Journal of Petrology - recent issues

Garnetites of the Cardigan Pluton, New Hampshire: Evidence for Peritectic Garnet Entrainment and Implications for Source Rock Compositions

Dorais, M. J., Pett, T. K., Tubrett, M. — Wed, 18 Nov 2009 06:20:39 PST

The Cardigan Pluton, located in southwestern New Hampshire, USA, is a strongly peraluminous granodiorite that contains distinctive, meter-sized pods consisting of 50–70 modal % garnet (+sillimanite + biotite + plagioclase + quartz). The presence of fibrolitic mats and flat, unzoned major and trace-element garnet profiles provide evidence for prograde metamorphism and single-stage garnet growth from biotite dehydration melting. Melt-depleted, bulk-rock compositions indicate that the garnetites are either fragments of restite or melt-depleted xenoliths. Comparison of the Nd and Sr isotopic compositions of the garnetites and the Cardigan granitic rocks indicates an equilibrium relationship as required in the restite model. Additionally, plagioclase and garnet compositions are the same in the garnetites and the most mafic host rocks, also permissive of a restite origin for the garnetites. Temperature and pressure calculated using garnet–biotite thermometry and garnet–aluminum silicate–quartz–plagioclase (GASP) barometry yield estimates of 800°C and 6–7 kbar. These temperatures are probably lower than the peak melting temperature because major element modeling subtracting a granodiorite composition from an average calc-pelite metasediment requires greater than 45% melting to generate garnetite sample CP-23G. Such high melting percentages require temperatures of ~900°C, near the biotite-out curve. We infer that the heat required for such high amounts of melting was obtained from asthenospheric upwelling and basaltic underplating. Inherited zircons with 600 Ma U–Pb ages suggest that the Cardigan Pluton does not have a Laurentian source, consistent with thermobarometric calculations that place the depth of melting below the décollement between the basement and the Central Maine Trough metasediments. A peri-Gondwanan basement source is inferred. Calculated ascent rates of >1000 km/yr allowed preservation of restite in the Cardigan Pluton whereas slower rates in other peraluminous plutons could account for the paucity of restite in most peraluminous plutons.

Continental Reworking during Overprinting Orogenic Events, Southern Prince Charles Mountains, East Antarctica

Phillips, G., Kelsey, D. E., Corvino, A. F., Dutch, R. A. — Wed, 18 Nov 2009 06:20:40 PST

In situ electron microprobe monazite dating and mineral equilibria modelling of amphibolite–granulite-facies metapelites from the southern Prince Charles Mountains, East Antarctica has been carried out to unravel the P–T conditions, spatial extent and structural style of two overprinting orogenic records. This study shows that: (1) rocks of the northern Palaeoproterozoic Lambert Complex were pervasively reworked at peak conditions (6·5–7·1 kbar and 790–810°C) during the Early Neoproterozoic Rayner orogenic event; (2) rocks of the southern Lambert Complex experienced pervasive deformation and metamorphism at peak conditions (5·8–6·1 kbar and 625–635°C) during Early Palaeozoic Prydz orogenic activity; (3) in regions of the Lambert Complex reworked during the Rayner orogenic event, Prydz-aged orogenesis was highly localized. The distribution of orogenic activity pertaining to the Rayner and Prydz orogenic events in the southern Prince Charles Mountains can be attributed to (1) the development of a southward directed (current coordinates) orogenic front that propagated from an Early Neoproterozoic collision between India and Antarctica, and (2) rock fertility (i.e. availability of free fluid) during Early Palaeozoic intraplate orogenesis that was driven by far-field stresses generated by a collision of India–Antarctica with the Mawson Craton.

The High P-T Stability of Hydroxyl-apatite in Natural and Simplified MORB--an Experimental Study to 15 GPa with Implications for Transport and Storage of Phosphorus and Halogens in Subduction Zones

Konzett, J., Frost, D. J. — Wed, 18 Nov 2009 06:20:40 PST

Experiments have been conducted in the range 3–15 GPa and 850–1800°C to investigate the P–T stability field of OH-apatite in an average mid-ocean ridge basalt (MORB) and a model Mg-basalt, to study the compositional evolution of apatite and its breakdown products and the partitioning of P between phosphates and silicates. In the bulk compositions investigated OH-apatite is stable to <7·5 GPa at 950°C in a typical eclogite assemblage garnet + omphacite + SiO₂ + TiO₂. This is ~5 GPa below the breakdown P of pure OH-apatite. The high-P breakdown product is tuite [-Ca₃(PO₄)₂]. Both apatite and tuite are stable in a wide range of subduction zone T regimes but not along an average mantle adiabat. This precludes apatite or tuite stability in the asthenospheric mantle. Apatite may be stable in cold continental lithosphere (40 mW/m²) but is restricted to P < ~4–5 GPa. The apatite breakdown reaction is an important limit for the crust–mantle transport of Cl in subduction zones and can contribute to the Cl depletion of subducted cust. Both apatite and tuite are important storage sites for large ion lithophile elements (LILE) and rare earth elements (REE), therefore apatite breakdown does not greatly affect LILE or REE transport in subduction zones. In an eclogite assemblage only garnet can accommodate significant P. In the presence of apatite or tuite, P₂O₅ contents in garnet range from ~0·2 to 0·6 wt % between 3 and 11 GPa and increase to ~0·8 wt % at 15 GPa in the absence of a detectable phosphate phase. The P-storage capacity of clinopyroxene is limited to ~250 ppm. Because of the extreme preference of P for the garnet structure, virtually the entire P budget of subducted MORB will be locked up in garnet well into the lower mantle provided fO₂ is high enough to prevent the stability of a metal phase.

