Showing papers on "Phenocryst published in 1995"

Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge

Abstract: The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip–dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocene–late Pleistocene–age volcanic rocks in the western Aleutians. These are termed the “Adak-type” and “Piip-type” magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (∼59% SiO2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks. Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age.

An experimental study of the effects of melt composition on plagioclase - melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

Abstract: An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H2O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H2O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H2O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H2O content can notably increase the An content of plagioclase, up to 10 mol% for H2O-undersaturated melts, and 20 mol% for H2O-saturated melts. Exceptionally calcic plagioclases (up to An100) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H2O (up to 3 wt%). Rather, they must be due to the involvement of extremely refractory (CaO/Na2O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na2O=12–15, or from melts with exceptionally high Al2O3(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H2O contents (∼6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An95 in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na2O values (>8) may account for the presence of highly anorthitic plagioclases in these rocks.

Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption

Abstract: Zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing Unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing. The dacite was formed by mixing of relatively high- and low-temperature ( T ) end-member magmas. The magnetite phenocrysts derived from the low- T magma are reversely zoned by the mixing with high- T magma. A diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching. For the mixed dacite erupted from May 1991 to May 1993, the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr. This indicates that the mixing was continuous during the effusion. The invariability of the other mixing signatures, such as the thickness of reaction rims around biotite phenocrysts, also supports the continuous mixing model. Low-T end-member magma is estimated by mass-balance calculation to be a crystal-rich mush of dacitic composition. These observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion. The proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes.

Stable Isotope Characteristics of Recent Natrocarbonatites from Oldoinyo Lengai

Abstract: Carbon and oxygen isotopic compositions of nyerereite and gregoryite phenocrysts and wholerock samples of natrocarbonatite lavas from the June 1988 eruption of Oldoinyo Lengai lie within restricted ranges of δ13C −6.3 to −7.1 and 5.8 to 6.7 for δ18O. These δ18O and δ13C values from unaltered natrocarbonatites and their carbonate phenocrysts support the conclusion that the carbonatitic magma was derived from the mantle and that their isotope composition was not changed by secondary isotopic exchange. Exposure to alteration under atmospheric conditions, weathering and hydration of the alkali carbonates at the surface produce distinctly higher δ18O, and also heavier δ13C values. A recent natrocarbonatite exposed to weathering for only several weeks shows δ18O and δ13C values of 17.4 and −3.3‰, respectively.

Crystallization of microlites during magma ascent: the fluid mechanics of 1980–1986 eruptions at Mount St Helens

Abstract: Eruptions of Mount St Helens (Washington, USA) decreased in intensity and explosivity after the main May 18, 1980 eruption. As the post-May 18 eruptions progressed, albitic plagioclase microlites began to appear in the matrix glass, although the bulk composition of erupted products, the phenocryst compositions and magmatic temperatures remained fairly constant. Equilibrium experiments on a Mount St Helens white pumice show that at 160 MPa water pressure and 900°C, conditions deduced for the 8 km deep magma storage zone, the stable plagioclase is An47. The microlites in the natural samples, which are more albitic, had to grow at lower water pressures during ascent. Isothermal decompression experiments reported here demonstrate that a decrease in water pressure from 160 to 2 MPa over four to eight days is capable of producing the albitic groundmass plagioclase and evolved melt compositions observed in post-May 18 1980 dacites. Because groundmass crystallization occurs over a period of days during and after decreases in pressure, microlite crystallization in the Mount St Helens dacites must have occurred during the ascent of each magma batch from a deep reservoir rather than continuously in a shallow holding chamber. This is consistent with data on the kinetics of amphibole breakdown, which require that a significant portion of magma vented in each eruption ascended from a depth of at least 6.5 km (∼160 MPa water pressure) in a matter of days. The size and shape of the microlite population have not been studied because of the small size of the experimental samples; it is possible that the texture continues to mature long after chemical equilibrium is approached. As the temperature, composition, crystal content and water content of magma in the deep reservoir remained approximately constant from May 1980 to at least March 1982, the spectacular decrease in eruption intensity during this period cannot be attributed to changes in viscosity or density of the magma. Simple fluld mechanical considerations indicate, however, that the observed changes in mass flux of magma can be modelled by a five-fold decrease in conduit radius from 35 to 7 m, produced perhaps by plating of magma along the conduit walls. The decreased ascent rates which accompanied the decrease in conduit radius can explain the change from closed-system to open-system degassing and the shift from explosive to effusive eruptions during 1980.

