Log fO2 Research Articles

Temporal Evolution of Proto-Izu–Bonin–Mariana Arc Volcanism over 10 Myr: Constraints from Statistical Analysis of Melt Inclusion Compositions

AbstractInternational Ocean Discovery Program (IODP) Expedition 351 ‘Izu–Bonin–Mariana (IBM) Arc Origins’ drilled Site U1438, situated in the northwestern region of the Philippine Sea. Here volcaniclastic sediments and the igneous basement of the proto-IBM volcanic arc were recovered. To gain a better understanding of the magmatic processes and evolution of the proto-IBM arc, we studied melt inclusions hosted in fresh igneous minerals and sampled from 30–40 Myr old deposits, reflecting the maturation of arc volcanism following subduction initiation at 52 Ma. We performed a novel statistical analysis on the major element composition of 237 representative melt inclusions selected from a previously published dataset, covering the full age range between 30 and 40 Ma. In addition, we analysed volatiles (H2O, S, F and Cl) and P2O5 by secondary ion mass spectrometry for a subset of 47 melt inclusions selected from the dataset. Based on statistical analysis of the major element composition of melt inclusions and by considering their trace and volatile element compositions, we distinguished five main clusters of melt inclusions, which can be further separated into a total of eight subclusters. Among the eight subclusters, we identified three major magma types: (1) enriched medium-K magmas, which form a tholeiitic trend (30–38 Ma); (2) enriched medium-K magmas, which form a calc-alkaline trend (30–39 Ma); (3) depleted low-K magmas, which form a calc-alkaline trend (35–40 Ma). We demonstrate the following: (1) the eruption of depleted low-K calc-alkaline magmas occurred prior to 40 Ma and ceased sharply at 35 Ma; (2) the eruption of depleted low-K calc-alkaline magmas, enriched medium-K calc-alkaline magmas and enriched medium-K tholeiitic magmas overlapped between 35 and 38–39 Ma; (3) the eruption of enriched medium-K tholeiitic and enriched medium-K calc-alkaline magmas became predominant thereafter at the proto-IBM arc. Identification of three major magma types is distinct from the previous work, in which enriched medium-K calc-alkaline magmas and depleted low-K calc-alkaline magmas were not identified. This indicates the usefulness of our statistical analysis as a powerful tool to partition a mixture of multivariable geochemical datasets, such as the composition of melt inclusions in this case. Our data suggest that a depleted mantle source had been replaced by an enriched mantle source owing to convection beneath the proto-IBM arc from >40 to 35 Ma. Finally, thermodynamic modelling indicates that the overall geochemical variation of melt inclusions assigned to each cluster can be broadly reproduced either by crystallization differentiation assuming P = 50 MPa (∼2 km deep) and ∼2 wt% H2O (almost saturated H2O content at 50 MPa) or P = 300 MPa (∼15 km deep) and ∼6 wt% H2O (almost saturated H2O content at 300 MPa). Assuming oxygen fugacity (fO2) of log fO2 equal to +1 relative to the nickel–nickel oxide (NNO) buffer best reproduces the overall geochemical variation of melt inclusions, but assuming more oxidizing conditions (log fO2 = +1 to +2 NNO) probably reproduces the geochemical variation of enriched medium-K and calc-alkaline melt inclusions (30–39 Ma).

Geochemical factors revealing the differences between the Xitian and Dengfuxian composite plutons, middle Qin-Hang Belt: Implications to the W–Sn mineralization

The Xitian and Dengfuxian ore fields, located in the middle Qin-Hang Belt, are known for hosting large-scale W–Sn polymetallic deposits. The Xitian ore field hosts a series of small to large scale Sn–W deposits, including skarn-, quartz vein- and structurally altered rock-type, whereas the Dengfuxian ore field mainly contains a medium-scale quartz vein-type W deposit. These granitic rocks are composed of ore-barren Triassic and ore-related Jurassic granites. In this study, LA–ICP–MS zircon U–Pb dating of three representative samples from Dengfuxian Triassic granites (DTG), Xitian Jurassic granites (XJG) and Dengfuxian Jurassic granites (DJG) yields weighted mean 206Pb/238U ages of 231.2 ± 2.5 Ma, 152.5 ± 1.2 Ma and 155.9 ± 1.6 Ma, respectively. Chemical compositions of the Xitian Triassic granites (XTG) and DTG indicate that they belong to relatively lower fractionated granites, whereas the XJG and DJG are highly fractionated granites. The XTG and DTG have similar Lu–Hf isotopic compositions, which have peak εHf(t) values of ca. −5.9 and TDMC ages of ca. 1.57 Ga, and peak εHf(t) values of ca. −5.7 and TDMC ages of ca. 1.57 Ga, respectively. However, the XJG have higher peak εHf(t) values of ca. −5.4 and younger TDMC ages of ca. 1.52 Ga than those of the DJG with peak εHf(t) values of ca. −7.6 and TDMC ages of ca. 1.70 Ga, respectively. Furthermore, the XTG have εNd(t) values of −10.5 to −9.40 and Nd model ages (TDM2) of 1.86 to 1.76 Ga, which are similar to those of the DTG with εNd(t) values of −11.7 to −9.67 and Nd model ages (TDM2) of 1.95 to 1.79 Ga (Wu et al., 2016; Cai et al., 2015). The XJG have εNd(t) values of −11.0 to −9.1 and younger TDM2 ages of 1.69–1.84 Ga, which are fairly distinct to the reported Nd isotopes of the DJG (Cai, 2013). It is proposed that the XTG and DTG were originated from the similar magma chamber and formed from partial melting of the Proterozoic basement rocks of the South China Block (SCB) at low temperatures (ca. 690 °C) and oxygen fugacity (log(fO2) values of ca. −17), involved with a certain amount of mantle derived magma; Magmas of the XJG and DJG might be mainly originated from the partial melting of Proterozoic basement rocks of the SCB. In contrast, the formation of XJG and DJG are involved with a considerable and little mantle-derived magma, respectively. In addition, the magma temperatures and log(fO2) values of XJG are of ca. 856 °C and ca. −15, respectively, which are higher than those of the DJG (ca. 716 °C and ca. −17). That the Xitian and Dengfuxian Triassic granites did not induce W–Sn mineralization is likely attributed to their low fractionated signatures. The key factors leading to the Sn–W mineralization in the Xitian ore field and W mineralization in the Dengfuxian ore field might be magma source and temperature, since additional Sn could be provided by the mantle and/or released by melting of those Sn-bearing minerals at high temperatures (over than 780 °C).

