Etienne Deloule

Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano

Caldera-forming volcanic eruptions are low-frequency, high-impact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the ‘Minoan’ caldera-forming eruption of Santorini volcano, Greece, which occurred in the late 1600s bc. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption. Despite the large volume of erupted magma (40–60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicic magma batches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems.

Behavior of Li and its isotopes during serpentinization of oceanic peridotites

Analyses of Li and Li isotopes in serpentinized peridotites have been performed using Thermo‐Ionisation Mass Spectrometry (TIMS) and Secondary Ion Mass Spectrometry (SIMS) techniques on samples collected from the southwest Indian Ridge (SWIR). In the bulk samples, Li concentrations range from 0.6 to 8.2 ppm, while whole rock δ6Li values range from −2.9 to −14‰. In situ analyses display a greater range in both Li concentration (0.1–19.5 ppm) and Li isotopic composition (−27 to +19‰), with the serpentinized portions having higher Li concentrations than the associated relict phases. These variations may reflect changes in Li partitioning and isotopic fractionation between serpentine and fluid with temperature and water/rock ratio. They may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization. As the serpentine forms by interaction with a circulating fluid, it preferentially removes 6Li, causing the Li in the fluid to become isotopically heavier. The isotopic composition of the initial hydrothermal fluid is dominated by basalt‐derived Li, which easily overwhelms the very low Li content originally present in seawater. As this fluid circulates through ultramafic rocks, it induces the formation of serpentine that incorporates this mantle‐derived Li. Hence, Li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves Li recycling within this crust. Consequently, Li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid. These results imply that serpentinized peridotites are probably only a minor sink of oceanic Li.

Pressure and temperature-dependence of water solubility in Fe-free wadsleyite

Abstract The effects of temperature and pressure on water solubility in wadsleyite in the system MgO-SiO2- H2O were investigated. Experiments were carried out using a 1000 ton multi-anvil press. One series of experiments was performed at a fixed pressure of 15 GPa and at various temperatures and in a second series the temperature was fixed at 1200 °C and pressure was varied from 13 to 18 GPa. The starting material was a mixture of oxides and hydroxides equivalent to the composition Mg2SiO4 + 5 wt% H2O. Run products consisted of wadsleyite, quenched hydrous melt, and minor amounts of clinoenstatite. The water content of wadsleyite was quantified by ion probe. Results show that at 15 GPa, the water solubility in wadsleyite decreases significantly with increasing temperature from ~2.2 wt% H2O at 900 °C down to ~0.9 wt% H2O at 1400 °C; the corresponding Mg/Si ratios increase from 1.80 to 1.91 over this temperature range. This effect appears to be largely due to changes in the water activity in the coexisting melt. The partition coefficient of water between wadsleyite and coexisting melt is nearly independent of temperature with Dwaterwadsleyite/melt ≈ 0.08. No significant effect of pressure on water solubility was observed at 1200 °C. Our data suggest that the water storage capacity of wadsleyite in the transition zone is much lower than previously suggested. Together with previous results on ringwoodite, our data imply a strong decrease of the water partition coefficient between wadsleyite and ringwoodite with temperature. This decrease could have two important consequences: (1) The width of the 520 km discontinuity may vary strongly as a function of temperature. (2) During cooling of the Earths mantle since the Hadean, water may have increasingly partitioned from the lower into the upper part of the transition zone.

Hydrogen isotope heterogeneities in the mantle from ion probe analysis of amphiboles from ultramafic rocks

Abstract An ion probe analytical procedure has been established to measure in-situ DH ratios in hydroxylated minerals on the scale of a few tens of μm with a precision of ±10‰. This technique has been applied to determine the DH ratio of amphiboles occurring as rare disseminated minerals in peridotites. These amphiboles include pargasite from the Lherz peridotite, Pyre´ne´es (France), and kaersutite and pargasite in ultramafic xenoliths and megacrysts from the Massif Central (France), Nunivak Island (Alaska) and Salt Lake Crater (Hawaii). The δD value measured in Lherz (−92 and −65) and Nunivak Island (−93 to −69) are in the accepted range for upper mantle minerals. The higher δD (−59 to −28) from the Massif Central xenoliths may involve contamination of the subcontinental lithosphere by subduction of seawater-altered oceanic crust. The very low δD (down to −125) observed for Hawaiian xenoliths could reflect the presence of a previously unidentified deep hydrogen component in the mantle. For the Massif Central and Salt Lake Crater, δD smow variations up to 70‰ were measured both on the scale of single crystals (≤200 μm) and among crystals of the same sample. Chemical variations are minor and not correlated with δD . Combining the diffusion data for hydrogen in amphiboles with these results implies that the time interval between the onset of exchange and volcanism was shorter than a few hundred years. The δD heterogeneity observed for the latter two localities probably results from incomplete equilibration between xenoliths and fluids or hydrous magmas rather than from fractionation processes.

