Showing papers on "Phenocryst published in 2011"

Identification of Source Lithology in the Hawaiian and Canary Islands: Implications for Origins

Abstract: Results are reported of an exploration of mantle source lithology for intraplate magmas using whole-rock and olivine phenocryst compositions.This analysis includes modern mid-ocean ridge basalts and Archean komatiites as low- and high-temperature reference frames. It is shown that the Ni, Ca, Mn, and Fe/Mn contents of olivine phenocrysts in modern mid-ocean ridge basalts and Archean komatiites are consistent with a normal peridotite source. In contrast, olivine phenocrysts in shield-building lavas on Hawaii are higher in Ni and Fe/ Mn, and lower in Mn and Ca than those expected to crystallize from meltsofa normal peridotite source, and point tothe importance ofpyroxenite as proposed by Sobolev and co-workers. Hawaiian shield stage lavas and their olivine phenocrysts are similar to those expected from partial melts of a 100% stage 2 pyroxenite source. Such a source might form from a variety of melt^rock, melt^melt, and rock^rock reactions. Primary pyroxenite-derived magmas have a range of SiO2 contents that are positively correlated with 187 Os/ 188 Os and negatively correlated with 3 He/ 4 He.These results are consistent with a Hawaiian plume containing recycled crust within a peridotite matrix.Variable amounts of free silica are inferred in Hawaiian pyroxenite sources, which contribute to the production of SiO2-rich magmas. In contrast, peridotite and olivine pyroxenite melting are inferred to produce SiO2-poor pre-shield magmas at Loihi.The interaction of SiO2-rich and -poor magmas in the Hawaiian plume will trigger crystallization, not mixing. Mixing is permitted at low pressures in melt conduits and magma chambers, and work on olivine-hosted melt inclusions will be useful to evaluate its importance. In contrast to Hawaii, many ocean island basalts in localities such as the Canary Islands are deficient in SiO2, and may have been generated by partial melting of olivine pyroxenites that formed by solid-state reaction between recycled crust þ peridotite in the lower mantle. There is likely to be a wide range of whole-rock pyroxenite compositions in the mantle, as well assignificant variability in Mn and Fe/Mn in both peridotite partial melts and their olivine phenocrysts. In general, there are not likely to be welldefined end-member peridotite and pyroxenite sources in the mantle. Nevertheless, taxonomical difficulties encountered in source lithology identification may yield rich rewards, such as a better understanding of the relationship between lithological diversity in the lower mantle and its petrological expression in intraplate magmatism.

HIGH Sr/Y ARC MAGMAS AND PORPHYRY Cu ± Mo ± Au DEPOSITS: JUST ADD WATER

Abstract: Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere. While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H 2 O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content.

Water-saturated magmas in the Panama Canal region: a precursor to adakite-like magma generation?

Abstract: Amphibole, while uncommon as a phenocryst in arc lavas, is increasingly recognized as a key constituent in the petrogenesis of arc magmas. Fractional crystallization of water-saturated arc magmas in the lower crust can yield substantial volumes of amphibole cumulates that, depending on the pressure of crystallization, may also contain garnet. Fractionation of this higher pressure assemblage has been invoked as a possible mechanism in the production of magmas that contain an adakitic signature. This study examines newly dated Late-Oligocene (25.37 ± 0.13 Ma) hypabyssal amphibole-rich andesites from Cerro Patacon in the Panama Canal region. These andesites contain nodules of amphibole cumulates that are ~4–6 cm in diameter and are almost entirely composed of 5–10-mm amphibole crystals (dominantly ferri-tschermakite). Geochemical variations, optical and chemical zoning of the Cerro Patacon amphiboles are consistent with their evolution in a crystal mush environment that had at least one recharge event prior to entrainment in the host andesite. Amphiboles hosted within the cumulate nodules differ from those hosted in the Cerro Patacon andesite and contain consistently higher values of Ti. We suggest these nodules represent the early stages of fractionation from a water-saturated magma. Cerro Patacon andesites have REE concentrations that plot at the most depleted end of Central American Arc magmas and exhibit a distinctive depletion in the middle REE. These geochemical and petrographic observations strongly support significant amphibole fractionation during formation of the Cerro Patacon andesite, consistent with the petrographic evidence. Fractionation of water-saturated magmas is a mechanism by which adakitic compositions may be produced, and the Cerro Patacon andesites do exhibit adakite-like geochemical characteristics (e.g., elevated Sr/Y; 28–34). However, the relatively elevated concentrations of Y and HREE indicate garnet was not stable in the fractionating assemblage during this early stage of arc development.