Zircon U–Pb and Sm–Nd geochronology and geochemistry of the Sn–W deposits in the northern Guposhan ore field, Nanling Range, southern China

The Guposhan ore field, located in the central part of the Nanling Range and close to the giant Central South Peninsula tin ore belt, is well-known for its large-scale Sn (W) mineralization and hosts a series of middle to large scale Sn (W) deposits. In this study, medium to fine grained biotite granite, outcropped in the mining area, have zircon U–Pb ages of 162.8 ± 1.3 Ma (MSWD = 0.54) and 162.7 ± 1.2 Ma (MSWD = 0.66), respectively. These zircon ages are consistent to the Sm–Nd isotopic age of minerals (garnet, hornblende, clinopyroxene and orthopyroxene) from the skarn-type ores, which is 160.4 ± 4.8 Ma (MSWD = 1.3). These granites are characterized by high content of SiO2, K2O, Na2O and REEs, and high Ga/Al ratios, indicating that they belong to highly fractionated A-type granites with high-K calc-alkaline and aluminous-peraluminous signatures. The zircon Lu–Hf isotopes of these granites have εHf(t) values and TDMC ages of −2.66 to −1.71 and 1.32–1.38 Ga, respectively. Combined with the previous studies, it was proposed that the magma was formed by the mixing of crust-mantle components (ca. 50 and ca. 50%, respectively). In addition, magmas are characterized by low temperatures of ca. 710 °C and oxygen fugacity (log(fO2) values of −18.5 to −16.4. Formation of these granites were likely related to an intraplate extensional tectonic setting, triggered by roll-back of the subducted Paleo-Pacific plate beneath the South China Block (SCB). These granites are in favor of the Sn (W) mineralization, making the Guposhan ore district a large-scale Sn (W) mineralization ore field in the Nanling Range.

Origin and evolution of uraniferous pegmatite: A case study from the Xiaohuacha granite–pegmatite system and related country rocks in the Shangdan uranium mineralization district of North Qinling Orogenic Belt, China

ABSTRACT The Shangdan district in the North Qinling Orogenic Belt hosts the largest pegmatite-type uraniferous province in China. The Xiaohuacha U mineralization in this district occurs along the contact zone between biotite pegmatite and its host rock of biotite gneiss, adjacent to the composite Huichizi batholith, which is composed mainly of biotite monzogranite and syenogranite. Zircon U Pb dating of these granitoids reveals two-stage magmatism, namely, the early Silurian for biotite monzogranite (438.5 ± 4.4 Ma) and the early Devonian for syenogranite (417.8 ± 3.2 Ma). The biotite pegmatite shows an age of 415.8 ± 3.8 Ma for the mineralized marginal zone, and similar ages of 414.6 ± 4.3 Ma and 413.2 ± 3.5 Ma are obtained for the barren marginal and intermediate zones, respectively. The biotite monzogranite exhibits a restricted range of positive zircon eHf(t) values of +5.3 to +7.8, with TDM2 ages of 0.93–1.08 Ga, suggesting magma derivation from mainly pre-existing basaltic rocks in the orogenic root. Magmatic oxygen fugacity (fO2) was estimated by Ti-in-zircon thermometry and Ce incorporation within zircons. The biotite monzogranite was emplaced at a temperature of 736 ± 24 °C with log fO2 values of −22.28 to −14.19. In contrast, the syenogranite zircons have eHf(t) values of −1.4 to +2.4 and TDM2 ages of 1.25–1.49 Ga, indicating a derivation from the Mesoproterozoic crust with some mantle input. Pristine zircon domains from different zones of the biotite pegmatite display eHf(t) values of −2.8 to +1.8 and TDM2 ages of 1.29–1.58 Ga, similar to those of the syenogranite, indicating they share a common magma source. Combination of petrographical, geochronological, isotopic, and mineral chemical data supports a petrogenetic relationship between the high-T (842 ± 14 °C) syenogranite and biotite pegmatite (756 ± 21 °C and 709 ± 26 °C for the uncontaminated marginal and intermediate zones respectively) through magmatic differentiation from the same magma source during a period of tectonic extension. Processes of segregation and emplacement of the pegmatitic magma are consistent with open-system assimilation and fractional crystallization (AFC) leading to decreasing log fO2 values from the syenogranite (−11.23 to −7.02) to biotite pegmatite through a fractionation trajectory. However, log fO2 values increase from the uncontaminated pegmatite marginal (−15.88 to −10.9) to intermediate (−17.86 to −6.14) zones with closed-system pegmatite internal evolution. The interaction (hybridization) between magma and gneiss induced decrease of oxidation state and (K + Na)/Al, as well as modifications to zircon compositions in the mineralized pegmatite marginal zone. We propose a model with decreasing of fO2 and (K + Na)/Al during AFC being the main controls on the formation of the U deposit.

