Evidence For Magma Mixing Research Articles

Laser-ablation ICP-MS analysis of silicate and sulfide melt inclusions in an andesitic complex II: evidence for magma mixing and magma chamber evolution

Laser-ablation microanalysis of a large suite of silicate and sulfide melt inclusions from the deeply eroded, Cu-Au-mineralizing Farallón Negro Volcanic Complex (NW Argentina) shows that most phenocrysts in a given rock sample were not formed in equilibrium with each other. Phenocrysts in the andesitic volcano were brought together in dominantly andesitic—dacitic extrusive and intrusive rocks by intense magma mixing. This hybridization process is not apparent from macroscopic mingling textures, but is clearly recorded by systematically contrasting melt inclusions in different minerals from a given sample. Amphibole (and rare pyroxene) phenocrysts consistently contain inclusions of a mafic melt from which they crystallized before and during magma mixing. Most plagioclase and quartz phenocrysts contain melt inclusions of more felsic composition than the host rock. The endmember components of this mixing process are a rhyodacite magma with a likely crustal component, and a very mafic mantle-derived magma similar in composition to lamprophyre dykes emplaced early in the evolution of the complex. The resulting magmas are dominantly andesitic, in sharp contrast to the prominently bimodal distribution of mafic and felsic melts recorded by the inclusions. These results severely limit the use of mineral assemblages to derive information on the conditions of magma formation. Observed mineral associations are primarily the result of the mixing of partially crystallized magmas. The most mafic melt is trapped only in amphibole, suggesting pressures exceeding 350 MPa, temperatures of around 1,000 °C and water contents in excess on 6 wt%. Upon mixing, amphibole crystallized with plagioclase from andesitic magma in the source region of porphyry intrusions at 250 MPa, 950 °C and water contents of 5.5 wt%. During ascent of the extrusive magmas, pyroxene and plagioclase crystallized together, as a result of magma degassing at low pressures (150 MPa). Protracted extrusive activity built a large stratovolcano over the total lifetime of the magmatic complex (>3 m.y.). The mixing process probably triggered eruptions as a result of volatile exsolution.

Accessory titanite: an important carrier of zirconium in lamprophyres

The chemical composition of minerals from a suite of Variscan lamprophyres (kersantite, minette, spessartite and vogesite) from the Saxothuringian Zone of the Variscan orogen, Germany, were investigated by electron microprobe. These lamprophyres usually contain small grains (<50 μm) of Zr-bearing titanite as the only primary mineral containing traceable zirconium. A late-stage residual alkaline melt within the lamprophyric magma is considered to have crystallized the Zr-rich titanite. The titanite typically has euhedral Zr-rich cores, mantled by Zr-poor overgrowths. This compositional change is explained by rapid decrease of temperature during magma uprise and emplacement. Relatively high Zr contents and compositional and textural heterogeneities (e.g. syenitic schlieren, veins and globules) within the lamprophyric host rocks are evidence of magma mixing. Incorporation of Zr into titanite apparently takes place as a coupled substitution (Ti 4++Al 3+⇔Zr 4++Fe 3+) rather than as a simple 1:1 replacement of Ti by Zr. A ZrO 2 content of about 6 wt.% (equivalent to ∼0.10 atoms per formula unit Zr) is implied as the upper concentration limit in titanite. Locally occurring higher values are attributed to tiny inclusions of secondary Zr phases. In carbonate-rich portions of lamprophyres, titanite is transformed into an unindentified TiO 2 phase. High-(Al,F) titanite ( X Al up to 0.54), restricted in occurrence to the contact between tonalite porphyry and kersantite, formed under the conditions of high fluorine activity connected with the rhyolitic magmatism. The results show that the Zr budget of lamprophyres, and possibly of other mafic igneous rocks lacking zircon, is strongly governed by titanite. Element distribution patterns in titanite are a key to the better understanding of magmatic evolution and petrogenesis of lamprophyres.

