Journal of Petrology - current issue

Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska

2008-04-16

Okmok volcano is situated on oceanic crust in the central Aleutian arc and experienced large (~15 km³) caldera-forming eruptions at ~12 000 years bp and 2050 years bp. Each caldera-forming eruption began with a small Plinian rhyodacite event followed by the emplacement of a dominantly andesitic ash-flow unit, whereas effusive inter- and post-caldera lavas have been more basaltic. Phenocryst assemblages are composed of olivine + pyroxene + plagioclase ± Fe–Ti oxides and indicate crystallization at 1000–1100°C at 0·1–0·2 GPa in the presence of 0–4% H₂O. The erupted products follow a tholeiitic evolutionary trend and calculated liquid compositions range from 52 to 68 wt % SiO₂ with 0·8–3·3 wt % K₂O. Major and trace element models suggest that the more evolved magmas were produced by 50–60% in situ fractional crystallization around the margins of the shallow magma chamber. Oxygen and strontium isotope data (¹⁸O 4·4–4·9, ⁸⁷Sr/ ⁸⁶Sr 0·7032–0·7034) indicate interaction with a hydrothermally altered crustal component, which led to elevated thorium isotope ratios in some caldera-forming magmas. This compromises the use of uranium–thorium disequilibria [(²³⁰Th/ ²³⁸U) = 0·849–0·964] to constrain the time scales of magma differentiation but instead suggests that the age of the hydrothermal system is ~100 ka. Modelling of the diffusion of strontium in plagioclase indicates that many evolved crystal rims formed less than 200 years prior to eruption. This addition of rim material probably reflects the remobilization of crystals from the chamber margins following replenishment. Basaltic recharge led to the expansion of the magma chamber, which was responsible for the most recent caldera-forming event.

Petrogenesis of Ultramafic Rocks from the Ultrahigh-pressure Metamorphic Kimi Complex in Eastern Rhodope (NE Greece)

Baziotis, I., Mposkos, E., Asimow, P. D. — 2008-04-16

Widespread bodies of garnet–spinel metaperidotites with pyroxenitic layers occur in the ultrahigh-pressure metamorphic Kimi Complex. In this study we address the origin of such peridotite–pyroxenite associations in the context of polybaric melting regimes. We conduct a detailed geochemical investigation of major and trace element relations and compare them with a range of major element modelling scenarios. With increasing bulk-rock MgO content, the garnet–spinel metaperidotites exhibit decreasing CaO, Al₂O₃, TiO₂, and Na₂O along with increasing Ni and a gradually increasing Zr/Zr* anomaly, consistent with an origin as residues after variable degrees of melt extraction. The major element modelling further suggests a polybaric adiabatic decompression melting regime beginning at high to ultrahigh pressure, with an intermediate character between pure batch and fractional melting and a mean extent of melting of 9–11%. The pyroxenites exhibit major element compositions that cannot be reproduced by experimental or calculated melts of peridotite. Moreover, the Kimi pyroxenites have highly variable Ni and Sc contents and a wide range of Mg-number (0· 76–0· 89), inconsistent with an origin as frozen melts or the products of melt–peridotite interaction. However, both the major element systematics and the observed rare earth element patterns, with both convex and concave shapes, can be explained by an origin as clinopyroxene-rich, high-pressure cumulates involving garnet and/or Cr-spinel.

Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C-O-H-S) on Phase Equilibria and Mineral Composition