Zircon-hosted melt inclusion record of silicic magmatism in the Mesoproterozoic St. Francois Mountains terrane, Missouri: Origin of the Pea Ridge iron oxide-apatite-rare earth element deposit and implications for regional crustal pathways of mineralization

Voluminous silicic magmatism was coeval with iron ore mineralization in the St. Francois Mountains terrane in southeast Missouri, part of the broader Mesoproterozoic Granite-Rhyolite province along the eastern margin of Laurentia. Some of the iron deposits contain extraordinary endowments of critical elements, such as the Pea Ridge iron oxide-apatite (IOA) deposit, which has an average grade of ∼12 wt% rare earth oxides in breccia pipes that flank the ore body. To assess the role of silicic magmatism in the genesis of the Pea Ridge deposit, we present a high-spatial resolution study of zircon-hosted melt inclusions from rhyolitic ash-flow tuffs. Melt inclusion data are combined with textural, geochemical, and geochronological analyses of zircon hosts to elucidate the magmatic-hydrothermal evolution of the Pea Ridge system. Two contemporaneous silicic igneous centers in the St. Francois Mountains terrane, Bourbon and Eminence, were studied for comparison. Pea Ridge melt inclusions are trachydacitic to rhyolitic (∼63–79 wt% SiO2, ∼5.6–11.7 wt% Na2O + K2O) with very high Cl in the least-evolved and most alkaline melt inclusions (∼2000–5000 ppm Cl). Rare earth elements (REE) in melt inclusions have identical chondrite-normalized patterns to the mineralized breccia pipes, but with systematically lower absolute concentrations. Haplogranite ternary pressures range from ∼0.5 to 10 kbar, with an average of ∼2–3 kbar (7–12 km depth), and liquidus temperatures are ∼850–950 °C, with an average of ∼920 °C. Silicate and phosphate mineral inclusions have compositions that overlap minerals from the iron ore body and breccia pipes, recording a transition from igneous to hydrothermal zircon growth. Igneous iron oxide inclusions have compositions that indicate Pea Ridge magmas were reduced to moderately oxidized (log fO2 of −0.8 to −1.84 ΔNNO). Zircons from two Pea Ridge samples have 207Pb/206Pb concordia ages of 1456 ± 9 Ma and 1467 ± 13 Ma that overlap published ages for the breccia pipes and iron ore zones of the Pea Ridge deposit. A population of texturally and chemically disrupted zircons have discordant domains that correspond to high Fe, U, and REE concentrations, consistent with the unique geochemical attributes of the IOA-REE ore body. Inherited cores in Pea Ridge and Bourbon zircons have concordant 207Pb/206Pb dates of 1550–1618 Ma, providing direct evidence of cratonic basement beneath these centers. Oxygen isotope data for inherited and autocrystic igneous zircons span from mantle to crustal values (δ18Ozircon = 5.5–7.9‰). Our data are consistent with a model in which metasomatized mantle components were mixed with cratonic and accreted crustal material in a back-arc or rifted segment of a volcanic arc, with ore fluids derived from Cl-rich melts to transport Fe and REE in a long-lived (tens of Myr), pulsed, magmatic-hydrothermal system. Bourbon, which also possesses IOA mineralization, shares key petrologic similarities with the Pea Ridge system, whereas Eminence, which is not mineralized, has disparate geochemical and isotopic signatures that indicate it formed in a different crustal setting. The location of Pea Ridge and Bourbon along a cratonic margin may have been important in focusing silicic melts and mineralization in the upper crust, serving as a guide for future exploration efforts.

Geology and Geochronology of the Maozaishan Sn Deposit, Hunan Province: Constraints from Zircon U–Pb and Muscovite Ar–Ar Dating

The Maozaishan Sn deposit, located south of the Dayishan ore field in the Nanling Range, is a newly explored greisen-type Sn deposit. Two muscovite samples from tin-bearing ores yielded 40Ar/39Ar plateau ages of 154.7 ± 1.1 Ma (Mean standard weighted deviation (MSWD) = 0.48) and 152.6 ± 0.7 Ma (MSWD = 0.25), respectively. Zircon U–Pb dating result of fine-grained biotite monzogranite in the Maozaishan mining area shows that these zircon grains can be subdivided into two populations, with ages of 154.2 ± 2.0 Ma (MSWD = 0.51) and 159.6 ± 1.9 Ma (MSWD = 0.09), respectively, indicating that the monzogranite is formed by a multi-stage magmatic event. It is indicated that formation of the Maozaishan Sn deposit is closely related to the Middle Jurassic granitic magmatism. Based on the trace element compositions of zircon grains, the calculated magma temperatures and oxygen fugacity (log(fO2)) values range from 638 °C to 754 °C (mean = 704 °C) and from −18.9 to −15.8 (mean = −17.1), respectively. In addition, these intrusive rocks in the Dayishan ore field belong to highly fractionated granites and are characterized by low oxygen fugacity and crust–mantle origin, which are consistent to these tin-bearing granites in the Nanling Range and in favor of the Sn mineralization.

Oxygen Fugacity and Water Activity during Thermal Peak and Retrogression of Granulites around the Sarvapuram Area, Karimnagar Granulite Terrane, Andhra Pradesh, India

The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity (XH2O) conditions obtained with the winTWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log fO2 values ranging from −11.38 to −14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.