The north?south propagating spreading center of the North Fiji Basin. Modeling of the geochemical evolution in periodically replenished and tapped magma chambers

Two main numerical approaches have been previously used to model the behavior of replenished and tapped magma chambers from geochemical data: 1) iterative computations, in which the magma evolution within steady-state reservoirs is modeled cyclically (P-RTF models); each cycle involves adding recharge magma, mixing the remaining liquid together, crystallizing the mixed product, then expelling part of the residual liquid (Model A); the expulsion can also take place after the mixing event but before crystallization (Model B); 2) continuous models (C-RTF models): in the corresponding time-dependent equations, the magma undergoes fractional crystallization with simultaneous replenishment of fresh liquid (Models C and D). A pertinent test of these models requires a cogenetic magmatic series having geochemical data that are not consistent with closed-system fractional crystallization. The northern tip of the north–south propagating spreading center, located in the North Fiji Basin between 18° and 19° S (NS-PSC 18–19° S), responds to this requirement. The lava ages range from 0 to 1 Ma. The dredged volcanic rocks studied are cogenetic in a broad sense (constant isotopic and incompatible trace elements ratios). While no petrographic indications of wall-rock assimilation have been found, evidence of magma mixing has been observed in one basaltic sample (ribbon structures). The lavas, which are normal mid-ocean ridge basalts (N-MORBs), are distributed between three homogeneous compositional groups spatially ordered. The most differentiated lavas have a Fe-Ti basalt composition. We find that one version of open-system fractionation in a periodically replenished reservoir (Model B) is consistent with both the petrologic and geochemical data in explaining the formation of the two most mafic lava groups (Group 1, 64≥mg#≥61; Group 2, 59≥mg#≥52). In our model, the liquids expelled from a first magma chamber at the end of each cycle (Group 1 magmas) feed a second reservoir, which in its turn expels cyclically Group 2 liquids. A part of these expelled liquids are then stocked in a third closed-system magma chamber, where the Fe-Ti basalts (Group 3 lavas: 50≥mg#≥46) are generated through additional crystallization. Thus, the NS-PSC 18–19° S lavas seem to have been produced by three magma chambers interconnected by a sill (and/or pipe) network, ending in the last 18 km of the northern tip. Consequently, only a small fraction of magma expelled from each open-system magma chamber reaches the surface as lava flows, because a fraction of it migrates from one reservoir to another. The off-axis sampling provides evidence for the persistence of open-system fractionation over time.

Petrology and Geochemistry of the Bandas del Sur Formation, Las Cañadas Edifice, Tenerife (Canary Islands)

The Bandas del Sur Formation preserves a Quaternary extra-caldera record of central phonolitic explosive volcanism of the Las Canadas volcano at Tenerife. Volcanic rocks are bimodal in composition, being predominantly phonolitic pyroclastic deposits, several eruptions of which resulted in summit caldera collapse, alkali basaltic lavas erupted from many fissures around the flanks. For the pyroclastic deposits, there is a broad range of pumice glass compositions from phonotephrite to phonolite. The phonolite pyroclastic deposits are also characterized by a diverse, 7-8-phase phenocryst assemblage (alkali feldspar + biotite + sodian diopside + titanomagnetite + ilmenite + nosean-hauyne + titanite + apatite) with alkali feldspar dominant, in contrast to interbedded phonolite lavas that typically have lower phenocryst contents and lack hydrous phases. Petrological and geochemical data are consistent with fractional crystallization (involving the observed phenocryst assemblages) as the dominant process in the development of phonolite magmas. New stratigraphically constrained data indicate that petrological and geochemical differences exist between pyroclastic deposits of the last two explosive cycles of phonolitic volcanism. Cycle 2 (0.85-0.57 Ma) pyroclastic fall deposits commonly show a cryptic compositional zonation indicating that several eruptions tapped chemically, and probably thermally stratified magma systems. Evidence for magma mixing is most widespread in the pyroclastic deposits of Cycle 3 (0.37-0.17 Ma), which includes the presence of reversely and normally zoned phenocrysts, quenched mafic glass blebs in pumice, banded pumice, and bimodal to polymodal phenocryst compositional populations. Syn-eruptive mixing events involved mostly phonolite and tephriphonolite magmas, whereas a pre-eruptive mixing event involving basaltic magma is recorded in several banded pumice-bearing ignimbrites of Cycle 3. The periodic addition and mixing of basaltic magma ultimately may have triggered several eruptions. Recharge and underplating by basaltic magma is interpreted to have elevated sulphur contents (occurring as an exsolved gas phase) in the capping phonolitic magma reservoir. This promoted nosean-hauyne crystallization over nepheline, elevated SO3 contents in apatite, and possibly resulted in large, climatologically important SO2 emissions.

