The quest for the solar g modes

Solar gravity modes (or g modes)—oscillations of the solar interior on which buoyancy acts as the restoring force—have the potential to provide unprecedented inference on the structure and dynamics of the solar core, inference that is not possible with the well-observed acoustic modes (or p modes). The relative high amplitude of the g-mode eigenfunctions in the core and the evanesence of the modes in the convection zone make the modes particularly sensitive to the physical and dynamical conditions in the core. Owing to the existence of the convection zone, the g modes have very low amplitudes at photospheric levels, which makes the modes extremely hard to detect. In this article, we review the current state of play regarding attempts to detect g modes. We review the theory of g modes, including theoretical estimation of the g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the techniques that have been used to try to detect g modes. We review results in the literature, and finish by looking to the future, and the potential advances that can be made—from both data and data-analysis perspectives—to give unambiguous detections of individual g modes. The review ends by concluding that, at the time of writing, there is indeed a consensus amongst the authors that there is currently no undisputed detection of solar g modes.     

Long-term variation in the anticyclonic ocean circulation over the Zapiola Rise as observed by satellite altimetry: Evidence of possible collapses

The Zapiola Rise (ZR) is a singular sedimentary deposit about 1200 m in height and 1500 km in width located in the Argentine Basin. In situ and satellite observations have revealed the presence of an intense counterclockwise circulation around the feature, with a volume transport comparable to those of the major ocean currents. The existence of a very low-frequency variability of the transport associated with the anticyclonic circulation is documented for the first time. As the Zapiola anticyclonic circulation plays a significant role in the mixing of the strongly contrasted water masses of the South Atlantic, variations in the anticyclonic transport can have a major impact on the mixing, hence a role in global climate variability. The circulation was clearly anticyclonic in the periods 1993–1999 and 2002–2007. In contrast, the 1999–2001 period did not show evidence of an anticyclonic flow in the mean surface velocity field. Moreover, the analysis of the weekly fields during that period of time revealed a cyclonic pattern from time to time. Previous work has shown that the flow can be considered as purely barotropic over the ZR region. A 15-year time-series of the transport was produced using absolute altimeter-derived geostrophic velocities. The estimated transport presents high-frequency variability associated with mesoscale activity superimposed on a low-frequency signal. The amplitude of the estimated transport is in good agreement with the only in situ-derived estimation available (80 Sv, January 1993). The low-frequency signal presents a minimum during the period 1999–2001, further suggesting that at times the Zapiola anticyclonic flow may have significantly decreased in strength or even vanished. Possible causes of the low-frequency variability are discussed.     

Investigation on numerical accuracy of ZAMS models of one solar mass

Helioseismology requires solar models computed with great accuracy. Results of investigations about the incidence in ZAMS models of one solar mass of the numerical errors arising from the interpolation of opacity tables and from the integration of the atmosphere are reported.Though the numerical accuracy reached by some actual codes is of the order of 10^–4 (Christensen-Dalsgaard, 1989), it looks optimistic to expect a numerical accuracy better than few percent for models without mesh refinements in opacity tables and more consistency between atmospheric and internal structure models.Since most of the calculations were made with a new code based on spline/collocation, the numerical method used is outlined in an Appendix.     

Mineralogical and geochemical evolution of the 1982–1983 Galunggung eruption (Indonesia)