Solubility of carbon and nitrogen in a sulfur-bearing iron melt: Constraints for siderophile behavior at upper mantle conditions

Abstract Carbon solubility in a liquid iron alloy containing nitrogen and sulfur has been studied experimentally in a carbon-saturated Fe-C-N-S-B system at pressures of 5.5 and 7.8 GPa, temperatures of 1450 to 1800 °C, and oxygen fugacities from the IW buffer to log fO2 ΔIW-6 (ΔIW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of iron and wüstite). Carbon saturation of Fe-rich melts at 5.5 and 7.8 GPa maintains crystallization of flaky graphite and diamond. Diamond containing 2100–2600 ppm N and 130–150 ppm B crystallizes in equilibrium with BN within the diamond stability field at 7.8 GPa and 1600 to 1800 °C, while graphite forms at other conditions. The solubility of carbon in the C-saturated metal melt free from nitrogen and sulfur is 6.2 wt% C at 7.8 GPa and 1600 °C and decreases markedly with increasing nitrogen. A 1450–1600 °C graphite-saturated iron melt with 6.2–8.8 wt% N can dissolve: 3.6–3.9 and 1.4–2.5 wt% C at 5.5 and 7.8 GPa, respectively. However, the melt equilibrated with boron nitride and containing 1–1.7 wt% sulfur and 500–780 ppm boron dissolves twice less nitrogen while the solubility of carbon remains relatively high (3.8–5.2 wt%). According to our estimates, nitrogen partitions between diamond and the iron melt rich in volatiles at DNDm/Met=0.013−0.024. The pressure increase in the Fe-C-N system affects iron affinity of N and C: it increases in nitrogen but decreases in carbon. The reduction of C solubility in a Fe-rich melt containing nitrogen and sulfur may have had important consequences in the case of imperfect equilibration between the core and the mantle during their separation in the early Earth history. The reduction of C solubility allowed C supersaturation of the liquid iron alloy and crystallization of graphite and diamond. The carbon phases could float in the segregated core liquid and contribute to the carbon budget of the overlying silicate magma ocean. Therefore, the process led to the formation of graphite and diamond, which were the oldest carbon phases in silicate mantle.

Reduced fluids in porphyry copper-gold systems reflect the occurrence of the wall-rock thermogenic process: An example from the No.1 deposit in the Xiongcun district, Tibet, China

High fO2 conditions characterize the majority of global porphyry copper deposits and contain highly oxidized minerals, such as magnetite and anhydrite. In contrast, the No. 1 porphyry Cu–Au deposit in the Xiongcun district (Tibet, China) has abundant pyrrhotite, reduced fluids (CH4 ≫ CO2), and a relative lack of highly oxidized minerals, which are indicative of low fO2 conditions. Scanning electron microscopy, fluid inclusion, C–H–O–He–Ar isotopes, and whole-rock organic carbon contents and isotope analysis were used to constrain the evolution of ore-forming fluid, the origin of CH4 and metal deposition mechanisms for the No. 1 deposit. The He–Ar isotopic compositions (3He/4He = 0.11–0.96 Ra, 40Ar/36Ar = 418.7–2920.2) suggest that the ore-forming fluids predominantly derived from crust source with minor mantle input. The H–O isotopic analysis results (δ18OH2O = −1.8 to +5.2‰, δD = −106 to −89.9‰) indicate that the ore-forming fluids were derived from a magmatic source that mixed with some meteoric waters. The element compositions of zircons and fluid/melt inclusions from the mineralized Middle Jurassic quartz diorite porphyry reveal that the primary magma was characterized by high log fO2 (>NNO) conditions. The quartz diorite porphyry intruded into the carbon-bearing wall rocks produced abundant CH4 by thermal decomposition of organic matter (δ13CCH4 = −26.3 to −28.5‰), which changed the redox state of the porphyry copper system from oxidized to reduced condition. Ore elements were deposited via fluid boiling as a consequence of the rapid decrease in temperature and pressure.

An experimental calibration of a sulfur-in-apatite oxybarometer for mafic systems

The incorporation of sulfur (S) into the apatite structure and the partitioning of S between apatite and silicate melt (DSap/m) have been proposed to vary systematically as a function of prevailing redox conditions. In this study, we experimentally equilibrated apatite with mafic silicate melt at 1000 °C, 300 MPa and a range of oxygen fugacity (fO2; log fO2 [ΔFMQ] = −1, 0, +0.3, +1.2, and +3 where FMQ is the fayalite-magnetite-quartz mineral redox buffer) to explore the partitioning behavior of S, including different oxidation states of S, between apatite and silicate melt. The data reveal that DSap/m values increase systematically with increasing fO2, from 0.02± 0.01 at log fO2 [ΔFMQ] of −1 to 3.20 ± 0.19 at log fO2 [ΔFMQ] of +3. The bulk S content (∼0.37 and ∼0.28 wt.% S added) imparts a minor influence on DSap/m at reducing conditions.Micro X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge was used to measure, in situ, the oxidation states of S in experimentally crystallized apatite. The S-XANES analyses reveal that with increasing fO2, apatite progressively incorporates S6+ ≫ S2− > S4+ > S1+. The integrated S6+/ΣS peak area ratios and centroid energies (eV) were determined for apatite crystals in apatite from experiments at all fO2 conditions. The orientation effects occurring during S-XANES analyses of apatite were considered by merging spectra from multiple grains with random crystallographic orientation. The S-XANES data reveal that S6+/ΣS peak area ratios and the centroid energies increase systematically with fO2, demonstrating that the S6+/ΣS ratio in apatite can be used as an oxybarometer.The results demonstrate that both the S6+/ΣS and CeV calibration methods are highly sensitive in the redox range of ∼FMQ to ∼FMQ + 1.2 at the conditions and compositions evaluated in this study. As a result, the S-in-apatite oxybarometer is particularly applicable to mafic systems such as mid ocean ridge basalts (MORB), relatively reduced ocean island basalts (OIB), and back-arc basin basalt (BABB). Owing to the ubiquity of apatite in magmatic and magmatic-hydrothermal systems, measuring the concentration and oxidations state(s) of S-in-apatite has the potential to serve as a powerful sulfo- and oxy-barometer for a broad range of natural systems.