In Situ U-Pb and Trace Element Analysis of Accessory Minerals in the Kiruna District, Norrbotten, Sweden: New Constraints on the Timing and Origin of Mineralization

Smith, M. P., Storey, C. D., Jeffries, T. E., Ryan, C. — Wed, 18 Nov 2009 06:20:40 PST

Northern Norrbotten, Sweden is a key part of Baltic Shield and provides a record of magmatic, tectonic and related, superimposed, Fe oxide–apatite and iron oxide–copper–gold (IOCG) mineralization, during the Svecokarelian orogeny. Titanite and allanite from a range of mineral deposits in the area have been analysed for U–Pb isotope systematics and trace element chemistry using laser ablation quadrupole inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses of a single sample from the regional scapolite–albite alteration give an age of 1903 ± 8 Ma (2) and may be contemporaneous with the early stages of Fe mineralization (1890–1870 Ma). Analyses of titanite and allanite from undeformed IOCG deposits indicate initial alteration at 1862 ± 16 Ma. In many deposits subsequent metamorphic effects reset titanite isotope systematics from 1790 to 1800 Ma, resulting in a spread of U–Pb isotope analyses along concordia. In some instances core regions may record evidence of early thermal events at around 2050 Ma. Titanite and allanite from deformed IOCG deposits on major shear zones record ages from 1785 ± 21 Ma to 1777 ± 20 Ma, corresponding to deformation, metamorphism and secondary hydrothermal alteration as a result of late orogenic movements. The lack of intracrystalline variations in titanite and allanite trace element chemistry suggests that hydrothermal fluid chemistry and metal source were the main controls on mineral trace element chemistry. Titanite from undeformed Fe oxide–apatite and IOCG deposits is typically light rare earth element (LREE) enriched, and shows low U/Th ratios and low Ni in both intermediate to acid and basic volcanic-hosted deposits. This is consistent with a granitic source for metals. Minor variations in trace element patterns are consistent with the influence of aqueous complex formation on relative REE solubility. Deposits related to the Nautanen Deformation Zone have relatively heavy REE (HREE)-enriched titanite, and LREE-depleted allanite, with high U/Th ratios and elevated Ni contents, consistent with leaching of metals from the local basic volcanic rocks. All hydrothermal titanites are high field strength element enriched (Nb, Ta, Zr) indicating their transport as a result of either high salinities or high F contents, or both. The data overall support models of IOCG-type mineralization as a result of regional circulation of saline hydrothermal fluids in association with major crustal structures, with at least some metallic components derived from the granitoid rocks of the area. All the deposits here show evidence of subsequent metamorphism, although penetrative fabrics are restricted to regional-scale deformation zones.

Magma Recharge and Crystal Mush Rejuvenation Associated with Early Post-collapse Upper Basin Member Rhyolites, Yellowstone Caldera, Wyoming

Girard, G., Stix, J. — Wed, 18 Nov 2009 06:20:40 PST

The Upper Basin Member rhyolites are the oldest known post-collapse rhyolites of the third Yellowstone caldera. They were erupted near the caldera's resurgent domes between 516 ± 7 and 473 ± 9 ka and at 257 ± 13 ka. An unusual characteristic is their low ¹⁸O signature. Few data are available on their mineralogy and glass geochemistry, and this study fills an important gap in understanding their petrogenesis. We report new mineralogical observations and plagioclase, whole-rock and glass compositional data. Based on our observations, we describe a new lava flow for which we propose the name East Biscuit Basin flow. This unit is a quartz- and sanidine-free low-silica rhyolite (71–72% SiO₂ in the whole-rock) in which the dominant mineral, plagioclase, comprises two populations: (1) small fresh euhedral crystals of An_20–48 (average of An₃₁) composition, commonly part of aggregates with pyroxenes and Fe–Ti oxides; (2) large sieve-textured isolated crystals, which are slightly more sodic in composition (An_19–34, average of An₂₇). Plagioclase compositions in most other Upper Basin Member rhyolites are similar. The range of compositions for trace elements such as Rb, Th, Y and the rare earth elements is small (e.g. 158–189 ppm Rb, 20–25 ppm Th, 52–63 ppm Y, 60–82 ppm La in the whole-rock), and there is no systematic variation of these elements as a function of SiO₂ content or mineralogy. Certain trace element signatures and ratios are specific to each of these rhyolites, allowing us to propose that the Upper Basin Member rhyolites originate from six independent magma batches. The coexistence of the two types of plagioclase and their progressive disappearance in the more evolved rhyolites suggest the following petrogenetic model for each magma batch. A low-¹⁸O rhyolitic protolith is heated by replenishing magmas, which initiate melting, forming a crystal mush. Replenishment by buoyant silicic magma may enhance melting and cause mixing with the mush material. As a consequence, crystal-poor eruptible magma batches are formed, which contain small, more calcic plagioclase crystals (aggregates) formed during cooling and mixing of the replenishing silicic melt, and larger, lower temperature crystals exhibiting dissolution features inherited from the protolith.

