2O Content Research Articles

Second Harmonic Generation induced by optical poling in new TeO 2–Tl 2O–ZnO glasses

New tellurite glasses with a large glass forming domain were elaborated within the TeO 2–Tl 2O–ZnO ternary system. The evolution of the glass transition ( T g) and onset crystallization ( T 0) temperatures for such tellurite glasses was studied, in particular, as a function of the Tl 2O addition. A decrease of both T g and T 0 temperatures was observed; the former being more affected. Structural modifications induced by the addition of the modifiers were studied by Raman spectroscopy. For a fixed ZnO concentration, the increase in the Tl 2O content leads to a destruction of the glass framework, characterized by the transformation of TeO 4 disphenoids into isolated TeO 3 2− trigonal pyramid-like ortho-groups. For a fixed Tl 2O concentration, the ZnO addition induces similar effects on the glass structure. The optical transmission of the ((80 − x)TeO 2– xTl 2O–20ZnO) ( x = 10, 20 and 30 mol%) glasses was measured in the 300–2000 nm range. Their good transparency was evidenced and a clear reduction of the optical band-gap was noticed with the increase in the Tl 2O content. Finally, Second Harmonic Generation was unambiguously detected for each glass composition. The second order non-linearity amplitude is found to be increasing as a function of the Tl 2O concentration, in the tested range.

Structural and electrical properties of Li 2O–ZnO–P 2O 5 glasses

Lithium zinc phosphate glasses of the composition xLi 2O–(40 − x)ZnO–60P 2O 5, (0 < x < 40), in mol%, were prepared by melting and analyzed by different thermal analysis (DTA), Raman and impedance spectroscopy in the frequency range from 0.01 Hz to 4 MHz and temperature range from 303 to 473 K. The systematic change in the variety of properties at 20 mol% Li 2O is explained by changes in the nature of the oxygen bonds in glass network. The Raman spectra predominantly show the metaphosphate structure with barely detectable pyrophosphate units for all glasses in the series. However, replacing ZnO with Li 2O in these glasses leads to the compositional dependent changes. This dependence is explained by different structural cross-linking of phosphate chains with increasing Li 2O, which is correlated with the unusual T g minimum at 20 mol% Li 2O. The electrical conductivity as a function of Li 2O concentration measured at lower temperatures (298 and 363 K) passes through a minimum at 20 mol% whereas linearly increases at higher temperatures. This suggests that the conductivity enhancement is directly related to the thermally stimulated mobility of Li + ions in the high temperature region. The observed minimum in electrical conductivity at 20 mol% Li 2O is due to the structural reorganization of the glass network. With increasing Li 2O content up to 40 mol% the values of electrical conductivity increases for five orders of magnitude indicating ionic conductivity.

Characteristics of a fast low-temperature zinc phosphating coating accelerated by an ECO-friendly hydroxylamine sulfate

A fast low-temperature phosphating processing accelerated by an ECO-friendly hydroxylamine sulfate (HAS) is developed. The zinc phosphate coating was fast formed on high-carbon steel in a low-temperature phosphating bath. Growth stages and characteristics of the phosphate coating were investigated by open circuit potential (OCP), SEM, EDS and XRD techniques. The phosphating process can be divided into three stages, namely amorphous precipitation, anodic depolarization and growth of phosphate coating. The phosphate coating consists of Zn 3(PO 4) 2 · 4H 2O and Zn 2Fe(PO 4) 2 · 4H 2O phases. The addition of HAS makes the three stages' time shorten to 53%, 31% and 50%, respectively, while markedly reduces the size of phosphate crystals from 100 µm to about 50 µm, and increases the Zn 2Fe(PO 4) 2 · 4H 2O content from 30% to 44% in the coating. HAS would be widely used as a low-temperature phosphating accelerator to replace the traditional nitrite, due to its less pollutant, higher phosphating rate and better quality of the coating.

