Journal of Petrology - recent issues

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.

Foreword: The Roles of Petrology and Experimental Petrology in Understanding Global Tectonics

Yaxley, G. M., Brey, G. P. — 2008-03-25

The Composition of Near-solidus Partial Melts of Fertile Peridotite at 1 and 1{middle dot}5 GPa: Implications for the Petrogenesis of MORB

Falloon, T. J., Green, D. H., Danyushevsky, L. V., McNeill, A. W. — 2008-03-25

We have determined the near-solidus melt compositions for peridotite MM-3, a suitable composition for the production of mid-ocean ridge basalt (MORB) by decompression partial melting, at 1 and 1·5 GPa. At 1 GPa the MM-3 composition has a subsolidus plagioclase-bearing spinel lherzolite assemblage, and a solidus at ~ 1270°C. At only ~ 5°C above the solidus, 4% melt is present as a result of almost complete melting of plagioclase. This melting behaviour in plagioclase lherzolite is predicted from simple systems and previous experimental work. The persistence of plagioclase to > 0·8 GPa is strongly dependent on bulk-rock CaO/Na₂O and normative plagioclase content in the peridotite. At 1·5 GPa the MM-3 composition has a subsolidus spinel lherzolite assemblage, and a solidus at ~ 1350°C. We have determined a near-solidus melt composition at ~ 2% melting within 10°C of the solidus. Near-solidus melts at both 1 and 1·5 GPa are nepheline normative, and have low normative diopside contents; also they have the highest TiO₂, Al₂O₃ and Na₂O, and the lowest FeO and Cr₂O₃ contents compared with higher degree partial melts. Comparison of these near-solidus melts with primitive MORB glasses, which lie in the olivine-only field of crystallization at low pressure, indicate that petrogenetic models involving aggregation of near-fractional melts formed during melting at pressures of 1·5 GPa or less are unlikely to be correct. In this study we use an experimental approach that utilizes sintered oxide mix starting materials and peridotite reaction experiments. We also examine some recent studies using an alternative approach of melt migration into, and entrapment within ‘melt traps’ (olivine, diamond, vitreous carbon) and discuss optimal procedures for this method.

Origin of the Oceanic Lithosphere

Presnall, D. C., Gudfinnsson, G. H. — 2008-03-25

In a global examination of mid-ocean ridge basalt (MORB) glass compositions, we find that Na₈–Fe₈–depth variations do not support modeling of MORBs as aggregates of melt compositions generated over a large range of temperature and pressure. However, the Na₈–Fe₈ variations are consistent with the compositional systematics of solidus melts in the plagioclase–spinel lherzolite transition in the CaO–MgO–Al₂O₃–SiO₂–Na₂O–FeO (CMASNF) system. For natural compositions, the P–T range for melt extraction is estimated to be ~1·2–1·5 GPa and ~1250–1280°C. This P–T range is a close match with the maximum P–T conditions for explosive pressure-release vaporization of carbonate-bearing melts. It is proposed that fracturing of the lithosphere induces explosive formation and escape of CO₂ vapor. This provides the vehicle for extraction of MORBs at a relatively uniform T and P. The upper portion of the CO₂-bearing and slightly melted seismic low-velocity zone flows toward the ridge, rises at the ridge axis to lower-lithosphere depths, melts much more extensively during this rise, and releases MORB melts to the surface driven by explosively escaping CO₂ vapor. The residue and overlying crust produced by this melting then migrate away from the ridge axis as new oceanic lithosphere. The entire process of oceanic lithosphere creation involves only the upper ~140 km. When lithospheric stresses shift fracture formation to other localities, escape of CO₂ ceases, the vehicle for transporting melt to the surface disappears, and ridges die. Inverse correlations of Na₈ vs Fe₈ for MORB glasses are explained by mantle heterogeneity, and positive variations superimposed on the inverse variations are consistent with progressive extraction of melts from short, ascending melting columns. The uniformly low temperatures of MORB extraction are not consistent with the existence of hot plumes on or close to ocean ridges. In this modeling, the southern Atlantic mantle from Bouvet to about 26°N is relatively homogeneous, whereas the Atlantic mantle north of about 26°N shows significant long-range heterogeneity. The mantle between the Charlie Gibbs and Jan Mayen fracture zones is strongly enriched in FeO/MgO, perhaps by a trapped fragment of basaltic crust. Iceland is explained as the product of this enrichment, not a hot plume. The East Pacific Rise, Galapagos Ridge, Gorda Ridge, and Juan de Fuca Ridge sample mantle that is heterogeneous over short distances. The mantle beneath the Red Sea is enriched in FeO/MgO relative to the mantle beneath the northern Indian Ocean.