Conditions and Timing of Pumpellyite-Actinolite-facies Metamorphism in the Early Mesozoic Frontal Accretionary Prism of the Madre de Dios Archipelago (Latitude 50{degrees}20'S; Southern Chile)

Willner, A. P., Sepulveda, F. A., Herve, F., Massonne, H.-J., Sudo, M. — Wed, 18 Nov 2009 06:20:40 PST

The Madre de Dios Metamorphic Complex (MDMC) in southern Chile is a fossil frontal accretionary prism, which is mainly composed of metapsammopelitic rocks, intercalations of oceanic rocks (greenstone and metachert) and platform carbonate. We concentrated on the metabasite to decipher the metamorphic evolution. This rock type contains assemblages of the pumpellyite–actinolite facies: pumpellyite ± actinolite–chlorite ± grandite ± phengite ± epidote–albite–quartz–titanite ± K-feldspar ± calcite. The metamorphic phases mainly grew by prograde hydration reactions during various episodes of restricted fluid influx. Fundamental phase relations of the pumpellyite–actinolite facies and adjacent facies were reproduced by pseudosections calculated for the system K₂O–Na₂O–CaO–FeO–O₂–MgO–Al₂O₃–TiO₂–SiO₂–H₂O–CO₂ at 200–400°C and 1–9 kbar. The calculated stability fields of the metamorphic assemblages as realized in the MDMC metabasite indicate highest metamorphic conditions restricted to 290–310°C, 4–6 kbar for the MDMC, presumably as a result of the main fluid influx at these conditions. Nevertheless, earlier local equilibria are still preserved as a result of strongly kinetically controlled mineral reactions and a lack of recrystallization and compositional homogenization at thin-section scale. Hence, thermodynamic calculations of local multivariant mineral equilibria using the entire compositional variation of minerals in the MDMC show that the prograde PT path evolved from 4 ± 1 kbar, 200–220°C to 5 ± 1 kbar, 290–330°C. The prograde PT path reflects nearly horizontal particle paths after reaching the maximum depth typical for frontal accretionary prisms. Long residence at maximum depth resulted in thermal re-equilibration. ⁴⁰Ar/³⁹Ar spot ages were measured by in situ UV laser ablation of local phengite concentrations in a deformed metapelite at 233·2 ± 1·8 Ma and in an undeformed metabasite at 200·8 ± 2·4 Ma. Whereas the first age represents an age of accretion, the latter age can be attributed to mineral growth either during a younger stage of accretion or during a retrograde stage. ⁴⁰Ar/³⁹Ar isotopic analyses of two further metabasite samples reflect a prominent resetting of ages at 152·0 ± 2·2 Ma and white mica growth during external fluid access triggered by either a local intrusion or a late Jurassic extensional episode.

Lithospheric Removal as a Trigger for Flood Basalt Magmatism in the Trans-Mexican Volcanic Belt

Wed, 18 Nov 2009 06:20:41 PST

The voluminous succession of tholeiitic basalts, calc-alkaline andesites and minor high-K basalts that form the Late Miocene Altos de Jalisco mafic province of the western Trans-Mexican Volcanic Belt is interpreted as the magmatic manifestation of a lithospheric dripping event, which removed mantle lithosphere and lower crustal lithologies beneath the study area. During this process, the release of fluids from the foundering materials, coupled with mantle upwelling around the sinking mass, promoted abundant melting of a spinel peridotite and the production of large volumes of tholeiitic magma with low La/Yb and Gd/Yb ratios. Negative correlations of these ratios with MgO contents, Nd isotopes and Rb/Nd ratios indicate that the parental basalts subsequently experienced high-pressure fractional crystallization and contamination with a newly exposed felsic continental crust, thus producing the more evolved calc-alkaline compositions. Stronger garnet signatures and marked enrichments in highly incompatible elements in the high-K suite support derivation from a garnet- and phlogopite-bearing pyroxenitic source, presumably formed by reaction of mantle peridotites with hydrous silicic melts derived from the foundering lithologies. This new petrogenetic model for the Altos de Jalisco volcanic district suggests that the loss of mafic lower crust during lithospheric dripping might be balanced by production of abundant flood basalts within continents, and thus indicates that additional mechanisms may be required for the stabilization of andesitic crust on Earth.

The Pressure-Temperature Path and the Origin of Phlogopite in Spinel-Garnet Peridotites from the Blansky Les Massif of the Moldanubian Zone, Czech Republic

Naemura, K., Hirajima, T., Svojtka, M. — Thu, 01 Oct 2009 02:25:09 PDT

A new lithotype of peridotite, phlogopite- and apatite-bearing spinel–garnet peridotite, associated with leucocratic granulite, has been recognized at the Plesovice quarry in the Gföhl Unit within the Moldanubian Zone of the Bohemian Massif, Czech Republic. There are three equilibrium stages in the Plesovice peridotite. The existence of Stage I, the precursor spinel ± garnet peridotite stage, is supported by the presence of an aluminous (Al₂O₃ ~ 3·0 wt %) orthopyroxene megacryst in the matrix. The minimum temperature of Stage I was estimated to be 1020 ± 15°C. Stage II is defined by the cores of relatively large (<3 mm long) grains of olivine, low-Al orthopyroxene (Al₂O₃ ~ 1·3–1·7 wt %), clinopyroxene, and chromian spinel [Cr/(Cr + Al) = 0·50–0·57], along with relatively small (<1 mm long) Ba-rich phlogopite (BaO = 1·0–4·0 wt %), Sr-rich apatite (SrO ~1·7 wt %) and rare potassic (K₂O ~0·9–1·2 wt %) amphibole. Garnet generally occurs as large spheroidal grains (up to 20 mm in diameter). It contains inclusions of olivine, orthopyroxene, chromian spinel, and phlogopite, all of which have similar compositions to their matrix counterparts. Therefore, garnet appears to be in equilibrium with the matrix phases at Stage II. Application of appropriate geothermobarometers to the assemblage at Stage II yielded temperatures of 850–1030°C and pressures of 2·3–3·5 GPa. Stage III is defined by aluminous orthopyroxene (Al₂O₃ ~ 2·1–4·0 wt %), aluminous clinopyroxene and aluminous spinel along with pargasitic amphibole and Ba-rich phlogopite in kelyphite; temperature conditions at this stage were estimated to be 730–770 (± 27)°C at 0·8–1·5 GPa. Multiphase solid inclusions, mainly composed of phlogopite, dolomite, apatite and calcite with minor amounts of chlorite and magnesiohornblende, are present only within large grains of chromian spinel, which are surrounded by kelyphites. The idiomorphic outline of the multiphase solid inclusions suggests that frozen remnants of carbonatite melts or supercritical fluids were trapped in the spinel. The mineral assemblage in the multiphase solid inclusions suggests relatively low-P and low-T conditions (T < 750°C; P < 1·6 GPa) for its crystallization. Furthermore, the timing of the crystallization of the multiphase solid inclusions appears to predate Stage II, as most multiphase solid inclusions are completely surrounded by the host chromian spinel. These data suggest that the Plesovice peridotite experienced cooling after Stage I and was then transformed to spinel–garnet peridotite by subsequent subduction processes.