Pyroclastic deposits from the 1982–1983 eruption of Galunggung volcano (Java, Indonesia) reflect preeruptive magmatic evolution which is of interest because of: (1) its duration of nine months, compared to a few hours or days for most historical eruptions; (2) the diversity of eruptive styles, from ash and scoria flows to phreatomagmatic explosions, and to the strombolian activity that marked the end of the eruption; and (3) the progressive variation in chemical composition with time, from andesite (58 wt.% SiO₂) to high-Mg basalt (47 wt.% SiO₂). The 1982–1983 Galunggung basalts are rather primitive: 10 to 12 wt% MgO, 180 to 200 ppm Ni and 550 to 700 ppm Cr. Despite the presence of about 40% phenocrysts, they may represent the most primitive basalts recognized in western Java. Basalts contain phenocrysts of olivine (Fo_90-80), diopside-salite, and plagioclase (An_95-75). Andesites contain plagioclase (An_80–60), augite, hypersthene (En_67-64), and titanomagnetite. The distribution of mineral compositions in each petrographic type is nearly unimodal, although scarce plagioclase and olivine xenocrysts have been observed. Abundance of gabbroic cumulates associated with the pyroclastic flows and evolution of mineral compositions from high-Mg basalts to andesites support crystal fractionation as the main differentiation mechanism, although magma mixing of basaltic andesite and andesite cannot be excluded. Major and trace element trends, which display rough decreases of MgO, CaO, Ni, Cr with increasing degree of differentiation and also linear positive correlations of hygromagmaphile elements, are compatible with both processes. However, some discrepancies are observed between major and trace element modelling, which may be explained to some extent by the influence of in situ crystallization and/or magma mixing. The constancy of ¹⁴³Nd/¹⁴⁴Nd (0.51286±3), ²³⁰Th/²³²Th (0.65±0.02), Th/U (4.08±0.07) ratios, and to a lesser extent ¹⁸O values (+5.8 to +6.4 % SMOW) and ⁸⁷Sr/⁸⁶Sr ratios (0.70440 to 0.70468) is compatible with a magmatic evolution through fractional crystallization without significant crustal contamination. Nevertheless low-¹⁸O and high ⁸⁷Sr/⁸⁶Sr values in basaltic andesites may be due to the introduction of meteoric fluids into the Galunggung magma.     

Kinetics of heterogeneous bubble nucleation in rhyolitic melts: implications for the number density of bubbles in volcanic conduits and for pumice textures

We performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic magma in a volcanic conduit. The starting materials were bubble-free rhyolites water-saturated at 200 MPa–800°C under oxidizing conditions: they contained 6.0 wt% dissolved H₂O and a dense population of hematite crystals (8.7 ± 2 × 10⁵ mm⁻³). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800°C); the rate of decompression was either 1,000 or 27.8 kPa/s. In all experiments, we observed a single event of heterogeneous bubble nucleation beginning at a pressure P _N equal to 63 ± 3 MPa in the 1,000 kPa/s series, and to 69 ± 1 MPa in the 27.8 kPa/s series. Below P _N, the degree of water supersaturation in the liquid rapidly decreased to a few 0.1 wt%, the nucleation rate dropped, and the bubble number density (BND) stabilized to a value strongly sensitive to decompression rate: 80 mm⁻³ at 27.8 kPa/s vs. 5,900 mm⁻³ at 1,000 kPa/s. This behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite-H₂O system and in numerical simulations of vesiculation in ascending magmas. Similar degrees of water supersaturation were measured at 27.8 and 1,000 kPa/s, implying that a faster decompression rate does not result in a larger departure from equilibrium. Our experimental results imply that BNDs in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate and the number density of phenocrysts, especially the number density of magnetite microphenocrysts (1–100 mm⁻³), which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation. Very low BNDs (≈1 mm⁻³) are predicted in the case of effusive eruptions ( ≈ 0.1 kPa/s). High BNDs (up to 10⁷ mm⁻³) and bimodal bubble size distributions are expected in the case of explosive eruptions: (1) a relatively small number density of bubbles (1–100 mm⁻³) will first nucleate in the lower part of the conduit ( ≈ 10 kPa/s), either at high pressure on magnetite or at lower pressure on quartz and feldspar (or by homogeneous nucleation in the liquid) and (2) then, extreme decompression rates near the fragmentation level ( ≈ 10³ kPa/s) will trigger a major nucleation event leading to the multitude of small bubbles, typically a few micrometers to a few tens of micrometers in diameter, which characterizes most silicic pumices.     