U, Th, and K partitioning between metal, silicate, and sulfide and implications for Mercury's structure, volatile content, and radioactive heat production

AbstractThe distribution of heat-producing elements (HPE) potassium (K), uranium (U), and thorium (Th) within planetary interiors has major implications for the thermal evolution of the terrestrial planets and for the inventory of volatile elements in the inner solar system. To investigate the abundances of HPE in Mercury’s interior, we conducted experiments at high pressure and temperature (up to 5 GPa and 1900 °C) and reduced conditions (IW-1.8 to IW-6.5) to determine U, Th, and K partitioning between metal, silicate, and sulfide (Dmet/sil and Dsulf/sil). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and that partitioning is enhanced with decreasing FeO and increasing O contents of the silicate and sulfide melts, respectively. Also, at low oxygen fugacity (log fO2 < IW-5), U and Th are more efficiently partitioned into liquid iron metal and sulfide than K. Dmet/sil for U, Th, and K increases with decreasing oxygen fugacity, while DUmet/sil and DKmet/sil increase when the metal is enriched and depleted in O or Si, respectively. We also used available data from the literature to constrain the concentrations of light elements (Si, S, O, and C) in Fe metal and sulfide. We calculated chemical compositions of Mercury’s core after core segregation, for a range of fO2 conditions during its differentiation. For example, if Mercury differentiated at IW-5.5, its core would contain 49 wt% Si, 0.02 wt% S, and negligible C. Also if core-mantle separation happened at a fO2 lower than IW-4, the bulk Mercury Fe/Si ratio is likely to be chondritic. We calculated concentrations of U, Th, and K in the Fe-rich core and possible sulfide layer of Mercury. Bulk Mercury K/U and K/Th were calculated taking all U, Th, and K reservoirs into account. Without any sulfide layer, or if Mercury’s core segregated at a higher fO2 than IW-4, bulk K/U and K/Th would be similar to those measured on the surface, confirming more elevated volatile K concentration than previously expected for Mercury. However, Mercury could fall on an overall volatile depletion trend where K/U increases with the heliocentric distance if core segregation occurred near IW-5.5 or more reduced conditions, and with a sulfide layer of at least 130 km thickness. At these conditions, the bulk Mercury K/Th ratio is close to Venus’s and Earth’s values. Since U and Th become more chalcophile with decreasing oxygen fugacity, to a higher extent than K, it is likely that at an fO2 close to, or lower than, IW-6 both K/U and K/Th become lower than values of the other terrestrial planets. Therefore, our results suggest that the elevated K/U and K/Th ratios of Mercury’s surface should not be exclusively interpreted as the result of a volatile enrichment in Mercury, but could also indicate a sequestration of more U and Th than K in a hidden iron sulfide reservoir, possibly a layer present between the mantle and core. Hence, Mercury could be more depleted in volatiles than Mars with a K concentration similar to or lower than the Earth’s and Venus’s, suggesting volatile depletion in the inner solar system. In addition, we show that the presence of a sulfide layer formed between IW-4 and IW-5.5 decreases the total radioactive heat production of Mercury by up to 30%.

Geochemistry of Magmatic and Xenocrystic Spinel in the No.30 Kimberlite Pipe (Liaoning Province, North China Craton): Constraints on Diamond Potential

There are two genetic types of spinel (magmatic spinel crystallizing directly from kimberlite magma and xenocrystic spinel derived from mantle xenoliths) in the No.30 kimberlite pipe (Liaoning Province, North China Craton). Their geochemistry is investigated to reveal processes of diamond capture and resorption during kimberlite magmatism to constrain the diamond potential. Magmatic spinels are mostly euhedral to subhedral, 20 to 60 µm in size, and have compositional zones: the cores are classified as chromite with high Cr and Mg contents, and the rims are classified as magnetite with low Cr and high ferric Fe. The compositional trends suggest that magmatic spinel and olivine phenocrysts are crystallized contemporaneously during the early stages of kimberlite crystallization. During this period, temperature (T) and oxygen fugacity (fO2) values calculated at an assumed pressure of 1 GPa are in the range of 994–1274 °C and 1.6–2.4 log fO2 units below the nickel-nickel oxide (NNO) buffer, respectively. The high values of fO2 suggest heavy diamond resorption during kimberlite magmatism. Estimated temperatures of xenocrystic spinel range from 828 to 1394 °C, and their distributions indicate that only a small proportion of xenocrystic spinels are derived from the diamond stabilization field, which suggests a low potential of diamond capture. The low diamond capture and heavy diamond resorption during kimberlite magmatism contributed to the low diamond grade of the No.30 kimberlite.

