Magmatic Water Contents Research Articles

Magmatic initial and saturated water thresholds determine copper endowments: Insights from apatite F-Cl-OH compositions

Magmatic volatiles (H2O, F, Cl), especially water, are critical in the formation of porphyry copper deposit, for its significance as a carrier for metals. However, accurately quantifying the water contents of deep ore-forming magma remain a challenge. Here, we used apatite and forward modelling methods to reconstruct magmatic water evolution histories, with special concern on the control of initial magmatic H2O contents and water saturation threshold to porphyry mineralization. Samples investigated include granitoid rocks and apatite from highly copper-mineralized and barren localities. Generally, our research suggested that both ore-related and ore-barren magma systems are hydrous, the modeled magmatic water contents vary significantly among systems whether mineralized or not, and the major difference lies in the threshold of water saturation (6.0 wt% for barren, and up to 10.0 wt% for highly mineralized). Combined with whole rock geochemistry data (high K2O and Sr/Y contents) and modeling result (high modeled water thresholds), we think the ore-related magmas are stored at deeper depth with higher water solubility. In conclusion, we propose that the level of magmatic water saturation plays a crucial role in the formation of porphyry copper systems. Fertile magma has higher water solubility to which deeper storage depth is a critical contributing factor, and can get significantly water enriched upon saturation.

Ages, petrogenesis and metallogenesis implications of the Miocene adakite-like igneous rocks in the Beimulang porphyry Cu Deposit, southern Tibet

Understanding the petrogenesis of Miocene adakite-like igneous rocks is crucial for unraveling the formation processes of post-collision porphyry copper deposits within the Gangdese orogenic belt. This study focuses on the mineralogical, geochronological, and geochemical characteristics of the Beimulang Miocene adakite-like igneous rock series, with particular emphasis on two types of diorite porphyry (DP1 and DP2), to constrain their petrogenesis. Zircon U-Pb dating indicates that the monzogranite (MG), monzogranite porphyry (MGP), DP1, DP2 and granite porphyry (GP) were emplaced at 14.8 ± 0.1 Ma, 14.6 ± 0.1 Ma, 14.6 ± 0.1 Ma, 13.2 ± 0.1 Ma and 12.5 ± 0.1 Ma, respectively. The major element compositions of the Beimulang Miocene intrusions exhibit a differentiation trend. The Sr-Nd isotopic compositions of the MG, MGP, and DP1 align with the mixing line between the juvenile lower crust and the potassic lamprophyres. Conversely, the Sr-Nd isotopic compositions of DP2 and GP fall precisely on the Sr-Nd mixing line between DP1 and the ultrapotassic lamprophyres. Considering the reverse zoning of plagioclase in the DP1 and presence of high-Mg phlogopite in the DP2, we suggest that the Miocene magmatic series at Beimulang likely derived from the mixing of partial melts of Gangdese juvenile lower crust and mantle-derived potassic-ultrapotassic magmas. The increase in Th/Yb ratios from DP1, MG, MGP to DP2 and GP suggests that the mantle metasomatism material may have evolved from slab-derived fluids to melts derived from Indian crustal sediments. The magmatic water content and oxidation calculated from zircon trace elements, are nearly identical among the ore-causative MG and MGP, and the barren GP. However, the decrease in V/Sc ratios from the MG and MGP to GP indicates that magnetite-driven magma sulfide saturation leads to the depletion of chalcophile elements, significantly reducing the Cu mineralization potential of the GP. We suggest that the higher degree of magmatic evolution leads to the barren nature of Beimulang granite porphyry.

Excess Alumina of Plagioclase Related to Copper-Gold Enrichment: Study Case at the Humpa Leu East (HLE) Porphyry CU-AU Prospect in HU’U District, Sumbawa Island, Indonesia

The formation of porphyry copper deposits is controlled by water available in magma to form a deposit since Cu ligands and S have finite solubilities. The high amount of water in magma allows the magma to reach saturation earlier and causes more Au to be drawn from the melt into volatile aqueous phases. High water content in magma is conducive to the migration and enrichment of metal elements, which are very important for mineralization. This study is aimed to examine trace element compositions of plagioclase related to early, intermediate and late causative intrusions of the Humpa Leu East (HLE) prospect by applying the mineral chemistry to assess magma fertility for copper and gold mineralization in term of magma water content. The presence of 'extra' alumina in plagioclase minerals has been linked to high water content in magma. Plagioclase may become enriched because 'excess' alumina excludes copper, enriching the remaining melts. Our findings show that the excess of alumina (Al*) in mineralized/fertile samples of the HLE porphyry Cu-Au prospect exhibits more pronounced (Al*) than the infertile samples of Mt. Kelud. The 'excess' alumina of plagioclase can be linked to high water content in the magma at the HLE porphyry copper prospect. Furthermore, Concentric zones in plagioclase phenocryst may represent fluid injections into the sub-porphyry magma chamber. Plagioclases in the intermediate intrusion shows a higher of 'excess' alumina than those in the early and late causative intrusive rocks, which indicates that the intrusion is genetically related to the high grade mineralization. This study proves that the chemical signatures of plagioclase could be an exploration indicator in term of magma fertility forming porphyry copper-gold deposits.