Global Correlations of Ocean Ridge Basalt Chemistry with Axial Depth: a New Perspective

Niu, Y., O'Hara, M. J. — 2008-03-25

The petrological parameters Na₈ and Fe₈, which are Na₂O and FeO contents in mid-ocean ridge basalt (MORB) melts corrected for fractionation effects to MgO = 8 wt%, have been widely used as indicators of the extent and pressure of mantle melting beneath ocean ridges. We find that these parameters are unreliable. Fe₈ is used to compute the mantle solidus depth (P_o) and temperature (T_o), and it is the values and range of Fe₈ that have led to the notion that mantle potential temperature variation of T_P = 250 K is required to explain the global ocean ridge systematics. This interpreted T_P = 250 K range applies to ocean ridges away from ‘hotspots’. We find no convincing evidence that calculated values for P_o, T_o, and T_P using Fe₈ have any significance. We correct for fractionation effect to Mg^# = 0·72, which reveals mostly signals of mantle processes because melts with Mg^# = 0·72 are in equilibrium with mantle olivine of Fo_89·6 (vs evolved olivine of Fo_{88·1–79·6} in equilibrium with melts of Fe₈). To reveal first-order MORB chemical systematics as a function of ridge axial depth, we average out possible effects of spreading rate variation, local-scale mantle source heterogeneity, melting region geometry variation, and dynamic topography on regional and segment scales by using actual sample depths, regardless of geographical location, within each of 22 ridge depth intervals of 250 m on a global scale. These depth-interval averages give Fe₇₂ = 7·5–8·5, which would give T_P = 41 K (vs ~250 K based on Fe₈) beneath global ocean ridges. The lack of Fe₇₂–Si₇₂ and Si₇₂–ridge depth correlations provides no evidence that MORB melts preserve pressure signatures as a function of ridge axial depth. We thus find no convincing evidence for T_P > 50 K beneath global ocean ridges. The averages have also revealed significant correlations of MORB chemistry (e.g. Ti₇₂, Al₇₂, Fe₇₂, Mg₇₂, Ca₇₂, Na₇₂ and Ca₇₂/Al₇₂) with ridge axial depth. The chemistry–depth correlation points to an intrinsic link between the two. That is, the ~5 km global ridge axial relief and MORB chemistry both result from a common cause: subsolidus mantle compositional variation (vs T_P), which determines the mineralogy, lithology and density variations that (1) isostatically compensate the ~5 km ocean ridge relief and (2) determine the first-order MORB compositional variation on a global scale. A progressively more enriched (or less depleted) fertile peridotite source (i.e. high Al₂O₃ and Na₂O, and low CaO/Al₂O₃) beneath deep ridges ensures a greater amount of modal garnet (high Al₂O₃) and higher jadeite/diopside ratios in clinopyroxene (high Na₂O and Al₂O₃, and lower CaO), making a denser mantle, and thus deeper ridges. The dense fertile mantle beneath deep ridges retards the rate and restricts the amplitude of the upwelling, reduces the rate and extent of decompression melting, gives way to conductive cooling to a deep level, forces melting to stop at such a deep level, leads to a short melting column, and thus produces less melt and probably a thin magmatic crust relative to the less dense (more refractory) fertile mantle beneath shallow ridges. Compositions of primitive MORB melts result from the combination of two different, but genetically related processes: (1) mantle source inheritance and (2) melting process enhancement. The subsolidus mantle compositional variation needed to explain MORB chemistry and ridge axial depth variation requires a deep isostatic compensation depth, probably in the transition zone. Therefore, although ocean ridges are of shallow origin, their working is largely controlled by deep processes as well as the effect of plate spreading rate variation at shallow levels.

Insights into Petrological Characteristics of the Lithosphere of Mantle Wedge beneath Arcs through Peridotite Xenoliths: a Review

Arai, S., Ishimaru, S. — 2008-03-25

The petrological characteristics of peridotite xenoliths exhumed from the lithospheric mantle below the Western Pacific arcs (Kamchatka, NE Japan, SW Japan, Luzon–Taiwan, New Ireland and Vanuatu) are reviewed to obtain an overview of the supra-subduction zone mantle in mature subduction systems. These data are then compared with those for peridotite xenoliths from recent or older arcs described in the literature (e.g. New Britain, Western Canada to USA, Central Mexico, Patagonia, Lesser Antilles and Pannonian Basin) to establish a petrological model of the lithospheric mantle beneath the arc. In currently active volcanic arcs, the degree of partial melting recorded in the peridotites appears to decrease away from the fore-arc towards the back-arc region. Highly depleted harzburgites, more depleted than abyssal harzburgites, occur only in the frontal arc to fore-arc region. The degree of depletion increases again to a degree similar to that of the most depleted abyssal harzburgites within the back-arc extensional region, whether or not a back-arc basin is developed. Metasomatism is most prominent beneath the volcanic front, where the magma production rate is highest; silica enrichment, involving the metasomatic formation of secondary orthopyroxene at the expense of olivine, is important in this region because of the addition of slab-derived siliceous fluids. Some apparently primary orthopyroxenes, such as those in harzburgites from the Lesser Antilles arc, could possibly be of this secondary paragenesis but have been recrystallized such that the replacement texture is lost. The Ti content of hydrous minerals is relatively low in the sub-arc lithospheric mantle peridotites. The K/Na ratio of the metasomatic hydrous minerals decreases rearward from the fore-arc mantle as well as downward within the lithospheric mantle. The lithospheric mantle wedge peridotites, especially metasomatized ones from below the volcanic front, are highly oxidized. Shearing of the mantle wedge is expected beneath the volcanic front, and is represented by fine-grained peridotite xenoliths.

