Subduction Zone Magmas Research Articles

Oxidising agents in sub-arc mantle melts link slab devolatilisation and arc magmas

Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO4) and dissolved SO42− in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe3+/∑Fe in spinel record a S6+–Fe2+ redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δ34S (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.

Generation of alkaline magmas in subduction zones by partial melting of mélange diapirs—An experimental study

Generation of alkaline magmas in subduction zones by partial melting of mélange diapirs—An experimental study

The partitioning of sulfur between multicomponent aqueous fluids and felsic melts

Sulfur partitioning between melt and fluid phase largely controls the environmental impact of volcanic eruptions. Fluid/melt partitioning data also provide the physical basis for interpreting changes in volcanic gas compositions that are used in eruption forecasts. To better constrain some variables that control the behavior of sulfur in felsic systems, in particular the interaction between different volatiles, we studied the partitioning of sulfur between aqueous fluids and haplogranitic melts at 200 MPa and 750–850 °C as a function of oxygen fugacity (Ni–NiO or Re–ReO2 buffer), melt composition (Al/(Na + K) ratio), and fluid composition (NaCl and CO2 content). The data confirm a first-order influence of oxygen fugacity on the partitioning of sulfur. Under “reducing conditions” (Ni–NiO buffer), Dfluid/melt is nearly one order of magnitude larger (323 ± 14 for a metaluminous melt) than under “oxidizing conditions” (Re–ReO2 buffer; 74 ± 5 for a metaluminous melt). This effect is likely related to a major change in sulfur speciation in both melt and fluid. Raman spectra of the quenched fluids show the presence of H2S and HS− under reducing conditions and of SO42− and HSO4− under oxidizing conditions, while SO2 is undetectable. The latter observation suggests that already at the Re–ReO2 buffer, sulfur in the fluid is almost completely in the S6+ state and, therefore, more oxidized than expected according to current models. CO2 in the fluid (up to xCO2 = 0.3) has no effect on the fluid/melt partitioning of sulfur, neither under oxidizing nor under reducing conditions. However, the effect of NaCl depends on redox state. While at oxidizing conditions, Dfluid/melt is independent of xNaCl, the fluid/melt partition coefficient strongly decreases with NaCl content under reducing conditions, probably due to a change from H2S to NaSH as dominant sulfur species in the fluid. A decrease of Dfluid/melt with alkali content in the melt is observed over the entire compositional range under reducing conditions, while it is prominent only between the peraluminous and metaluminous composition in oxidizing experiments. Overall, the experimental results suggest that for typical oxidized, silicic to intermediate subduction zone magmas, the degassing of sulfur is not influenced by the presence of other volatiles, while under reducing conditions, strong interactions with chlorine are observed. If the sulfur oxidation state is preserved during an explosive eruption, a large fraction of the sulfur released from oxidized magmas may be in the S6+ state and may remain undetected by conventional methods that only measure SO2. Accordingly, the sulfur yield and the possible climatic impact of some eruptions may be severely underestimated.

An overview of anorthosite-bearing layered intrusions in the Archaean craton of southern West Greenland and the Superior Province of Canada: implications for Archaean tectonics and the origin of megacrystic plagioclase

Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (>An70) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth.

Petrogenesis of an Early Cretaceous lamprophyre dike from Kyoto Prefecture, Japan: Implications for the generation of high-Nb basalt magmas in subduction zones

