Journal of Petrology - recent issues

Petrogenesis of Lavas along the Solomon Island Arc, SW Pacific: Coupling of Compositional Variations and Subduction Zone Geometry

2009-06-08

The Solomon island arc, SW Pacific, is of particular interest for understanding subduction zone volcanism, as magmatism in the active part of the arc is dominated by mafic melts, thus permitting direct insights into mantle processes. Along the Solomon island arc, the Indian–Australian plate is subducting at present beneath the Pacific plate. However, until at least c. 12 Myr ago, the Pacific plate was subducting beneath the Indian–Australian plate until the Cretaceous Ontong Java Plateau collided with the northern Solomon island arc. To evaluate the effects of the changes in tectonic regime on lava compositions, we present a comprehensive Sr–Nd–Hf–Pb isotope, major element and trace element dataset, covering lavas erupted along the entire island arc (c. 1000 km). Basalts and andesites represent the most abundant rock types. Picrites and ankaramites occur in the New Georgia Group of the Solomon Islands, where they erupted above the subducting Woodlark spreading center, and also in the Santa Cruz archipelago, north of Vanuatu, where the Rennell Fracture Zone is subducting. Recent work has also identified the presence of adakites (Sr/Y up to c. 200), and high-Mg andesites (MgO > 5 wt %, Sr/Y c. 11–46). Most of the high-Mg andesites are genetically linked to the adakites, but some of the high-Mg andesites show affinities to boninitic compositions. Large ion lithophile element abundances in most Solomon island arc magmas indicate a strong source overprint by subduction components. ⁸⁷Sr/⁸⁶Sr and Nd values along the arc range from 0·7029 to 0·7052 and from +5·8 to +8·3, respectively. The Sr–Nd values partially overlap the compositions of oceanic basalts from the Indian–Australian plate. Measured Hf values range from +10·5 to +14·6. If corrected for contributions from subduction components, combined Hf–Nd systematics also indicate that most of the studied Solomon arc lavas were generated within the Indian-type mantle domain. However, a few samples display Hf–Nd compositions resembling those of the Pacific-type mantle domain. These samples either originate from older Pacific basement (basalts) or represent melts derived from subducted Pacific crust (adakites). Lead isotope compositions, controlled by subduction components, can be used to identify the presence of two distinct types of subduction components that originate (1) from the Pacific plate including Ontong Java Plateau material (> 6 Myr old) and (2) from the more recently subducted Indian–Australian plate. Combined Hf–Nd–Pb isotope data also reveal that lower parts of the Ontong Java Plateau entered the mantle wedge, as previously postulated by geophysical models.

Differentiation and Compaction in the Skaergaard Intrusion

Tegner, C., Thy, P., Holness, M. B., Jakobsen, J. K., Lesher, C. E. — 2009-06-08

Igneous differentiation processes are constrained from bulk compositions, densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the Layered Series of the Skaergaard intrusion, East Greenland. The stratigraphic distribution of P, U and Rb in cumulates and residual magma, modeled by Rayleigh fractionation, constrains the final porosities or trapped liquid contents to 30–52% in LZa troctolites, decreasing to 4–12% at the top of LZb olivine-gabbros and remaining low (1–13%; 4·6% on average) in the oxide-gabbros of LZc, MZ and UZ. Local variations in trapped liquid content are associated with modal layering: leucocratic, low-density rocks have higher proportions of trapped melt than adjacent melanocratic, high-density rocks. These observations are explained by varying degrees of compaction. Compaction was most important after the onset of Fe–Ti oxide crystallization because of the high densities of the crystal matrix. Here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick. In the basal section (LZa) the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor. In the overlying section (LZb) the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation. The modeling constrains the P₂O₅ content of the residual magma to ~1·7 wt % at the level of apatite-in, suggesting that the magma contained ~49 wt % SiO₂ and followed a trend of iron enrichment. Compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix.

Generation of Tonalitic and Dioritic Magmas by Coupled Partial Melting of Gabbroic and Metasedimentary Rocks within the Deep Crust of the Famatinian Magmatic Arc, Argentina

2009-06-08

The source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the Famatinian arc (Sierras Pampeanas of western Argentina). The source zones of intermediate igneous rocks are located at the transition between a gabbro-dominated mafic unit and a tonalite-dominated intermediate unit. In the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall-rocks, crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments. In turn, the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks. The gabbroic rocks became mafic migmatites comprising amphibole-rich pyroxene-bearing mesosomes and leucotonalitic veins. Significantly, most of the mafic migmatites have isotopic compositions [⁸⁷Sr/⁸⁶Sr(T) < 0·7063 and _Nd(T) = –0·94 to +2·24] similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins [⁸⁷Sr/⁸⁶Sr(T) = 0·7075–0·7126 and _Nd(T) < –2·65], providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts [⁸⁷Sr/⁸⁶Sr(T) = 0·715 and _Nd(T) = –6·21]. Mass-balance calculations show that the model reaction plagioclase + amphibole + leucogranitic melt -> leucotonalitic melt + clinopyroxene ± orthopyroxene can better explain the partial melting of the gabbroic rocks. Based on field observations, we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt-dominated leucotonalitic pools. However, the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically (elemental and isotopic) more evolved than the dioritic and tonalitic rocks. We envisage that once detached from their source region the leucotonalitic magmas were able to react, commingle and mix with entrained fragments of both mafic and metasedimentary rocks. This process gave rise to melts that became tonalitic and dioritic magmas. This study concludes that the generation of intermediate magmas is a multistage process with three critical steps: (1) influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks; (2) physically and chemically coupled melting of mafic and metasedimentary rocks, leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies; (3) retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors. The dimensions of the source zone seem to be insufficient to generate crustal-scale volumes of intermediate igneous rocks. However, the Famatinian paleo-arc crust would expose only those magma source zones that were still active during the tectonic closure of the arc. Ultimately, a time-integrated perspective indicates that early active source zones were cannibalized during the downward expansion of the plutonic bodies already dominated by intermediate plutonic rocks.

Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons

Wilson, C. J. N., Charlier, B. L. A. — 2009-06-08

New and/or enlarged datasets of U–Th disequilibrium model ages from secondary ionization mass spectrometry (SIMS) analyses of zircons in eight eruptive units from the area of Taupo volcano, New Zealand, highlight the behavioural contrasts of two closely adjacent, contemporaneous but independent magma chambers. One yielded closely similar crystal-poor (‘Oruanui-type’) rhyolites, sampled in three small precursor eruptions (Tihoi, ‘New plinian’, Okaia) from ~45 to 30 ka, then the major 27 ka Oruanui eruption. Three of the four eruptions had vents within the modern Lake Taupo, whereas the fourth (‘New plinian’) was sourced ~20 km NNE of the other vents, fed by lateral magma migration. Samples from all four eruptions share a common model-age peak at ~95 ka of antecrystic zircons. However, three of the four differ in younger pre-eruptive model-age peaks that require their parental melt-dominant bodies to have been physically extracted independently from a common mush zone represented by the ~95 ka peak. A sample from a fifth eruption (‘New phreatoplinian’, also at ~45 ka) shares an older 80–100 ka peak but has numerous older grains and distinctly contrasting Sr-isotopic characteristics to the ‘Oruanui-type’ magmas. The 530 km³ Oruanui melt-dominant body was produced in at most ~3000 years as shown by differences in zircon model-age spectra and average ages between it and the 30 ka Okaia eruption, despite their coincidence in vent locations. The second suite of eruptions at ~47, 28 and 16 ka ejected moderately crystal-rich biotite rhyolites from a second source chamber, which vented over a 15 km wide area NE of Taupo (overlapping with Maroa volcano). This second chamber is inferred to have comparable horizontal dimensions to the vent spacing. The three biotite rhyolites show unimodal model-age spectra that peak at 30, 15–25 and 6 kyr prior to each eruption, respectively, and underwent single cycles of melt generation and eruption with no recycling of significantly older antecrysts or xenocrysts (< 1% equiline grains). Crystallization peaks defined by probability density function curves are not in phase between the two magma chambers and they had wholly independent thermal and chemical histories, despite their close geographical proximity. Post-Oruanui activity involved recycling of Oruanui-age zircons, but these crystals are xenocrystic, as the host melts show no lineage towards or mixing with the Oruanui compositions. Magma chambers at Taupo accumulated melt-dominant bodies as rapidly as > 5 m³/s (Oruanui) and effectively drained the mush of melt in doing so (Oruanui vs post-Oruanui activity), probably mediated by active rifting processes and tectonic disruption of the mush pile. Comparisons of ‘magma residence times’ and discussion of the growth histories of large silicic chambers represented by volcanic or plutonic rocks are self-limited by the uncertainties in the respective SIMS analyses. Growth times of Miocene and older plutons dated by SIMS U–Pb techniques are comparable with the 2 Myr lifetime of the whole Taupo Volcanic Zone, and the associated 1 SIMS analytical uncertainties exceed the lifetime of a volcano such as Taupo. Subtle details that indicate the rapidity of magma accumulation and recycling of crystals in the young Taupo system cannot be discerned in most pre-300 ka silicic systems. Averaging of SIMS model-age data further obscures subtle details that would allow discrimination of newly crystallized versus recycled zircons. Discussions of volcano–plutonic relationships and accumulation rates for large silicic melt-dominant bodies cannot rely on age data in isolation, but require knowledge of the stratigraphic and compositional settings.

An Experimental Determination of the Effect of Bulk Composition on Phase Relationships in Metasediments at Near-solidus Conditions

Ferri, F., Poli, S., Vielzeuf, D. — 2009-06-08

Phase relationships in metapelites were experimentally investigated in the model system CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O on four synthetic compositions differing in their K₂O content and K₂O/Al₂O₃ ratio. Experiments were carried out at pressures ranging from 0·8 to 1·4 GPa and temperatures from 620 to 740°C under different fluid-buffered conditions. The stability of the assemblage garnet–biotite–staurolite/Al₂SiO₅ at the investigated conditions largely controls the absence of muscovite in K-poor compositions. At 1·2 GPa and 700°C staurolite is present with garnet + biotite + muscovite, replaced by Al₂SiO₅ at lower pressures through the reaction staurolite + muscovite + quartz = garnet + biotite + Al₂SiO₅ + H₂O. In K-poor bulk compositions, garnet + biotite + staurolite + gedrite coexist up to 730°C. A compositional reversal of the Mg–Fe partitioning between garnet and staurolite is observed with decreasing pressure and is responsible for the singular equilibrium staurolite + quartz = Al₂SiO₅ + garnet + H₂O that governs the high-temperature breakdown of staurolite. Melting is found to depend both on bulk composition and on fluid speciation. The wet solidus is represented by the reaction muscovite + anorthite + garnet + quartz + H₂O = melt + biotite + Al₂SiO₅. Even though staurolite is never recovered at supersolidus conditions, the large pressure–temperature stability field of staurolite + muscovite and of staurolite + gedrite suggests that staurolite can be directly involved in melt production through fluid-present or fluid-absent reactions.