Geochronological and geochemical constraints on the formation of Chizhou Cu-Mo polymetallic deposits, middle and lower Yangtze metallogenic belt, eastern China

The Cu-Mo polymetallic deposits are widespread in the Chizhou region, the Middle and Lower Yangtze River metallogenic belt, eastern China, but the formation of Cu-Mo polymetallic deposits remains unclear. Geochronological, geochemical and Sr-Nd-Hf isotopic compositions of the Cu-Mo polymetallic deposits in the Chizhou region are systemically investigated to reveal the formation of Cu-Mo polymetallic deposits and petrogenesis of the granodiorite (porphyry). The geochronological dating results show the formation ages of Cu-Mo polymetallic deposits (molybdenite Re-Os age, 151–148 Ma) and the granodiorites (porphyries) (zircon U-Pb age, 150–146 Ma) in the Chizhou region are simultaneous (within error). The Chizhou granodiorites (porphyries) (SiO2 = 62.9–65.7 wt%) are high-K calc-alkaline peraluminous I-type granodiorites. They have high Mg# (mainly >40) values and low Na2O (<4.3 wt%) contents, negative whole-rock εNd(t) (−5.83 to −5.71) and zircon εHf(t) (−12.7 to −3.03) values, high apatite Cl contents (>0.2 wt%), and high log fO2 values (−28.3 to −6.22). The distinctly chemical and isotopic compositions indicate that the Chizhou granodiorites formed from metasomatized mantle-derived magmas mixing with Neoproterozoic crustal components. The results indicate that the formation of the Cu-Mo polymetallic deposits in the Chizhou region may depend on the slab-derived Cl-enriched fluids, an oxidizing environment and a mantle-crust mixed source.

Experimental exploration of volcanic rocks-atmosphere interaction under Venus surface conditions

This study presents an inventory of possible chemical reactions affecting, or having affected, the Venus surface. Fluid rock reactions are simulated using experiments under conditions close to the present surface. Slabs or powder of several natural and synthetic silicate material (crystalline fresh basalt, altered basalt, obsidian, pumice and basalt glass) were reacted at 475 °C in CO2-H2O-H2S-SO2-CO gas mixture. Most of the runs were carried out at roughly 90 bars with a duration of one week, some experiments having longer (one month) or shorter (one day) durations. The role of H2O content was explored through a wide range of water pressure: from dry gas for the current Venus conditions up to 590 bars (86%H2O) for early Venus (or other early terrestrial planets). The gas phase was sampled before the completion of the runs for chemical analysis of major gas components (CO, H2S, SO2) as well as trace elements possibly released by the rocks. The altered samples were examined by a suite of mineralogical and chemical techniques (scanning and transmission electron microscopy, X-ray diffraction and spectroscopy).In dry atmosphere, the redox potential of the gas was close to the Ni/NiO buffer (−21.3 to −27.3 log fO2), thus close to the current Venus conditions. The sample alteration is tenuous and limited to surface oxidation of glasses and coating of olivine by iron oxides, as well as the general deposition of (Ca,Na)SO4 at the sample surface. The oxidation of glass is reflected in the formation of magnesioferrite under the surface and is accompanied by the release of Ca, Mg and Na into the gas phase or mineralized as sulfate at the surface. In wet atmosphere, obsidian recrystallizes into a mixture of plagioclase and amphibole while basaltic glass produced non-expandable clays minerals: chlorite-type (2:1:1) at the surface and likely celadonite (2:1) below the surface. Olivine is preserved. Using obsidian (the most alterable material) as a proxy of aluminosilicates, we discuss the surface reactions operating under supercritical conditions, and we used a shrinking-core equation for modeling the long-term reactions.These parametric exploration offer new insights into processes having affected the surface of Venus and contribute to the discussion of open questions such as the fate of water or the lifetime of vitreous dust or fine grain material if present in the current or past Venusian environment. Longer duration experiments will provide more kinetic parameters that can be extrapolated to the geologic history of Venus.

Redox state of the Baogutu reduced porphyry Cu deposit in the Central Asian Orogenic belt

Reduced porphyry Cu deposits are scarce and their origins remain enigmatic. The Late Carboniferous Baogutu reduced porphyry Cu deposit in the western Junggar Terrane, NW China, provides an opportunity to address this issue. In this study, we conducted systematic analyses in terms of petrography, mineral chemistry, major elements, trace elements and Fe isotopes to investigate the redox state of its primary magma and further constrain the origin of Baogutu copper deposit. Based on the petrological observation and mineral chemistry, two magmatic stages have been recognized during the formation of ore-forming diorite. The early stage is characterized by the crystallization of magnetite and high Mg# [Mg/(Mg + Fe2+)] hornblende under oxidized condition (ΔNNO > 2.36, calculated by the equation given by Scaillet and Evans (1999). The late stage is characterized by the crystallization of ilmenite and low Mg# biotite under reduced condition (ΔNNO <−0.6). The large decrease of oxygen fugacity (log fO2, more than 3 units) between the two stages may result from the contamination of organic carbon during the emplacement of magma. In addition, major and trace element compositions of the Baogutu ore-forming diorites show the affinity of arc magmas which are like those that host oxidized porphyry Cu deposits.In this study, we first report the Fe isotopic compositions of reduced porphyry copper deposit and discuss its application for the origin of ore deposit. The δ56Fe values of less altered diorites and granodiorites (+0.083‰ and +0.054‰) fall into the field of arc magmas, consistent with their major and trace elements results. The δ56Fe values of sulfides (chalcopyrite and pyrite) show a large span (−0.228 to +0.614‰) and the early formed disseminated sulfides are enriched in light Fe isotopes relative to the late formed vein-hosted sulfides, which indicates that hydrothermal fluid evolves toward heavy Fe isotope enrichment. The average δ56Fe of all hydrothermal sulfides (chalcopyrite and pyrite) is much higher than that of Baogutu silicate rocks, which demonstrates that the hydrothermal fluid are Fe isotopically heavy and it well explains the positive correlations between δ56Fe and indices of hydrothermal alteration for altered plutonic rocks and wall rocks.In summary, major elements, trace elements and Fe isotopes of the Baogutu porphyries show the affinity of arc magmas, same as the typical oxidized porphyry Cu deposits. Both petrological observations and Fe isotope features of the Baogutu porphyry Cu deposit indicate that the primary magma of Baogutu porphyries is oxidized and its reduced features are secondary.