Rift-Related Transition from Andesite to Rhyolite Volcanism in the Taupo Volcanic Zone (New Zealand) Controlled by Crystal–melt Dynamics in Mush Zones with Variable Mineral Assemblages

Abstract: The Taupo Volcanic Zone (TVZ), located in the North Island of New Zealand, represents part of a magmatic arc that is at present undergoing active extension. Around 0·9 Myr ago, an acceleration in rifting was followed by a progressive transition in the composition of volcanic products (until 0·7 Ma) from typical arc-type andesite into overwhelmingly large, caldera-forming rhyolitic eruptions with subordinate basalt and dacite in the Central TVZ. Despite an obvious compositional gap in the erupted products in the Central TVZ within the last 0·7 Myr (little to no erupted products with SiO2 contents between 55 and 65 wt %), phenocryst minerals (plagioclase, amphibole, pyroxene) show an uninterrupted compositional record that suggests crystallization from a continuum of melt compositions. Coupled with radiogenic isotope evidence, the whole-rock and mineral chemistry data are consistent with magmatic differentiation controlled by crystal fractionation of primary mantle-derived magmas accompanied by some assimilation of local wall-rocks. In the Southern TVZ and in the early part of the Central TVZ, magmatic differentiation was dominated by the lower crustal evolution of relatively dry ( 1wt % H2O) arc basalts, crystallizing a pyroxene^plagioclase-dominated assemblage. However, the conditions of crystallization in the lower crust appear to have changed within the last million years in the Central TVZ, with amphibole and oxides appearing earlier in the crystallization sequence. In this framework and using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs, we show that melts extracted from crystal mushes within an optimal ‘extraction window’ ( 50 and 80 vol. % crystals) match those erupted at the surface. Lower crustal mushes fed by basalt with 1wt % H2O (dominated by a pyroxene^plagioclase assemblage) release andesitic melts at the extraction window. These melts then erupt at the surface to form the observed andesitic part of the arc.With a slightly higher water content ( 2 wt %) in the basalt, the melt composition at the extraction window from lower crustal mushes is dacitic rather than andesitic. Although some dacitic melts will reach the surface, most will be trapped in the upper crust and crystallize to form a silicic mush. Extraction of the interstitial liquid after450% crystallization from this upper crustal reservoir produces the large volumes of rhyolitic magma erupted over the past 0·7 Myr (44000 km from ignimbrite-forming eruptions).

Plagioclase zoning as an indicator of magma processes at Bezymianny Volcano, Kamchatka

Abstract: Back-scattered electron (BSE)-derived zoning patterns of plagioclase phenocrysts are used to identify magma processes at Bezymianny Volcano, Kamchatka, based on the 2000–2007 sequence of eruptive products. The erupted magmas are two-pyroxene andesites, which last equilibrated at ~915°C temperature, 77–87 MPa pressure, and a water content of ~1.4 wt%. Textural and compositional zoning of individual plagioclase phenocrysts typically includes a repeated core-to-rim sequence of oscillatory zoning (An50–60) truncated by a dissolution surface followed by an abrupt increase in An content (up to An85), which then gradually decreases rimward. This zoning pattern is interpreted to be the result of frequent replenishments of the magma chamber which cause both thermal and chemical interaction between resident and recharge magmas. The outermost 70- to 150-μm-wide zoning patterns of plagioclase phenocrysts are composed of dissolution surface with a subsequent increase in An and Fe contents. Zoning patterns of the rims exhibit correlation among plagioclase phenocrysts within one eruption. Rims are interpreted as a result of crystallization of a batch of magma in the conduit after recharge event.

Sphene and zircon in the Highland Range volcanic sequence (Miocene, southern Nevada, USA): elemental partitioning, phase relations, and influence on evolution of silicic magma

Abstract: Sphene is prominent in Miocene plutonic rocks ranging from diorite to granite in southern Nevada, USA, but it is restricted to rhyolites in coeval volcanic sequences. In the Highland Range volcanic sequence, sphene appears as a phenocryst only in the most evolved rocks (72–77 mass% SiO2; matrix glass 77–78 mass% SiO2). Zr-in-sphene temperatures of crystallization are mostly restricted to 715 and 755°C, in contrast to zircon (710–920°C, Ti-in-zircon thermometry). Sphene rim/glass Kds for rare earth elements are extremely high (La 120, Sm 1200, Gd 1300, Lu 240). Rare earth elements, especially the middle REE (MREE), decrease from centers to rims of sphene phenocrysts along with Zr, demonstrating the effect of progressive sphene fractionation. Whole rocks and glasses have MREE-depleted, U-shaped REE patterns as a consequence of sphene fractionation. Within the co-genetic, sphene-rich Searchlight pluton, only evolved leucogranites show comparable MREE depletion. These results indicate that sphene saturation in intruded and extruded magmas occurred only in highly evolved melts: abundant sphene in less silicic plutonic rocks represents a late-stage ‘bloom’ in fractionated interstitial melt.