High-3He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

Abstract: An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a 40Ar/39Ar plateau age of 253.3 ± 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed 3He/4He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-3He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

Oxygen isotope evidence against bulk recycled sediment in the mantle sources of Pitcairn Island lavas

Abstract: The hypothesis that subducted sediments survive dehydration and/or melting in subduction zones to become long-lived geo-chemical reservoirs in the mantle has gained support in recent years. Evidence for such reservoirs is found in the geochemistry of ocean island basalts (OIBs), some of which have isotopic and trace-element characteristics plausibly associated with ancient sedimentary components. In particular, the EM1 mantle end-member has been identified, principally on the basis of strontium, neodymium and lead isotopes, and has been proposed to carry a large sediment fraction. Oxygen isotopes should be sensitive indicators of subducted sediment in the sources of OIBs because minerals that interact with water at low temperatures near the Earth's surface (during weathering, for example) become enriched in ^(18)O relative to ^(16)O (ref. 6). We report here the ^(18)O:^(16)O ratios of phenocrysts from basalts from Pitcairn Island (southeast Pacific Ocean), which, together with the nearby Pitcairn seamounts, contain among the most extreme EM1 signatures known. We find the oxygen isotope ratios of the phenocrysts to be indistinguishable from the average for mantle peridotite. These results show that the end-member EM1 signature can be produced in the absence of substantial (>l-2%) recycled sediment in the mantle.

Volatile transport in a convecting magma column: Implications for porphyry Mo mineralization

Abstract: We propose that convection in a column of silicic magma allows transport of volatile components from large (>50 km3) magma chambers to the sites of shallow (≈3 km) porphyry-type Mo deposits. Using constraints from the Henderson and Pine Grove systems, we show that even at temperatures as low as 700 °C, a granitic magma with 30 vol% phenocrysts can flow through a magma column with a 150 m radius at a rate >10 km3/yr—enough magma to contribute the Mo necessary to form an ore deposit in a geologically reasonable time frame. This process requires an efficient mechanism for bubble separation at the top of the magma column to produce degassed magma that can descend down through the column. It also requires slow rates of crystallization in the silicic magma, consistent with experimental studies. Hydrothermal fluids released from the convecting magma column can explain many geologic features of the deposits.

Mafic magma batches at Vesuvius: a glass inclusion approach to the modalities of feeding stratovolcanoes

Abstract: Glass inclusions in olivine and diopside phenocrysts from pyroclasts of various eruptions of Vesuvius are representative of the magmas that supplied the volcano in the last 4–5000 years. During this interval the volcano alternated between open conduit activity (e.g. 1944 and 1906 eruptions) with long pauses interupted by Plinian and sub-Plinian eruptions (e.g. 3360 B.P. “Avellino”, A.D. 79 “Pompei”, A.D. 472 “Pollena”). The eruptive behaviour was conditioned in all cases by the presence of shallow reservoirs: two cases are distinguished: (1) small and very shallow, 1906-type; (2) large and deeper Plinian-sub-Plinian magma chamber. Lapilli of 1906 lava fountains contain olivine (Fo89.5–90.4) including Cr-spinel [Cr/(Cr+Al)] (Cr#>75) and volatile-K-rich tephritic glasses, which represent the first recognized Vesuvius primary magmas. Mg-poorer olivine (Fo83–89) also occurs in 1906 and 1944 products; it formed within the shallow reservoir, together with pyroxene and leucite, between 1200 and 1130°C, from K-tephritic melts (MgO=6–8 wt%). The Plinian and sub-Plinian pumices contain diopside, phlogopite and minor olivine (Fo85–87) representing adcumulates wrenched from the chamber walls. Glass inclusions in diopside (and some olivine) range from K-basalt to K-tephrite (MgO=6–8 wt%), with homogenization temperature of 1130–1170°C. They have been regarded as representative of the magmas supplying the Plinian-sub-Plinian chamber(s). The Avellino glass inclusions have K-basaltic compositions, contrasting with the mostly K-tephritic Pompei and Pollena inclusions. They display lower C1 and P contents with respect to the younger tephritic melts, and these variations should reflect primary features of the mantle-derived magmas. The primary and the near-primary Vesuvius magmas, as illustrated by melt inclusions, emphasize high K, P and volatile (H2O, Cl, F, S) contents, with high K2O/H2O (2–2.5), Cl/F (2.5) and Cl/S (2–3) ratios, consistent with a metasomatized mantle source, and distinguishing the Vesuvius potassic primary magmas from those of the northern part of the Roman Province.