Archean Accretion and Crustal Evolution of the Kalahari Craton--the Zircon Age and Hf Isotope Record of Granitic Rocks from Barberton/Swaziland to the Francistown Arc

Zeh, A., Gerdes, A., Barton, J. M. — 2009-06-08

U–Pb and Lu–Hf isotope analyses, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry on zircon grains from 37 granitoid samples indicate that the Kalahari Craton consists of at least five distinct terranes—Barberton South (BS), Barberton North (BN), Murchison–Northern Kaapvaal (MNK), Limpopo Central Zone (LCZ), and Francistown—which underwent different crustal evolutions, and were successively accreted at c. 3·23 Ga, 2·9 Ga and 2·65–2·7 Ga. The investigated granitoids were emplaced over a period of c. 1.5 billion years, and are exposed along a c. 1000 km long traverse from the Barberton Mountain Land/Swaziland to the Francistown arc complex, Botswana. The presented datasets reveal that most granitoids of the BS (3·45–3·10 Ga), MNK (2·93–2·67 Ga), Francistown (2·70–2·65 Ga) and LCZ terranes (3·2–2·03 Ga) show near-chondritic to subchondritic Hf_t (BS = –1·7 to + 0·5; MNK = –3·4 to + 0·7; Francistown = –0·5 to + 1·1; LCZ = –12·4 to –1·8), indicating that crustal recycling—perhaps by mixing of an older crust with a depleted mantle reservoir—played an important role during their formation and growth. Higher, superchondritic Hf_t values, as indicative for an important depleted mantle influence, were obtained only from some granitoids of the BN terrane (Hf_3·23Ga = +2·5 ± 0·8), the Gaborone Granite Suite (Hf_2·80Ga = +2·0 ± 1·6), and from a few detrital zircons from the Mahalapye complex of the Limpopo Belt. In addition, the datasets show that the internal Hf isotope variation of magmatic zircon domains from most granitoids is commonly less than ±1·5 -units, and only in rare cases up to ±3·1 -units. The rare significant Hf_t variations may be ascribed to incomplete mixing of different sources during magma crystallization. It is also shown that the combined approach of cathodoluminescence imaging with U–Pb and Lu–Hf isotope analysis provides a powerful tool to distinguish zircon domains formed and/or altered at different times.

The Complex Hydrothermal History of Granitic Rocks: Multiple Feldspar Replacement Reactions under Subsolidus Conditions

Plumper, O., Putnis, A. — 2009-06-08

Recurring subsolidus re-equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid–rock interactions that affect large volumes of the Earth's continental crust. This has a direct bearing on the interpretation of the present-day granitic rock mineralogy and geochemistry. We examine Palaeoproterozoic grey and red-stained granitoids from the Simpevarp and Laxemar areas in SE Sweden, particularly focusing on consecutive feldspar replacement reactions, to provide an in-depth understanding of subsolidus re-equilibration of granitic rocks with hydrothermal fluids. The apparently most unaltered grey granitoids contain highly porous oligoclase grains that enclose crystallographically continuous microcline relicts. This texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline. Oligoclase is progressively replaced by albite (~An₉) along polysynthetic twinning and intragranular fractures. The features of this replacement are characteristic of a dissolution–reprecipitation mechanism. Fine-grained mica (sericite) is closely associated with the albite porosity within micron-sized pores observable with scanning electron microscopy as well as in nanopores imaged with transmission electron microscopy. The reddening phenomenon in the vicinity of fractures is contemporaneously related to the K-feldspathization of sericite, which is restricted to the altered oligoclase. Submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration. The complex associations between the fluid–feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re-equilibration, changing the original magmatic mineralogy. Therefore, the effects of large-scale re-equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered.

Zircon Behaviour during Low-temperature Metamorphism

Hay, D. C., Dempster, T. J. — 2009-05-08

Zircon in greenschist-facies metasedimentary rocks from the Scottish Highlands displays a range of complex textures that reflect low-temperature alteration of original detrital grains. In situ back-scattered electron, cathodoluminescence, electron backscatter diffraction and chemical analyses show that altered zircon is porous, weakly luminescent, enriched in non-formula elements such as Al and Fe, and is associated with fractures within the host zircon. The low-temperature zircon appears to be nano-crystalline and to replace U-rich zircon via modification of whole grains or selective alteration of parts of grains, and is linked to the development of zircon outgrowths. The altered zircon is also associated with epitaxial xenotime outgrowths and inclusions. Low-temperature zircon is abundant in slates and other mica-rich samples and its formation is linked to a dissolution–reprecipitation mechanism. Zircon within quartz-rich host rocks typically shows evidence of deformation and the resulting fractures enhance its dissolution, creating rounded embayed morphologies. In contrast, zircon from phyllosilicate-rich rocks contains more new low-temperature growth. Zircon alters during both prograde and retrograde metamorphic events and its development is controlled by both the progressive accumulation of radiation damage in the host grain and the access of metamorphic fluids to the metamict zircon.