Preeruptive magma viscosity: An important measure of magma eruptibility

Abstract: [1] Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas The basaltic-to-rhyolitic magmas have preeruptive viscosities over the range 101 to 108 Pa s Although bulk SiO2 content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO2 content shows a weak correlation with magma viscosity (correlation coefficient r = 05) Because of a wide range of phenocryst contents from 0 to ∼50 vol %, andesitic magmas have viscosities ranging from 102 to 107 Pa s, which are lower or higher than those of phenocryst-poor rhyolitic magmas with 105 to 106 Pa s Focusing on andesitic to rhyolitic magmas, the r between bulk SiO2 contents and magma viscosities changes to −01 In contrast, the melt-only SiO2 content from a basaltic-to-rhyolitic melt shows a good linear correlation with melt-only viscosity (r = 09) Although most of the calculated viscosities of erupted magmas fall below ∼106 Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >106 Pa s, in most cases, composed of mixtures of high-silica rhyolitic melt (75–79 wt % SiO2) and abundant phenocrysts (30–55 vol %) In these highly viscous magmas, 9 examples have erupted following the precursory eruption of less viscous magma, suggesting that precursory dike propagation and conduit formation by the less viscous magma with <106 Pa s induced the following eruption of less eruptible, highly viscous magmas

Discovery of Paleoproterozoic rapakivi granite on the northern margin of the Yangtze block and its geological significance

Abstract: The Huashanguan rapakivi pluton in Zhongxiang, Hubei Province, China, is the first discovered Proterozoic rapakivi pluton in the Yangtze block. Based on field and petrographical observations, a typical rapakivi texture was found in the northern portion of the Huashanguan granitic pluton. Almost all the K-feldspar phenocrysts were round to oval in shape and most had plagioclase coatings known as rapakivi phenocrysts. Alkali feldspars and quartz had two or more generations. Petrochemically, the Huashanguan rapakivi granites were characterized as having high values of Si, K, Fe, Th, U, La, Ga, Ce, Sm and LREE, low values of Ca, Mg, Sr, Nb, Y and HREE, and a negative Eu anomaly. These geochemical characteristics of the Huashanguan granites were concordant with typical rapakivi granites, and had an affinity to A-type granites. LA-ICP-MS U-Pb zircon dating also was conducted. The dating yielded a 207Pb/206Pb weighted mean age of 1851±18 Ma (MSWD =1.2), which represents the age of the pluton emplacement. The age of 803±170 Ma at the lower intercept in the concordia diagram corresponds to the age of a later deformation event which affected the pluton, and suggests that the Huashanguan pluton was influenced by Neoproterozoic thermo-tectonic events after its formation. The discovery of Paleoproterozoic Huashanguan rapakivi granites indicates continental rifting or a post-orogenic extensional event that took place in the Paleoproterozoic in the Yangtze block. These events may be related to the breakup of the Paleoproterozoic Columbia supercontinent.

Geochemical and Sr-Nd Isotopic Characteristics of Post-Collisional Calc-Alkaline Volcanics in the Eastern Pontides (NE Turkey)

Abstract: Major, trace element, K-Ar age and Sr-Nd isotopic data are presented for the Eocene Torul volcanics in the eastern Pontide orogenic belt (NE Turkey). The studied rocks are composed of basaltic andesitic, andesitic, trachyandesitic, and minor trachydacitic lavas associated with their pyroclastics. These rocks contain plagioclase (An2-44), hornblende (Mg#= 0.78-0.98), clinopyroxene (Wo43-46 En41-43 Fs10-15), biotite, quartz, and minor sanidine phenocrysts. K-Ar ages on hornblendes ages range from 43.99 (±2.59) to 33.45 (±2.32) Ma, within the Middle to Late Eocene. The volcanic rocks show calc-alkaline affinities and have medium to high K contents. They are enriched in large ion lithophile (LILE) and light rare earth elements (LREE), with pronounced depletion of high field strength elements (HFSE). The chondrite-normalized REE patterns (Lacn/Lucn= 4.0-9.8) show low to medium enrichment, indicating similar sources for the rock suite. Initial 87Sr/86Sr values vary between 0.70457 and 0.70511 and initial 143Nd/144Nd values between 0.51264 and 0.51278. The main solidification processes involved in the evolution of the volcanics consist of fractional crystallization with minor amounts of crustal contamination ± magma mixing. All evidence supports the conclusion that the parental magma(s) of the rocks probably derived from an enriched upper mantle, previously modified by subduction- induced metasomatism in a post-collisional geodynamic setting.