Magma Mixing and Crustal Recycling Recorded in Ternary Feldspar from Compositionally Zoned Peralkaline Ignimbrite ‘A’, Gran Canaria, Canary Islands

Abstract Miocene Ignimbrite ‘A’ on Gran Canaria contains three compositional endmember fiamme types(two rhyolites and one trachyte) each of which crystallized distinct feldspar. Various textural and compositional criteria are interpreted as reflecting a complex scenario within the magma chamber in which the crystals formed. About 25–30% of the feldspar phenocrysts contain evidence for magma mixing in the form of (1) partial to severe dissolution–resorption rims, (2) distinct zones of drastically different compositions and (3) overgrowth textures on formerly resorbed crystals. Four major types of zoning in the oligoclase to anorthoclase feldspars of ignimbrite ‘A’ include a normal and a reversely zoned type and two complexly zoned types. The feldspars with normal and reverse zonation show only minor compositional amplitudes between individual zones (ΔAb, Or ∼4%), whereas the complexly zoned types show compositional differences between zones of up to 18 mol % Ab and 20 mol % Or and are commonly associated with an internal dissolution surface. Complex zoning with large compositional amplitudes and dissolution textures is taken as evidence of crystal movements within the magma and across compositional boundaries between magma batches. A multiple ‘step-cycle’ model, involving growth and transport of a crystal into another magma batch and its return to the original host magma, is suggested by the data. Moreover, feldspars from one rhyolite compositional group are found to be substantially elevated in δ18O, suggesting an input of a high δ18O component to this rhyolite. The other endmember rhyolite appears to be related to the endmember trachyte by mainly crystal fractionation of anorthoclase feldspar. This observation is consistent with trace element and rare earth element concentrations for the magma endmembers and their feldspars, where contamination led to a depletion in incompatible trace elements and light rare earth elements in the contaminated rhyolite and its feldspar phenocrysts. We suggest that the combination of textural and compositional variation in ternary feldspar of peralkaline rhyolitic systems is well suited to reconstruct dynamic processes such as magma mixing and contamination in evolving rhyolitic magma chambers.

Petrographic and Geochemical Evidence for Magma Mixing and Crustal Contamination in the Post-Collisional Calc-Alkaline Yozgat Volcanics, Central Anatolia, Turkey

Petrographic, major-oxide, and trace-element data are presented for the Yozgat volcanics. These rocks range in composition from basalts through basaltic andesites and andesites to dacites. Major-oxide variations are largely explicable in terms of fractional crystallization, involving removal of observed phenocrysts and microphenocrysts. However, complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas, and observed epitaxitic pyroxene growth around quartz xenocrysts imply that they are hybrids formed by a mixing process. In addition, observed enrichments in crustal elements such as K, Rb, Ba, Sr, and P provide clear evidence for the crustal assimilation of granitoid and metasedimentary xenoliths. The following model is suggested for the evolution of the Yozgat volcanics. The primitive magma underwent fractionation in an intracrustal magma chamber to yield more evolved liquids. Influx of hot, primitive magma into the magma chamber promoted vigorous convection-crustal assimilation and eruption of the volcanic rocks in the study area.

Field, geochemical, and isotopic evidence for magma mixing and assimilation and fractional crystallization processes in the Quottoon Igneous Complex, northwestern British Columbia and southeastern Alaska

The quartz dioritic Quottoon Igneous Complex (QIC) is a major Paleogene (65-56 Ma) magmatic body in northwestern British Columbia and southeastern Alaska that was emplaced along the Coast shear zone. The QIC contains two different igneous suites that provide information about source regions and magmatic processes. Heterogeneous suite I rocks (e.g., along Steamer Passage) have a pervasive solid-state fabric, abundant mafic enclaves and late-stage dikes, metasedimentary screens, and variable color indices (25-50). The homogeneous suite II rocks (e.g., along Quottoon Inlet) have a weak fabric developed in the magmatic state (aligned feldspars, melt-filled shears) and more uniform color indices (24-34) than in suite I. Suite I rocks have Sr concentrations &lt;750 ppm, average LaN/YbN = 10.4, and initial 87Sr/86Sr ratios that range from 0.70513 to 0.70717. The suite II rocks have Sr concentrations &gt;750 ppm, average LaN/YbN = 23, and initial 87Sr/86Sr ratios that range from 0.70617 to 0.70686. This study suggests that the parental QIC magma (initial 87Sr/86Sr approximately 0.706) can be derived by partial melting of an amphibolitic source reservoir at lower crustal conditions. Geochemical data (Rb, Sr, Ba, and LaN/YbN) and initial 87Sr/86Sr ratios preclude linkages between the two suites by fractional crystallization or assimilation and fractional crystallization processes. The suite I rocks are interpreted to be the result of magma mixing between the QIC parental magma and a mantle-derived magma. The suite II rocks are a result of assimilation and fractional crystallization processes.