We studied a 107Ma vogesite (a kind of lamprophyre with alkali-feldspar>plagioclase, and hornblende±clinopyroxene±biotite) dike in the Kinki district of the Tamba Belt, Kyoto Prefecture, SW Japan, using petrography, mineralogy, K–Ar ages, and geochemistry to evaluate its petrogenesis and tectonic implications. The dike has the very specific geochemical characteristics of a primitive high-Mg basalt, with 48–50wt.% SiO2 (anhydrous basis), high values of Mg# (67.3–72.4), and high Cr (~431ppm), Ni (~371ppm), and Co (~52ppm) contents. The vogesite is alkaline and ne-normative with high concentrations of large ion lithophile elements (LILEs: Sr=1270–2200ppm, Ba=3910–26,900ppm), light rare earth elements (LREEs) [(La/Yb)n=58–62), and high field strength elements (HFSEs: TiO2=1.5–1.8wt.%, Nb=24–33ppm, Zr=171–251ppm), and the vogesite can be classified as a high-Nb basalt (HNB). The vogesite was formed by the lowest degree of melting of metasomatized mantle in the garnet stability field, and it may also have been formed at higher melting pressures than other Kyoto lamprophyres. The low degree of melting is the primary reason for the high-Nb content of the vogesite, not mantle metasomatism, and a higher degree of melting would have changed the primary magma composition from a HNB to a Nb-enriched basalt (NEB). The vogesite magma was contaminated at an early stage of its development by melts derived from sediments drawn down a subduction zone, as indicated by some geochemical indices and the initial Nd isotope ratios. The vogesite exhibits positive correlations between εSr(107 Ma) values (5.4–50.9) and its high Ba and Sr concentrations, and it has a limited range of εNd(107 Ma) values (+0.97 to +2.4). The fact that the vogesite contains centimeter-sized xenoliths of chert, which are composed of polycrystalline quartz, calcite, barite, pyrite, and magnetite, indicates that the barium contamination took place during the ascent of the lamprophyric magma through the upper crust. The episode of magmatism at c. 107Ma extended regionally from the Kinki district, through the Chugoku district and North Kyushu in SW Japan, to Korea as a result of slab roll-back at the eastern margin of Asia.

Early Cretaceous Na-rich granitoids and their enclaves in the Tengchong Block, SW China: Magmatism in relation to subduction of the Bangong–Nujiang Tethys ocean

The Na-rich intermediate-to-felsic granitic rocks provide insights into the generation of magmas in subduction zones. This paper presents zircon LA-ICP-MS U–Pb ages as well as whole-rock geochemical, mineral chemical, and in situ zircon Hf isotopic data on Na-rich granitic rocks from the Tengchong Block, SW China. The granodiorites and associated mafic magmatic enclaves (MMEs) from the Menglian batholith yield zircon U–Pb ages of 116.1±0.8 to 117.8±0.6Ma and 117.7±0.7Ma, respectively. Both host granodiorites and enclaves show calc-alkaline and sodium-rich nature, enrichment in large-ion lithophile elements (LILEs), and variable depletion in zircon Hf isotopic compositions. Euhedral amphiboles in both granodiorites and associated enclaves are magnesian-hornblende with high Mg and Ca and contain euhedral plagioclase inclusions of labradorite to andesine (An36–57) composition. The granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources. The quartz monzodiorite–granodiorites and associated MMEs from the Xiaotang-Mangdong batholith yield zircon U–Pb ages of 120.3±1.3 to 122.6±0.8Ma and 120.7±1.5Ma. These rocks are also sodium-rich and show calc-alkaline trend with negative zircon Hf isotopic compositions (−5.55 to +0.58). The MMEs in the host intrusions are monzogabbro with variable and depleted zircon Hf isotopic compositions. The amphiboles in the both host intrusions and the enclaves show Al-rich ferro-tschermakite composition. We infer that the quartz monzodiorite–granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle-derived mafic magmas. The formation of the different types of Na-rich granitic rocks is correlated to the subduction of Bangong–Nujiang Tethyan ocean. A comparison with magmatism in the northern magmatic belt suggests that mantle-derived magmas played an important role in the genesis of Early Cretaceous intrusions from Tengchong to Lhasa Blocks, although crustal melting is the dominant contributor.

Crustal evolution in the Gyeongsang Arc, southeastern Korea: Geochronological, geochemical and Sr-Nd-Hf isotopic constraints from granitoid rocks