Magmatic water content in HIMU basalts from the Cook-Austral Islands: constraints on degassing processes and source composition from clinopyroxene phenocrysts

The amount of water recycled during subduction is unclear. Water/Ce estimates in HIMU (high-238U/204Pb) basalts are variable, ranging from < 100 to > 250 in glasses and melt inclusions. Because clinopyroxene (cpx) is a common early liquidus phase and the cpx/melt partitioning of water is well constrained, cpx phenocrysts provide an additional constraint on magmatic water contents. We present water and trace element concentrations in HIMU-basalt-hosted cpx phenocrysts from the Austral Islands. Calculated melt [H2O] (up to 4.3 wt.%) and H2O/Ce ratios (22–825) are higher than in olivine-hosted inclusions, as are estimated equilibration pressures. Correlation between estimated melt [H2O] and crystallization or entrapment pressure suggests significant water loss during magma ascent. Most ocean island basalts (OIBs) span a limited range in H2O (~ 1–1.5 wt.%), and low (< 100) H2O/Ce ratios are primarily observed in melts with unusually high [Ce] (up to 350 ppm). Additionally, [H2O] and H2O/Ce in some suites correlate with entrapment pressure despite having quench pressures high enough to prevent significant water loss from open- or closed-system degassing (< 100 MPa). Polybaric “sparging”, whereby low-P melts re-equilibrate with CO2-rich fluids exsolved at higher pressure, may result in water loss at pressures less than CO2 saturation. This may more accurately describe OIB degassing processes than open or closed system degassing. After correcting for degassing, primary Australs melts likely have H2O/Ce of ~ 300–600. If applicable to OIB sources in general, this limits the total water budget of the mantle, including the mantle transition zone, to < 2.4 ocean masses.

Detrital zircon REE and tectonic settings

Earth's long-term history is preserved in the rock archive, yet this record is incomplete, hindering our understanding of how tectonic processes have evolved and shaped our planet's evolution. The physio-chemical resilience of zircon and its ability to be rapidly analyzed for age, isotopic, and elemental data has made it a key phase in unraveling Earth's long-term evolution. The europium anomaly (Eu/Eu*) and the differentiation degree between light and heavy rare earth elements (LREEN¯/HREEN¯) of zircons positively correlate with the pressure of magma crystallization because high pressure inhibits plagioclase crystallization and promotes garnet growth. However, this correlation can be influenced by fluctuations in redox conditions, protolith composition, and water content of magma, whose stability varies in different tectonic settings. Therefore, the correlation coefficient between Eu/Eu* and LREEN¯/HREEN¯ of detrital zircons (rDz) provides a new proxy to help distinguish tectonic settings. We evaluate this new proxy using available global detrital zircon REE data from different tectonic settings in modern and deep times. The data from well-constrained simple tectonic settings suggest that rDz is higher in convergent settings (0.53–0.85) than in collisional settings (0.12–0.51). Convergent settings can also yield relatively low rDz (0.28–0.55) when there is a contribution of a mantle plume lying below the overriding plate of a subduction zone.When studying a detrital zircon archive derived from a large distributive province that includes sources from a variety of settings, known provenance information should be used to disentangle mixed source signals. Also, only considering detrital zircon grains that formed a short time (e.g., 50 or 100 Myr) prior to deposition can reduce the likelihood of multi-cycle distal mixing of contemporaneous but tectonically distinct zircons in a sample. Additionally, greater confidence in interpretations of changes in tectonic settings may be derived from evolution patterns of rDz instead of isolated values. A decreasing trend can likely be interpreted as a transition from convergent to collisional or mantle plume settings, whereas an increasing trend implies an opposite tectonic transition.

Genesis and metallogenic potential of the ca.90 Ma Jiangcisha magmatic rocks in the western Shiquanhe suture zone, Xizang, China