The generation of overpressure in felsic magma chambers by replenishment

Evidence of magma mixing is common in the products of explosive felsic eruptions and it is generally accepted as a common mechanism for triggering such events. In order to quantify the potential for magma mixing to trigger explosive eruptions, we have developed a simple analytical model, based on previous models of magma chamber replenishment, which considers the injection of volatile-rich mafic magma into a chamber occupied by a homogeneous, volatile-rich felsic magma. We assume that the overpressure caused by the injection of new magma is not sufficient to trigger an eruption, for which additional overpressure is required. Two mixing-related mechanisms have traditionally been considered to have the potential to generate the additional overpressure: (1) exsolution of volatiles from the felsic magma during forced convection, and (2) exsolution of volatiles from the mafic magma by oversaturation during cooling and subsequent crystallization. Our calculations suggest that exsolution of volatiles from the felsic magma is not an effective mechanism to generate additional overpressure. However, significant overpressure can be achieved by volatile exsolution from the mafic magma during its cooling and crystallization. The time scale between intrusion and eruption considered in our model is of the order of a few days to a few months, which coincides with petrological and geophysical evidence obtained from magma mixing related eruptions. We also suggest that replenishment of shallow felsic magma chambers by mafic magma in most cases does not lead to large scale mixing, as eruption will occur before thermal equilibrium between the two magmas is reached, so that density and viscosity contrasts between the two magmas will remain significant.

Evidence for magma mixing in the Mariana arc system

Volcanoes of the Mariana arc system produce magmas that belong to several liquid lines of descent and that originated from several different primary magmas. Despite differences in parental magmas, phenocryst assemblages are very similar throughout the arc. The different liquid lines of descent are attributed to differences in degree of silica saturation of the primary liquids and in the processes of magmatic evolution (fractional crystallization vs magma mixing). Pseudoternary projections of volcanic rocks from several arc volcanoes are used to show differences between different magmatic suites. In most of the arc, parental liquids were Ol‐ and Hy‐normative basalts that crystallized olivine, augite, and plagioclase (± iron‐titanium oxide) and then plagioclase and two pyroxenes, apparently at low pressure. Eruptive rocks follow subparallel liquid lines of descent on element–element diagrams and on pseudoternary projections. Magmas at North Hiyoshi are Ne‐normative and have a liquid line of descent along the thermal divide due to precipitation of olivine, augite, and plagioclase. Derived liquids are large ion lithophile element (LILE)‐rich. Magmas at other Hiyoshi seamounts included an alkaline component but had more complex evolution. Those at Central Hiyoshi formed by a process dominated by mixing alkaline and subalkaline magmas, whereas those at other Hiyoshi seamounts evolved by combined magma mixing and fractional crystallization. Influence of the alkaline component wanes as one goes south from North Hiyoshi. Alkaline and subalkaline magmas were also mixed to produce magmas erupted at the Kasuga seamounts that are behind the arc front. The alkaline magmas at both Hiyoshi and Kasuga seamounts had different sources from those of the subalkaline magmas at those sites as indicated by trace element ratios and by Nd.

Evidence of Magma Mixing and Hybrid Source on Calc-Alkaline Sarihan (Bayburt) Granitoid, NE Turkey

The study area is in the South Zone of Eastern Pontides (NE Turkey). The basement is Pre-PermoCarboniferous aged Pulur Massif which contains greenschist, micaschist, para-amphibole schist, ortoamphibolite, gneiss, marble, quarzite and intruding Middle-Carboniferous aged Saraycik Granodiyorite. Liassic aged Hamurkesen Formation overlying unconformably the massif begins with Dikmetas Conglomerate and continues with volcano-sedimentary rocks containing tuff, sandstone and andesite, all of which are cut by diabase dykes. Maim-Lower Cretaceous aged Hozbirikyayla Formation overlies conformably Hamurkesen Formation and contains limestone and sandy limestones. Hozbirikyayla Formation is overlied by conformably Otlukbeli Melange containing red-limestone, pyroclastics, serpentinite, radiolarite, limestone olistoliths and breciated basalt. Campanien aged Sarihan Granitoid cuts all these lithologies. Sarihan Granitoid covering an area of km 2 contains quartz-monzodiorite (60%), granodiorite (35%) and quartz-diorite (5%), and is cut by 5 to 10 cm width aplite dykes. These rocks show similar petrographical features. In the center of intrusion, 0.5-1 cm width cracks were filled by black tourmaline. The rocks of the intrusion shows medium-grained, poikilitic, anti-rapakivi and sometimes myrmekitic textures, and contains 43-64% plagioclase, 7-15% orthoclase, 10-29% quartz, 5-20% hornblende, 1-8% biotite and 1-6% opaque oxides. Plagioclase is subhedral, mainly oligoclase and rare andesite in composition (Anls-37 Ab55-79 Or22_9). Large crystals show generally oscillatory zoning and albite twinning. Crystals with oscillatory zoning range in composition from An33 in the core to Anls in the rim. Mirmekitic textm'es developed at the contact of orthoclase. Large plagioclase may contain plagioclase, apatite, magnetite and rare hornblende inclusions.

