Volume 105, Issue 10

Decades of study have documented several orders of magnitude variation in the oxygen fugacity $\left( {{f}_{{{\text{O}}_{2}}}} \right)$of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe 3+ /Fe 2+ ) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe 3+ /Fe 2+ ) can also lead to systematic variations in fO2${{f}_{{{\text{O}}_{2}}}}$in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2${{f}_{{{\text{O}}_{2}}}}$vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2${{f}_{{{\text{O}}_{2}}}}$decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2${{f}_{{{\text{O}}_{2}}}}$and increasing pressure in this facies has little influence on fO2${{f}_{{{\text{O}}_{2}}}}$(normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2${{f}_{{{\text{O}}_{2}}}}$(normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2${{f}_{{{\text{O}}_{2}}}}$reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇆ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe 3+ -bearing components in pyroxene and therefore to decreases in normalized fO2${{f}_{{{\text{O}}_{2}}}}$whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe 3+ -bearing components in pyroxene and to increases in normalized fO2${{f}_{{{\text{O}}_{2}}}}$(although this is counteracted to some degree by progressive partitioning of Fe 3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2${{f}_{{{\text{O}}_{2}}}}$inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2${{f}_{{{\text{O}}_{2}}}}$of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe 3+ /Fe 2+ or bulk O 2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2${{f}_{{{\text{O}}_{2}}}}$of peridotites of constant composition are likely to be superimposed on variations in fO2${{f}_{{{\text{O}}_{2}}}}$that reflect differences in the whole-rock Fe 3+ /Fe 2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

The investigation of the presence and role of sulfates in our solar system receives growing attention because these compounds play a crucial role in the water budget of planets such as Mars and significantly influence melting equilibria on the icy moons of Saturn and Jupiter, leading to the formation of subsurface oceans and even cryovolcanism. Despite the dominant presence of higher sulfate hydrates such as epsomite, MgSO 4 ·7H 2 O, and mirabilite, Na 2 SO 4 ·10H 2 O, on these moons’ surfaces, it is not excluded that lower-hydrated sulfates, such as kieserite, MgSO 4 ·H 2 O, are also present, forming from higher hydrates under pressures relevant to the mantle of the icy moons. Given the composition of the soluble fraction in C1 and C2 chondritic meteorites, which are high in Ni content and also considered to represent the composition of the rocky cores of the Jovian icy moons, the actual compositions of potentially present monohydrate sulfates likely lie at intermediate values along the solid-solution series between kieserite and transition-metal kieserite-group end-members, incorporating Ni in particular. Moderate Ni contents are also probable in kieserite on Mars due to the planet’s long-term accumulation of meteoritic nickel, although likely to a much lesser extent than Fe. Structural and spectroscopic differences between the pure Mg- and Ni-end-members have been previously documented in the literature, but no detailed crystal chemical and spectroscopic investigation along the Mg-Ni solid solution has been done yet. The present work proves the existence of a continuous (Mg,Ni)SO 4 ·H 2 O solid-solution series for the first time. It provides a detailed insight into the changes in lattice parameters, structural details, and positions of prominent bands in infrared (transmission, attenuated total reflectance, diffuse reflectance) and Raman spectra in synthetic samples as the Ni/Mg ratio progresses, at both ambient as well as low temperatures relevant for the icy moons and Mars. UV-Vis-NIR crystal field spectra of the Ni end-member also help to elucidate the influence of Ni 2+ -related bands on the overtone- and combination modes. The (Mg,Ni)SO 4 ·H 2 O solid-solution series shows Vegard-type behavior, i.e., lattice parameters as well as spectral band positions, change along linear trends with increasing Ni content. Infrared spectra reveal significant changes in the wavenumber positions of prominent bands, depending on the Ni/Mg ratio. We show that the temperature during measurement also has an influence on band position, mainly in the case of H 2 O-related bands. The changes observed for several absorption features in the IR spectra enable rough estimation of the Ni/Mg ratio in the monohydrate sulfate, which is applicable to present and future remote sensing data, as well as in situ measurements on Mars or the icy moons. The spectral features most diagnostic of composition are the vibrational stretching modes of the H 2 O molecule and a band unique to kieserite-group compounds at around 900 cm –1 in the IR spectra, as well as the pronounced ν 3 and ν 1 sulfate stretching modes visible in Raman spectra.