San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry

Abstract: The San Quintin Volcanic Field (SQVF) is unique for the Baja California peninsula as the only known location of intraplate-type mafic alkalic volcanism and the only known source of peridotitic and granulitic xenoliths. It consists of 10 distinct Quaternary volcanic complexes. The oldest cones mainly erupted primitive magmas (Mg # > 64)(Mg # = 100 × Mg/(Mg + (0.85 × FeTotal))), which carried occasional small xenoliths. As the SQVF evolved with time, differentiated magmas (Mg # < 64) became increasingly common, but primitive magmas, virtually devoid of xenoliths and unusually rich in olivine phenocrysts, dominanted at the youngest cones. Abundances of incompatible elements declined during evolution of the SQVF, implying a temporal increase in the extent of partial melting in the mantle, or progressive exhaustion of these elements in the source. Samples from two cones, Mazo and Ceniza, show relatively low Ce/Pb, eNd, and 206Pb/204Pb and high 87Sr/86Sr, which we interpret as evidence for crustal contamination of these magmas. Small isotopic variations for the other cones are collectively interpreted to reflect involvement of at least three mantle components beneath the SQVF. Ranges in isotopic composition overlap for primitive and differentiated rocks, supporting fractional crystallization as the mechanism for deriving the latter from the former. Most differentiated rocks can be successfully modeled by fractional crystallization of olivine, plagioclase, clinopyroxene, and spinel from primitive parents. The largest and most abundant xenoliths were carried by differentiated magmas, indicating that fractional crystallization took place within the mantle, below the level of peridotite entrainment, and reflecting the importance of fractionation-elevated volatile contents for driving these differentiated magmas rapidly to the surface. Primitive rocks of the SQVF are unusual compared to other reported intraplate-type mafic alkalic suites from around the world in having relatively high Al2O3 and Yb, as well as low La/Yb and CaO/Al2O3. These characteristics and trends of rising Al2O3 and falling CaO with decreasing incompatible element abundances are all consistent with origins for the SQVF primitive magmas by progressive partial melting of spinel lherzolite at unusually shallow levels in the mantle.

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

Abstract: The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and NbIY and low ZrINb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group 11 rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher ZrINb, and lower NbIY. Group I11 rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and I1 rocks, and contain elevated Ba contents. Groups 1-111 are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and I1 basalts. Groups I-111 are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

40Ar/39Ar laser probe ages of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States

Abstract: Laser probe Ar-40/Ar-39 dating of quartz phenocrysts with rhyolite glass inclusions from the Bishop Tuff air-fall and ignimbrite deposits reveals that the Long Valley magma system existed as a long-lived silicic magma chamber throughout most of the Pleistocene. Sanidine phenocryst and matrix glass analyses show that the Bishop Tuff eruption occurred at 759 +/- 1 to 761 +/- 1 ha, Initial and radiogenic Ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption. The quartz phenocrysts, in contrast, the most abundant phenocryst phase of the Bishop rhyolite, crystallized, trapped their glass inclusions, and became a closed system with respect to Ar at 1.89 +/- 0.03 to 2.3 +/- 0.3 Ma. Consequently, the Bishop rhyolite magma already resided in the Long Valley basement and had formed most of its quartz phenocrysts similar to 1.1 m.y. before its principal eruption, providing important constraints on the longevity of large silicic magma chambers.