Boninites and Adakites from the Northern Termination of the Tonga Trench: Implications for Adakite Petrogenesis

2008-03-25

Adakitic rocks were recovered by dredging from the northern termination of the Tonga Trench during the 1984 voyage of the R.V. Natsushima and the 1996 voyage of the R.V. Melville. These contain magmatic zircons that have been dated at 2·5 Ma by U–Pb methods, indicating that they are contemporaneous with boninite magmatism previously described from this area. This is the first time adakites and high-Ca boninites have been reported from the same active tectonic setting. The Tonga adakites are classified as high-SiO₂ adakites, and are compositionally consistent with an origin as partial melts of subducted Pacific oceanic crust and sediment. Geochemical modelling suggests that the adakites are not involved in the petrogenesis of the Tongan high-Ca boninites. However, the recovery of adakites and boninites from the termination of the northern Tonga Trench suggests that both magma types are related to the unique tectonic setting of this region, where a transition from steep subduction to a transform fault plate boundary has created a slab window with an associated slab edge. Boninites are generated as a result of hot Samoan plume mantle moving through the slab window and subsequently being fluxed by H₂O-rich fluids from the subducting Pacific oceanic crust. The Tonga adakites, in contrast, result from the direct melting of the slab edge as a result of the juxtaposition of the subducting slab against hot mantle derived from the Samoan plume.

Sediment Melts at Sub-arc Depths: an Experimental Study

Hermann, J., Spandler, C. J. — 2008-03-25

The phase and melting relations in subducted pelites have been investigated experimentally at conditions relevant for slabs at sub-arc depths (T = 600–1050°C, P = 2·5–4·5 GPa). The fluid-present experiments produced a dominant paragenesis consisting of garnet–phengite–clinopyroxene–coesite–kyanite that coexists with a fluid phase at run conditions. Garnet contains detectable amounts of Na₂O (up to 0·5 wt%), P₂O₅ (up to 0·56 wt%), and TiO₂ (up to 0·9 wt%) in all experiments. Phengite is stable up to 1000°C at 4·5 GPa and is characterized by high TiO₂ contents of up to 2 wt%. The solidus has been determined at 700°C, 2·5 GPa and is situated between 700 and 750°C at 3·5 GPa. At 800°C, 4·5 GPa glass was present in the experiments, indicating that at such conditions a hydrous melt is stable. In contrast, at 700°C, 3·5 and 4·5 GPa, a solute-rich, non-quenchable aqueous fluid was present. This indicates that the solidus is steeply sloping in P–T space. Fluid-present (vapour undersaturated) partial melting of the pelites occurs according to a generalized reaction phengite + omphacite + coesite + fluid = melt + garnet. The H₂O content of the produced melt decreases with increasing temperature. The K₂O content of the melt is buffered by phengite and increases with increasing temperature from 2·5 to 10 wt%, whereas Na₂O decreases from 7 to 2·3 wt%. Hence, the melt compositions change from trondhjemitic to granitic with increasing temperature. The K₂O/H₂O increases strongly as a function of temperature and nature of the fluid phase. It is 0·0004–0·002 in the aqueous fluid, and then increases gradually from about 0·1 at 750–800°C to about 1 at 1000°C in the hydrous melt. This provides evidence that hydrous melts are needed for efficient extraction of K and other large ion lithophile elements from subducted sediments. Primitive subduction-related magmas typically have K₂O/H₂O of ~0·1–0·4, indicating that hydrous melts rather than aqueous fluids are responsible for large ion lithophile element transfer in subduction zones and that top-slab temperatures at sub-arc depths are likely to be 700–900°C.

Phase Relationships of Hydrous Alkalic Magmas at High Pressures: Production of Nepheline Hawaiitic to Mugearitic Liquids by Amphibole-Dominated Fractional Crystallization Within the Lithospheric Mantle

Irving, A. J., Green, D. H. — 2008-03-25

Experimental melting studies were conducted on a nepheline mugearite composition to pressures of 31 kbar in the presence of 0–30% added water. A temperature maximum in the near-liquidus stability of amphibole (with olivine) was found for a water content of 3·5 wt % at a pressure of 14 kbar. This is interpreted to have petrogenetic significance for the derivation of nepheline mugearite magmas from nepheline hawaiite by amphibole-dominated fractional crystallization at depth within the lithospheric mantle. Synthetic liquids at progressively lower temperatures range to nepheline benmoreite compositions very similar to those of natural xenolith-bearing high-pressure lavas elsewhere, and support the hypothesis that continued fractional crystallization could lead to high-pressure phonolite liquids. Independent experimental data for a basanite composition modeled on a lava from the same igneous province (the Newer Basalts of Victoria) permit the inference that primary asthenospheric basanite magmas undergo polybaric fractional crystallization during ascent, and may evolve to liquids ranging from nepheline hawaiite to phonolite upon encountering cooler lithospheric mantle at depths of 42–50 km. Such a model is consistent with the presence in some evolved alkalic lavas of both lithospheric peridotite xenoliths indicative of similar depths and of megacryst suites that probably represent disrupted pegmatitic segregations precipitated from precursor alkalic magmas in conduit systems within lithospheric mantle.