Numerous mineral deposits related to the ca.90 Ma magmatic activity were found in the northern and central Lhasa terrane. Recently, many Late Cretaceous intermediate and felsic magmatic rocks were discovered in the Jiangcisha area in the western Shiquanhe-Nam Tso Mélange Zone, separating the Lhasa terrane into northern and central subterranes. In this study, zircon UPb dating, whole-rock major and trace element geochemistry, zircon Hf isotopes and trace element compositions are used to determine the petrogenesis and mineralization potential of the Jiangcisha intermediate and felsic magmatic rocks. The zircon UPb ages of the diorite porphyry, dacite porphyry, and diorite sample are 92.2 ± 1.6 Ma, 90.0 ± 1.3 Ma, and 85.8 ± 1.7 Ma, respectively. The diorite porphyry and diorite samples have A/CNK values ranging from 0.65 to 0.91 and show elevated Mg# (44.78–53.11), Cr (108–338 ppm) and Ni (53.2–119 ppm) concentrations. These rocks also have high Sr/Y values (30.37–58.03) and low Rb/Sr ratios (0.04–0.33), with weak negative Eu anomalies (δEu = 0.75–0.95). Based on these values, these samples are high-Mg# calc-alkaline and metaluminous adakitic rocks. The dacite porphyry has a high SiO2 content (73.54–74.07 wt%) and significant negative Eu (δEu = 0.69–0.73) and Sr anomalies. Their Sr contents are 10 times lower than those of the diorite porphyry and diorite samples. The zircon εHf(t) values of these three magmatic rocks are high (7.61–12.69), and the Hf isotope two-stage model ages are young (0.41–0.78 Ga). The geochemical characteristics mentioned above indicate that the diorite porphyry and diorite were derived from the partial melting of the delaminated juvenile lower crust. However, the dacite porphyry is the product of plagioclase and K-feldspar fractional crystallization from the diorite porphyry. The zircons in the diorite porphyry and diorite samples have low Ce/Nd (3.66–29.04), logfO2 (−16.022-–12.976), and ΔFMQ (0.08–1.99) values and moderate Dy/Yb (0.22–0.37), 10,000 × δEu/Y (1.48–9.66), and (Ce/Nd)/Y (0.0025–0.0224) ratios. Compared to typical large porphyry-skarn CuAu deposits with dioritic rocks developed in the Tibetan Plateau, the relatively low oxygen fugacity and magma water content of the Jiangcisha dioritic rocks indicate their limited potential to form large porphyry and skarn CuAu deposits.

Geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of the Late Jurassic granitoids from the Shuangyuangou deposit, South Qinling, China: Implications for petrogenesis and porphyry Cu metallogeny

The Shuangyuangou copper deposit is located in the southern segment of the Qinling orogenic belt in central China. The ore‐related intrusions include quartz diorite porphyry, granite porphyry and lamprophyre. In this study, we present whole‐rock geochemistry, zircon U–Pb geochronology, trace elements and Lu–Hf isotope data on the Shuangyuangou intrusions which provide critical constraints on the emplacement age, magma origin, petrogenesis and implications for porphyry Cu mineralization in this newly discovered deposit. Zircon U–Pb dating constrains the emplacement age of quartz diorite porphyry, lamprophyre and granite porphyry at 150–147 Ma. The quartz diorite porphyry and granite porphyry display high‐K calc–alkaline I‐type granite affiliation. The rocks show enrichment of LREE and LILE (Rb, Ba), depletion in HREE and HFSE (Nb, Ta, Ti) and absence of Eu anomaly. Zircon εHf(t) values of quartz diorite porphyry, lamprophyre and granite porphyry range from −7.5 to −1.7 with TDM2 ages of 1.3–1.6 Ga. Our data suggest that the parental magma was derived from partial melting of juvenile lower crust and metasomatic lithospheric mantle, with more involvement of mantle material for the lamprophyre. The zircon Eu/Eu* values of 0.55–0.78 suggest high magmatic water content which suppresses the fractionation of plagioclase. In addition, the ΔFMQ values calculated from zircon trace elements range from −0.90 to 2.96 which is a favourable factor for porphyry Cu mineralization at Shuangyuangou. The Late Jurassic Shuangyuangou intrusion and associated Cu mineralization were formed within an intracontinental setting after the collisional assembly of the Qinling orogenic belt.

Controls on metal fertility of dioritic intrusions in the Laiwu district, North China Craton: Insights from whole-rock geochemistry and mineral compositions