Central aims of IODP Expedition 352 were to delineate and characterize the magmatic stratigraphy in the Bonin forearc to define key magmatic processes associated with subduction initiation and their potential links to ophiolites. Expedition 352 penetrated 1.2 km of magmatic basement at four sites and recovered three principal lithologies: tholeiitic forearc basalt (FAB), high-Mg andesite, and boninite, with subordinate andesite. Boninites are subdivided into basaltic, low-Si, and high-Si varieties. The purpose of this study is to determine conditions of crystal growth and differentiation for Expedition 352 lavas and compare and contrast these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites. Cr# (cationic Cr/Cr+Al) vs. TiO 2 relations in spinel and clinopyroxene demonstrate a trend of source depletion with time for the Expedition 352 forearc basalt to boninite sequence that is similar to sequences in the Oman and other suprasubduction zone ophiolites. Clinopyroxene thermobarometry results indicate that FAB crystallized at temperatures (1142–1190 °C) within the range of MORB (1133–1240 °C). When taking into consideration liquid lines of descent of boninite, orthopyroxene barometry and olivine thermometry of Expedition 352 boninites demonstrate that they crystallized at temperatures marginally lower than those of FAB, between ~1119 and ~1202 °C and at relatively lower pressure (~0.2–0.4 vs. 0.5–4.6 kbar for FAB). Elevated temperatures of boninite orthopyroxene (~1214 °C for low-Si boninite and 1231–1264 °C for high-Si boninite) may suggest latent heat produced by the rapid crystallization of orthopyroxene. The lower pressure of crystallization of the boninite may be explained by their lower density and hence higher ascent rate, and shorter distance of travel from place of magma formation to site of crystallization, which allowed the more buoyant and faster ascending boninites to rise to shallower levels before crystallizing, thus preserving their high temperatures.

The evolutionary system of mineralogy relies on varied physical and chemical attributes, including trace elements, isotopes, solid and fluid inclusions, and other information-rich characteristics, to understand processes of mineral formation and to place natural condensed phases in the deep-time context of planetary evolution. Part I of this system reviewed the earliest refractory phases that condense at T > 1000 K within the turbulent expanding and cooling atmospheres of highly evolved stars. Part II considers the subsequent formation of primary crystalline and amorphous phases by condensation in three distinct mineral-forming environments, each of which increased mineralogical diversity and distribution prior to the accretion of planetesimals >4.5 billion years ago. (1) Interstellar molecular solids : Varied crystalline and amorphous molecular solids containing primarily H, C, O, and N are observed to condense in cold, dense molecular clouds in the interstellar medium (10 < T < 20 K; P < 10 –13 atm). With the possible exception of some nanoscale organic condensates preserved in carbonaceous meteorites, the existence of these phases is documented primarily by telescopic observations of absorption and emission spectra of interstellar molecules in radio, microwave, or infrared wavelengths. (2) Nebular and circumstellar ice : Evidence from infrared observations and laboratory experiments suggest that cubic H 2 O (“cubic ice”) condenses as thin crystalline mantles on oxide and silicate dust grains in cool, distant nebular and circumstellar regions where T ~100 K. (3) Primary condensed phases of the inner solar nebula : The earliest phase of nebular mineralogy saw the formation of primary refractory minerals that solidified through high-temperature condensation (1100 < T < 1800 K; 10 –6 < P < 10 –2 atm) in the solar nebula more than 4.565 billion years ago. These earliest mineral phases originating in our solar system formed prior to the accretion of planetesimals and are preserved in calcium-aluminum-rich inclusions, ultra-refractory inclusions, and amoeboid olivine aggregates.

White (Mg-rich) and green (Ni-rich) clay infillings (“deweylite”/“garnierite”) found in serpentine veins of faulted peridotite formations from New Caledonia consist of an intimate mixture of fine-grained and poorly ordered 1:1 and 2:1 layer silicates, commonly referred to as non-expandable serpentine-like (SL) and talc-like (TL) minerals. New data on the swelling and shrinking capacity of these layer silicates were gathered from X‑ray diffraction (XRD) after saturation of the clay fractions with different cations (Ca 2+ , Li + , K + ), ethylene glycol (EG) solvation, and heat treatments. Simultaneously, layer charge distribution and vacancy density, respectively, were investigated by FTIR spectroscopy on NH 4 -saturated clay fractions and XRD on Li-saturated clay fractions before and after heating (Hofmann Klemen treatment). Five clay infillings, with dominant 2:1 layer silicates and variable Ni contents, were selected for this study, from a large set of veinlets, according to their swelling capacity. The crystal chemistry of these samples was characterized by FTIR spectroscopy and bulk chemical analyses. The swelling ability of the clay infillings is attributed to the 2:1 layer silicates. It does not seem to be affected by the relative fraction of Mg and Ni in their octahedral sheets. In XRD patterns, the swelling ability is reflected by slight shifts of the basal reflection of the 2:1 layer silicates toward low angles for bulk samples and by splitting of the peak into two contributions for clay fractions saturated with Ca (or Li) and solvated with EG. The split increases with the swelling capacity of the sample. It originates mainly from octahedral-layer charge generated by vacant sites. Such results lead us to consider the 2:1 layer silicates of the infillings as an intimate mixture of non-expandable (TL) and expandable (stevensite) phases. In agreement with previous studies that suggested a contribution of hydrothermal processes in the alteration of serpentine species into 2:1 layer silicates, we propose that the proportion of expandable phases in the clay infillings (or vacancy sites in the octahedral sheets of the 2:1 layer silicates) could be used as an efficient means for assessing the temperature of their formation. Clay infillings mostly made of stevensite would have formed at ambient temperatures, whereas those consisting mainly of non-expandable TL would have formed at higher temperatures.