Pedogenetic and diagenetic fabrics in the Upper Proterozoic Sarnyëré formation (Gourma, Mali)

Several diagenetic fabrics are described in the uppermost member of the Sarnyéré formation, an Upper Proterozoic dolomite (700-600 Ma). Micronodular micrite, microsparite showing nodulisation processes, orbicular crusts and oncoids are thought to be due to the pedogenetic alteration of an algal (Porostromata) boundstone. Further evolution in a vadose zone creates a network of cavities which are later filled by “cave stromatolites”, micropopcorn micrite and multilayered cements. A rhythmical repetition of facies evidences a polyphased pedogenetic history, leading to a 30 m thick highly complex section. Such numerous and diversified structures are rarely preserved, especially in Proterozoic rocks, but each of them have equivalents in recent examples of caliche soil profiles.     

Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions

As the water concentration in magma decreases during magma ascent, olivine-hosted melt inclusions will reequilibrate with the host magma through hydrogen diffusion in olivine. Previous models showed that for a single spherical melt inclusion in the center of a spherical olivine, the rate of diffusive reequilibration depends on the partition coefficient and diffusivity of hydrogen in olivine, the radius of the melt inclusion, and the radius of the olivine. This process occurs within a few hours and must be considered when interpreting water concentration in olivine-hosted melt inclusions. A correlation is expected between water concentration and melt inclusion radius, because small melt inclusions are more rapidly reequilibrated than large ones when the other conditions are the same. This study investigates the effect of diffusive water loss in natural samples by exploring such a correlation between water concentration and melt inclusion radius, and shows that the correlation can be used to infer the initial water concentration and magma ascent rate. Raman and Fourier transform infrared spectroscopy measurements show that 31 melt inclusions (3.6–63.9 μm in radius) in six olivines from la Sommata, Vulcano Island, Aeolian Islands, have 0.93–5.28 wt% water, and the host glass has 0.17 wt% water. The water concentration in the melt inclusions shows larger variation than the data in previous studies (1.8–4.52 wt%). It correlates positively with the melt inclusion radius, but does not correlate with the major element concentrations in the melt inclusions, which is consistent with the hypothesis that the water concentration has been affected by diffusive water loss. In a simplified hypothetical scenario of magma ascent, the initial water concentration and magma ascent rate are inferred by numerical modeling of the diffusive water loss process. The melt inclusions in each olivine are assumed to have the same initial water concentration and magma ascent rate. The melt inclusions are assumed to be quenched after eruption (i.e., the diffusive water loss after eruption is not considered). The model results show that the melt inclusions initially had 3.9–5.9 wt% water and ascended at 0.002–0.021 MPa/s before eruption. The overall range of ascent rate is close to the lower limit of previous estimates on the ascent rate of basalts.     

Intercalibration of Mg Isotope Delta Scales and Realisation of SI Traceability for Mg Isotope Amount Ratios and Isotope Delta Values

The continuous improvement of analytical procedures using multi‐collector technologies in ICP‐mass spectrometry has led to an increased demand for isotope standards with improved homogeneity and reduced measurement uncertainty. For magnesium, this has led to a variety of available standards with different quality levels ranging from artefact standards to isotope reference materials certified for absolute isotope ratios. This required an intercalibration of all standards and reference materials, which we present in this interlaboratory comparison study. The materials Cambridge1, DSM3, ERM‐AE143, ERM‐AE144, ERM‐AE145, IRMM‐009 and NIST SRM 980 were cross‐calibrated with expanded measurement uncertainties (95% confidence level) of less than 0.030‰ for the δ^25/24Mg values and less than 0.037‰ for the δ^26/24Mg values. Thus, comparability of all magnesium isotope delta (δ) measurements based on these standards and reference materials is established. Further, ERM‐AE143 anchors all magnesium δ‐scales to absolute isotope ratios and therefore establishes SI traceability, here traceability to the SI base unit mole. This applies especially to the DSM3 scale, which is proposed to be maintained. With ERM‐AE144 and ERM‐AE145, which are product and educt of a sublimation–condensation process, for the first time a set of isotope reference materials is available with a published value for the apparent triple isotope fractionation exponent θ_app, the fractionation relationship ln α(^25/24Mg)/ln α(^26/24Mg).