Several large high-grade iron skarn deposits in the Laiwu district of Shandong Province (North China Craton) are developed along the contact zone between the Early Cretaceous Kuangshan diorite pluton and Middle Ordovician dolomitic limestone. In sharp contrast, no iron skarn mineralization has been recognized in association with other contemporaneous dioritic intrusions (Jiaoyu, Jinniushan, Tietonggou) in this district. The reason for this contrast remains elusive. Here we present a comparative study of the ore-related Kuangshan pluton and its barren equivalents using whole-rock and mineral (zircon, apatite) geochemistry combined with existing Sr-Nd isotope data to provide new insights into the critical controls on iron skarn mineralization and shed light on future mineral exploration. Geochemical and Sr-Nd isotope data suggest that both the ore-related and barren intrusions were derived from a common enriched lithospheric mantle source following variable degrees of fractional crystallization and crustal assimilation. Modeling based on whole-rock Sr-Nd isotopes implies that the magmas responsible for the Kuangshan and Jinniushan diorite plutons assimilated significant quantities of the Ordovician evaporite-bearing carbonates. Both the fertile and barren diorites show high whole-rock Sr/Y ratios, high zircon Eu/Eu* and Eu/Eu*/Y ratios, and low zircon Dy/Yb ratios. These data, combined with high magmatic water contents (4.4−8.0 wt%) calculated from amphibole compositions, indicate that those four plutons crystallized from hydrous magmas. High whole-rock Fe2O3/FeO, and high ΔFMQ values (mostly &amp;gt;1; fayalite-magnetite-quartz oxygen buffers) estimated from zircon, apatite, and amphibole suggest that the magmas from which the diorite intrusions crystallized were all oxidized. Using available partitioning coefficients for Cl between apatite and magma, the parental melts of the Kuangshan and Jinniushan diorite are estimated to have had higher Cl contents (0.15−0.77 wt%) than the Jiaoyu and Tietonggou diorites (0.06−0.34 wt%). Two types of apatite from the intrusions studied are recognized based on their paragenetic relations: Type 1 apatite (Ap1) is an early-formed phase enclosed in euhedral amphibole, plagioclase, and biotite, whereas type 2 (Ap2) is a late-formed variety hosted in, or intergrown with, anhedral K-feldspar (± anhedral amphibole ± subhedral to anhedral quartz). In the fertile Kuangshan diorite, Cl and OH contents of apatite decrease, but Sr contents increase from Ap1 to Ap2. However, those parameters show reverse trends or remain constant from Ap1 to Ap2 in barren diorites. Such distinctive apatite geochemical signatures are best interpreted as reflecting more efficient fluid exsolution from the parental magma of the Kuangshan diorite than from the barren intrusions. This view is supported by the presence of abundant fluid inclusions in apatite and quartz exclusively from the mineralized intrusion, indicating its crystallization from volatile oversaturated melts. The results presented here indicate that elevated magma Cl contents and efficient separation of magmatic hydrothermal fluid from cooling magmas have been the critical factors controlling iron skarn mineralization in the Laiwu district. Mass balance constraints on Cl budget indicate that a minimum of 44−60 km3 of magma was required to form the Fe deposits associated with the Kuangshan pluton. Our preliminary results suggest that decreasing OH and Cl contents from early to late apatite in intermediate rocks can be used as an indicator for efficient fluid exsolution and consequently metal fertility of those rocks.

Mo endowment of porphyry deposits in the southern margin of the North China Craton controlled by magmatic water content

Mo endowment of porphyry deposits in the southern margin of the North China Craton controlled by magmatic water content

Determining the impact of magma water contents on porphyry Cu fertility: Constraints from hydrous and nominally anhydrous mineral analyses

Most large-scale porphyry-style mineralization is genetically associated with oxidized, sulfur-rich, and hydrous magmas, the sources of which were enriched in mantle-derived or juvenile crust−derived materials. Geochemical indices for these features can distinguish fertile magmas and barren magmas. However, fertile magmas that generate different-sized porphyry deposits are usually geochemically indistinguishable, and the factors controlling the size of porphyry mineralization are poorly understood. Here, we used zircon H2O contents and the compositions of apatite hosted in zircon, phenocrysts, and groundmass to compare the water contents and volatile evolution of fertile magmas in the Yulong ore belt, Tibet, which generated giant-, large-, and medium-sized porphyry deposits. We found that these porphyries have comparable major- and trace-element and zircon Hf-O isotope compositions, with high oxygen fugacity and sulfur contents. A negative correlation between [see PDF for equation] versus [see PDF for equation] of apatites suggests that the magmas consistently achieved H2O saturation before zircon crystallization, and zircon hydroxyl contents reveal that the water contents of the melt positively correlate with the size of the deposits, consistent with larger deposits achieving H2O saturation at deeper crustal levels. The deeper H2O saturation also caused larger amounts of Cl extraction, as indicated by the trends of increasing [see PDF for equation] and decreasing [see PDF for equation] with larger deposit sizes. These data sets can be reconciled by magmas forming larger deposits having higher H2O contents and suggest that the H2O content is a key control on their fertility. Our study highlights the utility of integrated hydrous and nominally anhydrous mineral analyses for constraining enigmatic magmatic volatile processes in magmatic ore-forming systems.

Water in clinopyroxene from the 2021 Geldingadalir eruption of the Fagradalsfjall Fires, SW-Iceland

Water content plays a significant role in magma genesis, ascent rate, and, ultimately, in the style and intensity of volcanic eruptions, due to its control on the density, viscosity and melting behaviour of silicate melts. A reliable method for determining the pre-eruptive magmatic water content is to use phenocrysts of nominally anhydrous minerals (NAMs) which can preserve water as hydrogen configurations in structural defects. The advantage of this method is that eruptive changes such as water loss during magma degassing may be experimentally reconstructed and analysed by infrared spectroscopy. Applying this to clinopyroxene crystals (n=17) from lava samples (n=7) from April 2021 of the Geldingadalir eruption, SW-Iceland, reveals parental water contents of 0.69 ± 0.07 to 0.86 ± 0.09 wt. % H2O. These values are higher than those expected for typical mid-ocean ridge basalts (MORB 0.3–0.5 wt. % on average) indicating a significant plume (OIB) contribution to the magma source. Moreover, such water concentrations would imply that water saturation in the ascending Geldingadalir magmas was attained only at very shallow levels within the plumbing system. This could explain the at times pulsating behaviour within the uppermost conduit system as being the result of shallow episodic water vapour exsolution in addition to the deep-sourced CO2 flux.