Apatite Composition: Tracing Petrogenetic Processes in Transhimalayan Granitoids

Thu, 01 Oct 2009 02:25:09 PDT

Apatites crystallized from different types of igneous rocks show significant variations in the abundances of some minor and trace elements. In this study, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to determine the concentrations of 25 minor and trace elements in apatite separated from three principal rock types of the Transhimalayan igneous plutonic suite: S-type granites, the I-type Gangdese batholith and post-collisional adakites. F, Mn, Sr and rare earth elements (REE) in apatite vary systematically with the composition of the host magma and thus have high potential as petrogenetic tracers. More specifically, the F and Mn contents of apatite can be used as an indicator of magma aluminosity or differentiation index. Combined with Sr and REE data, which show significant variations in apatite from different rock types, these elements are useful for constructing ‘discrimination diagrams’. This study also reveals that apatite has the capacity to retain geochemical information about the host magma through the course of magmatic evolution. Systematic variations of Sr and REE in apatite with bulk-rock aluminosity are the results of partition competition with pre-existing and coexisting major and accessory minerals in silicate melts, and thus are useful for more detailed investigations of petrogenetic processes such as fractional crystallization and magma mixing, which is signaled by inconsistent Eu anomalies, Sr abundances and REE patterns relative to bulk-rock compositions.

Temporal Evolution of the Lithospheric Mantle beneath the Eastern North China Craton

Thu, 01 Oct 2009 02:25:09 PDT

It is now well established that lithospheric thinning of the eastern North China Craton (NCC) occurred during the Phanerozoic. The cause and extent of the thinning, however, remains highly debated. In this study, mantle xenoliths from the Paleozoic Mengyin kimberlites, along with xenoliths from the Cenozoic Penglai and Shanwang basalts in Shandong Province, are investigated via traditional petrographic and elemental, Sr–Nd–Hf–Os isotopic and platinum-group element (PGE) analyses. Late Archean Os model ages of c. 2·5 Ga for the Mengyin peridotites provide confirmation that refractory, Archean lithospheric mantle existed beneath the easternmost portion of the NCC during the Paleozoic. Some Paleoproterozoic lithospheric mantle fragments may also be present in the Mengyin xenolith suite. In contrast, the spatially associated Penglai and Shanwang peridotite xenoliths are more fertile, and have Sr–Nd isotopic compositions similar to the depleted mantle. They differ dramatically from the Archean peridotites sampled by the Mengyin kimberlites. Osmium model ages for single samples range from mid-Proterozoic to modern, similar to variations observed in Phanerozoic convecting upper mantle as sampled by modern abyssal peridotites. This suggests that the present lithospheric mantle beneath the eastern NCC formed in the Phanerozoic, despite the fact that Os model ages extend back to the Proterozoic. Some samples from the Penglai suite yield Proterozoic Lu–Hf clinopyroxene mineral isochron ages that are consistent with the Os model ages, suggesting that Hf isotopes in modern convective upper mantle can preserve evidence for ancient melt depletion, similar to Os isotopes. Our results are consistent with thinning of the eastern NCC as a result of foundering of the deep crust and lithospheric mantle, but are inconsistent with stretching or refertilization models, as remnants of Archean mantle are expected to be present in these scenarios. The match between ¹⁸⁷Os/¹⁸⁸Os in convective upper mantle and the eastern China lithopspheric mantle also precludes models that seek to explain the present lithosphere as being due to lateral translation of Proterozoic lithosphere.

Origin of Nepheline-normative High-K Ankaramites and the Evolution of Eastern Srednogorie Arc in SE Europe