A loosely assembled patchwork of Cretaceous to Paleogene granitoid plutons comprise the platform of the Gyeongsang Arc in southeastern Korea and represent a high-flux pulse of magmatism in Northeast Asia. The present study uses new and published zircon U-Pb ages, whole-rock geochemical and Sr-Nd isotopic data, and zircon Hf isotopic data of the plutons to decipher the crustal evolution in the Gyeongsang Arc and compare it with adjacent accretionary terranes. The ion microprobe zircon U-Pb ages range from 91 to 27 Ma, revealing the selective occurrence of Paleogene plutons in the eastern coastal area, to the east of the NNE-trending Yangsan fault system. Paleoproterozoic to Paleogene xenocrystic cores are occasionally observed in the zircon grains. The plutons are composed of dominantly magnesian, high- to medium-K calc-alkaline granites-granodiorites exhibiting typical geochemical characteristics of subduction zone magma, and less abundant ferroan alkaline granites. Their trace element patterns require residual amphibole and feldspar, but not garnet in the source. The whole-rock Sr-Nd and zircon Hf isotopic compositions of the granitoids (initial 87Sr/86Sr = 0.7043 to 0.7069; eNd(t) = –4.8 to +2.9; eHf(t) = –5.0 to +19.7) are distinctly more primitive than those of inland granitoids distributed outside of the Gyeongsang backarc basin. An important role of relatively young, most likely Paleozoic juvenile crust in the formation of the Late Cretaceous granitoids is suggested by the time-eHf trend of high-eHf zircons that converges toward data points of the Late Permian Yeongdeok adakite composing the arc basement. The asthenospheric mantle input is highlighted by significantly high (>+17) eHf(t) values of some zircons from the early Eocene alkaline plutons. Subsequent reworking of the rejuvenated crust yielded granitoid plutons possessing slightly but recognizably higher eNd(t) and eHf(t) values than the older plutons. These isotopic features demonstrate that the Cretaceous-Paleogene calc-alkaline granitoids in the Gyeongsang Arc are not “juvenile” (sensu stricto) despite their mostly positive epsilon Nd and Hf values, but are basically a product of crustal reworking. The Sr-Nd-Hf isotopic compositions of Late Cretaceous granitoids in the Gyeongsang Arc are comparatively more primitive than those in adjacent accretionary terranes such as Southwest Japan and Fujian province in southeastern China, reflecting differences in the formation age of the basement on which the arc system was built.

Systematic variations in magmatic sulphide chemistry from mid-ocean ridges, back-arc basins and island arcs

Immiscible sulphide liquids preserved as magmatic sulphide globules are hosted in igneous rocks of highly variable composition formed during magmatic processes at different tectonic settings. Here we report on compositional in situ data of magmatic sulphides from mid-ocean ridge, back-arc and island arc magmatic systems. Iron-Ni-rich monosulphide solid solutions (mss) that mainly consist of pyrrhotite and pentlandite are the dominant phases in magmatic sulphide globules. In subduction-related magmas mss are most abundant in relatively evolved melts and characterised by low Ni and high Fe contents, as well as low Ni/Cu ratios (<1). In contrast, mss from mid-ocean ridges and back-arcs without subduction input occur in mafic rocks and have high Ni/Cu ratios (>1). Thus, mss chemistry varies systematically in lavas from the different tectonic settings in terms of their Fe-Ni distribution and Ni/Cu ratio. The observed mineralogical and chemical variations between mss from the different settings reflect early S saturation accompanied by olivine fractionation along mid-ocean ridges compared to later stage S saturation in arc systems associated with Fe-Ti oxide fractionation. These systematics are probably related to the relatively oxidised character of subduction zone magmas opposed to more reduced mid-ocean ridge melts. Large Ni/Cu variations in mss from the same locality and in individual samples suggest that S saturation is a multistage or continuous process leading to inclusions of magmatic sulphides that represent different fractionation stages in the ascending magma.

Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas

Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation.Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith (‘hybrid mantle wedge’) composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe (‘calc-alkaline’) andesite-dacite-rhyolite suites, key lithologies forming the continental crust.

Zircon U-Pb age and geochemical constraints on the origin and tectonic implication of Cadomian (Ediacaran-Early Cambrian) magmatism in SE Turkey