Water-rich magmas optimise volcanic chalcophile element outgassing fluxes

Magmatic-hydrothermal fluids transport chalcophile metals to the atmosphere as volcanic gases; and to the crust, where they may play a role in the formation of ore deposits. Global volcanic gas datasets show considerable variability in the flux and composition of metals outgassed between volcanoes, but the controls on this variability are unclear. Magmatic chloride is a key ligand for metal transport but magmatic water dominates the exsolved fluid reservoir into which metals partition during crystallisation and decompression. Here we develop models simulating decompression-driven degassing (‘first boiling’) and isobaric crystallisation-driven degassing (‘second boiling’) of magmas to show that while moderate concentrations of chlorine are essential for metal partitioning into the magmatic fluids, magmatic water contents have the greatest potential to control the mass yield of metals carried by exsolving fluids. Our models explain why water-rich magmatic-volcanic systems like Mount Etna (Italy) deliver the largest mass fluxes (per unit of degassing magma) of metals to the atmosphere, whereas relatively dry magmatic-volcanic systems like Yasur (Vanuatu) deliver the smaller mass fluxes. Our results establish the important role of magmatic water in generating large reservoirs of metal-rich aqueous fluids in the shallow crust.

Petrogenesis and Physicochemical Conditions of Fertile Porphyry in Non‐arc Porphyry Mineralization: A Case from Habo Porphyry Cu‐Mo Deposits, SW China

AbstractThe Habo deposit is a typical porphyry Cu‐Mo deposit in the Ailaoshan–Red River metallogenic belt. Ore minerals in the Habo deposit typically occur as veins in the monzonite porphyry. Zircon U‐Pb dating suggests that the monzonite porphyry formed at 35.07 ± 0.38 Ma. The monzonite porphyry is characterized by high SiO2, Al2O3, K2O and Na2O contents, with A/CNK ratios ranging from 0.97 to 1.02. All samples exhibit fractionated REE patterns, characterized by high (La/Yb)N ratios (9.4–13.6, average of 11.2). They show adakite‐like geochemical features, high Sr concentrations (627–751 ppm, average of 700 ppm), low Y concentrations (15.13–16.86 ppm, average of 15.81 ppm) and high Sr/Y values (39.5–47.4, average of 44.3). These samples have high initial 87Sr/86Sr ratios (0.7074–0.7076) and negative εNd(t) values (–5.1 to −3.7), whereas the zircon εHf(t) values range from −2.2 to +0.4, suggesting that the monzonite porphyry was derived from the partial melting of a thickened juvenile lower crust. The oxygen fugacity, calculated on the basis of the chemical composition of the amphiboles, shows ΔNNO values ranging from +1.65 to +2.16 (average of 1.94) and lg(fO2) ranging from −12.72 to −11.99 (average of −12.25), indicating that the monzonite porphyry has high oxygen fugacity. Zircons have high Ce4+/Ce3+ ratios (29.29–164.24, average of 84.92), with high ΔFMQ values ranging from +0.50 to +1.51 (average of 0.87) and high lg(fO2) values ranging from −14.72 to −12.85 (average of −14.07), which also indicates that the oxygen fugacity of the magma was high. The dissolved water content of the Habo monzonite porphyry is 9.5–11.5 wt%, according to the geochemical characteristics, zircon‐saturation thermometry (692–794°C) and the mineral phases (amphibole, no plagioclase) in the deep magma chamber. Combined with previous studies, we propose that the high oxygen fugacity and high water content of magma played key roles in controlling the formation of the Habo and other Cu‐Mo‐Au deposits in the Ailaoshan–Red River metallogenic belt.

Petrogenesis of the Alubaogeshan granite in the Maodeng Mo-Bi-Sn-Cu deposit, southern Great Xing’an Range, NE China: Constraints from apatite, plagioclase, magnetite and ilmenite geochemistry