Molybdenite Saturation in Silicic Magmas: Occurrence and Petrological Implications

Abstract: We identified molybdenite (MoS2) as an accessory magmatic phase in 13 out of 27 felsic magma systems examined worldwide. The molybdenite occurs as small (520 m) triangular or hexagonal platelets included in quartz phenocrysts. Laser-ablation inductively coupled plasma mass spectrometry analyses of melt inclusions in molybdenite-saturated samples reveal 1^13 ppm Mo in the melt and geochemical signatures that imply a strong link to continental rift basalt^rhyolite associations. In contrast, arc-associated rhyolites are rarely molybdenite-saturated, despite similar Mo concentrations. This systematic dependence on tectonic setting seems to reflect the higher oxidation state of arc magmas compared with within-plate magmas. A thermodynamic model devised to investigate the effects of T, fO2 and f S2 on molybdenite solubility reliably predicts measured Mo concentrations in molybdenite-saturated samples if the magmas are assumed to have been saturated also in pyrrhotite. Whereas pyrrhotite microphenocrysts have been observed in some of these samples, they have not been observed from other molybdenitebearing magmas. Based on the strong influence of f S2 on molybdenite solubility we calculate that also these latter magmas must have been at (or very close to) pyrrhotite saturation. In this case the Mo concentration of molybdenite-saturated melts can be used to constrain both magmatic fO2 and f S2 if temperature is known independently (e.g. by zircon saturation thermometry). Our model thus permits evaluation of magmatic f S2, which is an important variable but is difficult to estimate otherwise, particularly in slowly cooled rocks.

Trace element variations in olivine phenocrysts from Ugandan potassic rocks as clues to the chemical characteristics of parental magmas

Abstract: Olivine phenocrysts in ugandite and leucite basanite from the western branch of the East African Rift have been analysed for up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1 ppb A combination of point analyses with varying ablation crater diameters and line scans allow the identification of subtle zonations from core to rim, as well as characterization of the chemical effects of contamination along cracks Trace element concentrations are remarkably uniform between large and small phenocrysts; fractionated leucite basanites (Mg# 59) have higher D Ca and D Al, and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75–80) Minor zonation is seen in elements with cation charges from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and show correlation between Ti and Al, but not P Early phenocryst cores with high Li or Ni, low Mn, or enrichments in many trace elements can be identified, whereas xenocrysts have exceptionally low Na, Cr, Ti, V and Co Partition coefficients for Ni are 31–35, less than in lamproites, with which they demonstrate an approximately linear correlation with K2O content, K2O/Al2O3 and K2O/Na2O of the melt, but none with SiO2 content or Mg# D-values for Cr, Mn and Co overlap with those of basalts, whereas those for Sc (0011–0018), Zn (044–049) and Ga (0006–0007) are lower D V of various potassic rocks (0015 in the Ugandan rocks) confirms the dependence on fO2 calibrated by the Fe3+/(Fe3++Fe2+) of spinels; the Ugandan potassic rocks crystallized at fO2 = FMQ to FMQ + 1 The ugandite olivines have some trace element characteristics reminiscent of those in metasomatized Kaapvaal peridotites, but not ocean islands Line scan analyses are contaminated in Al, Ca, Cu, Ga, Sr, Zr, Nb, La and Ce, elements that are also concentrated in microcracks between subgrains, indicating smearing out during polishing, and demonstrating that large spot analyses produce the best results