Significance of crustal and source region processes on the evolution of compositionally similar calc-alkaline lavas, Mt. Hood, Oregon

Mt. Hood, Oregon, in the Cascade Range volcanic arc has erupted predominantly andesite lava and pyroclastic-flow deposits over the last 700,000 years. Most lavas belong to the medium-K, calc-alkaline series and show a restricted range of composition (53–63 wt.% SiO 2). Least-squares mixing calculations show that fractional crystallization of observed phenocryst phases can account for most major-oxide variation displayed by Mt. Hood lavas. AFC modeling indicates that small amounts of assimilation ( M a/M c = 0.1–0.15) of high-K 2O crustal rock occurred during certain eruptive episodes, but did not have a significant effect on magma composition. Evidence of magma mixing (partially resorbed olivine, plagioclase and pyroxene phenocrysts, magmatic inclusions) is found in lavas erupted throughout the volcano's history. Trace-element mixing calculations indicate that repeated cycles of mixing resulted in the eruption of compositionally similar lavas throughout the history of the volcano. Mt. Hood lavas are unusual in that they do not exhibit depletion of high field strength elements (HFSE) relative to large ion lithophile elements (LILE). Depletion of HFSE relative to LILE is considered a common geochemical characteristic of arc lavas, and is usually attributed to modification of the upper mantle source region by interaction with slab-derived fluids. Calculations of pre-eruptive water contents using an H 2O-dependent plagioclase thermometer indicate that Mt. Hood lavas contained up to 6 wt.% H 2O prior to eruption. Therefore, the absence of HFSE depletions relative to LILE cannot be attributed to lack of interaction between slab-derived fluids and the source region. An alternate source of such depletions in arc magmas is subducted sediment. Both pelagic and continental margin sediments potentially subducted along the Cascades trench do exhibit depletion in certain HFSE (Nb, Zr) relative to LILE. The absence of HFSE depletions in lavas erupted at Mt. Hood, therefore, appears to reflect negligible subduction of sediment and/or negligible mixing between subducted sediment and the upper mantle source region.

Petrology and sulfur and chlorine emissions of the 1963 eruption of Gunung Agung, Bali, Indonesia

The 1963 eruption of Gunung Agung produced 0.95 km3 dense rock equivalent (DRE) of olivine±hornblende-bearing, weakly phyric, basaltic andesite tephra and lava. Evidence for magma mixing in the eruptive products includes whole-rock compatible and incompatible trace element trends, reverse and complex compositional zoning of mineral phases, disequilibrium mineral assemblages, sieve-textured plagioclase phenocrysts, and augite rims on reversely zoned orthopyroxene. Basalt magma mixed with pre-existing andesite magma shortly before eruption to yield basaltic andesite with a temperature of 1040–1100 °C at an assumed pressure of 2 kb, f O2>NNO, and an average melt volatile content (H2O±CO2) of 4.3 wt.%. Magma-mixing end members may have provided some of the S and Cl emitted in the eruption. Glass inclusions in phenocrysts contain an average of 650 ppm S and 3130 ppm Cl as compared with 70 ppm and 2220 ppm, respectively, in the matrix glass. Maximum S and Cl contents of glass inclusions approach 1800 and 5000 ppm, respectively. Application of the petrologic method to products of the 1963 eruption for estimating volatile release yields of 2.5×1012 g (Mt) of SO2 and 3.4 Mt of Cl released from the 0.65 km3 of juvenile tephra which contributed to stratospheric injection of H2SO4 aerosols on 17 March and 16 May, when eruption column heights exceeded 20 km above sea level. An independent estimate of SO2 release from atmospheric aerosol loading (11–12 Mt) suggests that approximately 7 Mt of SO2 was injected into the stratosphere. The difference between the two estimates can be most readily accounted for by the partitioning of S, as well as some Cl, from the magma into a water-rich vapor phase which was released upon eruption. For other recent high-S-release eruptions of more evolved and oxidized magmas (El Chichon, Pinatubo), the petrologic method gives values two orders of magnitude less than independent estimates of SO2 emissions. Results from this study of the Agung 1963 magma and its volatile emissions, and from related studies on eruptions of more mafic magmas, suggest that SO2 emissions from eruptions of higher-S-solubility magma may be more reliably estimated by the petrologic method than may those from more-evolved magma eruptions.