The Maodeng deposit, located in Xilinhot, Inner Mongolia, is a typical Mo-Bi-Sn-Cu deposit in the southern segment of the Great Xing’an Range. It is genetically related to the Alubaogeshan granitic complex, which is composed of porphyritic monzogranite (G1), granite porphyry (G2), and alkali feldspar granite (G3). However, the comparison of physical–chemical condition and magmatic evolution of different lithofacies has not been conducted. We present in-situ major-trace elements and Sm-Nd isotopes of apatite, in-situ major-trace elements of plagioclase, as well as in-situ major elements of magnetite and ilmenite from the G1 and G2, with the goal of discussing magma genesis and its constraints on mineralization. The apatites from the two granites are characterized by relative enrichment in F and light rare earth elements (LREEs) but depletion in Cl, S, and heavy rare earth elements (HREEs), as well as obviously negative Eu, Sr, Nb, Ba, Zr, and Hf and positive Y anomalies, indicating that the two granites have similar sources and experienced significant crystallization differentiation. When compared to G1, apatites from G2 exhibit more significant Eu, Sr, and Y anomalies, as well as lower (La/Sm)N and (La/Yb)N ratios, likely due to intensive crystallization of plagioclase and exsolution of Cl-bearing magmatic fluids. The presence of core-rim zoning patterns with BSE-bright and BSE-grey domains in some G2 apatite crystals indicates extensive hydrothermal alteration. The plagioclases within G1 belong to andesine–oligoclase series corresponding to magmatic water contents of 1.4 to 1.9 wt%, and those in G2 belong to oligoclase–albite series with calculated magmatic water contents of 2.1 to 2.2 wt%. The plagioclase crystals from G1 are characterized by prominently positive Eu and moderately positive Sr and Ba anomalies, with crystals from G2 showing strongly negative Eu, Ba and Sr anomalies, indicating more significant fractionating plagioclase and apatite in G2 than G1. The magnetite compositions are distinguished by high FeO, TiO2, and MnO contents. Ilmenite usually cuts and dissolves magnetite, and contains a lot of FeO, TiO2, MnO, and Nb2O3. The magnetite and ilmenite from the two granites have a coherent negative correlation between Ti and Fe3+ but a consistent positive correlation between Ti and Fe2+, implying that they crystallized from a homologous silicate magma. During the crystallization of apatites, magmatic oxygen fugacities of the two granites were slightly above the ΔNNO buffer line. However, these were near the FMQ buffer line when the magnetite and ilmenite were in equilibrium at temperatures ranging from 561 to 666 °C, indicating a decreasing trend in oxygen fugacity during magma evolution. The apatites of both G1 and G2 have high εNd(t) values (+0.59 to + 1.94 and − 2.13 to + 0.42, respectively), and young two-stage Nd model ages (tDM2 = 775–884 Ma and 897–1106 Ma, respectively), suggesting the predominant origin of these rocks by the partial melting of a juvenile mafic lower crust and “old” pre-existing crustal components, having a substantial proportion of continental crust for G2. The decline in oxygen fugacity, enrichment of F element, highly crystallization differentiation, intense melt/fluid interaction, and the strong hydrothermal alteration for the ore-related granite could contribute to the formation of the Maodeng deposit.

Degassing, Crystallization and Rheology of Hawaiitic Lava Flows: the Case of the 1669 AD Eruption of Mount Etna (Italy)

Abstract Understanding lava flow dynamics during major effusive events is of paramount importance in volcanic areas characterized by a high risk of lava invasion. Mount Etna volcano (Sicily, Italy) has a long history of eruptions characterized by the emplacement of kilometer-wide lava fields, which have often reached the distal parts of the volcanic edifice, nowadays the location of numerous population centers. The 1669 eruption was one of the volcano’s most important events in historic times due to the low altitude of the eruptive vent and the high volume of emitted products (607 ± 105 × 106 m3), with lava flows that destroyed numerous villages located along their path. The flows reached the city of Catania, at a distance of &amp;gt;16 km from the emission point. In this work, we investigate the products of the 1669 eruption through geochemical, mineralogical, 2D and 3D textural analyses with the aim of reconstructing the degassing, crystallization and rheological history of the magma and lavas in pre- to post-eruptive conditions. Combining geothermobarometric and hygrometric models allowed us to estimate magmatic water content (4.1 wt.%) before the eruption, whereas the syn-eruptive crystal content (10 vol.% at the onset of flowing) was retrieved through the textural analysis of pyroclasts sampled from the near-vent fallout. Finally, crystallization and degassing occurring at surface conditions were reconstructed using the textural and mineralogical analysis of lavas. Results were integrated in a three-phase (melt + crystal + bubble) rheological model indicating that lava viscosity, at the onset of the eruption, was low enough (&amp;lt;3.51 log Pa s) to permit the development of a complex and extensive lava field. Flows reached numerous kilometers in length, thanks to the joint effects of lava tunneling, delayed crystal nucleation and growth, and the presence (up to 21.35 vol.%) of deformed bubbles. The combination of these processes maintained the high fluidity of the melt suspension, allowing the flow to arrive at considerable distances from the vent. Lastly, the results of this study highlight the necessity of an accurate real-time multi-analytical petrological characterization of active lavas during the monitoring of effusive eruptions, essential for reliable viscosity modeling of advancing flow units and therefore vital in predicting the direction of lava flows, especially in densely populated areas such as the southern flank of Mount Etna.

