Volatile Budget Research Articles

Differences between the impact regimes of the terrestrial planets: Implications for primordial D:H ratios

It is often assumed that the terrestrial worlds have experienced identical impact regimes over the course of their formation and evolution, and, as a result, would have started life with identical volatile budgets. In this work, through illustrative dynamical simulations of the impact flux on Venus, the Earth, and Mars, we show that these planets can actually experience greatly different rates of impact from objects injected from different reservoirs. For example, we show scenarios in which Mars experiences far more asteroidal impacts, per cometary impactor, than Venus, with the Earth being intermediate in value between the two. This difference is significant, and is apparent in simulations of both quiescent and highly stirred asteroid belts (such as could be produced by a mutual mean-motion resonance crossing between Jupiter and Saturn, as proposed in the Nice model of the Late Heavy Bombardment). We consider the effects; such differences would have on the initial volatilisation of the terrestrial planets in a variety of scenarios of both endogenous and exogenous hydration, with particular focus on the key question of the initial level of deuteration in each planet's water budget. We conclude that each of the terrestrial worlds will have experienced a significantly different distribution of impactors from various reservoirs, and that the assumption that each planet has the same initial volatile budget is, at the very least, a gross over-simplification.

Geochemistry of volcanic and hydrothermal fluids and volatile budget of the Kamchatka–Kuril subduction zone

A data base for the composition and emission rates of more than 100 thermal manifestations including boiling geothermal systems and 23 volcanoes along the 1900 km long Kamchatka–Kuril (KK) arc is presented. These results were used to estimate mean fluxes of volatiles from the KK arc. The fluxes from the KK arc are compared with the fluxes from the best studied Central American (CA) arc and with the compiled literature data on global fluxes. The error ranges and the OUT/IN (in)balance calculations are also discussed. The estimated fluxes of volatiles from volcanic fumaroles and the observed, normalized to the Cl content, fluxes from hydrothermal systems are very close, with the higher hydrothermal flux from Kuril Islands due to a larger number of the acidic Cl–SO 4 springs on the Islands and their outflow rates. The total volcanic SO 2 flux from the whole KK arc is estimated to be higher than 3000 t/d. The measured S and C fluxes from hydrothermal systems are much lower than the volcanic output due to the loss of these components in the upper crust (mineral precipitation). The Cl/ 3He ratio is inferred to be a stable indicator of the arc setting for hydrothermal and volcanic fluids with a mean value of (2 ± 4) × 10 9. Comparison of the obtained volcano–hydrothermal fluxes with fluxes calculated from the erupted solid volcanic products at Kamchatka and Kurils during Holocene time reveals that the total estimated volatile output from the KK arc is compatible with the total magmatic output if the intruded to erupted ratio is close to 7, i.e. almost the same as assumed for the Central American arc. Calculated fluxes as well as the ratios for OUT/IN fluxes (volcanic + hydrothermal output/slab + mantle input) for CO 2, S, H 2O, Cl, N 2, 4He and 3He from the KK arc normalized to the arc length are in general close to the global estimates. The fractions of CO 2 and S in the total volatile output at KK arc derived directly from the mantle wedge are 18% and 16% (mole basis), respectively. Fractions of mantle derived H 2O, N 2 and Cl are much lower, less that 5% of their output.

Total volatile flux from Mount Etna

The Total Volatile (TV) flux from Mount Etna volcano has been characterised for the first time, by summing the simultaneously‐evaluated fluxes of the three main volcanogenic volatiles: H2O, CO2 and SO2. SO2 flux was determined by routine DOAS traverse measurements, while H2O and CO2 were evaluated by scaling MultiGAS‐sensed H2O/SO2 and CO2/SO2 plume ratios to the UV‐sensed SO2 flux. The time‐averaged TV flux from Etna is evaluated at ∼21,000 t·day−1, with a large fraction accounted for by H2O (∼13,000 t·day−1). H2O dominates (≥70%) the volatile budget during syn‐eruptive degassing, while CO2 and H2O contribute equally to the TV flux during passive degassing. The CO2 flux was observed to be particularly high prior to the 2006 eruption, suggesting that this parameter is a good candidate for eruption prediction at basaltic volcanoes.