Jurassic plutonism and crustal evolution in the central Mojave Desert, California

Middle to Late Jurassic plutonic rocks in the central Mojave Desert represent the continuation of the Sierran arc south of the Garlock fault. Rock types range from calc-alkaline gabbro to quartz monzonite. Chemical and isotopic data indicate that petrologic diversity is attributable to mixing of crustal components with mantle melts. Evidence for magma mixing is scarce in most plutons, but emplacement and injection of plutons into preexisting wallrocks (e.g. pendants of metasedimentary rocks) suggests that assimilation may be locally important. Field and petrographic evidence and major and trace element data indicate that the gabbros do not represent pure liquids but are, at least partly, cumulates. The cumulate nature of the gabbros coupled with field evidence for open-system contamination means that trace element contents of gabbros cannot be used to fingerprint the Jurassic mantle source, nor can isotopic data be unequivocally interpreted to reflect the isotopic composition of the mantle. Correlation of Sr and Nd isotropic composition with bulk composition allows some constraints to be placed on the mantle isotopic signature. Gabbros and mafic inclusions from localities north of Barstow, CA have the most depleted mantle-like isotopic signatures (Sr(i)≈0.705 and ɛNd(t)=≈0 to +1). However, these rocks have likely seen some contamination as well, so the mantle source probably has an even more depleted character. Gabbros with the lowest Sr((i) and highest ɛNd(t) are also characterized by the highest 207Pb/204Pb and 206Pb/204Pb in the entire data set. This may be a feature of the mantle component in the Jurassic arc indicative of minor source contamination with subducted sediment as has been observed in modcrn continental arcs. Locally exposed Precambrian basement and metasedimentary rocks have appropriate Sr, Nd and Pb isotopic signatures for the crustal end members and are possible contaminants. Incorporation of these components through combined anatexis and assimilation can explain the observed spread in isotopic composition. Evidence for a depleted mantle component in these gabbros contrasts with the enriched subcontinental mantle component in Jurassic arc plutons further to the east and suggests there may have been a major mantle lithosphere boundary between the two areas as far back as the Late Jurassic. Crustal boundaries and isotopic provinces defined on the basis of initial isotopic composition (Sr(i)=0.706 isopleth) are difficult to delineate because of the correlation of bulk composition with Sr and Nd isotopic composition and because values may differ depending on the age of the rocks sampled within a given area. Data from plutons intruded into rocks known or inferred to be Precambrian are, however, shifted dramatically (highest Sr(i) and lowest ɛNd(t) toward Precambrian values. The least isotopically evolved rocks (lowest Sr(i) and highest ɛNd(t)) occur within the eugcoclinal belt of the Mojave Desert. This zone has been previously identified as a Precambrian rift zone but more likely represents a zone where mantle magmas have been intruded into isotopically similar crustal rocks of the eugeocline with minor input from old Precambrian crust.

Eruptive history and petrology of Mount Drum volcano, Wrangell Mountains, Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80x200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occured in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO2, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in 87Sr/86Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks.

秋田焼山火山の地質学的・岩石学的研究 ２ マグマ組成の変化

Magmatic evolution of Akita-yakeyama volcano, northeastern Japan, has been investigated based on bulk chemistry (major and trace), Sr isotopic features and mineral chemistry in combination with stratigraphical study. The main body of the volcano was built by eruptions of a large amount of hypersthene-augite andesite magma and a minor amount of olivine bearing hypersthene-augite andesite magma as lava flows and pyroclastic falls, and an edifice of stratovolcano was built. No evidence of magma mixing or crustal assimilation can be recognized in the olivine free andesite, but olivine-pyroxene disequilibrium in the olivine bearing andesite indicates magma mixing. After a repose period, a series of eruptions began from the central vent and erupted hypersthene-augite andesite magma as fine ash fall and hypersthene-augite-olivine andesite magma as lithic-rich pyroclastic falls. As stated above the olivine-free andesite has no evidence of magma mixing, but the olivine bearing andesite has the mineralogical and chemical evidence of magma mixing, such as disequiliblium phenocryst assemblage, reverse zoning of pyroxene and dusty plagioclase. Flank eruptions followed and erupted quartz bearing olivine-hypersthene-augite andesite magma, quartz bearing augite-hypersthene andesite magma and augite-hypersthene dacite magma as lithic-rich pyroclastic falls, lava flows and pumice fall. Among them quartz bearing andesites have high 87Sr/86Sr ratios, suggesting the involvement of crustal assimilation. At the end of these eruptions, augitehypersthene dacitic lava dome and olivine bearing augite-hypersthene dacitic lava dome intruded into central crater. Dacite magma of the lava domes has no or little evidence of magma mixing or crustal assimilation. Chemical variation of trace elements (Zr, Rb, Y) from the hypersthene-augite andesite of the main body to the augite-hypersthene dacite of the domes cannot be accounted for by fractional crystallization, and difference of degree of partial melting of the source material is required.