The Petrology of the Kaiserstuhl Volcanic Complex, SW Germany: The Importance of Metasomatized and Oxidized Lithospheric Mantle for Carbonatite Generation

Abstract The Miocene Kaiserstuhl Volcanic Complex (Southwest Germany) consists largely of tephritic to phonolitic rocks, accompanied by minor nephelinitic to limburgitic and melilititic to haüynitic lithologies associated with carbonatites. Based on whole-rock geochemistry, petrography, mineralogy and mineral chemistry, combined with mineral equilibrium calculations and fractional crystallization models using the Least Square Fitting Method, we suggest that the Kaiserstuhl was fed by at least two distinct magma sources. The most primitive rock type of the tephritic to phonolitic group is rare monchiquite (basanitic lamprophyre) evolving towards tephrite, phonolitic tephrite, phonolitic noseanite, nosean phonolite and tephritic phonolite by fractional crystallization of variable amounts of clinopyroxene, amphibole, olivine, spinel/magnetite, garnet, titanite, plagioclase and nosean. During this evolution, temperature and silica activity (aSiO2) decrease from about 1100°C and aSiO2 = 0·6–0·8 to 880°C and aSiO2 = ∼0·2. At the same time, oxygen fugacity (fO2) increases from ΔFMQ* = +2–3 to ΔFMQ* = +3–5, with ΔFMQ* being defined as the log fO2 deviation from the silica activity-corrected FMQ buffer curve. Nephelinitic rocks probably derive by fractionation of mostly olivine, spinel/magnetite, melilite, perovskite and nepheline from an olivine melilititic magma. The nephelinitic rocks were formed at similarly high crystallization temperatures (&amp;gt;1000°C) and evolve towards limburgite (hyalo-nepheline basanite) by an increase of silica activity from about aSiO2 = 0·4–0·5 to aSiO2 = 0·5–0·9, whilst redox conditions are buffered to ΔFMQ* values of around +3. Haüyne melilitite and the more evolved (melilite) haüynite may equally be derived from an olivine melilitite by more intense olivine and less melilite fractionation combined with the accumulation of haüyne, clinopyroxene and spinel. These rocks were crystallized at very low silica activities (aSiO2 ≤0·2) and highly oxidized conditions (ΔFMQ* = +4–6). Even higher oxygen fugacities (ΔFMQ* = +6–7) determined for the carbonatite suggests a close genetic relation between these two groups. The assemblage of carbonatites with highly oxidized silicate rocks is typical of many carbonatite occurrences worldwide, at least for those associated with melilititic to nephelinitic silicate rocks. Therefore, we suggest that the existence of highly oxidized carbonate-bearing sublithospheric mantle domains is an important prerequisite to form such complexes.

Formation of the Qixiashan Pb–Zn deposit in Middle-Lower Yangtze River Valley, eastern China: Insights from fluid inclusions and in situ LA-ICP-MS sulfur isotope data

The Qixiashan Pb–Zn deposit, Jiangsu Province (1212011220678), is hosted in Huanglong Formation limestones and is one of the largest Pb–Zn deposits in the Lower Yangtze region, China, which is an important Cu–Pb–Zn metallogenic belt. Here we report in situ sulfur isotopic analyses of sulfides using LA-ICP-MS method, together with fluid inclusion data. Three types of pyrite mineralization were identified in the host rocks and ores, corresponding to three genetic stages: (1) fine-grained framboidal or colloform pyrite hosted in limestone (Py1); (2) fine-grained euhedral–subhedral pyrite (Py2) associated with brown sphalerite (Sph1), occurring as massive sulfide ores; and (3) coarse-grained euhedral–subhedral pyrite (Py3) associated with yellow–brown sphalerite (Sph2) in breccia-type or massive ores. Stages 2 and 3 constitute the main Pb–Zn-forming event. Depending on the stage in which they were formed, the sulfides exhibit different sulfur isotope values, implying different ore-forming processes. The Stage 1 Py1 was formed by bacterial sulfate reduction and yields extremely low δ34S values, in the range −14.39‰ to −3.08‰. Stage 2 Py2 and Sph1 have δ34S values ranging from 6.5‰ to 9.4‰ and from 5.0‰ to 8.3‰, respectively. Stage 3 Py3 and Sph2 have δ34S values of 1.3‰ to 3.2‰ and −0.05‰ to 2.5‰, respectively. The δ34S value of pyrite is greater than that of sphalerite for each coexisting pyrite–sphalerite pair, indicating that sulfur isotope equilibrium was reached between these minerals. The total δ34S values and formation temperatures of the ore fluids in Stages 2 and 3 were 9.4‰ and 173 °C–279 °C, and 2.3‰ and 195 °C–328 °C, respectively. The high total δ34S in Stage 2 ore fluids was a result of the interaction of magmatic fluids with marine sulfur, whereas the low total δ34S in Stage 3 ore fluids reveal that these fluids were derived from magmatic water only. Two-phase liquid-rich aqueous inclusions in Stage 2 sphalerite (Sph1) yield homogenization temperatures and salinities of 182 °C–284 °C and 0.9–8.8 wt% NaCl equivalent, respectively. Such inclusions in Stage 3 sphalerite (Sph2) yielded higher homogenization temperatures (207 °C–306 °C) and higher salinities (0.4–12.3 wt% NaCl equivalent), suggesting an increase in magmatic–hydrothermal activity from Stage 2 to 3. Such high-salinity fluids may be produced by magmatic water exsolving from a concealed intrusion beneath the Qixiashan ore district. On the basis of ore fluid temperatures and total δ34S values, sulfides in Stage 2 (δ34S = 9.4‰, T = ~250 °C) are inferred to have precipitated under conditions of pH 2–6 and log fO2 −38 to −33, and sulfides in Stage 3 (δ34S = 2.3‰, T = ~350 °C) are inferred to have precipitated under conditions of pH 2–6 and log fO2 −31 to −27. The textural, fluid inclusion, and sulfur isotopic evidence indicate that the Qixiashan Pb–Zn orebody is a hybrid deposit (rather than a SEDEX or magmatic–hydrothermal deposit) that formed by syngenetic sedimentary deposition and hydrothermal overprinting.