Petrology and Geochemistry of Cretaceous Ultramafic Volcanics from Eastern Kamchatka

Abstract: The origin, evolution and primary melt compositions of late Cretaceous high-K ultramafic volcanics and associated basalts of Eastern Kamchatka are discussed an the basis of a study of the mineralogy and geochemistry of the rocks and magmatic inclusions in phenocrysts. The exceptionally primitive composition of the phenocryst assemblage [olivine-Fo(88-95), Cr-spinel-Cr/(Cr + Al) up to 85] provides direct evidence of the mantle origin of primary melts, which were highly magnesian compositions (MgO 19-24 wt%). The rocks and melts are characterized by strong high field strength element (HFSE) depletion in comparison with rare earth elements, and high and variable levels of enrichment in large ion lithophile elements (LILE), P, K and H2O (0.6-1.2 wt% in picritic to basaltic melts). epsilon(Nd) values lie in a narrow range (+10.7 to +9.1), typical of N-MORB (mid-ocean ridge basalt), but Sr-87/Sr-86 (0.70316-0.70358) is slightly displaced from the mantle array. High-K ultramafic melts from Kamchatka are considered as a new magma type within the island-are magmatic spectrum; basaltic members of the suite resemble are shoshonites. The primary melts were produced under high-pressure (30-50 kbar) and high-temperature (1500-1700 oC) conditions by partial melting of a refractory peridotitic mantle.

Diffusional Gradients at the Crystal/Melt Interface and Their Effect on the Compositions of Melt Inclusions

Abstract: Melt inclusions are widely believed to represent melts from which host crystals grew Melt inclusions represent melt adjacent to growing crystals, where compositional gradients exist due to preferential incorporation or exclusion of components by the crystallizing mineral The possibility arises, therefore, that melt in inclusions may differ significantly from melt which was more remote from growing crystals at the time the crystals grew We have tested this possibility by analyzing 45 major, minor, and trace components in 50 to 400 (µm diameter melt inclusions in phenocrysts from the rhyolitic Bishop Tuff, California The following observations indicate that the effects of compositional gradients on chemical compositions of melt inclusions are negligible: (1) melt inclusions in quartz and sanidine phenocrysts have indistinguishable compositions; (2) no correlation is observed between sizes of melt inclusions and their chemical compositions; (3) ten melt inclusions in four quartz phenocrysts from a single

Potassic primary melts of Vulsini (Roman Province): evidence from mineralogy and melt inclusions

Abstract: The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO=13.5 wt%, NiO=340 ppm; Cr=1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo<90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo92-90.5) and the Cr-spinel (Cr#=100. Cr/(Cr+Al)=63-73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H2O-rich (≈ 1 wt%) primary melts (MgO=8.4-9.7 wt%,FeOtotal=6-7.5 wt%). They evidence a wide compositional range (in wt%: SiO2=46.5-50, K2O=5.3-2.8, P2O5=0.4-0.2, S=0.26-0.12; Cl=0.05-0.03, and CaO/Al2O3= 0.8-1.15), with negative correlations between SiO2 and K2O, Al2O3 and CaO, as well as positive correlations between K2O, and P2O5, S, Cl, with nearly constant ratios between these elements. These results are discussed in terms of segregation of various mantle-derived melts. The high and constant Mg# [100.Mg/(Mg+Fe2+)] 73-75 of studied melts and their variable Si, K, P, Ca, Al, S contents could be explained by the melting of a refractory lithospheric mantle source, heterogeneously enriched in phlogopite and clinopyroxene (veined mantle source).