The role of mantle-derived volatiles in the petrogenesis of Palaeoproterozoic ferropicrites in the Pechenga Greenstone Belt, northwestern Russia: Insights from in-situ microbeam and nanobeam analysis of hydromagmatic amphibole

New in-situ trace element and hydrogen isotopic data of hydromagmatic amphiboles from ferropicritic gabbro–wehrlite intrusions and lava flows of the Palaeoproterozoic Pechenga Greenstone Belt (Russia) provide insights into the controversial source of volatiles that are implicated in Precambrian ultramafic magmatism. The intrusions mainly contain oikocrystic kaersutites and pargasites, whereas lava flows generally include idiomorphic Fe-kaersutites and Fe-pargasites. All orthomagmatic amphiboles display slightly LREE-enriched chondrite-normalised patterns. Primitive mantle-normalised patterns of amphiboles from the intrusions display positive Nb–Ta, moderate negative Th–U and distinctive negative Sr–Li anomalies. Conversely, Fe-kaersutites and Fe-pargasites from the lava flows display undepleted Th, U, Sr and Li signatures. Boron and Cl enrichment is primarily observed in secondary edenites, which locally rim orthomagmatic kaersutites. In-situ δD measurements of hydromagmatic amphiboles from the gabbro–wehrlite intrusions display values ranging between −70‰ and −170‰, and total water contents between 1 and 3 wt.%. Conversely, primary hydromagmatic amphiboles from the Pechenga flows display lighter δD compositions ranging between −100‰ and −270‰, and lower (<2 wt.%) total water contents.The Ti– and Nb–Ta-enriched nature of hydromagmatic amphiboles from the intrusions and lava flows indicates that the source of volatiles in the Pechenga ferropicrites could be either a metasomatised subcontinental mantle or a volatile-bearing plume. Positive Nb–Ta/La anomalies in amphiboles from the intrusions rule out crustal contamination with granitoid crust during petrogenesis and emplacement as the main source of the volatiles. Local B–Cl enrichment suggests that some samples may have interacted with hydrothermal fluids. The range of in-situ δD values in hydromagmatic amphiboles is consistent with the presence of magmatic water. The lighter hydrogen isotopic composition and lower water contents of the primary igneous amphiboles from the lava flows compared to amphiboles from the intrusions indicate that the lava flows degassed upon emplacement.Previous studies investigating the role of volatiles in Precambrian ultramafic magmatism preferentially collected data from thin lava flows, whereas thicker flows or intrusions have been generally neglected. We emphasise the necessity to re-evaluate the controversial role of volatiles in the petrogenesis of ultramafic magmas by investigating evidence from thicker flows or intrusions, which may have retained their full volatile budget.

Noble gas signature of the Late Heavy Bombardment in the Earth's atmosphere

The Lunar cratering record is consistent with the occurrence of a late heavy bombardment (LHB), which marked the end of terrestrial planet accretion 3.8 billion years ago. However, clear evidence of a LHB on Earth has not yet been identified. Based on a volatile budget of the terres- trial mantle, the atmosphere and hydrosphere we propose that the LHB did indeed occur on Earth and that we are breath- ing its aftermaths. The terrestrial atmosphere and hydro- sphere is enriched in noble gases relative to the abundance of volatiles in the mantle. This enrichment is consistent with the mass delivered to Earth during the LHB, as recently proposed from dynamical modelling (Gomez et al., 2005), if this mate- rial comprised a few Kuiper-belt (cometary) objets (KBOs) mixed in with a population of largely chondritic (i.e. aster- oidal) impactors. The fraction of KBOs necessary to account for the atmospheric composition is, however, much lower (<1%) than the one ( 50%) inferred from modelling.