Oxygen Isotope Evidence for Chemical Interaction of Ki lauea Historical Magmas with Basement Rocks

Garcia, M. O., Ito, E., Eiler, J. M. — 2008-03-25

Kilauea historical summit lavas have a wide range in matrix ¹⁸O_VSMOW values (4·9–5·6) with lower values in rocks erupted following a major summit collapse or eruptive hiatus. In contrast, ¹⁸O values for olivines in most of these lavas are nearly constant (5·1 ± 0·1). The disequilibrium between matrix and olivine ¹⁸O values in many samples indicates that the lower matrix values were acquired by the magma after olivine growth, probably just before or during eruption. Both Mauna Loa and Kilauea basement rocks are the likely sources of the contamination, based on O, Pb and Sr isotope data. However, the extent of crustal contamination of Kilauea historical magmas is probably minor (< 12%, depending on the assumed contaminant) and it is superimposed on a longer-term, cyclic geochemical variation that reflects source heterogeneity. Kilauea's heterogeneous source, which is well represented by the historical summit lavas, probably has magma ¹⁸O values within the normal mid-ocean ridge basalt mantle range (5·4–5·8) based on the new olivine ¹⁸O values.

Phase Relations and Melting of Anhydrous K-bearing Eclogite from 1200 to 1600{degrees}C and 3 to 5 GPa

Spandler, C., Yaxley, G., Green, D. H., Rosenthal, A. — 2008-03-25

To investigate eclogite melting under mantle conditions, we have performed a series of piston-cylinder experiments using a homogeneous synthetic starting material (GA2) that is representative of altered mid-ocean ridge basalt. Experiments were conducted at pressures of 3·0, 4·0 and 5·0 GPa and over a temperature range of 1200–1600°C. The subsolidus mineralogy of GA2 consists of garnet and clinopyroxene with minor quartz–coesite, rutile and feldspar. Solidus temperatures are located at 1230°C at 3·0 GPa and 1300°C at 5·0 GPa, giving a steep solidus slope of 30–40°C/GPa. Melting intervals are in excess of 200°C and increase with pressure up to 5·0 GPa. At 3·0 GPa feldspar, rutile and quartz are residual phases up to 40°C above the solidus, whereas at higher pressures feldspar and rutile are rapidly melted out above the solidus. Garnet and clinopyroxene are the only residual phases once melt fractions exceed 20% and garnet is the sole liquidus phase over the investigated pressure range. With increasing melt fraction garnet and clinopyroxene become progressively more Mg-rich, whereas coexisting melts vary from K-rich dacites at low degrees of melting to basaltic andesites at high melt fractions. Increasing pressure tends to increase the jadeite and Ca-eskolaite components in clinopyroxene and enhance the modal proportion of garnet at low melt fractions, which effects a marked reduction in the Al₂O₃ and Na₂O content of the melt with pressure. In contrast, the TiO₂ and K₂O contents of the low-degree melts increase with increasing pressure; thus Na₂O and K₂O behave in a contrasted manner as a function of pressure. Altered oceanic basalt is an important component of crust returned to the mantle via plate subduction, so GA2 may be representative of one of many different mafic lithologies present in the upper mantle. During upwelling of heterogeneous mantle domains, these mafic rock-types may undergo extensive melting at great depths, because of their low solidus temperatures compared with mantle peridotite. Melt batches may be highly variable in composition depending on the composition and degree of melting of the source, the depth of melting, and the degree of magma mixing. Some of the eclogite-derived melts may also react with and refertilize surrounding peridotite, which itself may partially melt with further upwelling. Such complex magma-genesis conditions may partly explain the wide spectrum of primitive magma compositions found within oceanic basalt suites.

Experimental Melting of Carbonated Peridotite at 6-10 GPa

Brey, G. P., Bulatov, V. K., Girnis, A. V., Lahaye, Y. — 2008-03-25

Partial melting of magnesite-bearing peridotites was studied at 6–10 GPa and 1300–1700°C. Experiments were performed in a multianvil apparatus using natural mineral mixes as starting material placed into olivine containers and sealed in Pt capsules. Partial melts originated within the peridotite layer, migrated outside the olivine container and formed pools of quenched melts along the wall of the Pt capsule. This allowed the analysis of even small melt fractions. Iron loss was not a problem, because the platinum near the olivine container became saturated in Fe as a result of the reaction Fe₂SiO₄^Ol = Fe^{Fe–Pt alloy} + FeSiO₃^Opx + O₂. This reaction led to a gradual increase in oxygen fugacity within the capsules as expressed, for example, in high Fe³⁺ in garnet. Carbonatitic to kimberlite-like melts were obtained that coexist with olivine + orthopyroxene + garnet ± clinopyroxene ± magnesite depending on P–T conditions. Kinetic experiments and a comparison of the chemistry of phases occasionally grown within the melt pools with those in the residual peridotite allowed us to conclude that the melts had approached equilibrium with peridotite. Melts in equilibrium with a magnesite-bearing garnet lherzolite are rich in CaO (20–25 wt %) at all pressures and show rather low MgO and SiO₂ contents (20 and 10 wt %, respectively). Melts in equilibrium with a magnesite-bearing garnet harzburgite are richer in SiO₂ and MgO. The contents of these oxides increase with temperature, whereas the CaO content becomes lower. Melts from magnesite-free experiments are richer in SiO₂, but remain silicocarbonatitic. Partitioning of trace elements between melt and garnet was studied in several experiments at 6 and 10 GPa. The melts are very rich in incompatible elements, including large ion lithophile elements (LILE), Nb, Ta and light rare earth elements. Relative to the residual peridotite, the melts show no significant depletion in high field strength elements over LILE. We conclude from the major and trace element characteristics of our experimental melts that primitive kimberlites cannot be a direct product of single-stage melting of an asthenospheric mantle. They rather must be derived from a previously depleted and re-enriched mantle peridotite.