Thu, 01 Oct 2009 02:25:09 PDT

Eastern Srednogorie is part of the Apuseni–Banat–Timok–Srednogorie magmatic belt in SE Europe, the main arc related to the Late Cretaceous subduction and closure of the Tethys Ocean between Africa and Europe. Extrusive and shallow intrusive magmatism in the Eastern Srednogorie is abundant and extremely diverse in composition, covering a wide range from ultramafic volcanic rocks to granites; this provides a unique opportunity to study processes of primitive melt formation and magma evolution in an arc environment. In contrast to other parts of the belt, relatively mafic lavas predominate here. Three magmatic regions are distinguished within Eastern Srednogorie from south to north: Strandzha, Yambol–Burgas and East Balkan. Systematic differences exist between these regions, notably the increased alkalinity of samples from the Yambol–Burgas region in the central part. All rocks display a clear subduction-like signature in their trace-element patterns, particularly the enrichment in large ion lithophile elements and light rare earth elements relative to high field strength elements. A distinct primitive nepheline-normative ankaramite magma type is recognized among the mafic volcanic rocks from the Yambol–Burgas region and melt inclusions entrapped in olivine and clinopyroxene from a cumulitic rock. Lower crustal clinopyroxene and amphibole cumulates carried to the surface as xenoliths in a mafic dike represent a possible source for the ankaramite. Modeling of the melting process suggests that low degrees of batch melting of a clinopyroxene-rich, amphibole-bearing source similar to the cumulate xenoliths at 1 GPa, temperatures of 1240–1300°C, oxidized conditions and a water content of 0·2 wt % reproduce accurately most of the observed major- and trace-element characteristics of the studied ankaramites. The elevated Rb, K₂O, Th, Ba content and higher Pb isotope ratios of the predicted liquids compared with the ankaramites are explained by mixing of the ankaramite magma with lherzolite partial melts derived from the subduction-modified mantle wedge. Underplating of such mantle-derived magmas at the crust–mantle boundary in an extensional environment as a response to slab roll-back provides also the necessary heat to melt lower crustal cumulates. Fractional crystallization of mainly clinopyroxene plus olivine and Fe–Ti oxides in a deep (equivalent to 8 kbar pressure) magma chamber produced most of the observed range of shoshonitic basalts and basaltic andesites in Eastern Srednogorie. The more evolved intermediate varieties were probably formed by mixing and crystallization at lower temperatures in lower pressure magma chambers. Whole-rock Sr and Pb isotope compositions indicate variable degrees of admixing of basement rocks to generate the intermediate to acid Late Cretaceous magmas, but assimilation was minimal for magmas with less than 53 wt % SiO₂. The proposed model for the evolution of the magmatism in Eastern Srednogorie involves initial formation of the calc-alkaline and high-K arc magmatism in the Strandzha and East Balkan regions, followed by roll-back induced intra-arc rifting and the formation of high-K, shoshonitic and ultra-high-K magmatism, including primitive ankaramites in the Yambol–Burgas region.

Generating High Mg-numbers and Chemical Diversity in Tonalite-Trondhjemite-Granodiorite (TTG) Magmas during Melting and Melt Segregation in the Continental Crust

Getsinger, A., Rushmer, T., Jackson, M. D., Baker, D. — Thu, 01 Oct 2009 02:25:09 PDT

Major, trace, and rare earth element compositions of both tonalite–trondhjemite–granodiorite (TTG) and modern adakite-like magmas are typically used in conjunction with batch melting experiments and models to infer source rock composition, depth of melting and tectonic setting. However, batch melting does not capture the impact of melt segregation processes on magma geochemistry. We have used melting experiments in conjunction with numerical modelling to investigate the impact of melt segregation on TTG arc crust formation. Our melt segregation equilibrium (MSE) experiments are designed to reproduce the local changes in bulk composition that are predicted by the numerical model to occur as buoyant melt migrates upwards along grain boundaries and accumulates to form a magma that leaves the source region. The MSE experimental results show distinct differences in the melt and solid phase compositions and solid phase stability when compared with direct partial melting experiments. They yield a significant reduction in hornblende and plagioclase modal proportions at lower temperatures and partial melt compositions that are lower in An and have higher Mg-numbers. These results suggest that dynamic melt segregation and equilibrium processes may have a significant impact on modes, melt compositions and geochemical indicators such as Mg-numbers. Mantle wedge interaction may not be necessary to generate varying Mg-numbers in TTG and adakite magmas. Moreover, the use of batch melting models or experiments to interpret these geochemical signatures may not be appropriate.

Radiogenic Isotopic Ratio Variations in Carbonatites and Associated Alkaline Silicate Rocks: Role of Crustal Assimilation

Ray, J. S. — Thu, 01 Oct 2009 02:25:09 PDT

Carbonatites commonly occur together with alkaline silicate rocks. It is frequently argued in the literature that the varied ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and Pb isotopic ratios in the alkaline silicate rocks reflect source heterogeneity, and that in many cases bear no genetic significance to the coexistence of silicates and carbonatites. Such a hypothesis may not be universal as it fails to explain the observations that in numerous carbonatite–alkaline complexes both rock types are contemporaneous, they exhibit complementary trace element patterns, and most importantly their initial radiogenic isotopic ratios overlap, all of which when considered together suggest a common parentage. In addition, the alkaline silicate rocks have more variable and higher ⁸⁷Sr/⁸⁶Sr, and lower ¹⁴³Nd/¹⁴⁴Nd and Pb isotopic ratios compared with those of the carbonatites and show hyperbolic trends in isotopic ratio vs concentration plots, hinting at a possible contamination of their parental magma by crustal or lithospheric material. Using a mathematical model that quantifies the isotopic ratios and concentrations of an element during concurrent assimilation and fractional crystallization of silicate rocks combined with immiscible separation of carbonate melt (AFCLI), I propose that the isotopic ratio variations in most carbonatite–alkaline silicate complexes can be explained by assimilation of crustal material by parental carbonated silicate parental magmas. A highly plausible scenario that emerges from this exercise is that not only are the carbonate and associated silicate magmas derived from a single parental magma, but also that the lower crust plays an important role in their diversification.