New U-Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault

[1] Zircons and monazites from 6 samples of the North Ayilari dextral shear zone (NAsz), part of the Karakorum fault zone (KFZ), have been dated with the U-Th-Pb method, using both ID-TIMS and SIMS techniques. The ages reveal (1) inheritance from several events spanning a long period between the late Archean and the Jurassic; (2) an Eocene-Oligocene magmatic event (similar to 35-32 Ma); (3) an Oligo-Miocene magmatic event (similar to 25-22 Ma), at least partly synkinematic to the right-lateral deformation; and (4) a period of metamorphism metasomatism (similar to 22-14 Ma) interpreted as thermal and fluid advection in the shear zone. The Labhar Kangri granite located similar to 375 km farther Southeast along the KFZ is dated at 21.1 +/- 0.3 Ma. Such occurrence of several Oligo-Miocene granites along the KFZ, some of which show evidence for synkinematic emplacement, suggests that the fault zone played an important role in the genesis and /or collection of crustal melts. We discuss several scenarios for the onset and propagation of the KFZ, and offset estimates based on the main sutures zones. Our preferred scenario is an Oligo-Miocene initiation of the fault close to the NA range, and propagation along most of its length prior to similar to 19 Ma. In its southern half, the averaged long-term fault-rate of the KFZ is greater than 8 to 10 mm/a, in good agreement with some shorter-term estimates based on the Indus river course, or Quaternary moraines and geodesy. Our results show the KFZ cannot be considered as a small transient fault but played a major role in the collision history.

Interstellar water in meteorites

D/H ratios of two meteorites (Renazzo CR and Semarkona LL3), which are known to exhibit the largest departures from the terrestrial hydrogen isotopic ratios, have been determined with the CRPG Nancy ion microprobe. Correlations between the D/H ratios and the chemical compositions (H2O, K, Si, C/H) of plausible hydrogen carriers were observed. From these correlations, it is possible to show that, contrary to previous interpretations, phyllosilicates are the carriers of the deuterium-rich hydrogen in Semarkona and Renazzo: 870 x 10(-6) > or = D/H > or = 670 x 10(-6) (+4600 > or = deltaD > or = 3300%) and > or = 320 x 10(-6) (deltaD > or = 1050%), respectively. Hydrogen is also present in the chondrules of these two deuterium-rich meteorites. The large differences in D/H ratios between matrix (up to 700 x 10(-6), deltaD up to +3500%) and chondrules (from 120 x 10(-6) (deltaD = -230%) to 230 x 10(-6) (deltaD = +475%)) show that hydrogen in chondrules cannot originate from the matrix by simple contamination or diffusion processes. The high D/H ratios measured in water-bearing minerals could not have been produced thermally within a dense solar nebula. Chemical reactions (i.e., involving ions or radicals), taking place in interstellar space or in the outer regions of the nebula at 110-140K are presently the only conceivable mechanisms capable of yielding such isotopic enrichments. Water in these meteorites should no longer be considered as a simple product of nebular condensation under equilibrium thermodynamic conditions at T > or = 160K.

Primitive basaltic melts included in podiform chromites from the Oman Ophiolite

Abstract In an attempt to characterize the composition of the parental melts of ophiolitic chromitites and their tectonic setting, we have undertaken a study of polymineralic solid inclusions trapped in chromites from the Oman Ophiolite (Sumail nappe). High-temperature experiments performed on inclusions show that they result from post-entrapment crystallization of homogeneous basaltic melts with primitive compositions (Mg# = 63.5–66.8). The primary nature of the inclusions, demonstrated by their distribution outlining the crystallographic zones of mineral growth, indicates that the trapped melts represent small amounts of the parental liquids of the host chromites. Homogenised melt inclusions show depleted trace element patterns, with significant Nb depletions, characteristic of a subduction-related origin. These observations indicate that chromite deposits from Oman harzburgitic ophiolites may have formed in a geodynamic setting akin to present-day back-arc basins.