Olivine in the Udachnaya-East Kimberlite (Yakutia, Russia): Types, Compositions and Origins

2008-03-25

Olivine is the principal mineral of kimberlite magmas, and is the main contributor to the ultramafic composition of kimberlite rocks. Olivine is partly or completely altered in common kimberlites, and thus unavailable for studies of the origin and evolution of kimberlite magmas. The masking effects of alteration, common in kimberlites worldwide, are overcome in this study of the exceptionally fresh diamondiferous kimberlites of the Udachnaya-East pipe from the Daldyn–Alakit province, Yakutia, northern Siberia. These serpentine-free kimberlites contain large amounts of olivine (~50 vol.%) in a chloride–carbonate groundmass. Olivine is represented by two populations (olivine-I and groundmass olivine-II) differing in morphology, colour and grain size, and trapped mineral and melt inclusions. The large fragmental olivine-I is compositionally variable in terms of major (Fo_85–94) and trace element concentrations, including H₂O content (10–136 ppm). Multiple sources of olivine-I, such as convecting and lithospheric mantle, are suggested. The groundmass olivine-II is recognized by smaller grain sizes and perfect crystallographic shapes that indicate crystallization during magma ascent and emplacement. However, a simple crystallization history for olivine-II is complicated by complex zoning in terms of Fo values and trace element contents. The cores of olivine-II are compositionally similar to olivine-I, which suggests a genetic link between these two types of olivine. Olivine-I and olivine-II have oxygen isotope values (+ 5·6 ± 0·1 VSMOW, 1 SD) that are indistinguishable from one another, but higher than values (+ 5·18 ± 0·28) in ‘typical’ mantle olivine. These elevated values probably reflect equilibrium with the Udachnaya carbonate melt at low temperatures and ¹⁸O-enriched mantle source. The volumetrically significant rims of olivine-II have constant Fo values (89·0 ± 0·2 mol%), but variable trace element compositions. The uniform Fo compositions of the rims imply an absence of fractionation of the melt's Fe²⁺/Mg, which is possible in the carbonatite melt–olivine system. The kimberlite melt is argued to have originated in the mantle as a chloride–carbonate liquid, devoid of ‘ultramafic’ or ‘basaltic’ aluminosilicate components, but became olivine-laden and olivine-saturated by scavenging olivine crystals from the pathway rocks and dissolving them en route to the surface. During emplacement the kimberlite magma changed progressively towards an original alkali-rich chloride–carbonate melt by extensively crystallizing groundmass olivine and gravitational separation of solids in the pipe.

Seismic Properties of Anita Bay Dunite: an Exploratory Study of the Influence of Water

2008-03-25

As a pilot study of the role of water in the attenuation of seismic waves in the Earth's upper mantle, we have performed a series of seismic-frequency torsional forced-oscillation experiments on a natural (Anita Bay) dunite containing accessory hydrous phases, at high temperatures to 1300°C and confining pressure (P_c) of 200 MPa, within a gas-medium high-pressure apparatus. Both oven-dried and pre-fired specimens wrapped in Ni–Fe foil within the (poorly) vented assembly were recovered essentially dry after 50–100 h of annealing at 1300°C followed by slow staged cooling. The results for those specimens indicate broadly similar absorption-band viscoelastic behaviour, but with systematic differences in the frequency dependence of strain-energy dissipation Q^–1, attributed to differences in the small volume fraction of silicate melt and its spatial distribution. In contrast, it has been demonstrated that a new assembly involving a welded Pt capsule retains aqueous fluid during prolonged exposure to high temperatures—allowing the first high-temperature torsional forced-oscillation measurements under high aqueous fluid pore pressure P_f. At temperatures >1000°C, a marked reduction in shear modulus, without concomitant increase in Q^–1, is attributed to the widespread wetting of grain boundaries resulting from grain-scale hydrofracturing and the maintenance of conditions of low differential pressure P_d = P_c – P_f . Staged cooling from 1000°C is accompanied by decreasing P_f and progressive restoration of significantly positive differential pressure resulting in a microstructural regime in which the fluid on grain boundaries is increasingly restricted to arrays of pores. The more pronounced viscoelastic behaviour observed within this regime for the Pt-encapsulated specimen compared with the essentially dry specimens may reflect both water-enhanced solid-state relaxation and the direct influence of the fluid phase. The scenario of overpressurized fluids and hydrofracturing in the Pt-encapsulated dunite specimen may have some relevance to the high Q^–1 and low-velocity zones observed in subduction-zone environments. The outcomes of this exploratory study indicate that the presence of water can have a significant effect on the seismic wave attenuation in the upper mantle and provide the foundation for more detailed studies on the role of water.