The Bitlis-Pütürge Massifs and Derik volcanics that crop out in the Southeast Anatolian Belt are parts of the Cadomian domain in Anatolia where relicts of the oldest continental crust of Turkey are exposed. The Bitlis-Pütürge Massifs contain a Neoproterozoic basement, with overlying Phanerozoic rocks that were imbricated, metamorphosed and thrust over the edge of Arabia during the Alpine orogeny. The basement consists mainly of granitic to tonalitic augen gneisses and metagranites, associated with schists, amphibolites and paragneisses. Based on whole-rock geochemical data, the augen gneisses are interpreted to have protoliths crystallized from subduction zone magmas. This study conducted the first zircon dating on two augen gneisses that gave 206Pb/238U dates of 551±6 and 544±4Ma, interpreted as the formation ages of the Pütürge Massif, broadly coeval to those of the Bitlis metagranites and the Derik volcanics that occurred from ca. 581 to 529Ma (the Ediacaran-early Cambrian). The ɛHf(t) values (+1.2 to −5.3) of the dated zircons, with crustal model ages (TDMC) from 1.4 to 1.8Ga, indicate that formation of the Pütürge Massif involves an older, most likely the Mesoproterozoic, continental crust component. Similar to the Bitlis-Pütürge gneisses, coeval basement rocks are widespread in the Tauride-Anatolide platform (e.g., the Menderes Massif). All these dispersed Cadomian basement rocks are interpreted as fragments of the Ediacaran-Early Cambrian continental arcs bordering the active margin of northern Gondwana.

Aqueous fluids and sedimentary melts as agents for mantle wedge metasomatism, as inferred from peridotite xenoliths at Pinatubo and Iraya volcanoes, Luzon arc, Philippines

Mantle xenoliths entrained in subduction-zone magmas often record metasomatic signature of the mantle wedge. Such xenoliths occur in magmas from Iraya and Pinatubo volcanoes, located at the volcanic front of the Luzon arc in the Philippines. In this study, we present the major element compositions of the main minerals, trace element abundances in pyroxenes and amphiboles, and Nd–Sr isotopic compositions of amphiboles in the peridotite xenoliths from Pinatubo volcano. The data indicate enrichment in fluid-mobile elements, such as Rb, Ba, U, Pb, and Sr, and Nd–Sr isotopic ratios relative to those of mantle. The results are considered in terms of mixing of asthenospheric mantle and subducting oceanic crustal components. The enrichments observed in the Pinatubo mantle xenoliths are much less pronounced than those reported for the Iraya mantle xenoliths. This disparity suggests differences in the metasomatic agents contributing to the two suites; i.e., aqueous fluids infiltrated the mantle wedge beneath the Pinatubo volcano, whereas aqueous fluids and sediment-derived melts infiltrated the mantle wedge beneath the Iraya volcano.

Molybdenum isotope systematics in subduction zones

Abstract This study presents Mo isotope data for arc lavas from different subduction zones that range between δ 98 / 95 Mo = − 0.72 and + 0.07 ‰ . Heaviest isotope values are observed for the most slab fluid dominated samples. Isotopically lighter signatures are related to increasing relevance of terrigenous sediment subduction and sediment melt components. Our observation complements previous conclusions that an isotopically heavy Mo fluid flux likely mirrors selective incorporation of isotopically light Mo in secondary minerals within the subducting slab. Analogue to this interpretation, low δ 98 / 95 Mo flux that coincides with terrigenous sediment subduction and sediment melting cannot be simply related to a recycled input signature. Instead, breakdown of the controlling secondary minerals during sediment melting may release the light component and lead to decreasing δ 98 / 95 Mo influx into subarc mantle sources. The natural range between slab dehydration and hydrous sediment melting may thus cause a large spread of δ 98 / 95 Mo in global subduction zone magmas.

No effect of H2O degassing on the oxidation state of magmatic liquids

Abstract The underlying cause for why subduction-zone magmas are systematically more oxidized than those formed at mid-ocean spreading ridges is a topic of vigorous debate. It is either a primary feature inherited from the subduction of oxidized oceanic crust into the mantle or a secondary feature that develops because of H 2 O degassing and/or magma differentiation. Low total iron contents and high melt H 2 O contents render rhyolites sensitive to any effect of H 2 O degassing on ferric–ferrous ratios. Here, pre-eruptive magmatic Fe 2+ concentrations, measured using Fe–Ti oxides that co-crystallized with silicate phenocrysts under hydrous conditions, are compared with Fe 2+ post-eruptive concentrations in ten crystal-poor, fully-degassed obsidian samples; five are microlite free. No effect of H 2 O degassing on the ferric–ferrous ratio is found. In addition, Fe–Ti oxide data from this study and the literature show that arc magmas are systematically more oxidized than both basalts and hydrous silicic melts from Iceland and Yellowstone prior to extensive degassing. Nor is there any evidence that differentiation (i.e., crystal fractionation, crustal assimilation) is the cause of the higher redox state of arc magmas relative to those of Iceland/Yellowstone rhyolites. Instead, the evidence points to subduction of oxidized crust and the release of an H 2 O-rich fluid and/or melt with a high oxygen fugacity ( f O 2 ), which plays a role during H 2 O-flux melting of the mantle in creating basalts that are relatively oxidized.

