate and misleading; (2) the andesites were formed by crystal f...

Petrogenesis and secondary alteration of upper layer 2 basalts of the Nazca plate

Abstract: This paper examines the factors controlling the petrogenesis and chemical alteration of oceanic layer 2 basalts recovered from the margins and interior of the Nazca plate in the southeast Pacific. Specifically, the extent and nature of fractional crystallization, spreading-rate variations, proximity to mantle plumes and prominent fracture zones, mantle source-rock heterogeneities, and secondary alteration are evaluated as some of the more salient factors influencing the composition of upper oceanic lithsophere of the plate. A total of 274 analyses from 88 locations are available for this purpose, and many analyses are of crust generated at the fastest spreading part of the world’s mid-ocean ridge system, the East Pacific Rise. We find that oxidative, low-temperature sea-water alteration, extensive shallow-level fractional crystallization, and mantle source-rock inhomogeneities are the most important factors. The most pronounced effects of oxidative alteration (high Fe2O3/FeO ratios and K2O contents; low MgO abundances; preponderance of a celadonite-iron oxide secondary mineral assemblage) are observed in older basalts (<10 m.y.) dredged from topographic highs on the plate interior, whereas basalts dredged from the crestal portions of the East Pacific Rise and the Galapagos spreading center show only minimal chemical effects of alteration. Only where drilling or recent tectonism has made possible the recovery of basalts from lower in the crustal section are the effects of nonoxidative (hydrothermal) alteration preserved (low Fe2O3/FeO ratios and K2O contents; high MgO abundances; smectite-dominated secondary mineral assemblage). Extensive shallow-level fractional crystallization, involving plagioclase, clinopyroxene, and olivine, is clearly the most important process controlling the range in composition of basalts observed in many areas; Fe-Ti basalts are the end products of this process and are common on the plate. Clinopyroxene fractionation is required for the formation of many of these highly evolved basalts, and this is indicated by observed phenocryst assemblages, normative mineralogy, standard variation diagrams and petrogenetic modeling. First-order regional differences in the composition of mantle source rocks are required to explain the pronounced light-rare-earth-element enrichment and high 87Sr/86Sr ratios of basalts associated with suspected mantle-plume activity of Easter Island and the Galapagos Isalnds. Less pronounced mantle source-rock heterogeneity may be responsible for the occurrence of mid-ocean ridge basalts with similar major-element abundances but markedly different rare-earth-element patterns. Except for fracture-zone offsets near 85° and 95°W longitude along the Galapagos spreading center, we find little evidence supporting the concept that such prominent offsets may be compositional interfaces between opposing rise-crest segments of the East Pacific Rise. Continuous compositional variation exists along the East Pacific Rise, although basalts tend to become somewhat more primitive as the Nazca-Pacific-Cocos triple junction is approached. Compared to basalts from the slow-spreading Mid-Atlantic Ridge, basalts analyzed from the fast-spreading East Pacific Rise and the Galapagos Rise are significantly more evolved (higher FeO*/MgO ratios and TiCh contents; lower CaO and AI2O3 abundances). Such systematic differences in major-element abundances appear related to a near order-of-magnitude difference in spreading rates of the divergent plate margins and to the size and continuity of subaxial magma chambers that can be physically maintained beneath them. It is proposed that an enhanced thermal regime beneath a fast-spreading center favors the existence of a large, steady-state magma chamber. Primitive magma entering such a reservoir will mix with much larger volumes of highly evolved magma, thereby reducing the probability that primitive magma can be erupted on the sea floor. The observed extensive fractionation required to account for the highly evolved character of Nazca plate basalts would require a thicker layer of oceanic layer 3 cumulate; available seismic refraction data support the existence of such a layer for older crust of the Nazca plate.

Contrasted petrological relations between tholeiitic and cala-alkalic serues from funagata volcano, northeast japan

Abstract: Funagata volcano, on the volcanic front of northeast Japan arc, is situated at about 35km northwest of Sendai city, Miyagi prefecture. It is made up of three major volcanic bodies, consisting predominantly of lavas with subordinate pyroclastic rocks, of which about 95 vol. % is calc-alkalic andesite and about 5 vol. % is tholeiitic basalt and andesite. Chemical and mineralogical data are presented to show the contrasted petrological relations between tholeiitic and talc-alkalic series from Funagata volcano. The different compositional trends between both series in the 1/K2O-oxides diagram suggest that the calc-alkalic magma was not produced by continuous fractional crystallization of the tholefitic magma. Analysed mineral phases in the tholeiitic rocks display systematic compositional variations, obeying the process of simple fractional crystallization, On the other hand, mineralogical evidences of some calc-alkalic rocks, such as disequilibrium phenocryst assemblages and reverse zoning of phenocrysts, indicate that magma mixing may have played a key role in their formation. Calculation of magmatic temperatures by pyroxene geothermometer indicates that the tholeiitic magmas crystallized under about 1180-1000°C, whereas the calc-alkalic magmas did under almost constant 950°C. The liquidus temperatures of plagioclase at 1 atm were estimated to be nearly the same range (1070-1210°C) between tholeiitic and calc-alkalic series. The difference between the calculated 1 atm liquidus temperature and pyroxene equilibration temperature was supposed to be ascribed to the difference of H2O contents in magmas at the time of phenocrysts crystallization. Evaluated H20 contents on the basis of experimental data of Sekine et al. (1979) range from 0.5-1.6 wt. % in the tholeiitic magma and 2.0-3.8 wt. % in the calc-alkalic magma. The extremely low H2O contents of the tholeiitic magma imply that the primary magma of the tholeiitic series is unlikely to have originated by hydrous partial melting of mantle peridotite.