Evidence for magma mixing and a heterogeneous mantle on the West Valley Segment of the Juan de Fuca Ridge

We studied basaltic rocks of the northern end of the Juan de Fuca Ridge to compare the volcanology, geochemistry, and petrogenesis of spatially related rocks of an active spreading center (West Valley and West Ridge), the Sovanco Fracture Zone, the Middle Ridge, and the adjacent Endeavour and Northern Symmetrical ridges. Rocks were analyzed for mineral chemistry and major, trace, and rare earth elements (REE). Crystallization temperatures and equilibrium phase compositions were calculated. The complex zoning and disequilibrium compositions of plagioclase and olivine megacrysts and phenocrysts from all areas except West Ridge, Sovanco Fracture Zone (7123‐7), and Endeavour Ridge indicate mixing in shallow level magma chambers. The rocks range from primitive (up to Mg # = 68) in the West Valley south and West Ridge areas to evolved ferrobasalts (Mg # as low as 52) at West Valley central, West Valley north, and Northern Symmetrical Ridge. The rocks from the West Valley axis span the range of Mg #. Rocks from West Valley are transitional to enriched (e.g., [La/Ce]n= 0.89–1.06), as are rocks from Endeavour Ridge (e.g., [La/Ce]n = 0.88–1.06). Rocks from West Valley wall, Middle Ridge, Sovanco Fracture Zone, and Northern Symmetrical Ridge are light REE depleted (e.g., [La/Ge]n = 0.76–0.88), and the most depleted rocks are from West Ridge (e.g., [La/Ce]n = 0.62). The chemically heterogeneous character of the lavas exists both within site and within a single dredge haul (West Valley north and Endeavour Ridge rocks) and between sites, indicating that the mantle source is heterogeneous on a variety of scales. The range in incompatible element ratios can be modelled as binary mixing between an enriched source and a depleted source. Further, trace element systematics require variations in the degree of partial melting. Lava chemistry and ridge morphology are consistent with southward propagation of spreading in West Valley to form an overlap with the Endeavour Segment. West Valley (West Valley north, central and south) is dominated by extension over volcanism and is thus thermally and magmatically depressed. Rocks from the West Valley Segment span a broader compositional range than ridge systems to the south, reflecting the heterogeneity of the source and the lack of robust volcanism to homogenize the chemistry. Zero age rocks from West Valley are similarly enriched in [La/Ce]n as zero age rocks from the Endeavour and Explorer ridges. Rocks from the West Valley wall, the Middle Ridge, and the older Sovanco Fracture Zone rocks are depleted similar to rocks of the Juan de Fuca Ridge south of the Cobb Offset. This difference suggests that enrichment of the mantle source north of the Cobb Offset has been a recent phenomena.

Paleomagnetic and cooling constraints on the duration of the Pahute Mesa‐Trail Ridge Eruptive event and associated magmatic evolution, Black Mountain Volcanic Center, southwestern Nevada

Two successive ash‐flow cooling units of the Black Mountain volcanic center, the Pahute Mesa and Trail Ridge members of the late Miocene Thirsty Canyon Tuff, possess nearly indistinguishable reverse polarity remanent magnetization directions of about 178° declination and −10° inclination. These are interpreted to reflect thermoremanent magnetization either during a single geomagnetic field excursion or polarity transition, or possibly from a field oriented near the limit of secular variation. Regardless of the type of geomagnetic field behavior, the Pahute Mesa Member was apparently overlain by the Trail Ridge Member before appreciable shift of the anomalous geomagnetic field. This strongly suggests that the time between eruption of the two ash‐flow sheets was very short, perhaps less than 100 years, a conclusion that is independently supported by the local absence of a complete cooling break between the two units. The interval between eruption of the Pahute Mesa and Trail Ridge members was too brief for reestablishment of chemical stratification in the Black Mountain magma system. Nevertheless, a small volume of highly evolved comendite magma developed and was erupted as lava within the Black Mountain caldera and as a component of the Trail Ridge Member. The short repose interval and rapid development of the comendite, together with evidence for mixing of magmas of silicic to near‐mafic composition (Vogel et al., this issue), and the presence of air‐fall tuff at the base of the Trail Ridge, lead us to conclude that eruption of the Trail Ridge Member was probably triggered by injection of hot, relatively unevolved magma into the Black Mountain magma body.