A Comment on 'Two Distinctive Granite Suites in the SW Bohemian Massif and their Record of Emplacement: Constraints from Geochemistry and Zircon 207Pb/206Pb Chronology' by Siebel et al. Journal of Petrology 49, 1853-1872

Finger, F., Rene, M. — 2009-05-08

Two Distinctive Granite Suites in the Southwestern Bohemian Massif: Reply to F. Finger and M. Rene

Siebel, W., Shang, C. K., Reitter, E., Rohrmuller, J., Breiter, K. — 2009-05-08

Experimental Constraints on the Deep Magma Feeding System at Stromboli Volcano, Italy

Pichavant, M., Di Carlo, I., Le Gac, Y., Rotolo, S. G., Scaillet, B. — 2009-05-08

New experiments have been performed on a high-K basalt (PST-9) from Stromboli volcano, Italy, to constrain the physical conditions of golden pumice magmas at their storage level and discuss their petrogenesis. Fluid-present, H₂O- and CO₂-bearing, near-liquidus experiments were performed at 1150°C between 100 and 400 MPa and under oxidizing conditions. Glasses were analyzed by Fourier transform IR spectroscopy and their H₂O and CO₂ concentrations compared with those in glass inclusions. Most glass inclusions cluster near the 200 MPa isobar, suggesting entrapment at a depth of ~8 km. Golden pumice magmas have viscosities of 7–9 Pa s and densities of 2·48–2·57 g/cm³. Compositions of experimental fluids coexisting with melts along the liquidus have been estimated by mass balance. They range from CO₂-rich (XH₂O ~ 0·2) at 400 MPa to H₂O-rich (XH₂O ~ 0·8) at 100 MPa. The free fluid phase present at the reservoir level has an XH₂O of ~0·6, consistent with equilibration with a near-liquidus golden pumice magma below 300 MPa. Clinopyroxene is the liquidus phase in all high-pressure experiments, either fluid-absent (H₂O) or fluid-present (H₂O–CO₂). In contrast, at 0·1 MPa, cpx and ol appear together on the liquidus and olivines are more Fo-rich (up to 89·1) than those crystallizing at high pressures (up to 87·3). The composition of cotectic liquids multiply saturated in cpx and ol has been experimentally determined. Most pumices and a majority of melt inclusions have compositions of cotectic melts. In contrast, PST-9 plots in the cpx field and is representative of less evolved ankaramitic magmas parental to golden pumices. Melt inclusions trapped in Fo >87 olivines form a group of ultra-calcic compositions plotting in the cpx field, interpreted as boundary layer melts locally generated by cpx dissolution in the deep reservoir. Ankaramitic melts in the Stromboli feeding system are proposed to derive from primitive mantle melts by combined crystallization, mixing, wall-rock interaction and assimilation.

PGE Tenor and Metal Ratios within and below the Merensky Reef, Bushveld Complex: Implications for its Genesis

Naldrett, A. J., Wilson, A., Kinnaird, J., Chunnett, G. — 2009-05-08

The Merensky Reef is the consequence of several major influxes of magma into the Bushveld magma chamber that gave rise to at least two cyclic units, including the Pre-Merensky and Merensky cyclic units. Analysis of the S, Ni, Cu, platinum group element (PGE) and Au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the Reef to close to the UG-2 horizon of the Western Bushveld Complex provide important petrogenetic constraints on the origin of the Reef. Particular attention is paid to variations in the calculated PGE tenors and Cu/Pd ratios of sulfides, both vertically and laterally. All of the profiles show a decrease in Pt tenor and concomitant increase in Cu/Pd ratio upward across the pyroxenite of the Merensky unit and laterally away from certain centres that are interpreted as feeder zones. The data also document that more mafic horizons within the Merensky to UG-2 interval of the southwestern Bushveld are characterized by much lower Cu/Pd (<100–400) than the ambient ratio (~4000). This is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there. The favoured interpretation for the Merensky Reef is that the Bushveld chamber(s) was occupied by a resident magma crystallizing orthopyroxene and plagioclase. Pre-Merensky and, subsequently, Merensky magmas entered the chamber(s) intermittently from localized feeder structures, mixing or mingling with the resident magma and giving rise to the more mafic cumulates. At specific horizons, including the Merensky Reef, the mixing or mingling caused sulfide immiscibility. In these cases, as the new magma pulses flowed laterally away from the entry zones they deposited sulfide, becoming depleted in PGE. On coming to rest, the magma continued to segregate sulfide and crystallize pyroxene, so that the sulfides became progressively depleted in PGE and acquired a higher Cu/Pd upward through the pyroxenite. This model requires that the Merensky magmas could dissolve up to two orders of magnitude more PGE than current measurements on dry diopside–anorthite melts have indicated. It is suggested that very PGE-rich magmas can develop as a result of sulfide-unsaturated magma reacting with early formed sulfide in a staging chamber, dissolving FeS and enriching the remaining sulfide in PGE. Later magmas equilibrating with this enriched sulfide will themselves become highly enriched in PGE.