Parat, F., Holtz, F., Feig, S. — 2008-04-16

Crystallization experiments at 400 MPa, oxidized condition (logfO₂ = NNO + 1, where NNO is nickel–nickel oxide buffer) and over a range of temperatures (850–950°C) and fluid composition (XH₂O_in = 0·3–1) have been carried out to constrain the storage conditions of the sulphur-rich magma of the Huerto Andesite (an anhydrite, pyrrhotite, and S-rich apatite-bearing, post-Fish Canyon Tuff mafic lava). The results are used to evaluate the role of fluids released from the crystallization of magmas such as the Huerto Andesite on the remobilization of the largely crystallized dacitic Fish Canyon magma body. Experiments were performed using the natural andesitic bulk composition with and without added sulphur. The presence of sulphur slightly affects the phase equilibria by changing the phase proportions, stability fields of plagioclase, pyroxenes and ilmenite, and also affects the plagioclase composition. Phase equilibria and mineral composition data indicate that the magma may have contained 4·5 wt % water in the melt and that the pre-eruptive temperature was 875 ± 25°C. Assuming that the magma was in equilibrium with a fluid phase, the CO₂ concentration of the melt is estimated to be in the range 2000–4000 ppm (at 400 MPa). Before eruption, the andesite had an oxidation state very close to, or slightly within, the co-stability field of anhydrite–pyrrhotite at NNO + 1·1. At these conditions, the sulphur content in the melt is ~500 ppm. Assuming open-system degassing resulting from continuing crystallization at depth, most of the CO₂ dissolved in the andesitic melt should be released after the crystallization of <10 vol. % of the magma, corresponding to a cooling from 875 to 825–850°C. Thus, the fluids released owing to crystallization processes should be mainly composed of water at temperatures below 825°C.

Petrology and U-Pb Zircon Geochronology of Amphibole-rich Cumulates with Sanukitic Affinity from Husky Ridge (Northern Victoria Land, Antarctica): Insights into the Role of Amphibole in the Petrogenesis of Subduction-related Magmas

Tiepolo, M., Tribuzio, R. — 2008-04-16

A microanalytical trace element and geochronological study was carried out on mafic amphibole-rich cumulates (quartz diorites) cropping out in northern Victoria Land (Antarctica). Associated tonalites and basement rocks were also investigated. Rock textures and major and trace element mineral compositions reveal the presence in quartz diorites of two mineral assemblages: (1) clinopyroxene-I + brown amphibole ± dark mica; (2) clinopyroxene-II + green amphibole + plagioclase + quartz. Both mineral assemblages contain mafic phases with elevated Mg-number, but their trace element signatures differ significantly. In situ U–Pb zircon geochronology was carried out to support petrogenetic and geological interpretations. Quartz diorites were emplaced in the mid-crust probably at 516 ± 3 Ma. Parental melts of quartz diorites were computed by applying solid/liquid partition coefficients. The melt in equilibrium with the first mineral assemblage (melt-I) is extremely depleted in heavy rare earth elements (HREE), Y, Ti, Zr and Hf (at about 0·2 times normal mid-ocean ridge basalt) and enriched in B, Th, U, the large ion lithophile elements and light REE (LREE). It shares many similarities with sanukitic melts (e.g. Setouchi andesites), which originated by equilibration of subduction-derived sediment melts with a refractory mantle. The melt in equilibrium with the second mineral assemblage (melt-II) is characterized by a steep LREE enrichment (La_N/Yb_N up to 39), a U-shaped HREE pattern and low Ti, which is depleted relative to HREE. The trace element signature of melt-II can be acquired through amphibole crystallization starting from a sanukitic melt similar to melt-I, probably in a deeper magma chamber. Our results allow us to constrain that melts from the subducted slab were produced on a regional scale, in accordance with literature data, below a large sector of the east Gondwana margin during the mid-Cambrian. Implications for the role of amphibole in petrogenesis of subduction-related magmas are also discussed.

Oxygen Isotope Geochemistry of the Lassen Volcanic Center, California: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism

Feeley, T. C., Clynne, M. A., Winer, G. S., Grice, W. C. — 2008-04-16

This study reports oxygen isotope ratios determined by laser fluorination of mineral separates (mainly plagioclase) from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the Lassen Volcanic Center (LVC), northern California. Plagioclase separates from nearly all rocks have ¹⁸O values (6·1–8·4) higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back-arc regions of the southernmost Cascades during the late Cenozoic. Most LVC magmas must therefore contain high ¹⁸O crustal material. In this regard, the ¹⁸O values of the volcanic rocks show strong spatial patterns, particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust. Rhyodacitic magmas erupted from vents located within 3·5 km of the inferred center of the LVC have consistently lower ¹⁸O values (average 6·3 ± 0·1) at given SiO₂ contents relative to rocks erupted from distal vents (>7·0 km; average 7·1 ± 0.1). Further, magmas erupted from vents situated at transitional distances have intermediate values and span a larger range (average 6·8 ± 0·2). Basaltic andesitic to andesitic composition rocks show similar spatial variations, although as a group the ¹⁸O values of these rocks are more variable and extend to higher values than the rhyodacitic rocks. These features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas, followed by mixing or mingling with silicic magmas formed by partial melting of initially high ¹⁸O continental crust (~9·0) increasingly hybridized by lower ¹⁸O (~6·0) mantle-derived basaltic magmas toward the center of the system. Mixing calculations using estimated endmember source ¹⁸O values imply that LVC magmas contain on a molar oxygen basis approximately 42 to 4% isotopically heavy continental crust, with proportions declining in a broadly regular fashion toward the center of the LVC. Conversely, the ¹⁸O values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the LVC has been substantially modified by intrusion of mantle-derived basaltic magmas, with the degree of hybridization ranging on a molar oxygen basis from approximately 60% at distances up to 12 km from the center of the system to 97% directly beneath the focus region. These results demonstrate on a relatively small scale the strong influence that intrusion of mantle-derived mafic magmas can have on modifying the composition of pre-existing continental crust in regions of melt production. Given this result, similar, but larger-scale, regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction-related basaltic magmas.

Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

Bodinier, J.-L., Garrido, C. J., Chanefo, I., Bruguier, O., Gervilla, F. — 2008-04-16

The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclase–spinel websterites. The ‘asthenospherized’ part of the Ronda peridotite contains abundant layered websterites (‘group C’ pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitions towards their host peridotites. Previous studies have suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modelling study of a layered outcrop of group C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization melt–rock reactions. Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (~0·65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (>0·1 and > 1, respectively) to balance Mg–Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front. The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convex-upward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)_N vs (Sm/Yb)_N diagram (where N indicates chondrite normalized). Based on (Ce/Nd)_N values, two transient, enriched domains between the light REE (LREE)-depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxenite–peridotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions. The latter are probably volatile-rich, small-volume melt fractions residual after the refertilization reactions that generated group C pyroxenites, which migrated throughout the massif—including the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the ‘layered granular’ subdomain and ‘plagioclase-tectonite’ domain) are on average more fertile than the residual, ‘coarse granular’ subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.

Crystal-Melt Separation and the Development of Isotopic Heterogeneities in Hybrid Magmas

Beard, J. S. — 2008-04-16

If a magma is a hybrid of two (or more) isotopically distinct end-members, at least one of which is partially crystalline, separation of melt and crystals after hybridization will lead to the development of isotopic heterogeneities in the magma as long as some of the pre-existing crystalline material (antecrysts) retains any of its original isotopic composition. This holds true whether the hybridization event is magma mixing as traditionally construed, bulk assimilation, or melt assimilation. Once a magma-scale isotopic heterogeneity is formed by crystal–melt separation, it is essentially permanent, persisting regardless of subsequent crystallization, mixing, or equilibration events. The magnitude of the isotopic variability resulting from crystal–melt separation can be as large as that resulting from differential contamination, multiple isotopically distinct sources, or in situ isotopic evolution. In one model, a redistribution of one-third of the antecryst cargo yielded a crystal-enriched sample with ⁸⁷Sr/⁸⁶Sr of 0·7058, whereas the complementary crystal-poor sample has ⁸⁷Sr/⁸⁶Sr of 0·7068. In other models, crystal-rich samples are enriched in radiogenic Sr. Isotopic heterogeneities can be either continuous (controlled by the modal distribution of crystals and melt) or discontinuous (when there is complete separation of crystals and liquid). The first case may be exemplified by some isotopically zoned large-volume rhyolites, formed by the eruptive inversion of a modally zoned magma chamber. In the latter case, the isotopic composition of any (for example) interstitial liquid will be distinct from the isotopic composition of the bulk crystal fraction. The separation of such an interstitial liquid may explain the presence of isotopically distinct late-stage aplites in plutons. Crystal–melt separation provides an additional option for the interpretation of isotopically zoned or heterogeneous magmas. This option is particularly attractive for systems whose chemical variation is otherwise explicable by fractionation-dominated processes. Non-isotopic chemical heterogeneities can also develop in this fashion.