MagMin_PT: An Excel-based mineral classification and geothermobarometry program for magmatic rocks

ABSTRACTIgneous rock forming minerals carry valuable information from the deep earth that is not directly accessible at the surface. Each mineral represents the physico-chemical conditions at which various magmatic processes have occured over a wide range of depths from upper mantle to shallow crustal levels. These processes are cryptically inscribed in the whole-rock and mineral compositions (e.g. major elements, trace elements and isotopic ratios) and textures (equilibrium vs. disequilibrium features), together with intensive variables (e.g. pressure, P; temperature, T). Therefore, particular attention should be given to igneous minerals to understand better the processes that took place during their journey from the source through magma chambers and conduit systems to the Earth's surface.MagMin_PT is an Excel© based user-friendly program, designed to calculate mineral formulae and end-members, and to estimate pressure and temperature (e.g. geothermobarometry) from electron microprobe analytical data. The program operates using the most common igneous rock-forming minerals (olivine, pyroxene, amphibole, biotite, feldspar, magnetite, ilmenite, apatite and zircon), resulting in various classification diagrams and P–T diagrams. The program allows for whole-rock or glass composition to be entered together with the EPMA data to evalaute the equilibration status for most P–T calculation models. Fe2+ and Fe3+ estimation is routinely performed in MagMin_PT based on stoichiometric constraints, and to some extent using machine learning methods for different iron-bearing minerals. MagMin_PT is also able to carry calculations of fugacity, magmatic water content and saturation temperature. Graphical and numerical outputs produced by the program can be easily copied to other media for further processing.

Petrogenesis and metal fertility of Yinzu pluton in the Daye district, eastern China: Insights from whole-rock and mineral (zircon, apatite and amphibolite) geochemistry

The Daye district in eastern China is featured by numerous granitoid intrusions of Early Cretaceous ages that intrude Paleozoic to Early Triassic marine carbonate and clastic rocks. Several dozen skarn Cu-Fe-Au deposits occur along the intrusive contact zones of the Echeng, Tieshan, Jinshandian, Lingxiang, Tongshankou and Tonglushan intrusions. However, no economic skarn mineralization has been recognized in other intrusions and whether or not these intrusions are metal fertile remain unresolved. In this paper, we use whole-rock and mineral (zircon, apatite and amphibole) geochemistry to provide significant new insights into the petrogenesis and metal fertility of the Yinzu granitoid pluton.The Yinzu pluton mainly consists of quartz diorite and diorite phases that have zircon U-Pb ages of 148.4 ± 1 Ma and 146.6 ± 1.6 Ma, respectively. Whole-rock Sr-Nd isotopic compositions ((87Sr/86Sr)i = 0.70635 to 0.70741; εNd(t) = −8.74 to −3.73) and zircon hafnium isotopes (εHf(t) = −9.1 to −4.8) overlap with values of the Cu-fertile Tongshankou and Tonglushan granitoids, implying a common enriched lithospheric mantle source. The whole-rock geochemical data suggest that both the diorite and quartz diorite have experienced variable degrees of fractional crystallization of olivine, amphibole, titanite and magnetite. The Ce4+/Ce3+ ratios (16–445) and oxygen fugacity (logfO2) calculated using trace element concentrations of zircon (ΔFMQ = +0.2 to +2.4) suggest that both intrusive phases crystalized from oxidized melts similar to Cu-fertile magmas in the world, with the quartz diorite being well comparable to the Tongshankou and Tonglushan granitoids. Based on available melt-apatite partitioning models, the estimated S and Cl contents for the magmas parental to the Yinzu pluton are akin to those of the Cu-fertile granitoids in the Daye district. Rocks of the Yinzu pluton display high apatite Sr/Y (0.38–3.01) and Eu/Eu* (0.35–0.71) ratios as well as high zircon Eu/Eu* (0.51–0.86) and 10000*(Eu/Eu*)/Y (2–27) ratios, indicating significant fractionation of amphibole with limited plagioclase fractionation and high magmatic water contents (>4 wt% H2O). This view is consistent with the calculated melt H2O content of 4.3–6.1 wt% based on the amphibole compositions.Collectively, results presented here indicate that magmas generating the Yinzu pluton are hydrous, oxidized and S-rich, characters similar to the Cu-fertile granitoids in Daye and elsewhere. Thus, the Yinzu pluton is considered to be Cu-(Au) fertile. This consideration is partially supported by occurrences of several vein-type Cu prospects within and disseminated Au ores in Silurian clastic sedimentary rocks immediately to the southeast of this pluton. Based on the regional metallogeny and results of this study, we suggest that the Yinzu pluton is a promising target of future Cu-(Au) exploration.

Tracing the effects of fO2, pressure and H2O on the ore-forming magmas: Perspective from zircon REE composition

Efforts have been devoted to explore if there are independent zircon trace element ratios that could be proxy of oxidation state and magmatic water content, and can distinguish between mineralization and barren granites. In this study, I evaluated the difference of the zircon elemental signatures between fertile magmatic suites (which host porphyry Cu-Mo, Mo, Cu-Au, Cu, Cu-Mo-Au, Mo-Cu, and Au deposits) and infertile I-type granites. The “potential” magmatic suites, which means that they are supposed to develop porphyry deposits based on the previously proposed ore-forming criteria (e.g., zircon Eu/Eu* > 0.5) and yet no associated mineralization has been found, were also included in this comparison. Most of the trace elemental signatures cannot differentiate the I-type granite zircons from fertile zircons and “potential” zircons, except for the Eu anomaly (∼0.5 < Eu/Eu* < ∼0.8 for ore-related and “potential” suites; ∼0.2 < Eu/Eu* < ∼0.5 for I-type granites) and middle/heavy rare earth element variation diagrams (MREE and HREE; e.g., Eu/Gd versus Gd/Yb, Lu/Yb versus Gd/Yb, Sm/Ho versus Sm/Lu, and Yb/Lu versus Dy/Lu). The behavior of zircon REE possibly indicate a more MREE-depleted signature for the ore-forming and “potential” suites. Taken together, zircon REE features documented in this study highlight the fractional crystallization of hornblende and garnet, and the delay of plagioclase crystallization, revealing the combined effect of high overlying pressure, high fO2 and high H2O (or H+) concentration on the differentiation of fertile magmas.