Subduction zone evolution and low viscosity wedges and channels

Dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased. Such a decrease in viscosity could form a low viscosity wedge (LVW) or a low viscosity channel (LVC) on top of the subducting slab. Using numerical models, we investigate the influence of low viscosity wedges and channels on subduction zone structure. Slab dip changes substantially with the viscosity reduction within the LVWs and LVCs. For models with or without trench rollback, overthickening of slabs is greatly reduced by LVWs or LVCs. Two divergent evolutionary pathways have been found depending on the maximum depth extent of the LVW and wedge viscosity. Assuming a viscosity contrast of 0.1 with background asthenosphere, models with a LVW that extends down to 400 km depth show a steeply dipping slab, while models with an LVW that extends to much shallower depth, such as 200 km, can produce slabs that are flat lying beneath the overriding plate. There is a narrow range of mantle viscosities that produces flat slabs (5 to10 × 10 19 Pa s) and the slab flattening process is enhanced by trench rollback. Slab can be decoupled from the overriding plate with a LVC if the thickness is at least a few 10 s of km, the viscosity reduction is at least a factor of two and the depth extent of the LVC is several hundred km. These models have important implications for the geochemical and spatial evolution of volcanic arcs and the state of stress within the overriding plate. The models explain the poor correlation between traditional geodynamic controls, subducting plate age and convergence rates, on slab dip. We predict that when volcanic arcs change their distance from the trench, they could be preceded by changes in arc chemistry. We predict that there could be a larger volatile input into the wedge when arcs migrate toward the trench and visa-versa. The transition of a subduction zone into the flat-lying regime could be preceded by changes in the volatile budget such that the dehydration front moves to shallower depths. Our flat-slab models shed some light on puzzling flat subduction systems, like in Central Mexico, where there is no deformation within the overriding plate above the flat segment. The lack of in-plane compression in Central Mexico suggests the presence of a low viscosity shear zone above the flat slab.

Early Earth systems: a geochemical approach

1. The Earth System. Introduction. The Nature of the Early Geological Record. Archaean Lithological Associations. The Oldest Rocks. How Much do we Really Know About the Early Earth?. 2. The Origin and Differentiation of the Earth. The Origin and Early History of the Universe. Star Formation. The Condensation of the Solar System. Earth Differentiation - the First Earth System. 3. The Evolution of the Earth's Mantle. Understanding the Mantle. The Earth's Earliest Mantle. Mantle Models. 4. The Origin of the Continental Crust. Modern Crust Formation - Models and Mechanisms. First Order Constraints on the Origin of the Continental Crust. The Secular Evolution of the Earth's Continental Crust. Crustal Growth During the Archaean. Crust-Mantle Interactions: Reservoirs and Fluxes. 5. The Origin of the Earth's Atmosphere and Oceans. The Volatile Budget of the Modern Earth. The Origin of the Earth's Atmosphere and Oceans. The Nature of the Archaean Atmosphere. The Nature of the Archaean Oceans. 6. The Origin of Life. Setting the Scene for Life. Geochemical Signals of Biological Activity. The Geological Record of Life's Origins. The Microbial Record of Life's Origins. In the Beginning... 7. Postscript. References. Index

Recent volatile evolution in the magmatic system of Hekla volcano, Iceland

Estimates of the volatile budgets for volcanic eruptions are often based on volatile concentrations measured in melt inclusions (MIs) versus groundmass glass. Here, we present new measurements and estimates of pre-eruptive volatile concentrations in magma from the Hekla volcano in Iceland based on MIs in recent basalts and basaltic icelandites from the 2000 eruption. The last eruption at Hekla occurred from 26 February to 8 March, 2000. It began with a short sub-Plinian phase followed by lava effusion that produced 0.17 km 3 of basaltic icelandite. Volatile (S, Cl, F and H 2O) and major element concentrations were measured in MIs trapped in olivine and plagioclase phenocrysts contained in the basaltic icelandite from the 2000 eruption and in basalt erupted in 1913 from a fissure adjacent to the Hekla volcano. Same elements were measured in the groundmass glass from the 2000 tephra. The observed compositional range of MIs from the basalt (MgO ≤ 7.73%) to the basaltic icelandite (MgO ≥ 2.51%) is readily explained by fractional crystallisation, allowing the prediction of the evolution of dissolved volatile concentrations within the magma plumbing system beneath Hekla. The incompatible behaviour of the volatile elements controls their concentrations in MIs during the crystal fractionation, and consequently the highest concentrations are found in the most evolved basaltic icelandite MIs, with the notable exception of sulphur. Volatile concentrations in the basaltic MIs exhibit strong linear correlations with K 2O and this relationship can be used to estimate the theoretical “expected” volatile contents in the basaltic icelandite melt just prior to eruption. Such an approach avoids underestimation of the pre-eruptive volatile contents measured in MIs and improves constraints on the volatile mass release into the atmosphere. The difference of volatile contents between “expected” concentrations and those present in the groundmass, scaled to the mass of erupted magma, yields maximum atmospheric mass loading of 0.10 Mt HCl, 0.17 Mt HF and 3.77 Mt SO 2 during the 2000 Hekla eruption, whereas calculations using the measured values in the inclusions give a significant lower mass yields of 0.05 Mt HCl, 0.17 Mt HF and 0.6 Mt SO 2. These results are discussed and compared to the data derived from remote sensing and snow (proxy of the gas phase) measurements.