Interstellar hydroxyl in meteoritic chondrules: implications for the origin of water in the inner solar system

Abstract Ion probe determinations of water concentrations and D/H ratios have been performed on different phases (pyroxene, olivine, and mesostasis) of chondrules from the Bishunpur and Semarkona LL3 chondrites. Mean water concentrations average 1290 ppm in olivine (500 to 2100 ppm), 2400 ppm in pyroxene (400 to 9800 ppm), and up to 3950 ppm in the mesostasis (1000 to 16500 ppm). In one chondrule an iron oxide phase exhibits a mean water concentration of 1.70 wt% (1.21 to 2.15 wt%). Hydrogen isotopic measurements reveal the presence of two water sources with high and low D/H ratios: pyroxenes range from 74 to 479 × 10 −6 , with a mean value of 186 × 10 −6 ( n = 54); the mesostasis from 124 to 203 × 10 −6 , with a mean value of 152 × 10 −6 ( n = 8); olivines from 63 to 209 × 10 −6 , with a mean value of 130 × 10 −6 ( n = 13); the iron oxide phase from 157 to 360 × 10 −6 , with a mean value of 219 × 10 −6 ( n = 7). Hydroxyls in pyroxenes have been identified as submicroscopic, poorly crystallized amphibole lamellae, through high resolution transmission electron microscopy. Considering the broad range of D/H ratios in pyroxene chondrules, alteration in a unique meteoritic parent body seems to be excluded. An early hydroxylation of the pyroxene occurring during chondrule formation due to the presence of water-bearing minerals among chondrule precursors seems the only possible interpretation of the data. Accordingly, the distribution of D/H ratios in chondrules reflects that of their precursors. Such a distribution is quite similar to that found in matrix minerals. As recorded by water concentration and D/H ratio, the mesostasis has been subjected to hydrothermal alteration after the incorporation of the chondrules into the meteoritic parent body. This alteration took place with water having a mean D/H ratio reflecting the mixing of the D-depleted and D-rich sources. The D-rich component is interpreted as a preserved interstellar source, while the low D/H ratio reflects a water component that underwent isotopic exchange with protosolar molecular hydrogen.

Aqueous alteration in the Northwest Africa 817 NWA 817 Martian meteorite

Samples of a new Martian meteorite of the nakhlite family (NWA 817) contain traces of an iron-rich alteration product. Textural arguments indicate that this alteration product has been formed on the parent body of the meteorite (Mars). The chemical composition and structural data (X-ray diffraction, transmission electron microscopy and vibrational spectroscopy) show that the alteration mineral is a hydrous phase from the smectite family. Major elements and rare earth elements suggest that the formation of the alteration phase is related to the circulation of an aqueous fluid which composition is controlled by the dissolution of feldspars to account for a positive Eu anomaly, olivine and possibly apatite. Hydrogen isotope data display negative deltaD values ranging from -60 to -280parts per thousand in olivine and pyroxenes and from -140 to -181parts per thousand in the alteration phase. The values of deltaD for the alteration product show a small scatter with a mean value of -170 +/- 14parts per thousand. These values are lower than those previously obtained on other Martian meteorites, which give mainly positive deltaD values. These positive values have been interpreted as resulting from the interaction of the Martian meteorites with water from the Martian atmosphere. Ruling out the effect of terrestrial alteration, it is suggested that alteration in the NWA 817 meteorite was likely produced on Mars by the circulation of an aqueous fluid originating from a chemical reservoir, such as the Martian mantle, which has not equilibrated with a fractionated Martian atmosphere

Cathodoluminescence of quartz from sandstones; interpretation of the UV range by determination of trace element distributions and fluid-inclusion P-T-X properties in authigenic quartz

Abstract Quartz overgrowths from Keuper sandstones of the Paris basin were examined using cathodoluminescence (CL) microscopy and spectroscopy coupled with a scanning electron microscope (SEM). With the use of standard cold CL equipment, it was observed that the emission of authigenic quartz is much less intense than that of detrital quartz grains, but a reversal of intensity was observed with scanned CL in the 200-800 nm range. The main CL emission band of diagenetic quartz is at 330-340 nm in the UV range, with other bands in the visible range. The determination of trace element distributions in authigenic quartz by in situ analyses and by SIMS imagery reveals the correlation of the 330-340 nm emission band with the highest Al and Li contents. The cathodoluminescence emission of diagenetic quartz in the UV range appears to be influenced by the coupled substitution of Al and Li into the crystal. Two explanations are suggested: (1) Al, Li, or both serve as activators, and (2) the incorporation of Al and Li causes lattice defects that lead to an enhancement of the intrinsic luminescence. Trace element analyses and fluid-inclusion studies revealed that quartz overgrowths precipitated from a fluid that partially originated from an Li-enriched primary brine derived from eastern Triassic evaporites of the Paris basin. The specific CL emission band in the UV range can be linked to the diagenetic environment. The chemistry of the fluids appears to be the essential parameter, whereas precipitation temperatures do not influence the occurrence of such emissions.