Strontian melilite in a nephelinite lava from etinde, cameroon

Abstract: Strontium-rich (up to 16 wt ~o SrO) melilite oc- curs as microphenocrysts in a nosean-leucite-nephelinite lava from Etinde, Cameroon. Electron microprobe an- alyses show that the melilite is very strongly zoned with Sr, Fe, and Na enrichment towards the crystal rims. Sodium melilite and sodium ferrimelilite are the dominant end-member molecules and together account for up to 76 mole ~o. The crystals are optically negative and strongly birefringent with birefringence ranging from 0.014 in the cores to 0.032 at the crystal rims. Refractive indices measured at the rim of one crystal gave to = 1.656, e = 1.628. The cell dimensions of strontian melilite are a = 7.765A, c = 5.054A. ETINDE is a Quaternary nephelinite volcano on the coast of the Cameroon Republic, West Africa. Its lavas show continuous chemical variation from olivine melanephelinite to a leucocratic nosean- leucite-nephelinite (referred to as 'leucitite' by Esch, 1901). The petrology and geochemistry of the Etinde lavas will be described in a later paper where we shall show that these nosean-leucite nephelinites represent the end product of crystal fractionation of the Etinde nephelinite magma. They plot very close to the nepheline-leucite- diopside cotectic in the system nepheline-sanidi ne- diopside (Platt and Edgar, 1972). Feldspathic lavas (e.g. phonolites) have not been reported from Etinde. A sample of nosean-leucite-nephelinite (C154) forms the subject of this paper. Petrography of sample C154. C 154 is a green-grey aphanitic rock with sparse phenocrysts of nepheline, nosean, and augite. The augite phenocrysts are zoned from aluminous sahlite cores to aegirine- auglte rims. The groundmass is composed of very fine-grained nepheline, leucite, sodalite (identified by X-ray diffraction), aegirine-augite, and magnetite with microphenocrysts of leucite, nepheline, melilite, schorlomite, magnetite, and, rarely, apatite and t~ Copyright the Mineralogical Society sphene. A chemical analysis of this rock sample is presented in Table I. Optical properties of the melilite. Melilite occurs as small (up to 200 x 50/~m) tabular crystals show- ing slight alteration to a brown material around the edges. The crystals show unusually high bire- fringence for melilite and are strongly zoned optic- ally. Birefringence, measured with a Berek com- pensator, ranges from 0.014 in the cores to 0.032 TABLE L Chemical composition of sample C154"

Petrology of the 1979 eruption of Soufriere volcano, St. Vincent, Lesser Antilles

Abstract: Major element, trace element, and Sr isotope data are used to study the temporal variation in the chemistry of the ejecta from the 1979 eruption of Soufriere volcano, St. Vincent, and to compare the compositions of the 1979 and 1971/2 magmas. Both the 1971/2 and 1979 products were basaltic andesites almost identical in petrography. A small temporal variation in chemistry is apparent in the 1979 samples but these cannot be related to the 1971/2 lava by fractional crystallisation of phenocryst phases, and the two eruptions may therefore have sampled different batches of magma. 87Sr/86Sr ratios of the two magmas were identical within analytical error.

The Mt. Edgecumbe Volcanic Field, Alaska: An Example of Tholeiitic and Calc-Alkaline Volcanism

Abstract: Major element, trace element, and isotopic composition data for lava samples have been collected from a Holocene stratovolcano in southeastern Alaska. Results indicate that a wide variation in chemical compositions exist ($$48-71\% SiO_{2}; 10.14-.16\% MgO$$) and the suite may be divided into two series on the basis of chemical, mineralogical, and isotopic characteristics. The Tholeiitic Series lavas consist of generally phenocryst poor olivine basalts whose chemical and isotopic compositions are transitional between 'normal' MORB and LREE-enriched lavas of atypical areas such as the Azores platform. CaO is high, $$MgO/FeO > 1, Fe_{2}O_{3}, K_{2}O$$, Rb and Sr are low while MgO and Ni are high. Data show that the basalts are LREE depleted and $$Sr^{87}/Sr^{86} = .70304$$. The basalts compose 65% of the volcanic field and crystal fractionation appears to have produced the compositions of the more silicic basalts. The Calc-alkaline Series lavas are composed of basaltic andesites, andesites, and dacites, of ...