Petrology of a Late Archaean, Highly Potassic, Sanukitoid Pluton from the Baltic Shield: Insights into Late Archaean Mantle Metasomatism

2008-02-22

The late Archaean Panozero pluton in Central Karelia (Baltic Shield) is a multi-phase high-Mg, high-K intrusion with sanukitoid affinities, emplaced at 2·74 Ga. The magmatic history of the intrusion may be subdivided into three cycles and includes monzonitic and lamprophyric magmas. Compositional variations are most extreme in the monzonite series and these are interpreted as the result of fractional crystallization. Estimates of the composition of the parental magmas to the monzonites and lamprophyres show that they are enriched in light rare earth elements, Sr, Ba, Cr, Ni and P but have low contents of high field strength elements. Radiogenic isotope data indicate a low U/Pb, high Th/U, high Rb/Sr, low Sm/Nd source. The magmatic rocks of the Panozero intrusion are also enriched in H₂O and CO₂; carbon isotope data are consistent with mantle values, indicating a fluid-enriched mantle source. The similarity in trace element character of all the Panozero parental magmas indicates that all the magmas were derived from a similar mantle source. The pattern of trace element enrichment is consistent with a mantle source enriched by fluids released from a subducting slab. Nd-isotope data suggest that this enrichment took place at c. 2·8 Ga, during the main episode of greenstone belt and tonalite–trondhjemite–granodiorite formation in Central Karelia. Sixty million years later, at 2·74 Ga, the subcontinental mantle melted to form the Panozero magmas. Experimental studies suggest that the monzonitic magmas originated by the melting of pargasite–phlogopite lherzolite in the subcontinental mantle lithosphere at 1–1·5 GPa. The precise cause of the melting event at 2·74 Ga is not known, although a model involving upwelling of asthenospheric mantle following slab break-off is consistent with the geochemical evidence for the enrichment of the Karelian subcontinental mantle lithosphere by subduction fluids.

A Quartz-bearing Orthopyroxene-rich Websterite Xenolith from the Pannonian Basin, Western Hungary: Evidence for Release of Quartz-saturated Melts from a Subducted Slab

2008-02-22

An unusual quartz-bearing orthopyroxene-rich websterite xenolith has been found in an alkali basaltic tuff at Szigliget, Bakony–Balaton Highland Volcanic Field (BBHVF), western Hungary. Ortho- and clinopyroxenes are enriched in light rare earth elements (LREE), middle REE and Ni, and depleted in Nb, Ta, Sr and Ti compared with ortho- and clinopyroxenes occurring in either peridotite or lower crustal granulite xenoliths from the BBHVF. Both ortho- and clinopyroxenes in the xenolith contain primary and secondary silicate melt inclusions, and needle-shaped or rounded quartz inclusions. The melt inclusions are rich in SiO₂ and alkalis and poor in MgO, FeO and CaO. They are strongly enriched in LREE and large ion lithophile elements, and display negative Nb, Ta and Sr anomalies, and slightly positive Pb anomalies. The xenolith is interpreted to represent a fragment of an orthopyroxene-rich body that crystallized in the upper mantle from a hybrid melt that formed by interaction of mantle peridotite with a quartz-saturated silicate melt that was released from a subducted oceanic slab. Although the exact composition of the slab melt cannot be determined, model calculations on major and trace elements suggest involvement of a metasedimentary component.

Petrogenesis of Volcanic Rocks from Saipan and Rota, Mariana Islands, and Implications for the Evolution of Nascent Island Arcs

Reagan, M. K., Hanan, B. B., Heizler, M. T., Hartman, B. S., Hickey-Vargas, R. — 2008-02-22

An ⁴⁰Ar/³⁹Ar age of 45·1 Ma determined for lavas from northern Saipan confirms that these high-silica rhyolites erupted during the ‘proto-arc’ stage of volcanism in the Izu–Bonin–Mariana system, which is characterized elsewhere by eruption of boninitic lavas. Incompatible trace element concentrations and Sr, Hf, Nd, and Pb isotope ratios for these rhyolites are transitional between those of c. 48 Ma boninitic lavas and post-38 Ma ‘first-arc’ andesites and dacites from Saipan and Rota that have typical subduction-related compositions. These transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust. A second stage of Rayleigh fractionation in the upper crust is required by SiO₂ concentrations that exceed 77 wt % and near-zero compatible element concentrations. First-arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first-arc magmas themselves. The mantle sources of the proto-arc and first-arc lavas from Saipan and Rota are similar to those of Philippine back-arc basin basalts based on Nd and Hf isotopic compositions. The Pb isotope compositions of these lavas are between those of Pacific sea-floor basalts and Jurassic and younger cherty and clay-rich sediments. This contrasts with the boninitic proto-arc volcanic rocks from Guam and Deep Sea Drilling Project Sites 458 and 459 that have Pb isotope compositions similar to Pacific basin basalts and volcaniclastic sediments. The preferred explanation for the difference in the nature of proto-arc volcanism between Saipan and other fore-arc locations is that the crust ceased extending 3–4 Myr earlier beneath Saipan. This was caused by a change from mantle upwelling, fore-arc extension, and shallow melting to an environment dominated by more normal mantle wedge convection, stable crust, and deeper melting.