The A‐type Mount Scott Granite sheet: Importance of crystal magma traps

Abstract: The presence of rapakivi feldspar and of distinctive porphyritic texture of Mount Scott Granite indicates a period of crystallization prior to final emplacement beneath an extensive penecontemporaneous rhyolite volcanic pile. Final crystallization conditions are interpreted to have been <50 MPa at depths < <1.4 km based on stratigraphic constraints. However, geobarometry based on the Al content of amphibole phenocrysts and comparison of granite compositions with phase relations in the SiO2-NaAlSi3O8-KAlSi3O8 ternary system both yield pressure estimates of ≈200 MPa. These pressure estimates are interpreted as plumbing the depth of a temporary storage chamber at ≈7–8 km. This depth coincides, in this case, both with the probable Proterozoic basement-cover contact and with the calculated brittle-ductile transition at time of ascent of Mount Scott magma. Although rising magma that fed the preceeding voluminous Carlton Rhyolite apparently rose unimpeded past these horizontal anisotropies, rising magma that formed Mount Scott Granite temporarily paused at this depth. Based on magmastatic calculations, we suggest that horizontal anisotropies (e.g., brittle-ductile transition) become crustal magma traps where the magma driving pressure exceeds the lithostatic load when the anisotropy is encountered. During rifting, initial large influxes of magma may proceed passed crustal anisotropies but have the effect of increasing the relative magma driving pressure through reducing horizontal stress. Thus, magma driving pressure may eventually exceed the lithostatic load at the depth of the brittle-ductile transition thereby activating this crustal magma trap. Ponding of magma at the brittle-ductile transition chokes the eruption. Such a pause in magma supply rate may permit a return to initial stress conditions and deactivate the crustal magma trap. Once again magma will rise to the surface initiating a new magmatic cycle.

Petrologic diversity in Mount St. Helens dacites during the last 4,000 years: implications for magma mixing

Abstract: Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62–64 wt% SiO2) to felsic (65–67 wt% SiO2), temperature has varied by about 150°C (770°–920°C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period.

Early miocene post-collisional calc-alkaline magmatism along the easternmost segment of the periadriatic fault system (Slovenia and Croatia)

Abstract: Early Miocene calc-alkaline igneous rocks from the easternmost segment of the Periadriatic fault system can be subdivided into two series of different alkalinity: (1) Volcanics which occur in the vicinity of the Velenje-Rogatec Line range from high-alumina basalt via low-K and medium-K andesites to medium-K dacites. (2) In the Pohorje Mountains mafic rocks are lacking. A high-K tonalitic pluton crystallized at pressures of about 0.7 GPa as indicated by Al-in-hornblende barometry and accessory magmatic epidote. Rapid exhumation of the tonalite during dextral transtension along the Periadriatic fault system is indicated by tonalitic pebbles in Helvetian (?) clastic sediments. High-K andesitic to dacitic volcanics are interlayered with, and dikes cut, the clastic sediments. Compositional variations shown by the volcanics from the Velenje-Rogatec Line are consistent with fractionation of the observed phenocryst assemblages (olivine, plagioclase, clinopyroxene, orthopyroxene, titanomagnetite). Substantial fractionation of plagioclase is indicated by decreasing Sr/Nd and Sr/Y values and increasing negative Eu anomalies with increasing SiO2. All samples have chondrite-normalized HREE > 9.6 and low (Tb/Yb)n ratios (1.29−1.08). With increasing SiO2, the abundances of HREE and Y (18 to 39 ppm) increase and those of Sc (32.5 to 20.9 ppm) decrease slightly. These features, together with low Hf/Lu and Zr/Y values (10.1–5.7 and 5.7−3.6, respectively), rule out garnet as a major fractionating phase. Since (La/Yb)n values (3.24–6.59) do not increase with SiO2 and chondrite-normalized REE patterns do not show concave-upward shapes, fractionation of amphibole was probably insignificant. Although Rb/Cs values (⩽ 18) are generally low, a significant contribution by continental crustal materials to the magmas by an AFC process is suggested by negative correlations of Nb/U(7.16 to 3.14) and Nb/Th(2.14 to 0.87) values with SiO2, as well as by low Hf/Ta(7.8−3.4), and in part also by K/Rb (340-71) and Zr/Rb (5.0−1.7) values. Rocks from the Pohorje Mountains have high abundances of U. Values of Nb/U and Nb/Th are low (3.55 to 1.85 and 1.27 to 0.64, respectively) and are negatively correlated with SiO2. These features, in combination with high values of Ba/Nb (56 to 93), Ba/La (23–30), and Rb/Cs (19–56), as well as with previously published δ18O values (∼ 9‰) for the tonalite indicate a substantial contribution of the continental crust to these magmas. High Sr abundances (455 to 984 ppm) and moderate negative Eu anomalies suggest that fractionation of plagioclase was of minor importance. Although the rocks have relatively low Sc (9.3 to 3.8 ppm) and Y (21 to 14 ppm) contents, low (Tb/Yb)n values (1.67−1.14) indicate that garnet was not a major fractionating phase. Instead, fractionation of amphibole is a viable mechanism to explain the combination of high (La/Yb)n (21.8−13.1) and low (Tb/Yb)n.