Electrical conduction in the vitreous and crystallized Li 2O–V 2O 5–P 2O 5 system

The glass-forming region of the Li 2O–V 2O 5–P 2O 5 system was examined. The glass transition temperature and stability of glass were enhanced with increasing P 2O 5 content. Li 2O–V 2O 5–P 2O 5 glasses show electronic or ionic conduction depending on the glass composition. The electronic conductivity increases with increasing V 2O 5 content, while the ionic conductivity is proportional to the Li 2O content. The effect of crystallization on the electrical conduction was also investigated. The ionic conductivity decreased by thermal crystallization. Drastic increase of electronic conduction was observed in the crystallization process in glasses of 50Li 2O–25V 2O 5–25P 2O 5 and its neighboring compositions. It was clarified that high electronic conduction occurs in a metastable crystalline phase prior to the thermal equilibrium crystallization.

Petrology, geochemistry, and geochronology of the post-collisional Triassic mangerite and syenite in the Gwangcheon area, Hongseong Belt, South Korea

The Gwangcheon intrusive rocks occur in the Hongseong collision belt of South Korea, which is thought to represent the eastern extension of the Dabie–Sulu collision zone of China. The central part of the Gwangcheon intrusive complex consists of orthopyroxene-bearing monzonite (mangerite), and the marginal part is composed of syenite. North of the Gwangcheon mangerite, the post-collisional (233 ± 2 Ma) Haemi biotite granite contains syenite enclaves. SHRIMP U–Pb zircon analysis yields ages of 232 ± 3 Ma for the mangerite and 230 ± 3 Ma for a syenite enclave within the Haemi biotite granite. The mangerite–syenite complex and syenite enclaves exhibit a shoshonitic affinity, with a total alkalinity (Na 2O + K 2O) of 7.38–9.64 wt.%, high K 2O, Mg#, Ba, Sr, Cr, Ni, and LREE contents, and insignificant negative Eu anomalies. Geochemical data indicate that the mangerite–syenite intrusion and the syenite enclaves are post-collisional igneous rocks formed by the partial melting of an enriched lithospheric mantle. The heat for this melting was derived from asthenospheric upwelling following oceanic slab break-off. The SHRIMP age data suggest that both the Gwangcheon intrusives and the syenite enclaves formed after the Triassic continental collision of the North and South China blocks in Korea.

Petrographic and geochemical characteristics of upper Miocene Tekkedag volcanics (Central Anatolia—Turkey)

The Tekkedağ volcanic complex, which extends as a ridge in the direction of NW–SE, is one of the poorly known volcanic centers and is exposed to the southwest of Kayseri located within the Central Anatolian Volcanic Province (CAVP) of Turkey. The mineralogical composition of Tekkedağ volcanics reveals an assemblage of plagioclase (labradorite, bytownite)+pyroxene (augite, diopside and enstatite)+Fe–Ti oxide (magnetite, rutile)±olivine (forsterite) mineral composition having hypocrystaline porphryric, hypohyaline porphryric, gleomeroporphryric and seriate textures under the microscope. Confocal Raman Spectroscopy (CRS) has been used to define the mineral types. Tekkedağ volcanics have medium K 2O contents and are calc-alkaline in character. Geochemically, Tekkedağ volcanics show a narrow range of major element compositions and are classified as augite andesite/basaltic andesite. On the variation diagrams based on MgO versus major and trace elements, they show good positive and negative correlations, indicating fractional crystallization of plagioclase, clinopyroxene and Fe–Ti oxide. Tekkedağ volcanics display enrichment in large-ion lithophile elements (LILEs) relative to high field-strength elements (HFSEs) in chondrite, MORB, E-MORB and lower crust normalized multi-element diagrams. In all normalized multi-element diagrams, the trace element patterns of all samples are similar in shape and exhibit depletions in Ba, Nb, P and Ti as characteristics of subduction-related magmas. Rare earth element (REE) patterns for Tekkedağ volcanics show REE enrichment with respect to chondrite values. They exhibit marked enrichment in light rare earth elements (LREEs) ((La/Sm) N =4.13–4.62) relative to heavy rare earth elements (HREEs) ((Sm/Lu) N =1.34–1.92). Furthermore, all samples have negative Eu anomalies ((Eu/Eu *) N =0.77–0.90), indicating the significant role of plagioclase in the fractional crystallization. Elemental ratios such as K/P (15.46–21.69), La/Nb (2.01–4.26), Rb/Nb (8.74–10.59), Ba/Nb (38.54–75.77), Nb/Ta (1.16–2.14), Ce/P (2.13–4.32) and Th/U (1.77–3.97) propose that the magma was subjected to conceivable crustal contamination during the evolution of these Tekkedağ volcanics. This statement is supported by the AFC modeling based on the trace elements. As a result, despite the lack of isotopic data, the petrographic and geochemical results suggest a significant role of plagioclase, clinopyroxene and Fe–Ti oxide fractionation during the evolution of the Volcanic Arc Basalts (VAB) nature of the Tekkedağ volcanics. Furthermore, these results reveal that the volcanics of Tekkedağ were produced from a parental magma derived from an enriched source of mixed subduction and/or crustal products.