A Partial Record of Mixing of Mantle Melts Preserved in Icelandic Phenocrysts

Abstract: The record of mixing of mantle melts in magma chambers has previously been observed in the compositions of olivine-hosted melt inclusions from Borgarhraun, a primitive basalt flow from the Theistareykir volcanic system, northern Iceland. Borgarhraun also contains high Mg-number (85^92) clinopyroxenes, which exist in polycrystalline nodules and as phenocrysts. Coincident major and trace element analyses were made in compositional zones of these clinopyroxenes, and Ce/Yb ratios of the melts in chemical equilibrium with each of the clinopyroxene zones were calculated using carefully selected crystal^melt partition coefficients. These calculations allow direct comparison of clinopyroxene compositions with existing melt inclusion data. The range of Ce/Yb ratios in the crystals and in the equilibrium melts cannot be accounted for by crystallization alone, requiring simultaneous mixing and crystallization of compositionally variable mantle melts. However, the range in Ce/Yb for melts in equilibrium with these high Mg-number clinopyroxenes is smaller than that of melt inclusions hosted by olivines with equivalent Fo contents. Also, the mean composition of the melts from which clinopyroxene grew has significantly lower Ce/Yb than the olivine-hosted melt inclusions. The record of mantle melt variability in clinopyroxenes is thus biased towards more depleted (low Ce/Yb) melt compositions. This bias can be understood if the trace element variation in the Borgarhraun parental melts is coupled to major element variation, as expected from petrological parameterizations of mantle melting. The major element variation influences the phase relationships and controls the appearance of liquidus phases during fractional crystallization in near-Moho magma chambers. Small-degree, deep melts, formed in the presence of garnet, have high Ce/Yb ratios. On cooling, these melts have a longer olivine-only crystallization path than melts derived from the shallow mantle. When these deep-sourced melts eventually become clinopyroxene saturated, they have too low Mg-numbers to crystallize high Mg-number clinopyroxenes such as are found in Borgarhraun. In contrast, shallow, depleted melts saturate in clinopyroxene at high Mg-number. The delayed onset of clinopyroxene crystallization in the enriched melts, coupled with concurrent mixing and crystallization of melts generated at a range of depths in the mantle, can account for the difference in the distribution of the trace element composition of high Mg-number melts saturated in olivine and clinopyroxene. The trace element compositions of high Mg-number clinopyroxenes in Borgarhraun therefore provide only a partial and biased record of the mixing of mantle melts. As well as showing that melt mixing may be preserved in phenocryst compositions, the results illustrate that trace element disequilibrium between crystals and carrier melt can be a consequence of magma mixing, rather than necessitating a xenocrystic origin for the crystals. Furthermore, care must be taken when using clinopyroxene separates from primitive basalts to examine compositional heterogeneity, as they provide a record of the chemical evolution of the magmatic system that is biased towards depleted compositions and therefore incomplete.

Heterogeneous pumice populations in the 2.08-Ma Cerro Galán Ignimbrite: implications for magma recharge and ascent preceding a large-volume silicic eruption

Abstract: Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the ~2.08-Ma caldera-forming eruption of Cerro Galan volcano, Argentina, which produced distinct pumice populations of two colors: grey (5%) and white (95%) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e.g., ~150 ppm lower Ba at equivalent SiO2 content and 0.03 wt.% higher TiO2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44–57%), lack microlites, and have highly evolved groundmass glass compositions (76.4–79.6 wt.% SiO2), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35–49%), have abundant microlites, and have less evolved groundmass glass compositions (69.4–73.8 wt.% SiO2). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Galan eruption, grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.

Two Primary Basalt Magma Types from Northwest Rota-1 Volcano, Mariana Arc and its Mantle Diapir or Mantle Wedge Plume

Abstract: Primitive basalts are rarely found in arcs.The active NWRota-1 volcano in the Mariana arc has erupted near-primitive lavas, which we have sampled with ROV Hyper-Dolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51^52 wt % SiO2) with 7·5^9·5 wt % MgO and Mg-number of 61^67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo93 and contain 0·4 wt % NiO; the Cr/(CrþAl) values of spinels are mostly 0·5^0·8, indicating equilibrium with depleted mantle.There are three petrographic groups, based on phenocryst populations: (1) cpx^olivine basalt (COB); (2) plagioclase^olivine basalt (POB); (3) porphyritic basalt. Zr/Y and Nb/Yb are higher in POB (3·1^3·2 and 1·2^1·5, respectively) than in COB (Zr/Y1⁄4 2·8^3·0 and Nb/ Yb1⁄4 0·7^0·9), suggesting that POB formed from lower degrees of mantle melting, or that the COB mantle source was more depleted. On the other hand, COB have Ba/Nb (70^80) and Th/Nb (0·4^ 0·5) that are higher than for POB (Ba/Nb1⁄4 30^35 and Th/ Nb1⁄4 0·1^0·2), and also have steeper light rare earth element (LREE)-enriched patterns. Moreover, COB have enriched Sr/Sr and Nd/Nd, and higher Pb isotope values, suggesting that COB has a greater subduction component than POB. Hf/Hf between COB and POB are similar and Hf behavior in COB and POB is similar to that of Zr,Yand HREE, suggesting that Hf is not included in the subduction component, which produced the differences between COB and POB.The calculated primary basaltic magmas of NW Rota-1 volcano (primary COB and POB magmas) indicate segregation pressures of 2^1·5 GPa (equivalent to 65^50 km depth).These magmas formed by 24^18% melting of mantle peridotite having Mg-number 89·5. Diapiric ascent of hydrous peridotite mixed heterogeneously with sediment melts may be responsible for the NW Rota-1 basalts. These two basalt magma types are similar to those found at Sumisu andTorishima volcanoes in the Izu^Bonin arc, with COB representing wetter and POB representing drier magmas, where subduction zone-derived melt components are coupled with the water contents.