Metamorphism of High/ultrahigh-pressure Pelitic-Felsic Schist in the South Tianshan Orogen, NW China: Phase Equilibria and P-T Path

Wei, C., Wang, W., Clarke, G. L., Zhang, L., Song, S. — Thu, 01 Oct 2009 02:25:09 PDT

The South Tianshan high/ultrahigh-pressure (HP–UHP) metamorphic belt, NW China is characterized by extensive pelitic–felsic schists with numerous blocks of eclogite, ultramafic rock and marble. The pelitic–felsic schists preserve two generations of garnet-bearing mineral assemblages: (1) an early generation that involves garnet, phengite, glaucophane, and quartz/coesite with or without paragonite; (2) a late generation that involves albite, hornblendic amphibole, quartz and grossular-rich, pyrope-poor garnet. Early generation garnet has growth zoning involving core to rim increases in pyrope content, coupled with increased or constant grossular content. Pseudosection modeling of these growth zoning textures in three samples (T311, T314, and T316) reflects prograde paths and peak conditions of c. 32 kbar at 550–570°C, c. 22.5 kbar at 550°C, and c. 22–23 kbar at 540–550°C, respectively. These P–T estimates are consistent with those recovered from eclogite blocks hosted by the schists. Phase equilibrium modeling predicts that the early garnet would have mainly grown in mineral assemblages involving lawsonite, jadeite and chloritoid, with or without coesite or carpholite, distinct from the assemblages now in the matrix. The post-peak decompression of the pelitic–felsic schists is inferred to have involved two stages. The early stage decompression is characterized by dehydration reactions involving lawsonite and carpholite at P > 20 kbar, coupled with mode and compositional changes of garnet, glaucophane and phengite. The late-stage decompression after lawsonite disappearance led to the rocks being fluid-absent. Under such fluid-absent conditions, the solid transition of jadeite to albite occurs at P 14 kbar, and hornblendic amphibole forms at 12–13 kbar. Most garnet grains are somewhat changed in composition, to produce a pyrope-poor and grossular-rich outer rim. In contrast to above-solidus conditions, decompression of the pelitic–felsic schists at subsolidus conditions does not destabilize phengitic muscovite and tends to preserve their peak mineral assemblages if there is no intensive fluid infiltration. In the South Tianshan belt, buoyancy of subducted metasediments with respect to mantle rocks could be one of the major reasons for fast exhumation of the HP–UHP rocks.

Origin of Anorthosite and Magnetitite Layers in the Bushveld Complex, Constrained by Major Element Compositions of Plagioclase

Cawthorn, R. G., Ashwal, L. D. — Fri, 04 Sep 2009 08:07:56 PDT

The Bushveld Complex, a layered mafic intrusion in South Africa, shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite. In a continuous borehole core drilled through the uppermost 2·8 km of the intrusion, typical rocks range upwards from troctolite, through gabbronorite and ferrogabbronorite to ferrodiorite, with extreme examples of anorthosite, magnetitite and feldspathic pyroxenite. The An content of plagioclase has previously been determined for 420 samples and decreases upward from An₇₈ to An₃₆, with six minor, slow reversals. Variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole-rocks. Forty-five anorthosite layers have been identified, ranging from 1 to 23 m thick. None of these layers is associated with the above-mentioned reversals in An content in plagioclase and nearly all have leucocratic rocks below and above, with more than the likely cotectic proportions of plagioclase. These observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation. Rhythmically pulsed crystallization, possibly associated with pressure changes, followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations. Twenty layers of magnetitite have been identified. There is a significant reversal in An content in the overlying plagioclase compared with the underlying sample across only one such layer. Again, this observation challenges hypotheses that such layers result from magma addition, but is consistent with a pressure-change hypothesis for triggering magnetite crystallization. The upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting. Rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions, demonstrating that there is no downward crystallizing roof facies. Furthermore, this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions. Hence, we find no evidence supporting flotation or prolonged suspension of plagioclase.

Magma Evolution and Ascent at the Craters of the Moon and Neighboring Volcanic Fields, Southern Idaho, USA: Implications for the Evolution of Polygenetic and Monogenetic Volcanic Fields

Putirka, K. D., Kuntz, M. A., Unruh, D. M., Vaid, N. — Fri, 04 Sep 2009 08:07:56 PDT

The evolution of polygenetic and monogenetic volcanic fields must reflect differences in magma processing during ascent. To assess their evolution we use thermobarometry and geochemistry to evaluate ascent paths for neighboring, nearly coeval volcanic fields in the Snake River Plain, in south–central Idaho, derived from (1) dominantly Holocene polygenetic evolved lavas from the Craters of the Moon lava field (COME) and (2) Quaternary non-evolved, olivine tholeiites (NEOT) from nearby monogenetic volcanic fields. These data show that NEOT have high magmatic temperatures (1205 ± 27°C) and a narrow temperature range (< 25°C) at any given depth; NEOT parent magmas partially crystallize within the middle crust (14–17 km), but with little time for cooling or assimilation. In contrast, COME magmas partially crystallize at similar depths, but at any given depth exhibit lower temperatures (by ~40°C), and wider temperature ranges (>50°C). Prolonged storage of COME magmas allows them to evolve to higher ⁸⁷Sr/⁸⁶Sr and SiO₂, and lower MgO and ¹⁴³Nd/¹⁴⁴Nd. Most importantly, ascent paths control evolution: NEOT often erupt near the axis of the plain where high-flux (Yellowstone-related), pre-Holocene magmatic activity replaces granitic middle crust with basaltic sills, resulting in a net increase in NEOT magma buoyancy. COME flows erupt off-axis, where felsic crustal lithologies sometimes remain intact, providing a barrier to ascent and a source for crustal contamination. A three-stage ascent process explains the entire range of erupted compositions. Stage 1 (40–20 km): picrites are transported to the middle crust, undergoing partial crystallization of olivine ± clinopyroxene. COME magmas pass through unarmored conduits and assimilate 1% or less of ancient gabbroic crust having high Sr and ⁸⁷Sr/⁸⁶Sr and low SiO₂. Stage 2 (20–10 km): magmas are stored within the middle crust, and evolve to moderate MgO (10%). NEOT magmas, reaching 10% MgO, are positively buoyant and migrate through the middle crust. COME magmas remain negatively buoyant and so crystallize further and assimilate middle crust. Stage 3 (15–0 km): final ascent and eruption occurs when volatile contents, increased by differentiation, are sufficient (1–2 wt % H₂O) to provide magma buoyancy through the middle (and upper) crust.