The effect of oxygen fugacity on the rheological evolution of crystallizing basaltic melts

Storage and transport of silicate melts in the Earth's crust and their emplacement on the planet's surface occur almost exclusively at sub-liquidus temperatures. At these conditions, the melts undergo crystallization under a wide range of cooling-rates, deformation-rates, and oxygen fugacities (fO2). Oxygen fugacity is known to influence the thermodynamics and kinetics of crystallization in magmas and lavas. Yet, its influence on sub-liquidus rheology remains largely uncharted.We present the first rheological characterization of crystallizing lavas along natural cooling paths and deformation-rates and at varying fO2. Specifically, we report on apparent viscosity measurements for two crystallizing magmatic suspensions 1) at log⁡fO2 of −9.15 (quartz–fayalite–magnetite buffer, QFM, −2.1) and 2) in air. These fugacities span a range of reduced to oxidized conditions pertinent to magma migration and lava emplacement. We find that: 1) crystallization at constant cooling-rates results in a quasi-exponential increase in the apparent viscosity of the magmatic suspensions until they achieve their rheological cut off temperature (Tcutoff), where the melt effectively solidifies 2) the rheological departure and Tcutoff increase with increasing fO2 and 3) increasing fO2 results in decreased crystallization-rates. Based on the experimental results and by comparison with previous rheological isothermal studies we propose a generalisation of the effect of fO2 on the dynamic rheological evolution of natural magmatic and volcanic suspensions. We further discuss the implications for magmatic transport in plumbing and storage systems (e.g. conduits, dikes and magma chambers) and during lava flow emplacement.

Zircon trace element and isotopic (Sr, Nd, Hf, Pb) effects of assimilation-fractional crystallization of pegmatite magma: A case study of the Guangshigou biotite pegmatites from the North Qinling Orogen, central China

Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon UPb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415±2.6Ma; internal zone-413.5±2.5Ma), in line with a pyrite PbPb isochron age (413±22Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon (~413Ma) εHf(t) (−1.53−+3.24), low εNd(t) values (−2.4), and old Hf and Nd model ages (tDM2=1.5–1.19Ga, T2DM=1.35Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from −24.81 to −13.34 of log fO2 and 570°C to 793°C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (−3.9 to −2.8) and T2DM (1.47–1.37Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of magma. However, the heterogeneous Nd isotopic composition in individual pegmatites imply an incorporation of wallrock-derived melt with less radiogenic Nd.

Physicochemical model for the genesis of Cu-Ag-Au-Hg solid solutions and intermetallics in the rodingites of the Zolotaya Gora gold deposit (Urals, Russia)

In this contribution we examine the compositions of solid solutions and intermetallics of the system Cu-Ag-Au-Hg and the physicochemical conditions of their formation in rodingites from the Zolotaya Gora gold deposit (Southern Urals, Russia). Thermodynamic calculations, modeling the formation of mineral assemblages of rodingite and Cu-Ag-Au-Hg mineralization, were carried out using a “Selektor-C” software package. Two probable models for the genesis of Au-Ag-Cu-Hg solid solutions and Au-Cu intermetallics in rodingites are: 1) hydrothermal; the result of single-stage discharge in open space of deep-sourced gold-bearing fluid with the composition corresponding to rodingite, taking into account its interaction with host serpentinites. 2) metasomatic; deep-seated gold-bearing fluid (W) rising to the surface interacts with early formed rodingite (R) at different ratios (W/R). T and P-conditions of modeling: 450 °C, 3 kbar; 350 °C, 2 kbar; 250 °C, 1 kbar. Results of the calculations on the “hydrothermal” and “metasomatic” models showed different degrees of similarity of natural and theoretical model associations of rodingites. The metasomatic model is better for corresponding to real mineral compositions and mineral paragenesis in the natural Cu-Ag-Au-Hg system observed at the deposit. In this model the chlorite-garnet-pyroxene rodingite is replaced by a chlorite-rich rock with increasing W/R. In this case all gold minerals of Zolotaya Gora deposit (Au-Cu intermetallics and Au-Ag solid solutions) are formed at 250–450 °C. Gold-copper solid solution formed at a temperature of 450 °C (W/R >10). Au-Ag-Hg solid solutions and native copper are formed only at 250 °C. According to the hydrothermal model native copper and AuCu3 were absent phases, but other Au-Cu intermetallics (AuCu, Au3Cu) precipitate if gold concentration in the solution is higher than 0.5 ppm. Thermodynamic calculations proved the possibility of formation of equilibrium assemblages of rodingite minerals and gold-bearing minerals with the participation of water-chloride complexing and low CO2 fluids. At a temperature <350 °C the main status of gold in solution are Au(HS)2− and AuHS0, while at higher temperatures it occurs as AuOH0. Formation of Au-Cu intermetallics occurred under the effect of weak-acid hydrothermal solutions (pH = 3.5 ÷ 5) with low fugacity of O2 and S2: log fO2 = −26 ÷ −47, log fS2 = −8 ÷ −20. Both models (hydrothermal and metasomatic) explain the formation of Au-bearing rodingites and can be used for predicting potential gold-bearing rodingite targets.