Pressures of Crystallization of Icelandic Magmas

Kelley, D. F., Barton, M. — 2008-02-22

Iceland lies astride the Mid-Atlantic Ridge and was created by seafloor spreading that began about 55 Ma. The crust is anomalously thick (~20–40 km), indicating higher melt productivity in the underlying mantle compared with normal ridge segments as a result of the presence of a mantle plume or upwelling centered beneath the northwestern edge of the Vatnajökull ice sheet. Seismic and volcanic activity is concentrated in ~50 km wide neovolcanic or rift zones, which mark the subaerial Mid-Atlantic Ridge, and in three flank zones. Geodetic and geophysical studies provide evidence for magma chambers located over a range of depths (1·5–21 km) in the crust, with shallow magma chambers beneath some volcanic centers (Katla, Grimsvötn, Eyjafjallajökull), and both shallow and deep chambers beneath others (e.g. Krafla and Askja). We have compiled analyses of basalt glass with geochemical characteristics indicating crystallization of ol–plag–cpx from 28 volcanic centers in the Western, Northern and Eastern rift zones as well as from the Southern Flank Zone. Pressures of crystallization were calculated for these glasses, and confirm that Icelandic magmas crystallize over a wide range of pressures (0·001 to ~1 GPa), equivalent to depths of 0–35 km. This range partly reflects crystallization of melts en route to the surface, probably in dikes and conduits, after they leave intracrustal chambers. We find no evidence for a shallow chamber beneath Katla, which probably indicates that the shallow chamber identified in other studies contains silica-rich magma rather than basalt. There is reasonably good correlation between the depths of deep chambers (> 17 km) and geophysical estimates of Moho depth, indicating that magma ponds at the crust–mantle boundary. Shallow chambers (< 7·1 km) are located in the upper crust, and probably form at a level of neutral buoyancy. There are also discrete chambers at intermediate depths (~11 km beneath the rift zones), and there is strong evidence for cooling and crystallizing magma bodies or pockets throughout the middle and lower crust that might resemble a crystal mush. The results suggest that the middle and lower crust is relatively hot and porous. It is suggested that crustal accretion occurs over a range of depths similar to those in recent models for accretionary processes at mid-ocean ridges. The presence of multiple stacked chambers and hot, porous crust suggests that magma evolution is complex and involves polybaric crystallization, magma mixing, and assimilation.

Petrogenesis of Cogenetic Silica-Oversaturated and -Undersaturated Syenites by Periodic Recharge in a Crustally Contaminated Magma Chamber: the Kangerlussuaq Intrusion, East Greenland

Riishuus, M. S., Peate, D. W., Tegner, C., Wilson, J. R., Brooks, C. K. — 2008-02-22

The Palaeogene Kangerlussuaq Intrusion (~50 Ma) of East Greenland displays concentric zonation from quartz-rich nordmarkite (quartz syenite) at the margin, through pulaskite, to foyaite (nepheline syenite) in the centre; modal layering and igneous lamination are locally developed but there are no internal intrusive contacts. This is an apparent violation of the phase relations in Petrogeny's Residua System. We propose that this intrusion is layered, grading from quartz syenite at the bottom to nepheline syenite at the top. Mineral and whole-rock major and trace element data and Sr–Nd–Hf–Pb isotope data are presented that provide constraints on the petrogenesis of the intrusion. Radiogenic isotope data indicate a continuously decreasing crustal component from the quartz nordmarkites (⁸⁷Sr/⁸⁶Sr = 0·7061; _Ndi = 2·3; _Hfi = 5·2; ²⁰⁶Pb/²⁰⁴Pb_meas = 16·98) to the foyaites (⁸⁷Sr/⁸⁶Sr = 0·7043–0·7044; _Ndi = 3·8–4·9; _Hfi = 10·7–11·1; ²⁰⁶Pb/²⁰⁴Pb_meas = 17·78–17·88); the foyaites are dominated by a mantle isotopic signature. The average Mg-number of amphibole cores becomes increasingly primitive, varying from 26·4 in the nordmarkites to 57·4 in the pulaskites. Modal layering, feldspar lamination and the presence of huge basaltic xenoliths derived from the chamber roof, now resting on the transient chamber floor, demonstrate bottom-upwards crystallization. The intrusion cannot, therefore, have formed in a system closed to magmatic recharge. The lack of gneissic xenoliths in the nordmarkites suggests that most contamination took place deeper in the crust. In the proposed model, the nordmarkitic magma formed during crustal assimilation in the roof zone of a large, silica-undersaturated alkali basaltic/basanitic, stratified magma chamber, prior to emplacement in the uppermost crust. The more primitive syenites, terminating with foyaite at the top of the intrusion, formed as a consequence of repeated recharge of the Kangerlussuaq Intrusion magma chamber by tapping less contaminated, more primitive phonolitic melt from deeper parts of the underlying chamber during progressive armouring of the plumbing system.

Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes

Klimm, K., Blundy, J. D., Green, T. H. — 2008-02-22

Experimental phase equilibrium and trace element partitioning data are reported for H₂O-saturated mid-ocean ridge basalt at 2·5 GPa, 750–900°C and oxygen fugacities at the nickel–nickel oxide buffer. Garnet, omphacite and rutile are present at all temperatures. Amphibole and epidote disappear as residual phases above 800°C; allanite appears above 750°C. The Na–Al-rich silicate glass present in all run products is likely to have quenched from a supercritical liquid. Trace element analyses of glasses demonstrate the important control exerted by residual minerals on liquid chemistry. In addition to garnet, which controls heavy rare earth elements (HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanite buffers the light REE (LREE; La–Sm) contents of liquids to relatively low levels and preferentially holds back Th relative to U. In agreement with previous experimental and metamorphic studies we propose that residual allanite plays a key role in selectively retaining trace elements in the slab during subduction. Experimental data and analyses of allanite-bearing volcanic rocks are used to derive a model for allanite solubility in liquids as a function of pressure, temperature, anhydrous liquid composition and LREE content. The large temperature dependence of allanite solubility is very similar to that previously determined for monazite. Our model, fitted to 48 datapoints, retrieves LREE solubility (in ppm) to within a factor of 1· 40 over a pressure range of 0–4 GPa, temperature range of 700–1200°C and for liquids with anhydrous SiO₂ contents of 50–84 wt %. This uncertainty in LREE content is equivalent to a temperature uncertainty of only ± 27°C at 1000 K, indicating the potential of allanite as a geothermometer. Silicic liquids from either basaltic or sedimentary protoliths will be saturated in allanite except for Ca-poor protoliths or at very high temperatures. For conventional subduction geotherms the low solubility of LREE (+ Th) in liquids raises questions about the mechanism of LREE + Th transport from slab to wedge. It is suggested either that, locally, temperatures experienced by the slab are high enough to eliminate allanite in the residue or that substantial volumes of H₂O-rich fluids must pass through the mantle wedge prior to melting. The solubility of accessory phases in fluids derived from subducted rocks can provide important constraints on subduction zone thermal structure.

Lithospheric Origin of Oligocene-Miocene Magmatism in Central Chile: U-Pb Ages and Sr-Pb-Hf Isotope Composition of Minerals

Montecinos, P., Scharer, U., Vergara, M., Aguirre, L. — 2008-02-22

Establishing the petrogenesis of volcanic and plutonic rocks is a key issue in unraveling the evolution of distinct subduction-related tectonic phases occurring along the South American margin. This is particularly true for Cenozoic times when large volumes of magma were produced in the Andean belt. In this study we have focused on Oligo-Miocene magmatism in central Chile at 33°S. Our data include field and petrographic observations, whole-rock major and trace element analyses, U–Pb zircon dating, and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene, and zircon mineral separates. Combined with earlier dating results the new zircon ages define a 28·8–5·2 Ma period of plutonic and volcanic activity that ceased as a consequence of flattening subduction of the Nazca–Farallon plate. Rare earth elements patterns are variable, with up to 92 times chondrite concentrations for light rare earth elements yielding (La/Yb)_N between 3·6 and 7·0, and an absence of Eu anomalies. Initial Pb isotope signatures are in the range of 18·358–19·023 for ²⁰⁶Pb/ ²⁰⁴Pb, 15·567–15·700 for ²⁰⁷Pb/ ²⁰⁴Pb and 38·249–39·084 for ²⁰⁸Pb/ ²⁰⁴Pb. Initial ⁸⁷Sr/ ⁸⁶Sr are mostly in the range of 0·70369–0·70505, with two more radiogenic values at 0·7066. Initial Hf isotopic compositions of zircons yield exclusively positive Hf_i ranging between + 6·9 and + 9·6. The newly determined initial isotope characteristics of the Oligo-Miocene magmas suggest that the mantle source lithologies are different from both those of Pacific mid-ocean ridge basalt and ocean island basalt, plotting in the field of reference values for subcontinental lithospheric mantle, characterized by moderate large ion lithophile element–high field strengh element depletion and high ²³⁸U/ ²⁰⁴Pb. A Hf model age of 2 Ga is estimated for the formation of the subcontinental mantle–continental crust assemblage in the region, suggesting that the initial Sr and Pb isotope ratios inferred for the source of the Oligo-Miocene parental magmas are the result of later Rb and U enrichment caused by mantle metasomatism. A time-integrated model Rb/Sr of 0·039 and µ 16 are estimated for the source of the parental magmas, consistent with ratios measured in peridotite xenoliths from continental areas. Evolution from predominant (>90%) basaltic–gabbroic to andesitic–dioritic magmas seems to involve a combination of (1) original trace element differences in the metasomatized subcontinental mantle, (2) different degrees of partial melting and (3) fractional crystallization in the garnet- and spinel-peridotite stability fields. The genesis of more differentiated magmas reaching rhyolitic–granitic compositions most probably also includes additional crystal fractionation at both shallow mantle depths and within the crust, possibly leading to some very minor assimilation of crustal material.

Codification of Unnamed Minerals

Smith, D. G. W., Nickel, E. H. — 2008-02-22

The Subcommittee for Unnamed Minerals of the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC, formerly CNMMN) has developed a codification system that includes the year of publication and qualitative chemical composition for unnamed minerals reported in the literature. Such minerals are divided into two categories. Those regarded as being ‘valid as unnamed minerals’ are those that do not correspond to existing species, have not been reported previously, and whose published descriptions allow them to be recognized if found elsewhere. Unnamed minerals regarded as being ‘invalid as unnamed minerals’ are those whose published descriptions are inadequate for their confident recognition if found elsewhere, or that correspond to existing mineral species or unnamed minerals published previously.

Erratum

2008-02-22