Understanding State Government Appropriations for the Arts: 1976-1999

Using panel data analysis, we examine the relative importance of citizen and government characteristics on a highly discretionary and volatile budget item: state appropriations to arts agencies. Despite the unimportance of arts spending to most citizens, changes in arts spending appear to reflect citizen desires. Spending rises with per capita income, state revenues, and citizen political and social liberalism, but characteristics of state legislatures do not significantly affect spending.

Explosive basaltic volcanism of the Chikurachki Volcano (Kurile arc, Russia): Insights on pre-eruptive magmatic conditions and volatile budget revealed from phenocryst-hosted melt inclusions and groundmass glasses

Highly explosive eruptions of basaltic composition are relatively rare and poorly understood, yet they may be quite important in terms of atmospheric impact because of the generally much higher concentrations of S in basaltic systems compared with the typical explosive felsic eruptions. We have determined concentrations of H 2O, major, trace and volatile (S, Cl) elements by EPMA and SIMS from melt inclusions and groundmass glasses of the 1986, 1853 and prehistoric explosive eruptions of basaltic magmas from the Chikurachki volcano, northern Kurile arc. Melt inclusions are hosted by olivine (Fo 72–78), orthopyroxene (mg# = 72–75), clinopyroxene (mg# = 71–77) and plagioclase (An 74–96) phenocrysts. Estimated crystallization conditions were in the range from 910 to 1180 °C at less than 400 MPa total pressure and oxygen fugacity of NNO+ 1 to + 2 log units. Inclusion glasses are basaltic to andesitic in composition. Relative to N-MORB, the melts are enriched in LREE ([La / Sm] n = 1.8–2.6) and Sr ([Sr / Ce] n = 1.7–2.4), and show Nb, Zr and Ti depletions typical for arc magmas ([Nb / La] n = 0.2–0.3, [Zr / Sm] n = 0.6–0.9, [Ti / Gd] n = 0.4–1.1). The pre-erupted melts were volatile rich (up to 6.4 wt.% H 2O, 2900 ppm S and 1400 ppm Cl), which may explain the strongly explosive character of volcanic activity. Much lower volatile contents in the groundmass glasses compared to inclusions reflect the extent of eruptive magma degassing. The wide range of volatile contents in glass inclusions is interpreted as a result of complex patterns of magma ascent, phenocryst growth and partial degassing at high-level reservoirs before eruption. The most H 2O- and S-poor inclusions are close to groundmass values and may relate to magmas degassed in the summit reservoir, which drained back and mixed with less degassed magmas at depth. Estimations of total volatile release to the atmosphere, based on the difference between maximum melt inclusion contents and matrix glass, suggest that the 1986 and 1853 explosive eruptions released 3400 and 2500 metric tons H 2O, 300 and 200 tons SO 2 and 8 and 12 tons Cl, respectively. This is about ten times higher for SO 2 and ten times lower for Cl than for the 1989–1990 eruptions of the Redoubt volcano.

Determination of total H2O and CO2 budgets in evolving magmas from melt inclusion data

Numerical simulations of volcanic processes show that the preeruptive and eruptive dynamics depend substantially on the amount and composition of volatiles present in magma, either dissolved in the liquid or separated in a gas phase. Although the investigation of melt inclusions within crystals is providing increasingly abundant data on the amount of volatiles dissolved in magmatic liquids, classic approaches involving phase relationships and experimental petrology cannot provide the relative mass of gas and liquid phases in magma. This represents a serious limit to the simulation of the dynamics of magmatic and volcanic processes. Here, a theoretical framework based on mass balance equations for the volatile species water and carbon dioxide in crystallizing, degassing magmas is developed. The theory presented offers a systematic description of the mass relationships between dissolved and total volatiles in magma, and leads to the definition of the total volatile (TV) line concept. The TV line represents the locus of total volatile contents consistent with the amounts dissolved in the liquid and measured in melt inclusions. Appropriate expressions for TV lines for closed and open system conditions with respect to the gas phase, and for crystallizing and noncrystallizing magmas, are developed. The TV line approach allows discrimination between melt inclusions formed in closed versus open system conditions and determination of total volatile budgets for magmas evolving under closed system conditions, and it can constrain the degassing history of magmas evolving in open system conditions. Several examples of the use of the TV line approach to determine total H2O and CO2 budgets in evolving magmas starting from melt inclusion data are provided. Applications to real cases suggest that total (dissolved plus exsolved) carbon dioxide contents in magmas can be in some cases from 1 to more than 2 orders of magnitude greater than the dissolved amounts.