Petrology of the Paleocene Picrites and Flood Basalts on Disko and Nuussuaq, West Greenland

Larsen, L. M., Pedersen, A. K. — Fri, 04 Sep 2009 08:07:56 PDT

The c. 62–60 Ma rift-related Paleocene volcanic succession in West Greenland comprises voluminous picrites (the Vaigat Formation) overlain by flood basalts (the Maligât Formation). A detailed stratigraphy for the c. 3 km combined succession on Disko and Nuussuaq has been established that provides evidence of the evolution of the volcanic systems, processes and sources through time. Picrites constitute about one-third of the total erupted volume. The primary magmas for the whole succession were highly magnesian; the weighted average of 16·6 wt % MgO for the uncontaminated Vaigat Formation is a lower limit for the primary magmas. During the emplacement of the Vaigat Formation, the magmas fractionated olivine in the feeder channels before eruption but generally did not stop in magma chambers. Only minor magma batches stopped in high-level magma chambers, where many became crustally contaminated. At the transition to the Maligât Formation magma chambers developed, presumably in the lower crust. From then on, the main eruption products were basalts with compositions buffered by refilling–fractionation–tapping processes. Large-scale cyclicity can be seen, with more and less fractionated steady-state compositions. The upper part of the Maligât Formation is contaminated throughout, except for the three youngest flows. Repeated pulses of new picritic magmas can be seen in both formations. The asthenospheric mantle source was heterogeneous and the dominant component was depleted, mid-ocean ridge basalt (MORB)-like mantle with Nb/La <1. An additional, less depleted mantle component akin to the Iceland source is evident in the upper Vaigat Formation but disappeared again at the transition to the Maligât Formation. A third mantle component similar to the source for the E-type lavas on Baffin Island may be present in the earliest rocks. Melting took place in the garnet peridotite facies because the melting column was curtailed by the 100 km thick lithosphere. The picrites show coupled isotopic and elemental variations even within single units, indicating that the accumulated primary melts were not homogenized when they left the melting column. An incompatible-element-rich lithospheric mantle component is seen in the Manîtdlat Member of alkaline picrites and basalts. Single scattered tholeiitic lavas somewhat enriched in the same trace elements are considered to be slightly contaminated with this component. The volume of alkaline and enriched rocks is very small. The crustal contaminants are the sediments in the Nuussuaq Basin. The contamination processes included bulk assimilation, inmixing of partial melts of the sediments, assimilation–fractional crystallization processes, and exchange reactions. When carbon-rich shales were assimilated severe reduction led to formation of graphite or metallic iron. All rocks more evolved than basalt arose by contamination processes. The units of contaminated rocks constitute 5–9% of the Vaigat Formation and around 30% of the Maligât Formation.

Sub-millimeter Heterogeneity of Yttrium and Chromium during Growth of Semi-pelitic Garnet

Martin, A. J. — Fri, 04 Sep 2009 08:07:56 PDT

X-ray maps of yttrium and chromium in two garnets from a semi-pelitic schist from central Nepal reveal parallel, spiraling bands of high and low concentration ~10–30 µm wide. This zoning mimics similar concentration bands defined by foliation in the matrix outside the garnets. The garnets apparently incorporated the matrix zoning intact during growth at conditions up to lower amphibolite facies. These observations and interpretations indicate that Y and Cr concentrations remained heterogeneous at the scale of tens of micrometers during breakdown of matrix minerals and growth of garnet. This conclusion extends results of previous work, which showed that Y and Cr do not homogenize at the centimeter to millimeter scale during metamorphism to amphibolite facies. The width of the concentration bands in the garnets places a constraint on the maximum effective intracrystalline diffusion coefficient for both Y and Cr in garnet of about 10^–21 cm²/s for peak metamorphic conditions of about 9 kbar at 570°C. Different concentrations of Y and Cr at different positions around the rim of each garnet indicate disequilibrium for these elements between garnet and matrix phases. The presence of an internal foliation defined by the Y and Cr concentration bands rules out pre-kinematic garnet growth. Most useful in conjunction with other indicators, zoning of these elements in garnet is thus a tool for deciphering the relative timing of deformation and garnet growth.