Geochemical variation of Quaternary basaltic volcanics in the Turkana Rift, northern Kenya

There are five Quaternary volcanic edifices along the axis of the Turkana Rift of northern Kenya. From north to south these are the Korath Range, North Island, Central Island, South Island and the Barrier. Each of these edifices occurs in a structurally distinct segment of the active Turkana Rift. Preliminary studies of these Quaternary volcanics show that crystalliquid differentiation is an important process in all centers, though there is evidence of magma mixing, multiple intrusive episodes and incorporation of earlier lava flows into younger eruptives. The least-fractionated lavas from each volcano have distinctive geochemical signatures, defined by Nb/Zr, Ba/Ti, Ba/Sr and similar ratios. The Korath Range is the most alkalic series, with high Nb/Zr and Ba/Ti. These differences are attributed in large part to melting of a veined mantle beneath the individual rift segments; assimilation of continental crust may be important in the North Island magmas. Elemental abundance patterns in the Korath Range and Central Range magmas are very similar to those for enriched or plume-type ocean ridge basalts. The source for these rift volcanics may not be fundamentally different than the veined mantle that has been postulated as the source for normal and enriched ocean-ridge basalts. Quaternary rift volcanism, much like volcanism along spreading ridges, is segmented both tectonically and chemically. Comparison of these five Quaternary centers to a few analyses of Tertiary lavas along the Turkana Rift indicate that magmas have been produced from similar sources and processes for most of the development of the rift, but that there is no persistence of a particular geochemical signature with time for each magmatic cell.

Petrogenesis of Tauhara Dacite (Taupo Volcanic Zone, New Zealand) - Evidence for magma mixing between high-alumina andesite and rhyolite

Tauhara dacites have petrographic, geochemical and isotopic characteristics which indicate an origin by magma mixing between andesite and rhyolite. Phenocrysts typically exhibit strong zoning near their rims, are resorbed or display fusion textures. Assemblages are not in equilibrium with host lavas and compositions are bimodal: plagioclase An23–43 and An66–91; orthopyroxene En44–51 and En69–79. Chemical and isotopic trends pass through the bulk compositions of high-alumina andresite and rhyolite which crop out in the vicinity of the dacite domes. Least squares mixing models indicate 40–75% of a rhyolite endmember mixed with andesite can generate the full range of dacite compositions. Subtle geochemical differences between domes suggest that magma mixing may have proceeded as three or more general episodes, each punctuated by several events. These episodes may have catalyzed some of the larger pyroclastic flow eruptions of Taupo Volcanic Zone in the past 50,000 years.

Petrology of a leucogabbroic interval within basal layered gabbros at North Arm Mountain, Bay of Islands ophiolite

Data from detailed sample traverses in the layered gabbro unit of the North Arm Mauntain massif, Bay of Islands ophiolite, allow meter-scale resolution of magmatic processes in spreading ridge magma chambers. One suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase (An73.7–87.5; average modal abundance=75%), clinopyroxene (Mg#=80.3–86.0; 18%), and olivine (Fo76.6–82.1; 6%), with intergranular orthopyroxene (Mg#=78.0–83.3; 1%), and accessory Cr-Al spinel (Cr#=32.3–41.4). Ilmenite rims spinel in one sample. Whole rock Zr contents range from <6 to 15 ppm. Plots of stratigraphic height in the traverse versus petrogenetic indicators (e.g. Mg#'s of mafic phases and An in plagioclase) reveal both normal and reverse cryptic variation patterns; the patterns for all indices are generally correlated. The normal portions of the patterns formed during fractional crystallization of basalt batches. Ranges of mineral compositions in the normal trends suggest that ∼29–38% crystallization of each batch of basalt occurred before magmatic replenishment. The reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing. Other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends. Experiments and observations of natural assemblages indicate that ∼55% modal plagioclase crystallizes from basalts at the olivine+plagioclase+clinopyroxene+liquid piercing point. The average plagioclase content of this suite of leucogabbros from North Arm Mountain is too high to have formed from simple crystallization at the piercing point. Petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount (<10%) of plagioclase was resorbed during mixing; the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume. Other suites of layered gabbros from North Arm Mountain are not so plagioclase-rich as the leucogabbros described above. Crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers, or may occur episodically in the overall lifetimes of the magma chambers.