An estimate of TiO 2 activity (aTiO2melt−sat)$\left( a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}} \right)$is necessary for the application of trace-element thermobarometry of magmatic systems where melts are typically undersaturated with respect to rutile/anatase. Experiments were performed in the system SiO 2 -Na 2 O-TiO 2 to develop two independent methods of estimating aTiO2melt−sat$a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}}$—one based on the commonly applied rutile-saturation technique and another utilizing a novel Ti-in-tridymite thermometer. It is demonstrated that the rutile-saturation model can lead to an overestimate of aTiO2melt−sat$a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}}$relative to TiO 2 activity calculated using the solubility of Ti in tridymite (SiO 2 ) coexisting with rutile. Overestimation via the rutile-saturation technique is due to variations in the solubility mechanisms of Ti in the melt phase as a function of Ti content. In natural systems, overestimates of aTiO2melt−sat$a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}}$will lead to an underestimation of crystallization temperatures by Ti-based trace-element thermobarometers. Although this study is not directly applicable to natural systems, it lays the groundwork for future research on natural composition magmas to constrain TiO 2 activity in melts.

Circumstantial evidence for the sources of uranium in ore deposits may be drawn from the study of deposit geochemistry and mineralogy. However, direct evidence supporting uranium leaching from source rocks has rarely been found. This study investigates the source of uranium in the Baiyanghe deposit in the Xiemisitai Mountains, northwest China. The main uranium ore bodies occur as fracture-fillings along contact zones between the Yangzhuang granite porphyry and the Devonian volcanic rocks. Zircon, thorite, columbite-(Mn), and bastnäsite are the dominant accessory minerals that host uranium in the granite porphyry. In situ columbite-(Mn) LA-ICP-MS U-Pb dating yields a weighted mean 206 Pb/ 238 U age of 310 ± 4 Ma, suggesting that the Yangzhuang granite porphyry was emplaced during the Late Carboniferous. Backscattered electron (BSE) images reveal that various degrees of alteration of these same accessory minerals may be observed in the granite porphyry, and the altered domains of these minerals have lower BSE intensities compared to the unaltered domains. Results indicate that the altered domains of zircon grains have lower concentrations of Zr, Si, and U, and higher concentrations of Y, Fe, Ca, and P relative to the unaltered domains, and the altered domains of columbite-(Mn) grains are enriched in Ti and Fe and are depleted in Nb, Ta, Mn, U, and Zr. The altered domains of thorite grains have higher concentrations of Zr, Fe, Ca, Nb, and P, and lower Th and U compared to those of the relict domains. The petrochemical data indicate that the granite porphyry experienced losses in U, Be, F, Ba, Sr, Pb, Zr, Mo, Nb, Ta, and Hf during alteration. These results suggest that the past-magmatic hydrothermal fluids might be responsible for the mobilization of uranium form minerals in the granite porphyry. It is concluded that U-bearing accessory minerals in the granite porphyry were the primary source of uranium, and that post-magmatic hydrothermal processes caused remobilization and significant localized enrichment of the uranium to form high-grade ores as fracture-fillings along its contacts.

The genesis of high-silica igneous rocks is important for understanding the behavior of shallow magmatic systems. However, although many such studies have focused on the eruption of crystal-poor high-SiO 2 rhyolites, the origin of high-silica granites (HSGs) has received comparatively little attention. Here, we present a detailed study of HSGs from the Narusongduo volcanic complex, Gangdese arc. Combining zircon U-Pb geochronology with stratigraphic investigations, we show that the Narusongduo magmatic system was constructed over a period of ≥3.7 Myr with or without lulls. On the basis of zircon textures and ages, diverse zircon populations, including antecrysts and autocrysts, are recognized within the HSGs and volcanic rocks. All of the igneous rocks within the Narusongduo volcanic complex have highly radiogenic Sr–Nd isotopic compositions. Our results indicate the presence of an andesitic magma reservoir in the upper crust at a paleodepth of ~8 km. Ubiquitous zircon antecrysts in the HSGs, combined with compositional similarities between the HSGs and evolved melts of the andesitic magma reservoir, indicate that the Narusongduo HSGs represent melts extracted from the shallow magma reservoir. In addition, our results suggest that magma recharge promoted the escape of high-silica melts to form the Narusongduo HSGs. This work presents an excellent case that kilometer-scale high-silica granites are the differentiated products from an upper crustal magma reservoir. It would make a contribution to contemporary debates concerning the efficiency of crystal–melt separation in upper crustal magmatic systems.