Subduction-related Volatile Recycling and Magma Generation beneath Central Mexico: Insights from Melt Inclusions, Oxygen Isotopes and Geodynamic Models

Fri, 04 Sep 2009 08:07:56 PDT

The subduction-related Michoacán–Guanajuato Volcanic Field (MGVF) in central Mexico contains ~900 cinder cones and numerous larger shield volcanoes of Late Pliocene to Holocene age. We present data for major, trace and volatile (H₂O, CO₂, S, Cl) elements in olivine-hosted melt inclusions from eight calc-alkaline cinder cones with primitive magma characteristics and one more evolved alkali basalt tuff ring. The samples span a region extending from the volcanic front to ~175 km behind the front. Relationships between H₂O and incompatible trace elements are used to estimate magmatic H₂O contents for 269 additional volcanic centers across the MGVF and central Mexico. The results show that magmatic H₂O remains high (3–5·75 wt %) for large distances (~150 km) behind the front. Chlorine and S concentrations are strongly correlated with melt H₂O and are also high across most of the arc (700–1350 ppm Cl, 1500–2000 ppm S). The alkali basalt, located far behind the front (~175 km), has much lower volatile contents (<1·5 wt % H₂O, 200 ppm Cl, 500 ppm S), and is compositionally similar to other melts erupted in this region. Oxygen isotope ratios of olivine phenocrysts (5·6–6) from the calc-alkaline samples are higher than for typical mantle-derived magmas but do not vary systematically across the arc. Calc-alkaline samples have high large ion lithophile element concentrations relative to Nb and Ta, as is typical of subduction-related magmas, but alkali basalt samples far behind the front have high Nb and Ta and lack enrichments in fluid-mobile elements. Modeling based on volatiles and trace elements suggests that the calc-alkaline magmas were generated by 6–15% partial melting of a variably depleted mantle wedge that was fluxed with H₂O-rich components from the subducted slab. In contrast, the alkali basalts formed by small degrees of decompression melting of an ocean island basalt source that had not been fluxed by slab-derived components. Based on high ¹⁸O_olivine values and trace element characteristics, the H₂O-rich subduction components added to the mantle wedge beneath the MGVF are likely to be mixtures of oceanic crust derived fluids and sediment melts. Integrating these results with new 2-D thermo-mechanical models of the subduction zone beneath the MGVF, we demonstrate that the present-day plate configuration beneath the MGVF causes fluids to be released beneath the forearc and volcanic front, and that sediment melts can be produced beneath the volcanic front by the waning stages of fluid released from the oceanic crust percolating through already dehydrated sediments. Down-dragging of serpentine- and chlorite-bearing peridotite in the lowermost mantle wedge probably plays a role in fluid transport from the forearc to beneath the arc. H₂O-rich magmas located more than ~50 km behind the volcanic front can be explained by mantle hydration related to a shallower slab geometry that existed at ~3 Ma. Rollback of the slab over the last ~2 Myr has resulted in strong mantle advection that forms low-H₂O, high-Nb alkali basaltic magmas by decompression melting far behind the present-day volcanic front.

The Crystal/Melt Partitioning of V during Mantle Melting as a Function of Oxygen Fugacity Compared with some other Elements (Al, P, Ca, Sc, Ti, Cr, Fe, Ga, Y, Zr and Nb)

Mallmann, G., O'Neill, H. St. C. — Fri, 04 Sep 2009 08:07:56 PDT

Vanadium exists in multiple valence states in silicate and oxide systems, namely V²⁺, V³⁺, V⁴⁺ and V⁵⁺. This special characteristic has been exploited in several ways to estimate the redox conditions of high-temperature planetary processes, such as partial melting and core formation. However, the use of V as a universal redox indicator (i.e. suitable for the entire range of oxygen fugacities found in the inner Solar System) requires precise knowledge of the partitioning of all of its several valence states. Here we report the results of a series of 1 atm (1300°C) and high-pressure (1–3 GPa, 1315–1450°C) experiments carried out over a range of redox conditions sufficiently large (from QFM–13·3 to QFM+11·4, where QFM is the quartz–fayalite–magnetite oxygen buffer) to constrain the full panoply of V chemical behaviour. Partition coefficients between the major upper-mantle minerals (olivine, clinopyroxene, orthopyroxene, spinel and garnet) and silicate melt were precisely measured with laser ablation inductively coupled plasma mass spectrometry for V and other selected heterovalent and homovalent elements (Al, P, Ca, Sc, Ti, Cr, Fe, Ga, Y, Zr and Nb). The particularly large range of redox conditions investigated here enabled concentrations of V²⁺ and V⁵⁺ to be constrained along with V³⁺ and V⁴⁺, allowing modelling of the change in bulk V partition coefficient with oxygen fugacity to be performed in a robust thermodynamic fashion. As in previous studies, we found a trend of increasing incompatibility from V³⁺ to V⁴⁺ to V⁵⁺ for all phases. Partition coefficients for V²⁺ can be either higher or lower than for V³⁺, depending on the phase. Additionally, we found evidence for changes in the oxidation state of Cr (Cr²⁺ Cr³⁺), Fe (Fe²⁺ Fe³⁺) and Ti (Ti³⁺ Ti⁴⁺). There is also indication that P may behave as a heterovalent element, occurring as a trivalent cation at very reducing conditions (P³⁺ P⁵⁺). For all other trace elements, the oxidation state remained constant. The data presented here can be used to implement methods of estimating the redox state of mantle and mantle-derived planetary samples from bulk-rock concentration and crystal/melt partitioning with only minor extrapolation and bias, allowing better precision and accuracy than previously possible. The method of estimating the redox conditions of basalt suites from bulk-rock V concentrations relative to homovalent elements with similar compatibility (e.g. V/Sc and V/Ga) was tested using databases for mid-ocean ridge, ocean island and island arc basalts. Within the many assumptions involved in forward trace-element modelling (e.g. degree and style of melting, source composition, constant partition coefficients), we show that the redox states of the source regions of these different types of terrestrial basalts are indistinguishable from each other, having relative oxygen fugacities at ~QFM ± 1. The fact that arc magmas have higher Fe³⁺/Fe²⁺ ratios than other types of basalts, making them appear to be more oxidized, may be due to late-stage processes rather than derivation from a more oxidized part of the asthenosphere.