Plagioclase from planetary basalts: Chemical signatures that reflect planetary volatile budgets, oxygen fugacity, and styles of igneous differentiation

Plagioclase grains from 15 basalt suites from the Earth, Moon, Mars, and 4 Vesta were studied by electron and ion microprobe techniques. The results demonstrate that chemical signatures in plagioclase record three planetary aspects that affect basaltic magmatism: (1) Volatile element depletions and enrichments in the planetary bodies are recorded by the systematics of Na and K in plagioclase. Terrestrial and Martian plagioclase are enriched in the volatile elements relative to plagioclase from the Moon and Vesta, a result that mimics the overall volatile budgets of these planets. Furthermore, the volatile-depleted nature of both the Moon and Vesta imply some similarities in planetary origin and/or evolution of these bodies. (2) The overall oxygen fugacity conditions on a planet may be realized by the Eu/Sm ratios of planetary plagioclase grains. Our data show that plagioclase from the Moon and 4 Vesta has higher Eu/Sm relative to plagioclase from the Earth and Mars. These data are consistent with low oxygen fugacity conditions on the Moon and Vesta, where divalent Eu is more compatible within the plagioclase structure than other trivalent REE elements. (3) The incompatible element systematics of K, REE, Ba, Y, and Sr in plagioclase show that styles of igneous differentiation affect basaltic magmatism on planets and more specifically, within tectonic settings on each planet. Terrestrial plagioclase grains from continental flood, continental rift, and Hawaiian basalt suites are enriched in the incompatible elements K, Ce, Ba, and Sr compared with Ocean Floor and Island Arc suites. The enrichments in the former are caused by continental crust interaction with basaltic magmas, and/or derivation from undepleted (fertile) mantle sources. Lunar KREEP basalt plagioclase enrichment in incompatible elements reflect mare basalt assimilation of a KREEP component, or partial melting of a KREEP-rich source. The Martian incompatible element data show that plagioclase from Shergotty is enriched in incompatible elements relative to plagioclase from QUE 94201. The data are in agreement with the idea that most Martian shergottites (except for QUE) have assimilated an incompatible element-rich crustal component on their way to eruption on the Martian surface.

Survival of a proto-atmosphere through the stage of giant impacts: the mechanical aspects

When a giant impact occurs, atmosphere loss may occur due to global ground motion excited by a strong shock wave traveling in the planetary interior. Here, the relations between the ground motion and the amount of the lost atmosphere are systematically investigated through calculations of a spherically one-dimensional atmospheric motion for various initial atmospheric conditions. The fraction of the lost atmosphere to the total mass of the atmosphere is found to be controlled only by the ground velocity and, insensitive to the initial atmospheric conditions. Unlike the previous studies (Ahrens, 1990, Origin of the Earth, H.E. Newson, J.H. Jones (Eds.), pp. 211–227; Ahrens, 1993, Annu. Rev. Earth Planet. Sci. 21, 525–555; Chen and Ahrens, 1997, Phys. Earth Planet. Inter. 100, 21–26); the estimated loss fraction for the giant impact is only 20%. Significant escape occurs only when the ground velocity is close to the escape velocity. Thus, most of the atmosphere should survive the giant impact. The cause of the difference from previous estimates is discussed from energetic and dynamic points of view. Moreover, if our estimates are applied to the atmosphere of the impactor planet, a significant fraction of it is carried to the target planet. Survival of the proto-atmosphere has very important effects on the origin and evolution of the terrestrial planets' volatile budget.

Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis

Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6–15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8–4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust.