Crustal Contamination of Mantle-derived Magmas within Piton de la Fournaise Volcano, Reunion Island

Pietruszka, A. J., Hauri, E. H., Blichert-Toft, J. — 2009-05-08

Historical lavas from Piton de la Fournaise are ideally suited to investigate temporal changes in the magmatic processes at this active ocean-island volcano. Here we present a time series of the Sr, Nd, Hf, and U-series isotope ratios, and incompatible trace element abundances of lavas erupted from Piton de la Fournaise during the late 20th century. Most of the lavas display positive correlations on plots of ratios of trace elements that are more vs less incompatible during partial melting of the mantle (e.g. Th/Yb) against ⁸⁷Sr/⁸⁶Sr or other trace element ratios (e.g. Nd/Sm). These trends can be explained by (1) melting of two compositionally distinct components intrinsic to a heterogeneous mantle plume beneath Réunion island or (2) incorporation of the ambient Indian mantle asthenosphere into the upwelling plume. The ⁸⁷Sr/⁸⁶Sr and trace element (e.g. Nd/Sm) ratios of the lavas decreased systematically over three periods of time: 1953–1964, 1966–1973, and 1975–1991. Each of these trends may represent a single melting event within the mantle, such that the initial extraction of melt from a more fertile, trace-element enriched source (with relatively high ⁸⁷Sr/⁸⁶Sr) was followed (as melting progressed) by the later extraction of melt from a more refractory, trace-element depleted (or less enriched) source (with relatively low ⁸⁷Sr/⁸⁶Sr). The chemistry of some historical lavas from Piton de la Fournaise appears to be anomalous because (1) they depart from the trends defined by the majority of the lavas and (2) they have relatively low Th/U ratios and small ²³⁰Th–²³⁸U disequilibria. The most likely explanation for these anomalous lavas is contamination of mantle-derived magmas with small amounts (~3–6%) of low-degree (<1%) partial melts of young gabbro and wehrlite cumulates within the volcanic edifice. The cumulates were probably altered to become enriched in U prior to contamination (potentially as a result of recent hydrothermal alteration of the crust with U-rich fluids). Crustal contamination was most important between 1950 and 1964 and for the March 1998 lava from Hudson Crater, although moderate amounts of contamination may have been important throughout the late 20th century.

Silicic Magmas in the Izu-Bonin Oceanic Arc and Implications for Crustal Evolution

2009-05-08

Abundant rhyolite has erupted from the Izu–Bonin–Mariana volcanic arc (IBM arc) from its earliest stage (Eocene) to the present. Geochemically, three types of Quaternary rhyolites exist in the Izu–Bonin arc front, and they are closely related to volcano type and crustal structure. The dominantly basaltic islands of the volcanic front produce small volumes of rhyolites that we call R1. The submarine calderas of the volcanic front erupt mostly rhyolite that we call R2. Seamounts, knolls, and pillow ridges in the backarc extensional zone are mostly basaltic but also contain rhyolites that we call R3. The thickest total crust, and the thickest intermediate composition middle crust, occurs below the dominantly basaltic volcanoes, whereas the intermediate composition middle crust tends to be thinner beneath the submarine calderas. R1 rhyolites may be derived from Quaternary andesitic sources whereas R2 and R3 rhyolites may be derived from Oligocene ones. The higher CaO/Al₂O₃ in R1 compared with R2 and R3 rhyolites can be attributed to the same difference between Quaternary and Oligocene andesite sources, respectively. Light rare earth element (LREE)-depleted REE patterns of Quaternary andesites versus flat to slightly LREE-enriched patterns of Oligocene andesites are almost parallel to those of R1 and R2, respectively. Partial melts of Quaternary andesite will have low Zr/Y values such as in R1, whereas melts of Oligocene andesites will have the higher Zr/Y values of R2. Lavas from basalt-dominant volcanoes have higher Sr, Pb, and sometimes Nd isotope ratios than those from rhyolite-dominated volcanoes, which are closer to the isotope ratios of the Oligocene arc. Why and how do crustal sources differ systematically and alternately along and behind the Izu–Bonin arc? If locally developed regions within the mantle wedge (hot fingers) produce large basaltic volcanoes and remain stationary for millions of years, then basalt-dominant volcanoes eventually will overlie thicker crust. Remelting of middle crust to form rhyolite magmas takes place beneath both basaltic and rhyolitic volcanoes (R1 and R2 rhyolite, respectively). However, basalt volcanoes consume new middle crust to produce rhyolite magma whereas rhyolite volcanoes consume old Oligocene middle crust. Moreover, rhyolite volcanoes have no mantle roots beneath the crust. Instead, dikes from basalt volcanoes provide the heat source to partially melt the crust.