Paleoproterozoic continental crust generation events at 2.15 and 2.08 Ga in the basement of the southern Brasília Orogen, SE Brazil

New zircon U-Pb LA-ICP-MS data combined with Nd-Hf isotopes and whole-rock geochemistry provide constraints on timing and tectonic setting of Paleoproterozoic continental crust generation events in the basement of the Neoproterozoic southern Brasília Orogen, SE Brazil. The Paleoproterozoic basement, designated as the Pouso Alegre Complex, comprises mainly metatexitic tonalitic to granodioritic orthogneisses with lenses and boudins of metamafic rocks. Minor porphyroclastic granite bodies also occur. Two well-defined crystallization ages groups were recognized at 2.14–2.15Ga (6 samples; weighted average of 2146.7±6.6Ma) and 2.07–2.08Ga (5 samples; weighted average of 2078.7±6.7Ma). Inheritance of older zircon grains is almost absent. Whole-rock Nd data show juvenile signatures with TDM ages between 2.16 and 2.37Ga associated with positive ɛNd(t) values from +0.16 to +2.85. Zircon Hf LA-ICP-MS analysis yields positive ɛHf(t) values from +1.9 to +8.7. Whole-rock geochemical analyses show a wide range of SiO2 contents from 52 to 76wt% with felsic types predominating over mafic ones. The analyzed samples show high concentrations of Rb, Ba, Th, U and low Nb/La ratios, which are characteristics of subduction zone magmas and most of the samples show continental arc affinities. The geochemical signatures associated with the predominance of felsic rocks suggest a continental arc margin or an evolved accreted oceanic arc as the favored tectonic setting for the Pouso Alegre Complex. The Pouso Alegre Complex is interpreted as the southernmost and youngest recognized part of an arc complex emplaced at the southern edge of the São Francisco paleo-continent during the Paleoproterozoic. This part of the arc complex was deeply reworked by the collisional events related to the Neoproterozoic southern Brasília Orogen and its cratonic counterpart is the Mineiro Belt at the southern São Francisco craton. The Pouso Alegre Complex and the juvenile suites of the Mineiro Belt represent a major Paleoproterozoic continental crust generation event between 2.35 and 2.08Ga. This time period has not been recognized for extensive continental crust generation and preservation globally. However, it appears to be a major period of continental crust generation and preservation at the southern edge of the São Francisco paleo-continent.