SIGNIFICANCE OF MUD VOLCANISM

Mud volcanism and diapirism have puzzled geoscientists for ∼2 centuries. They have been described onshore and offshore in many places on Earth, and although they occur in various tectonic settings, the majority of the features known to date are located in compressional tectonic scenarios. This paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described. Mud volcanoes show variable geometry (up to tens of kilometers in diameter and several hundred meters in height) and a great diversity regarding the origin of the fluid and solid phases. Gas (predominantly methane), water, and mud may be mobilized at subbottom depth of only a few meters but, in places, can originate from several kilometers depth (with minor crustal or mantle input). The possible contribution of mud extrusion to global budgets, both from quiescent fluid emission and from the extrusive processes themselves, is important. In regions where mud volcanoes are abundant, such as the collision zones between Africa and Eurasia, fluid flux through mud extrusion exceeds the compaction‐driven pore fluid expulsion of the accretionary wedge. Also, quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and, hence, to greenhouse climate.

In search of lost planets – the paleocosmochemistry of the inner solar system

The depletion of moderately volatile elements in planetesimals and planets is generally considered to be a result of removal of hot nebula gases. This theory can be tested with Sr isotopes. The calculated initial 87Sr/ 86Sr of the angrite parent body (APB), eucrite parent body (EPB), the Moon and the Earth are significantly higher than the initial Sr isotopic composition of the solar system despite the volatile-depleted nature of all of these objects. Calculated time-scales required to accomplish these increases in 87Sr/ 86Sr with a solar Rb/Sr in a nebula environment are >2 Myr for the APB, >3 Myr for the EPB and >10 Myr for the Moon. These times are more than an order of magnitude longer than that expected for cooling the nebula in the terrestrial planet-forming region and correspond to the period during which most of the mass already should have been accreted into sizeable planetesimals and even planets. Therefore, incomplete condensation of the nebula does not provide an adequate explanation for the depletion in moderately volatile elements. The data are better explained by a protracted history of depletion via more than one mechanism, including processes completely divorced from the earliest cooling of the circumstellar disk. The Sr model ages are maximum formation ages of the APB and EPB and indicate that these are most probably secondary objects. With independent estimates of their minimum age, a time-integrated Rb/Sr can be calculated for the precursor materials from which they formed. These are consistent with accretion of the APB and EPB from objects that at one stage may have resembled carbonaceous chondrite parent bodies in terms of volatile budgets. At some late stage there were large losses of volatiles, the most likely mechanism for which is very energetic collisions between planetesimals and proto-planets that, in the case of the Asteroid Belt, have since been lost. The same applies to the Moon, which presently has Rb/Sr=0.006 even though the material from which it formed had a time-integrated Rb/Sr ratio of ∼0.07, consistent with a precursor planet (Theia) that was even less volatile element-depleted than the present Earth (Rb/Sr=0.03). The time-integrated Rb/Sr of Theia is similar to the present Rb/Sr of Mars (0.07). There is suggestive evidence of a similar time-integrated value for the proto-Earth (∼0.09). Therefore, prior to the later stages of planet formation involving giant impacts between large objects, the inner solar system may have had relatively uniform concentrations of moderately volatile elements broadly similar to those found in volatile-depleted chondrites. Correlations of the present Rb/Sr ratios in planets and planetesimals with ratios of other volatile elements to Sr can be used to infer the time-integrated composition of precursor materials. The time-integrated inferred K/U ratios of the proto-Earth, as well as Theia, were ∼20 000, so that early radioactive heat production may have been ∼40% greater than that calculated by extrapolating back from the Earth’s present K/U. Higher C and S bulk concentrations may have led to concentrations in proto-cores of 0.6–1.5% C and 4–10% S. These are significantly higher than those anticipated from the degree of volatile depletion of the present silicate Earth (∼0.12% C, ∼1.3% S). If the late history of accretion did not involve large-scale re-equilibration of silicates and metal, the present core may have inherited such high C and S concentrations. In this case, S would be the dominant light element in the present core.