Petrological evidence of an anhydrous carbonatitic peridotite source for the Xiaogulihe ultrapotassic volcanic rocks, northeastern China

This study presents new olivine and whole-rock geochemical data for the Xiaogulihe ultrapotassic volcanic rocks of northeastern China. These data provide insights into the petrogenesis of the rocks of the Erkeshan–Wudalianchi–Keluo–Xiaogulihe (EWKX) potassic–ultrapotassic volcanic belt. The petrography and whole-rock geochemical compositions of the Xiaogulihe volcanic rocks are indicative of formation from magmas that primarily underwent olivine + clinopyroxene fractionation with insignificant crustal contamination and post-magmatic alteration. The high-Fo (87–90) olivine within the Xiaogulihe volcanic rocks has high Ni (2050–3800 ppm) and Mn (1190–1750 ppm) contents, high 10000Zn/Fe values (12–15), low Ca (1310–1800 ppm) contents, and low Fe/Mn (56–72) and Mn/Zn (10–14) values. Their high Mn contents and low Fe/Mn values together with their low FC3MS (<0.65; FC3MS = FeO/CaO – 3MgO/SiO2), low FCKANTMS (<0.37; FCKANTMS = ln(FeO/CaO) − 0.08ln(K2O/Al2O3) − 0.052ln(TiO2/Na2O) − 0.036ln(Na2O/K2O) ln(Na2O/TiO2) − 0.062(ln(MgO/SiO2))3–0.641(ln(MgO/SiO2))2–1.871ln(MgO/SiO2) − 1.473) and low CaO–Al2O3–Fe2O3Total values are indicative of derivation from a mantle source containing refractory peridotite. The Xiaogulihe olivines have slightly higher Zn/Fe and lower Mn/Zn values than expected for olivines formed from typical peridotite-derived melts, suggesting that the source for these lavas was previously metasomatized by interaction with Zn-rich sedimentary carbonatitic melts. The low magmatic water contents of the Xiaogulihe lavas (<1 wt% determined by use of the Ca-in-olivine geohygrometer) is also indicative of derivation from an anhydrous mantle source. These data suggest that the Xiaogulihe ultrapotassic lavas represent partial melts of anhydrous refractory subcontinental lithospheric mantle peridotite. The K-rich nature of these volcanic rocks is indicative of derivation from the K-rich reservoir of the mantle, which is closely associated with carbonatitic metasomatism, indicating the role of this process in the generation of the K-rich EWKX volcanics.

Development of major element proxies for magmatic H2O content in oceanic basalts

Analysis of H2O concentrations in quenched glass has enabled significant improvements in our understanding of its role in mantle melting, magma differentiation, eruption dynamics, and the origin of mantle heterogeneity. Direct measurements of dissolved H2O in glass, however, are not always possible, and we lack robust methods of constraining magmatic H2O contents in aphyric, bulk rock samples that lack glass. Here, we present a major element hygrometer for mid-ocean ridge (MOR) and back-arc basin (BAB) basalt magmas based on the sensitivity of phenocryst phase assemblages to magmatic H2O contents, which translate into resolvable differences in liquid lines of descent (LLDs) as a function of magmatic H2O concentrations. Existing hygrometers lack sufficient resolution to be useful at the low H2O concentrations typical of MOR and BAB basalts (<1.0 wt%). We develop the major element proxy, Al2O3/FeO*(7.0) (fractionation-corrected to 7 wt% MgO), for determining magmatic H2O contents using cogenetic suites of oceanic basalts with well-defined LLDs and well-constrained H2O contents. H2O(7.0) positively correlates with Al2O3/FeO*(7.0) in the mid-ocean ridge basalt dataset, and this relationship is maintained in back-arc basin basalts with a broader range of water contents (up to 2.0 wt%). The main petrological control over this covariation is the role of H2O in suppressing plagioclase crystallization, while crystallization pressure and magmatic oxygen fugacity play lesser roles. Herein, we present an empirical model that uses Al2O3/FeO*(7.0) to estimate the magmatic water content in plagioclase-saturated oceanic basaltic magmas: H2O(7.0) = 1.109Al2O3/FeO∗(7.0) − 1.111. This model enables the estimation of magmatic H2O content using whole-rock major element data, which can be readily determined for aphyric or crystalline lavas that lack quenched glassy rinds, melt inclusions, or appropriate phenocryst assemblages.