Probing the Earth’s Deep Interior Through Geochemistry

Basaltic volcanism provides a window into the Earth’s mantle. Seeing through this window requires seeing through the processes of magma genesis and evolution that distort the view. Radiogenic isotope ratios, and to a lesser extent incompatible elements and stable isotope ratios, allow us to see through these distortions and infer the nature and evolution of various mantle magma sources. Geochemical studies of mid-ocean ridge basalts (MORB) reveal that the upper mantle was long ago depleted in incompatible elements through previous episodes of melt extraction, likely associated with crust formation. Oceanic island basalts (OIB) are the products of mantle plumes and hence provide us with a view of the deepest parts of the Earth’s mantle. These plumes can be divided into several genera, EMI, EM II, and HIMU, most of which contain several species. Little or no mixing is observed between these genera, but all mix with a prevalent mantle component, PREMA, that contributes to many if not all plumes. Although the mantle comprising plumes also show evidence of previous melt extraction events, the more prominent chemical signature is of material anciently recycled from the Earth’s surface through subduction of oceanic crust and sediment, subduction erosion, and foundering of lower continental crust. This is most unambiguously manifest in variations in stable isotope ratios that could only have been produced at or near the surface of the Earth. In addition to a recycled component, many plumes also contain a quite primitive component, manifested primarily in their noble gas isotope ratios. The primitive component seems most closely associated with PREMA. In contrast to hot spot and mid-ocean ridge volcanism, subduction-related volcanism allows an assessment of the flux into the Earth’s mantle. It was apparent from early Pb isotope data that sediment was subducted along with oceanic crust, an observation later dramatically confirmed by the discovery of cosmogenic 10Be in subduction zone magmas. Because of this sediment, the flux into the mantle at subduction zones is more incompatible-element enriched than MORB. Although some fraction of this material is extracted into subduction zone magmas, a variety of approaches reveal that the flux into the deep mantle remains incompatible-element enriched. Other studies, particularly geophysical ones, indicate that this subduction flux is substantially augmented by subduction erosion of the overriding plate. Over Earth’s history these processes together with continental crustal founding may have delivered a volume of material equal to the present volume of the continents. The discovery that all modern terrestrial rocks have an excess of 142Nd, which is the decay product of the extinct radionuclide 146Sm, compared to chondrites, reopens the question of the Earth’s composition. One important implication is that the concentrations of heat producing elements, U, Th, and K, may be 25 % or more lower than previously thought. Another implication is that the depleted source of MORB may constitute a very large fraction of the mantle, perhaps nearly all of it. On the other hand, the heat flow from the core may be substantially greater than previously thought. This implies that the lowermost mantle and the core-mantle boundary layer may play a dominant role in driving mantle convention, and ultimately all geologic processes. Geophysical evidence is revealing that the lowermost mantle is indeed a dynamic and interesting region. The challenge for mantle geochemists is to integrate their observations with those of geophysicists and progress is slowly being made in this respect.

Geochemistry and fluid characteristics of the Dalli porphyry Cu–Au deposit, Central Iran

The Miocene Dalli porphyry Cu–Au deposit in the central part of Urumieh–Dokhtar magmatic arc is the first reported Au-rich porphyry Cu deposit in the Zagros orogenic belt. The Cu–Au mineralization is mainly hosted in diorite and quartz diorite intrusions, presenting as numerous veinlets in the altered wall rocks, with potassic, phyllic, and propylitic alteration developed. Based on the mineral assemblages and crosscutting relations of veinlets, hydrothermal mineralization–alteration occurred in at least three stages, characterized by veinlets of (1) Qtz+Kfs+Mag±Ccp, (2) Qtz+Py+Ccp±Bn±Cv±Cc and, (3) Qtz+Chl+Bt. The ore-bearing intrusions exhibit typical geochemical characteristics of subduction zone magmas, including LREE fractionated pattern, strong enrichment in LILE (Cs, Rb, Ba, Pb, and U), and depletion of HFSE, with marked negative Ti and Nb anomalies. The adakite-like ore-hosting porphyry intrusions are characterized by a systematic gradual decreasing and increasing of Y and Eu/Eu∗ with increasing SiO2 content, respectively. Moreover, they exhibit a significant increasing trend of Sr/Y with decreasing of Y, which indicates progressive hornblende fractionation and suppression of plagioclase fractionation during the evolution toward high water content of parental magma. A relatively flat HREE pattern with low Dyn/Ybn and Nb/Ta values may represent that amphibole played a more important role than garnet in the generation of the adakitic melts in the thickened lower crust. Based on the phase assemblages confirmed by detailed laser Raman spectroscopy analyses and proportion of solid, liquid, and gaseous components, five types of fluid inclusions were recognized, which are categorized as; (1) liquid-rich two phase (liquidH2O+vaporH2O) (IIA), (2) vapor-rich two phase (vaporH2O/CO2+liquidH2O) (IIB), (3) high saline simple fluids (IIIA; liquidH2O+vaporH2O+Hl), (4) high saline opaque mineral-bearing fluids (IIIB; liquidH2O+vaporH2O+Hl+Hem+Ccp+Py) and (5) multi-phase type (IIIAB; liquidH2O+vaporH2O+Hl+Anh+Hem+Mag+Ccp). In early stage veins, the homogenization temperature of multiphase inclusions as high as 620°C, with corresponding salinities of up to 75wt.% NaCl equivalent represent the initial ore-forming fluids. From early to late stage veins, the gradual decrease of homogenization temperature of saline inclusions (IIIAB, IIIA, and IIIB) from 620 to 340°C (corresponding salinities of 75–35wt.% NaCl equivalent) may reflect fluid boiling and mixing of the early magmatic fluids with circulating groundwater. The common association of hematite and anhydrite daughter phases with the most primitive inclusions (IIIAB) in early-stage veins and the lack of CO2-bearing inclusions in the middle to late stage veins reveal CO2 content and oxygen fugacity of the fluids was significantly decreased from early to main stage of sulfide mineralization (second generation of veins). It seems that magnetite crystallization and CO2-escape have a decisive role in oversaturation of S2− and subsequent rapid and large-scale precipitation of sulfides in second generation of veins at the Dalli deposit.