Deep fluids in subduction zones

The fluid inclusions preserved in high and ultrahigh pressure rocks provide direct information on the compositions of fluid phases evolved during subduction zone metamorphism, and on fluid–rock interactions occurring in such deep environments. Recent experiments and petrologic studies of eclogite–facies rocks demonstrate that stability of a number of hydrous phases in all rock systems allows fluid transport into the mantle sources of arc magmas, as well as into much deeper levels of the Earth's mantle. In eclogite–facies rocks, the presence of large ion lithophile elements (LILE) and light rare earths (LREE)-bearing hydrous phases such as epidote and lawsonite, together with HFSE repositories as rutile and other Ti-rich minerals, controls the trace element budget of evolved fluids and fluid-mediated cycling of slab components into the overlying mantle. Studies of fluid inclusions in eclogite–facies terrains suggest that subduction mainly evolves aqueous solutions, melts being produced only locally. Eclogite-facies rocks diffusely record processes of fluid–melt–rock interactions that exerted considerable control on the element and volatile budget of subduction fluids. Trace element fractionation during such interactions needs to be tested and quantified in more detail to achieve the ultimate compositions actually attained by fluids leaving off the slab. Variably saline inclusions with minor CO 2 and N 2 are trapped in rock-forming high pressure minerals; brines with up to 50% by weight dissolved solute are diffusely found in veins. The latter inclusions are residues after fluid–rock interactions and deposition of complex vein mineralogies: this evidence suggests increased mineral solubility into the fluid and formation, at a certain stage, of silicate-rich aqueous solutions whose geochemical behaviour and transport capacity can approach that of a melt phase. This is supported by experimental work showing high solubility of silicate components in fluids at high pressures. However, natural examples of inclusions trapping such a fluid and quantitative analyses of its major and trace element composition are not yet available. Fluids in high and very high pressure rocks do not move over large scales and the channelways of fluid escape from the slab are not yet identified. This suggests that only part of the slab fluid is lost and returned to the surface via magmatism; the remaining trapped fraction being subducted into deeper levels of the upper mantle, to renew its budget of substances initially stored in the exosphere.

On the relationship between subducted slab age and arc basalt petrogenesis, Cascadia subduction system, North America

Olivine-normalized ≤2.0 Ma magnesian basalts erupted close to the volcanic axis of the Cascadia subduction system exhibit arc-parallel compositional variations compatible with a northward decrease in slab-derived components in the underlying mantle wedge. Inferred decreases in slab input correlate strongly with a systematic decrease in age of the oceanic crust along the convergent margin. Geochemical trends are most pronounced in southwestern British Columbia and northern Washington basalts, where the Garibaldi belt strikes obliquely to the trend of isochrons on the subducted plate. From southern Washington to northern California, High Cascades arc segments trend nearly parallel to subducted plate isochrons, and basalts show more subdued arc-parallel variations. These observations support a model in which temperature and depth of melting in the mantle wedge beneath the arc are influenced by the age of the subducted plate. Where the arc is underlain by relatively young and hot oceanic crust, only a minor amount of heating during subduction is required before dehydration reactions begin in the subducted plate. The slab loses much of its volatiles trenchward of the arc, and the volatile budget in the mantle wedge beneath the arc is relatively low. In the Garibaldi belt, which overlies very young oceanic crust, this produces progressively more alkalic basalts to the north. The High Cascades overlies older oceanic lithosphere which was cooler at the time of subduction. As a result, dehydration reactions are delayed and a greater amount of volatiles are released beneath the arc. This results in lower melt temperatures and higher degrees of melting, producing predominantly low-K tholeiite and LILE-enriched HFSE-depleted calc-alkaline basalt magmas.

Sulfur, chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland

The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated ∼250 Mt of H2SO4 aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and 15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods.

The characterization of glass inclusions through nuclear microprobe

In the last decade, nuclear microanalytical methods have been developed in order to study the distribution of elements of specific interest in natural glasses involving melt (or glass) inclusions trapped in minerals of magmatic origin. The study of these small objects represent a very original application of the microbeam techniques in general. With the nuclear microprobe, we have focused our research in two main directions: (1) the determination of light elements by nuclear reaction analysis (C, N, F, Li, B) and elastic recoil detection analysis in transmission mode (H), contributing to the knowledge of the magmatic thermodynamic conditions (pressure, temperature, gas solubility), to the quantification of volatile budget and to the volcanoes atmospheric contribution; (2) the determination of trace elements as geochemical markers with PIXE, contributing to the knowledge of magmatic processes. In this paper, after a brief review of the methods developed for light elements and their application in volcanology, we present with more details simultaneous use of microPIXE-microPIGE to characterize strontium and fluorine in melt inclusions. The experiments were carried out both with the Bruyères Le Châtel nuclear microprobe (of first generation) and with the new nuclear microprobe of the Pierre Süe laboratory.