Progressive Melt Extraction from Upwelling Mantle Constrained by the Kita-Matsuura Basalts in NW Kyushu, SW Japan

Sakuyama, T., Ozawa, K., Sumino, H., Nagao, K. — 2009-05-08

Systematic petrological, geochemical, and chronological studies of Cenozoic intra-plate volcanism in southwestern Japan, in the Kita-Matsuura area in northwestern Kyushu, reveal temporal and spatial changes in mantle melting processes on a time scale of ~2·5 Myr and a horizontal scale of ~35 km. The most extensive basaltic activity in the area occurred between 8·5 and 6·0 Ma. Four vertical sections through the volcanic sequence from the underlying basement to the top flows were studied to encompass the distribution of the Kita-Matsuura basalt. In the central and western sections (Hirado, Senryu, and Ishimori) there is a temporal variation in the chemical composition of the basalts: volcanism initiated eruption of mildly alkaline basalts (low- to medium-SiO₂ group) followed by major flows of sub-alkaline basalt (medium- to high-SiO₂ group). The eastern section (Kunimi section) is characterized by mildly alkaline basalts (low-SiO₂ group). Each SiO₂ group has a distinctive phenocryst assemblage and major element composition. The Zr/Y, Nb/Th, and Nb/Y ratios of the Kita-Matsuura basalts are positively correlated, which cannot be explained by assimilation of crustal materials but instead is linked to melting processes in the mantle. The average segregation depth of the inferred parental magmas of the three groups decreases in the order of SiO₂ enrichment, indicating a temporal decrease in melt segregation depth in the central and western sections. The correlation between Zr/Y and Nb/Y can be reproduced by a series of instantaneous melts resulting from decompressional critical melting of a primitive mantle source with ~2% trapped melt. In each section, both Zr/Y and Nb/Y decrease upwards, which implies an increase in the degree of melting with time. The temporal and spatial variations in basalt chemistry are most plausibly accounted for by progressive melt extraction during mantle upwelling, which started in the garnet stability field (~3·0 GPa) and ended in the spinel field (~1·5 GPa). Upwelling was centred beneath the Hirado section, in the westernmost area in Kita-Matsuura, with a diameter inferred to be greater than 70 km.

Onset and Progression of Serpentinization and Magnetite Formation in Olivine-rich Troctolite from IODP Hole U1309D

2009-04-09

Serpentinization of olivine-rich troctolite from core 227, Integrated Ocean Drilling Program (IODP) Hole U1309D ranges from <10% to >90%. Two episodes of serpentinization are recognized. The first, dominant in weakly serpentinized samples, is an approximately isochemical (except for water) replacement of olivine (Fo_84–85) by a mixture of serpentine (antigorite, Mg-number 92) and brucite (amakinite-rich; Mg-number 65). The compositions of the minerals in type 1 veins are a reflection of Fe–Mg exchange between olivine and the brucite + serpentine formed during early serpentinization. The early serpentinite veins (type 1) are thin (< 0·05 mm), irregular, and exploit pre-existing cracks in olivine. The presence of antigorite suggests that early serpentinization occurred at T > 300°C. Type 1 veins reflect rock-dominated serpentinization, became isolated early in their history, and persist as relicts in all but the most altered samples. The main episode of serpentinization is manifested by through-going lizardite (average Mg-number 96)–magnetite veins (type 2). Type 2 veins define an anastomosing foliation, may be several millimeters in width and appear to exploit pre-existing, favorably oriented type 1 veins. Type 2 veins reflect open-system serpentinization. Magnetite in these veins formed by oxidation of the Fe in brucite and serpentine, whereas addition of silica to the system converted the Mg-component of the brucite to serpentine. Magnetite forms one or more distinct bands in the interior of the vein and is never in direct contact with relict olivine. A brucite–serpentine mixture, similar to that found in type 1 veins, but with lizardite instead of antigorite, is commonly present at the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine). We interpret type 2 veins as a steady-state system where brucite continually forms at the olivine–vein contact and then reacts out in the interior of the vein. This continual formation and destruction of brucite imposes an exceptionally low a_SiO₂ on the system. Magnetite and olivine are never in contact in type 2 veins (or anywhere else) because the olivine-out reaction yields ferroan brucite and not magnetite. The desilication of serpentine in the type 2 veins is a reflection of the inherent instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine in equilibrium with magnetite in serpentinites is a function of serpentine–magnetite and not serpentine–olivine equilbria.

A Temporal Record of Magma Accumulation and Evolution beneath Nevado de Toluca, Mexico, Preserved in Plagioclase Phenocrysts

Smith, V. C., Blundy, J. D., Arce, J. L. — 2009-04-09

Plagioclase crystals from the 8 km³ 10·5 ka Upper Toluca Pumice (UTP) eruption from Nevado de Toluca provide a detailed temporal record of pre-eruptive magmatic processes. The crystals display a range of textures and major and trace element concentrations. A distinct feature of the crystals is the presence of several sharp increases in MgO, FeO and TiO₂, which occur at some of the numerous resorption horizons and coincide with increases in X_An and in some cases Ce and La. These abrupt compositional peaks, and the associated textural variations, reflect recharge events of more mafic melt. Three distinct recharge events can be recognized. The general compositional trends suggest that crystallization occurred within a common chamber. However, the crystal cores display a range of textures and a wide range of compositions suggesting that they are relics of earlier crystallization episodes within the upper crust. The temporal record of Sr and Ba melt contents recorded by the crystals, calculated using partition coefficients, fluctuates significantly within single crystals. Overall, two trends of Sr–Ba variation in the melt are apparent. The first trend involves decreasing Sr and slightly increasing Ba, consistent with plagioclase crystallization. The second trend involves an increase in both Sr and Ba that is not consistent with crystallization of plagioclase alone. This trend is ascribed to chemical variations within the deeper magma reservoirs from which the various magma batches entering the UTP shallow reservoir were ultimately sourced. The magmatic system under Nevado de Toluca was open and received intermittent, but relatively small, pulses of magma from a deeper source. Dacite was the predominant recharge composition, although some pulses were distinctly more mafic and hotter. These processes led to the accumulation of a large volume of dacitic magma in the upper crust. One of the more mafic pulses appears to have triggered the eruption.