Temporal evolution of mantle wedge oxygen fugacity during subduction initiation

Arc basalts have a higher proportion of Fe in an oxidized state (Fe^(3+)) relative to Fe^(2+) compared to mid-oceanic ridge basalts (MORBs), likely because slab-derived fluids oxidize the mantle wedge where subduction zone magmas originate. Yet, the time scales over which oxygen fugacity of the mantle wedge changes during subduction initiation and margin evolution are unknown. Fe speciation ratios show that magmas produced during the early stages of subduction in the Mariana arc record oxygen fugacities ∼2× more oxidized than MORB. Mantle wedge oxygen fugacity rises by ∼1.3 orders of magnitude as slab fluids become more involved in melt generation processes, reaching conditions essentially equivalent to the modern arc in just 2–4 m.y. These results constrain existing models for the geochemical evolution of the mantle wedge and suggest that oxidation commences upon subduction initiation and matures rapidly in the portions of the mantle wedge that produce melts. This further implies that sulfide or other reduced phases are not present in the mantle wedge in high enough abundance to prevent oxidation of the magmas that form upon subduction initiation. The arc mantle source is oxidized for the majority of a subduction zone’s lifetime, influencing the mobility of multivalent elements during recycling, the degassing of oxidized volcanic volatiles, and the mechanisms for generating continental crust from the immediate onset of subduction.

Quantification of the CO2 budget and H2O–CO2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H2O pressure

Reliable evaluation of CO2 contents in parental arc magmas, which can be preserved in melt inclusions in phenocrysts, is required to verify the proposed efficiency of CO2 recycling at convergent margins. Quantification of bulk CO2 concentration in melt inclusions requires their complete homogenization. Using samples from lavas from the Bulochka vent of Klyuchevskoy Volcano (Kamchatka), we applied a novel experimental approach to homogenize and re-equilibrate naturally dehydrated (<1 wt.% H2O) melt inclusions from high-Fo (85–91 mol.%) olivine. The experiments were performed at temperatures of 1150–1400 °C, pressures of up to 500 MPa, under dry to H2O-saturated conditions and with oxygen fugacity ranging from CCO to QFM+3.3. No homogenization was achieved at dry conditions. Complete dissolution of fluid bubbles (homogenization) in the melt inclusions was achieved at H2O pressures of 500 MPa and temperature of 1150 °C, when water content in the melt inclusions reached 4–5 wt.% H2O. The CO2 content in the homogenized inclusions is 3800±140 ppm and CO2/Nb = 3000 ± 420, which are the highest values reported so far for the typical middle-K primitive arc melts and fall within the range of values inferred from the magmatic flux and volcanic gas data for primary arc magma compositions. About 83% of the CO2 in Klyuchevskoy magmas is likely to be derived from the subducting slab and can be attributed to flux melting with a fluid having a CO2/H2O ratio of ∼0.06. The H2O and CO2 contents in the melt inclusions after hydrous experiments were found to correlate positively with each other and negatively with the volume of fluid bubble, reflecting increasing internal pressure in melt inclusions with increasing melt hydration. Therefore, similar trends observed in some natural sets of melt inclusions can be attributed to a partial dehydration of melts after entrapment, operating simultaneously with or following post-entrapment crystallization. Our study implies that the process of post-entrapment dehydration can be completely reversed under high pressure experimental conditions. If temperature, redox conditions and pressure of melt inclusion entrapment can be independently estimated, then our novel experimental approach (homogenization at high H2O pressure) can be used to reconstruct the initial CO2 content and also the entire composition of melt inclusions in olivine, including their initial H2O content, from any type of volcanic rock. With this approach volatiles in ancient lavas can also be determined, expanding our knowledge of volatile recycling further back in Earth history.