Complex Phenocryst Textures and Zoning Patterns in Andesites and Dacites: Evidence of Degassing-Induced Rapid Crystallization?

Abstract: A detailed petrological study is presented of seven phenocryst-poor andesites and dacites erupted from short-lived, monogenetic, periph- eral vents in the Mexican volcanic arc Despite low phenocryst (þ microphenocryst) abundances (55%), these magmas were each multiply saturated with five to seven mineral phases (plagio- clase þ orthopyroxene þ titanomagnetite þ apatite þ ilmeniteclino- pyroxenehornblendebiotite) Groundmass textures range from nearly complete glass to nearly complete microlite crystallization Eruptive temperatures, on the basis of Fe^Ti two-oxide thermom- etry, range from 872 (� 13) to 1085 (� 16) 8C for the andesites and 760 (� 18) to 957 (� 19)8C for the dacites Corresponding oxygen fugacities range from � 0· 9t oþ0·6 log units relative to the Ni^NiO bufferWith temperature known, the plagioclase^liquid hy- grometer was applied to the sparse plagioclase phenocrysts in each sample; the highest calculated water concentrations range from 6·4 to 2· 8w t %H 2O for the andesites and from 8· 3t o 4·1wt % H2O for the dacites These results require that the magmas were fluid-saturated at depths 4 9a nd412 km, respectively, for the most hydrous andesite and dacite The plagioclase population in each sample spans a wide range in composition (� 34 mol % An), which can be attributed to the effects of changing melt water concen- tration owing to degassing during magma ascentThe orthopyroxene population in each sample also spans a wide range in composition (� 17% Mg-number), and it is proposed on the basis of phase equi- librium experiments from the literature and data from the JANAF thermochemical tables that this also reflects variable melt water con- centrations, with higher water leading to more Fe-rich orthopyroxene Degassing leads to a progressive increase in melt viscosity, and the phenocrysts crystallized over an interval of � 2·4^4· 3l og 10 Pa s in these samples The phenocrysts often display diffusion-limited growth textures (eg skeletal and hopper crystals, large interior melt hollows) consistent with large undercoolings caused by degassing, which has produced a complicated petrography in these crystal-poor hydrous magmas, including reverse and patchy zoning in plagioclase and pyroxene

Extraction of crystal-poor rhyolite from a hornblende-bearing intermediate mush: a case study of the caldera-forming Matahina eruption, Okataina volcanic complex

Abstract: The Matahina Ignimbrite (~160 km3 rhyolite magma, 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, Taupo Volcanic Zone (TVZ), New Zealand. Juvenile clasts are divided into three groups: Group (1) the dominant crystal-poor rhyolite type, Group (2) a minor coarse-grained, mingled/mixed intermediate type, and Group (3) a rare fine-grained basalt. The ignimbrite consists of the Group 1 type and is divided into three members: a lower and middle member, which is high-silica, crystal-poor (<10 vol.%) rhyolite, and the upper member, which is low-silica and slightly more crystal-rich (up to 21 vol.%). Cognate, crystal-rich (up to 50 vol.%) basalt to intermediate pumice occurs on top of lag breccias and within lithic-rich pyroclastic density current deposits along the caldera margin (Groups 2 and 3). Several lines of evidence indicate that the intermediate clasts represent the cumulate complement to the melt-rich rhyolite: (1) continuity in the compositions of plagioclase, orthopyroxene, hornblende, and oxides and normal zoning of individual phenocrysts; (2) the silicic glass from the intermediate magma (interstitial melt) overlaps compositionally with the bulk rock rhyolite and glass; (3) high Zr and a slight positive Eu anomaly in the intermediate magma relative to quenched enclaves from other intermediate TVZ eruptions indicates zircon and plagioclase accumulation, respectively; (4) an increase in the Cl contents in glass from the least evolved to most evolved is consistent with the concentration of volatiles during magma evolution. Most of the compositional variations in the low- to high-silica rhyolites can be accounted for by continued Rayleigh fractionation (up to 15%), following melt extraction from the underlying mush, under varying fO2–fH2O conditions to form a slightly compositionally zoned rhyolitic cap. This link to the varying fO2–fH2O conditions is evidenced by the strong correlation between key geochemical parameters (e.g. Dy, Y), that qualitatively reflect fH2O conditions (presence or absence of hornblende/biotite), and fO2 estimated from Fe–Ti oxide equilibrium. Magma mingling/mixing between the basalt–andesite and the main slightly compositionally zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved rhyolite at the lower portion of the reservoir and effectively destroying any pre-eruptive gradients.