Partial Melting of Mantle and Crustal Sources beneath South Karakorum, Pakistan: Implications for the Miocene Geodynamic Evolution of the India-Asia Convergence Zone

Maheo, G., Blichert-Toft, J., Pin, C., Guillot, S., Pecher, A. — 2009-04-09

In south Karakorum, the western prolongation of southern Tibet, three distinct types of magmatic rocks were emplaced during the Neogene: (1) 22–24 Myr old lamprophyres, characterized by strong enrichment in large ion lithophile (LILE) and light rare earth elements (LREE), ⁸⁷Sr/⁸⁶Sr_(i) = 0·7096, _Nd(i) = –7, and _Hf = –9, interpreted to reflect partial melting of a previously metasomatized spinel-lherzolite mantle source; (2) the 21–26 Myr old Baltoro high Ba–Sr granitoids, likewise strongly enriched in LILE and LREE, with ⁸⁷Sr/⁸⁶Sr_(i) = 0·7034–0·7183, _Nd(i) = –6·5 to –11·0, and _Hf = –1·8 to –8·0, produced by partial melting of amphibole-bearing rocks in the lower crust, possibly the root of south Karakorum Cretaceous magmatic arc; (3) the 8–9 Myr old Hemasil syenite and its associated lamprophyre, also both enriched in incompatible elements but with isotopic compositions closer to those of depleted mantle (⁸⁷Sr/⁸⁶Sr_(i) = 0·7043–0·7055, _Nd(i) = +3·5 – + 4·3, and _Hf = + 10·4 – + 11·2). The Hemasil syenite is interpreted as the product of partial melting of a time-integrated depleted spinel-lherzolite source that was enriched in K and LREE during a recent metasomatic event. We propose that the lamprophyres were formed during partial melting of the South Asian mantle previously metasomatized by fluids derived from the subducted Indian continental crust. This melting episode is interpreted to be related to a break-off event that occurred within the subducting Indian continental lithosphere. Intrusion of the resulting lamprophyric melts into the previously thickened south Karakorum crust caused partial melting of calc-alkaline igneous protoliths and generation of the Baltoro granitoids. Late-stage syenitic magmas were produced by low-degree partial melting during upwelling and adiabatic decompression of depleted mantle along the Shigar strike-slip fault.

Crystal Size Distribution of Periclase in Contact Metamorphic Dolomite Marbles from the Southern Adamello Massif, Italy

Muller, T., Baumgartner, L. P., Foster, C. T., Bowman, J. R. — 2009-04-09

Crystal size distributions (CSD) of periclase in contact metamorphic dolomite marbles are presented for two profiles near the Cima Uzza summit in the southern Adamello Massif (Italy). The database was combined with geochemical and petrological information to deduce the controls on the periclase-forming reaction. The contact metamorphic dolomite marbles are exposed at the contact of mafic intrusive rocks and are partially surrounded by them. Brucite is retrograde and pseudomorphs spherical periclase crystals. Prograde periclase growth is the consequence of limited infiltration of water-rich fluid at T near 605°C. Stable isotope data show depletion in ¹³C and ¹⁸O over a narrow region (~40 cm) near the magmatic contact, whereas the periclase-forming reaction front extends up to 4 m from the contact. CSD analyses along the two profiles show that the median grain size of the periclase crystals does not change, but that there is a progressively greater distribution of grain sizes, including a greater proportion of larger grains, with increasing distance from the contact. A qualitative model, based on the textural and geochemical data, attributes these variations in grain size to changing reaction affinities along a kinetically dispersed infiltration front. This study highlights the need to invoke disequilibrium processes for metamorphic mineral growth and expands the use of CSDs to systems of mineral formation driven by fluid infiltration.

Melting History and Magmatic Evolution of Basalts and Picrites from the Accreted Wrangellia Oceanic Plateau, Vancouver Island, Canada