Volatile budget of Tenerife phonolites inferred from textural zonation of S-rich haüyne

Research Article| May 01, 2015 Volatile budget of Tenerife phonolites inferred from textural zonation of S-rich haüyne Lauren B. Cooper; Lauren B. Cooper 1Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Olivier Bachmann; Olivier Bachmann 1Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Christian Huber Christian Huber 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Lauren B. Cooper 1Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland Olivier Bachmann 1Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland Christian Huber 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA Publisher: Geological Society of America Received: 05 Dec 2014 Revision Received: 12 Feb 2015 Accepted: 18 Feb 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (5): 423–426. https://doi.org/10.1130/G36505.1 Article history Received: 05 Dec 2014 Revision Received: 12 Feb 2015 Accepted: 18 Feb 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Lauren B. Cooper, Olivier Bachmann, Christian Huber; Volatile budget of Tenerife phonolites inferred from textural zonation of S-rich haüyne. Geology 2015;; 43 (5): 423–426. doi: https://doi.org/10.1130/G36505.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Intermediate to silicic volcanic eruptions often emit more S than predicted by petrological models; this is known as the excess S problem. While most common minerals in these magmas are poor in volatile elements, the occurrence of large phenocrysts of S-rich haüyne (with as much as ∼13 wt% SO3) in phonolites holds much promise for better constraining volcanic volatile budgets in differentiated alkaline magmatic systems. We examined textural zonation patterns in haüyne separates from Tenerife (Spain), using mineral oil to enhance grain transparency. Included phases were characterized by energy dispersive spectroscopy, X-ray mapping, and Raman spectroscopy. Slow growth of phenocrystic haüyne, inferred from zones with few inclusions, probably represents cooling-induced crystallization from S-rich melt during storage in the upper crust. By contrast, rapid growth of haüyne, generating wispy zones containing Fe-rich haüyne laths and zones rich in melt inclusions, fluid inclusions, and Fe sulfide inclusions, is associated with magma recharge and/or upward percolation of a low-density fluid phase (i.e., gas sparging). Both processes can bring new pulses of S from deep within the magmatic system into the shallow reservoir. Zones containing thousands of fluid inclusions provide direct physical evidence that the melt was fluid saturated during periods of rapid haüyne growth. The transfer of S-rich fluid should occur in all volatile-rich magmatic systems, including dacitic-rhyolitic arc systems with large S excesses, but is difficult to document in subduction zone magmas devoid of a large S-rich mineral phase like haüyne. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Continental collision, orogenesis and arc magmatism of the Miocene Maramuni arc, Papua New Guinea

The Maramuni arc represents the only continuous record of the tectonic evolution of Papua New Guinea during the Miocene, and hence provides an opportunity to gain insight into subduction dynamics, orogenesis and crustal processes that operated throughout this dynamic period. We present an integrated U–Pb geochronology, Hf isotope and geochemical investigation of the Maramuni arc utilizing a suite of intrusive rocks from the Kainantu region of the eastern Papuan Highlands that span the Late Miocene from ca. 12Ma to 6Ma. The magmatic rocks formed from ca. 12–9Ma have compositional affinities of subduction-zone magmas, but record increasing incompatible trace element contents and decreasing εHf with time, which we interpret to reflect a progressive increase in the crustal component of the magmas. Porphyry suites emplaced at 7.5–6Ma are distinct from the older magmatic rocks by their marked HREE-depletion, which reflects a dramatic shift in arc-mantle dynamics. Based on these results we propose a revised geodynamic model for the tectonic evolution of Papua New Guinea involving the arrival of the Australian continent at a north-dipping Pocklington trough from ca. 12Ma. Continent collision then led to growth of the New Guinea Orogen from 12Ma aided by the underthrusting of the leading continental margin, which contributed crustal material to magma-genesis at ca. 9Ma. From ca. 7Ma slab break-off and lithospheric delamination are reflected in a second phase of orogenesis that produced the HREE-depleted geochemical signatures of the contemporaneous magmatic rocks.