Geochemical homogeneity of a long-lived, large silicic system; evidence from the Cerro Galán caldera, NW Argentina

Abstract: By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components, we show that the Cerro Galan volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr. The Cerro Galan system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides, apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro Galan ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice is interpreted to represent the parental magmas to the Galan system emplaced into the upper crust from a deeper storage zone. Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from 40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and Yb between the Galan ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galan system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts. A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma source in a subduction zone volcanic setting. At Cerro Galan, we propose that there is a zone between mantle magma input and upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures. This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation, storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve only to slightly modify the geochemistry of the magmas.

Magma storage and ascent during the 1995 eruption of Fogo, Cape Verde Archipelago

Abstract: The 1995 eruption of Fogo (Cape Verde Islands) differed from previous eruptions by the occurrence of evolved lavas, the SW-orientation of vents, and pre-eruptive seismicity between Fogo and the adjacent (~20 km) island of Brava. We have conducted a thermobarometric and chemical study of this eruption in order to reconstruct its magma plumbing system and to test for possible connections to Brava. The bimodal eruption produced basanites (5.2–6.7 wt% MgO) and phonotephrites (2.4–2.8 wt% MgO) that are related by fractional crystallization. Clinopyroxene-melt-barometry of phenocrysts yields pressure ranges of 460–680 MPa for the basanites and 460–520 MPa for the phonotephrites. Microthermometry of CO2-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yields systematically lower pressure ranges of 200–310 MPa for basanites and 270–470 MPa for phonotephrites. The combined data indicate pre-eruptive storage of the 1995 magmas within the lithospheric mantle between 16 and 24 km depth. During eruption, the ascending magmas stalled temporarily at 8–11 km depth, within the lower crust, before they ascended to the surface in a few hours as indicated by zonations of olivine phenocrysts. Our data provide no evidence for magma storage at shallow levels (<200 MPa) or lateral magma movements beneath the Fogo-Brava platform. Sr–Nd–Pb isotope ratios of samples from Brava differ significantly from those of the 1995 and older Fogo lavas, which rules out contamination of the 1995 magmas by Brava material and indicates different mantle sources and magma plumbing systems for both islands.

Phenocryst complexity in andesites and dacites from the Tequila volcanic field, Mexico: resolving the effects of degassing vs. magma mixing

Abstract: The petrology of five phenocryst-poor (2–5%) andesites and dacites, all of which were erupted from different short-lived, monogenetic vents, is compared to that of phenocryst-rich (10–25%) andesites erupted from the adjacent stratovolcano, Volcan Tequila, in the Mexican arc. Despite differences in phenocryst abundances, these magmas have comparable phase assemblages (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), and similarly wide variations in phenocryst compositions, coupled to complex zoning patterns. For the phenocryst-poor lavas, equilibrium pairs of two Fe–Ti oxides lead to a narrow range of calculated temperatures for each sample that range from 934 (±24) to 1,073 (±6)°C and oxygen fugacities that range from +0.1 to +0.7 log units relative to the Ni–NiO buffer. Application of the plagioclase-liquid hygrometer to each sample at these calculated temperatures leads to maximum melt water concentrations of 4.6–3.1 wt% during plagioclase crystallization, indicating that the magmas were fluid saturated at depths ≥6.4–4.5 km. There is a wide, continuous range in the composition of plagioclase (≤44 mol% An) and orthopyroxene (≤16% Mg#) phenocrysts in each sample, which is consistent with a loss of dissolved water (≤2.8 wt%) from the melt phase during degassing as the magmas ascended rapidly to the surface. Evidence is presented that shows the effect of dissolved water is to reduce the activity of MgO relative to FeO in the melt phase, which indicates that degassing will also affect the Mg# of pyroxene phenocrysts, with higher melt water concentrations favoring Fe-rich pyroxene. Both plagioclase and orthopyroxene commonly display diffusion-limited growth textures (e.g., skeletal and hopper crystals, large interior melt hollows, and swallow tails), which are consistent with large undercoolings produced by degassing-induced crystallization. Therefore, degassing is proposed as a possible cause for the phenocryst compositional diversity documented in the phenocryst-poor andesite and dacite lavas erupted from peripheral vents, including the coexistence of normally zoned plagioclase and reversely zoned orthopyroxene. Degassing-induced crystallization may also explain some of the phenocryst complexity in crystal-rich andesites erupted from large stratovolcanoes, including Volcan Tequila.