Petrology and Geochemistry of Oldoinyo Lengai Lavas Extruded in November 1988: Magma Source, Ascent and Crystallization

Abstract: Lavas erupted from Oldoinyo Lengai in November, 1988, carry phenocrysts of the alkali carbonates nyerereite and gregoryite, with inclusions of apatite. They are set in a matrix of microphenocrysts of nyerereite and gregoryite wih tiny grains of (Mn, Fe)S, MnFe spinel, sodic sylvite, Na- and Si-rich apatite, spurrite, and intergrowths of sylvite, fluorite and a phase similar to nyerereite. The matrix also contains an unidentified complex carbonate? of Ca, Ba, Sr, K and Na. All phases represent solid solutions (attributed to the high-temperature crystallization of the chemically complex dominantly carbonate liquid) that are very rare or not previously reported.

Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada

Abstract: High-temperature, high-silica rhyolites are commonly found near Cu-Zn volcanic-associated massive sulfide (VMS) deposits in the Archean Abitibi subprovince. They are characterized by anhydrous phenocrysts (feldspar, quartz) in concentrations of generally 73% SiO(2), high incompatible element contents, Rb/Sr > 1, Zr/Y

Petrology of lavas from Sierra Negra volcano, Isabela Island, Galápagos archipelago

Abstract: Sierra Negra volcano is a voluminous, active basaltic shield situated in the western Galapagos archipelago, a hotspot-related chain adjacent to the Galapagos spreading center. The volcano has erupted Fe-rich tholeiitic basalts of very limited compositional range. Comparison of lavas from each of five stratigraphic units covering the entire volcano indicates that the lavas are mineralogically similar and compositionally restricted (4.4 to 6.9 wt % MgO). The lavas are Fe-rich and hypersthene-normative and plot in both the alkaline and tholeiite fields of the alkali-silica diagram. Rare earth element (REE) patterns are steep and parallel. The major and trace element data indicate a comagmatic relationship by fractional crystallization of the observed phenocryst phases. Projections into pseudoternary phase diagrams suggest that the magmas cooled and crystallized at pressures between 1 and 3 kbar. Calculated melt densities range from 2.73 to 2.77 g/cm3, which are higher than those of typical mid-ocean ridge basalt (MORB), Hawaiian tholeiite, and other Galapagos lavas. The high calculated melt densities may be caused by fractionation at greater depths than is ordinary for the other basaltic magmas. Suppression of plagioclase and olivine crystallization in favor of augite may eliminate the minimum density “window” that has been proposed for MORB fractionation. Sierra Negra lavas have the most radiogenic lead and strontium isotopic ratios in the western Galapagos, indicating that the magmas have a relatively large contribution of plume material and have been minimally contaminated by entrainment of MORB-producing mantle. Magmatic 3He/4He isotopic ratios from Sierra Negra are approximately 15 times the atmospheric ratio; although these ratios clearly indicate plume helium, they are not the highest 3He/4He in the archipelago, suggesting that helium is decoupled from the heavier isotopes. High Sm/Yb ratios, light rare earth element (LREE) enrichment, and a steep REE slope are consistent with an origin by moderate extents (5–15 %) of partial melting of a garnet-lherzolite source with REE characteristics that are between chondrite and depleted Earth mantle sources. Sierra Negra lavas are compositionally monotonous because homogeneous plume-rich material has experienced long-term exposure to a well-regulated sublithospheric thermal environment with additional compositional restrictions imposed in the lithosphere by high magma flux.