Greene, A. R., Scoates, J. S., Weis, D., Nixon, G. T., Kieffer, B. — 2009-04-09

The accreted Wrangellia oceanic plateau in the Pacific Northwest of North America provides important insights into the volcanic architecture of major oceanic plateaus, as well as the nature of their mantle source, conditions of melting and subsequent magmatic evolution. The ~20 000 km² Karmutsen Formation flood basalts (Vancouver Island) were emplaced at c. 225–230 Ma onto Middle Triassic marine sediments and Late Devonian to Early Permian island-arc volcanic and sedimentary sequences, and are overlain by Late Triassic platformal carbonates. The basalts form an emergent sequence consisting of a basal sediment–sill complex (600–900 m thick), pillowed and massive submarine flows (>2·5 km), pillow breccia and hyaloclastite (<1·5 km), and massive subaerial flows (<2·5 km). Although the Karmutsen Formation is predominantly composed of tholeiitic basalt, the submarine part of the stratigraphy on northern Vancouver Island also contains picritic basalts. These high-MgO (9–20 wt %) lavas are depleted in light rare earth elements (LREE; La/Yb_CN = 0·5 ± 0·2), whereas the tholeiitic lavas (6–8 wt % MgO) are LREE-enriched (La/Yb_CN = 2·2 ± 0·3). Both lava groups have overlapping initial _Hf(+10·3 ± 2·1) and _Nd(+7·7 ± 1·3), indicating a common, plume-type Pacific mantle source with geochemical characteristics similar to the source of basalts from the Ontong Java and Caribbean plateaus. Major-element modeling results indicate that the picrites formed by extensive melting (23–27%) of anomalously hot mantle (~1500°C), which is consistent with a mantle plume initiation model for formation of the Karmutsen flood basalts on Vancouver Island. Trace element constraints indicate that the picrites require melting of a depleted spinel lherzolite source, whereas the tholeiitic basalts involved melting of garnet and spinel lherzolite. The tholeiitic basalts underwent significant fractional crystallization (>50%) and the fractionated residues may be represented by high-velocity rocks beneath Vancouver Island identified from seismic reflection studies.

Temporal Variations in U-series Disequilibria in an Active Caldera, Rabaul, Papua New Guinea

2009-04-09

Intra-caldera eruptions have been speculated to sample the last batches of magma remaining from earlier caldera-forming eruptions. Rabaul Caldera, New Britain, Papua New Guinea has erupted several times since the last caldera-forming eruption in ad 640, with the most recent intra-caldera eruptions in 1878, 1937–1941 and 1994–present from the Tavurvur and Vulcan vents. U-series isotopes, in conjunction with ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, were analyzed on 16 samples collected from 1994 to 2001 to monitor short-term changes in magma composition to model magmatic processes and to test whether there is evidence of recent fresh magma input. Inflections on MgO diagrams imply that fractional crystallization is an important process in long-term magma evolution, and the homogeneity in ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd shows that assimilation of isotopically distinct material has not occurred. A vertical array on a ²³⁰Th–²³⁸U isochron diagram requires open-system behavior and could support a model of differentiation of multiple magma batches over 26 kyr. However, the presence of (²²⁶Ra/²³⁰Th) excesses requires introduction of new magma within the past 8000 years and is permissible of a model in which the currently erupting magmas were emplaced at or since the last caldera-forming event. Other than the presence of mafic enclaves in the 1878 and 1937 eruptions, no evidence exists to suggest open-system magma injection. Systematic variation in U-series disequilibria between 1994 and 2001 is lacking, which may indicate that the system is broadly in steady state or that the processes acting to produce the limited compositional variation have time scales that are too short to be resolved by Ra isotopes (i.e. are less than a few hundred years).

Geochemical Architecture of the Lower- to Middle-crustal Section of a Paleo-island Arc (Kohistan Complex, Jijal-Kamila Area, Northern Pakistan): Implications for the Evolution of an Oceanic Subduction Zone

2009-04-09

The processes active in the deep crust above an oceanic subduction zone during its evolution have been constrained through a detailed geochemical study (major and trace elements and Sr, Nd and Pb isotopes) of representative samples through an ~30 km thick exhumed crustal section of the Cretaceous Kohistan oceanic island arc (Northern Pakistan). The use of both trace elements and radiogenic isotopes reveals two distinct geochemical suites (suites A and B) within the Jijal–Patan–Kiru–Kamila (JPKK) complex. Suite A is characterized by a progressive enrichment in ²⁰⁷Pb and a decrease in ¹⁴³Nd/¹⁴⁴Nd with increasing La_N/Sm_N. Suite B has higher ²⁰⁷Pb/²⁰⁴Pb and lower ¹⁴³Nd/¹⁴⁴Nd ratios with approximately constant La_N/Sm_N. By combining trace elements with different partitioning behaviour it is demonstrated that there is an increasing contribution of the subduction component in the magmas with time. It is also possible to distinguish a slab component imprint carried by aqueous fluids from one corresponding to sediment melts. Intrusive granites are abundant in the upper levels of the JPKK section. All were generated at the arc root level (Jijal crustal section) during dehydration-melting of hornblende-rich plutonic rocks. A three-stage geodynamic model is proposed for the evolution of the arc over a period of ~30 Myr. The first stage (~117–105 Ma) starts with the onset of subduction, which was followed by the building of the volcanic arc. The second stage (~105–99 Ma to ~96–91 Ma) corresponds to a major igneous event, which was characterized by abundant magma underplating and granulite-facies metamorphism at the arc base. Recycling of the residual–cumulative lower crust into the convective asthenospheric mantle was efficient during this stage, and was probably related to thermo-mechanical erosion of the base of the crust. The last stage (~95–85 Ma) corresponds to a period of low magmatic activity, which marked the end of the intra-oceanic subduction. This is related to the formation of a ‘cold blanket’ above the slab surface as a result of thermo-mechanical erosion of the cold walls of the subduction zone (i.e. the upper part of the slab and the base of the overriding plate), and corner flow dragging the cold material into the zone of melt generation. Ultimately, a voluminous magmatic pulse occurred around 85 Ma (forming the Chilas complex), before arc–continent collision.