Petrochemistry and U-Pb Zircon Ages of Adakitic Intrusions from the Pulur Massif (Eastern Pontides, NE Turkey): Implications for Slab Rollback and Ridge Subduction Associated with Cenozoic Convergent Tectonics in the Eastern Mediterranean

Abstract: The well-preserved magmatic arc in the eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic–early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we investigate the petrology, geochemistry, and U-Pb zircon chronology of adakitic intrusions from the Pulur region in the southern zone of the eastern Pontides belt. The intermediate to felsic intrusives in the Pulur region are characterized by the presence of abundant plagioclase, amphibole, and biotite phenocrysts. The rocks display extreme light rare earth element enrichment (); high Al2O3 (15.46–17.47 wt%), Na2O (3.57–8.43 wt%), Sr (324.8–1468 ppm), and La (13.9–55.8 ppm) concentrations; a high Sr/Y ratio (35–473); and low Y (3–12.6 ppm) and heavy rare earth element concentrations, closely comparing with the typical features of adakites. Zircons extracted from the Pulur adakites show euhedral crystal shapes, oscillatory zoning patterns, an...

´A´ā lava flows in the Deccan Volcanic Province, India, and their significance for the nature of continental flood basalt eruptions

Abstract: Newly identified ´a´ā lava flows outcrop intermittently over an area of ~110 km2 in the western Deccan Volcanic Province (DVP), India. They occur in the upper Thakurvadi Formation in the region south of Sangamner. The flows, one of which is compound, are 15–25 m thick, and exhibit well-developed basal and flow-top breccias. The lavas have microcrystalline groundmasses and are porphyritic or glomerocrystic and contain phenocrysts of olivine, clinopyroxene or plagioclase feldspar. They are chemically similar to compound pāhoehoe flows at a similar stratigraphic horizon along the Western Ghats. Petrographic and geochemical differences between ´a´ā flows at widely spaced outcrops at the same stratigraphic horizon suggest that they are the product of several eruptions, potentially from different sources. Their presence in the DVP could suggest relative proximity to vents. This discovery is significant because ´a´ā lavas are generally scarce in large continental flood basalt provinces, which typically consist of numerous inflated compound pāhoehoe lobes and sheet lobes. Their scarcity is intriguing, and may relate to either their occurrence only in poorly preserved or exposed proximal areas or to the flat plateau-like topography of flood basalt provinces that may inhibit channelization and ´a´ā formation, or both. In this context, the ´a´ā flow fields described here are inferred to be the products of eruptions that produced unusually high-effusion-rate lavas compared to typical flood basalt eruptions. Whether these phases were transitional to lower intensity, sustained eruptions that fed extensive low effusion rate pāhoehoe flow fields remains unclear.

Magma mixing/mingling in the Eocene Horoz (Nigde) granitoids, Central southern Turkey: evidence from mafic microgranular enclaves

Abstract: Mafic microgranular enclaves (MMEs) are widespread in the Horoz pluton with granodiorite and granite units. Rounded to elliptical MMEs have variable size (from a few centimetres up to metres) and are generally fine-grained with typical magmatic textures. The plagioclase compositions of the MMEs range from An18–An64 in the cores to An17–An29 in the rims, while that of the host rocks varies from An17 to An55 in the cores to An07 to An33 in the rims. The biotite is mostly eastonitic, and the calcic-amphibole is magnesio-hornblende and edenite. Oxygen fugacity estimates from both groups’ biotites suggest that the Horoz magma possibly crystallised at fO2 conditions above the nickel–nickel oxide (NNO) buffer. The significance of magma mixing in their genesis is highlighted by various petrographic and mineralogical characteristics such as resorption surfaces in plagioclases and amphibole; quartz ocelli rimmed by biotite and amphibole; sieve and boxy cellular textures, and sharp zoning discontinuities in plagioclase. The importance of magma mixing is also evident in the amphiboles of the host rocks, which are slightly richer in Si, Fe3+ and Mg in comparison with the amphiboles of MMEs. However, the compositional similarity of the plagioclase and biotite phenocrysts from MMEs and their host rocks suggests that the MMEs were predominantly equilibrated with their hosts. Evidence from petrography and mineral chemistry suggests that the adakitic Horoz MMEs could be developed from a mantle-derived, water-rich magma (>3 mass%) affected by a mixing of felsic melt at P >2.3 kbar, T >730°C.