In this comment, we show mathematically that the equations of Angel et al. (2017) and Guiraud and Powell (2006) are equivalent. The tiny difference is due to different definitions of strain.

We provide a further algebraic proof that the lines of entrapment conditions for inclusions calculated with the formula of Guiraud and Powell (2006) are not thermodynamic isomekes and therefore do not represent exactly lines of possible entrapment conditions.

Pb-dominant tourmaline was synthesized at 700 °C and 200 MPa in two hydrothermal experiments in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -PbO-H 2 O (run OV-4-2) and MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -PbO-CaO-Na 2 O-H 2 O (run OV-5-3), respectively. Run OV-4-2 forms needle-like (lengths up to 7 μm), lead-rich (up to 13.3 wt% PbO) crystals that are chemically homogeneous. Run OV-5-3 forms columnar (lengths up to 400 μm) crystals that are chemically zoned (Pb-rich cores, up to 14.7 wt% PbO, and Pb-poor rims, ~2 wt% PbO). Additional phases that form in trace amounts are Pb-feldspar, quartz, diaspore (in OV-4-2) and talc, mullite, spinel, quartz (in OV-5-3). Single-crystal structure refinement (SREF) of the central zone of Pb-rich tourmaline from the run OV-5-3 proves that Pb 2+ cations occupy the X -site in the tourmaline structure. The unit-cell parameters of the studied tourmaline are: a = 15.9508(10) Å, c = 7.2024(6) Å. The formula derived from SREF results of this Pb-rich tourmaline is X (Pb 0.63 ◻ 0.37 ) Y (Al 1.71 Mg 1.29 ) Z (Al 5.04 Mg 0.96 ) T (Si 6.00 O 18 ) (BO 3 ) 3 V (OH) 3.00 W (O 1.00 ). Accordingly, the studied crystal is a Pb-analog of hypothetical “oxy-uvite,” and thus referred to here as “Pb-oxy-uvite.” Similarities between (1) the paragenesis of Minh Tien tourmaline, and (2) the final experimental phase assemblages observed here, indicate comparable P-T conditions of formation.

Element partition coefficients play key roles in understanding various geological processes and are typically measured by performing high-temperature–pressure (HTP) experiments. In HTP experiments, samples are usually enclosed in capsules made of noble metals. Previous studies have shown that Fe, Ni, and Cu readily alloy with noble metals, resulting in significant loss of these elements from the experimental samples. The loss of elements could severely undermine phase equilibrium and compromise the validity and accuracy of the obtained partition coefficients. However, it remains unclear if other elements (in addition to Fe, Ni, and Cu) will also be lost from samples during HTP experiments, and how to minimize such losses. We performed a series of experiments at 1 GPa and 1400 °C to investigate which elements will be lost from samples and explore the influence of capsule materials and oxygen fugacity ( f O2 ) on the loss behavior of elements. The starting material is a synthesized basaltic glass consisting of 8 major elements and 37 trace elements. The sample capsules included platinum (Pt), graphite-lined Pt, and rhenium-lined Pt, and the experimental oxygen fugacity ( f O2 ) was buffered from <FMQ-2 to ~FMQ+5. Results show that: (1) 15 elements (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Cd, In, Sn, W, and Mo) were lost from the sample due to direct contacting and alloying with Pt under graphite-bufered conditions; (2) graphite and Re lining can physically isolate the starting material from Pt and prevent the loss of V, Cr, Mn, Fe, Zn, Ga, Ge, Cd, In, Sn, W, and Mo, but only slightly reduce the loss of Ni and Cu; and (3) element loss can be significantly reduced under oxidizing conditions, and all elements except Cu were retained in the samples under Ru–RuO 2 buffered conditions. These findings provide several viable capsule assemblies that are capable of preventing or reducing element loss, which may prove useful in determining accurate partition coefficients in HTP experiments.

In this issue This New Mineral Names has entries for 16 new minerals, including minerals of the volcanic fumarols: acmonidesite, elasmochloite, russoite, and sbacchiite; ammoniolasalite; minerals of Ca-Al-rich inclusions in Allende CV3 chondrite: beckettite, burnettite, and paqueite; new terrestrial phosphides: grammatikopoulosite, halamishite, murashkoite, negevite, transjordanite, tsikourasite, and zuktamrurite; kamenevite and new data on oyelite.