H 2O and CO 2 in kimberlitic fluid as recorded by diamonds and olivines in several Ekati Diamond Mine kimberlites, Northwest Territories, Canada

Surface dissolution features on diamonds and Fourier Transform Infra Red spectroscopy (FTIR) of phenocrystal and xenocrystal olivines from kimberlites contain a record of magmatic fluid in kimberlite magmas. We investigated composition and behavior of kimberlitic fluid and the effect of volatiles on the eruption style and geology of kimberlites using microdiamonds and olivine concentrates from six kimberlite pipes with different lithologies and the character of diamond resorption (Ekati Diamond Mine, Northwest Territories, Canada). The study showed a clear correlation between the resorption style of diamond population of the kimberlites and the type of infrared (IR) spectra of their olivines. Four kimberlites have high quality diamonds with smooth regular surface features and high H 2O content of the olivines indicating the presence of H 2O-rich fluid during the emplacement. Fast ascent rates of fluid-rich magma can explain explosive eruption and filling the pipes with volcaniclastic kimberlite facies. Conversely, Grizzly and Leslie kimberlites have diamonds with complex sharp features diminishing diamond quality and indicating loss of the fluid. The slower ascent rates and less explosive eruption of the fluid-free magmas produced kimberlite pipes filled with magmatic facies kimberlite. Distinctive peaks in olivine IR spectra at 3356 and 3327 cm − 1 were found to correlate with the presence of hydrous magmatic fluid. Character of diamond morphology suggests that during the whole ascent of all six kimberlites, the magmatic fluid when present had a high H 2O:CO 2 ratio.

Hydrogen incorporation into forsterite in Mg 2SiO 4–K 2Mg(CO 3) 2–H 2O and Mg 2SiO 4–H 2O–C at 7.5–14.0 GPa

Experiments on water solubility in forsterite in the systems Mg 2SiO 4–K 2Mg(CO 3) 2–H 2O and Mg 2SiO 4–H 2O–C were conducted at 7.5–14.0 GPa and 1200–1600 °C. The resulting crystals contain 448 to 1480 ppm water, which is 40–70% less than in the forsterite–water system under the same conditions. This can be attributed to lower water activity in the carbonate-bearing melt. The water content of forsterite was found to vary systematically with temperature and pressure. For instance, at 14 GPa in the system forsterite–carbonate–H 2O the H 2O content of forsterite drops from 1140 ppm at 1200 °C to 450 ppm at 1600 °C, and at 8 GPa it remains constant or increases from 550 to 870 ppm at 1300–1600 °C. Preliminary data for D-H-bearing forsterite are reported. Considerable differences were found between IR spectra of D-H- and H-bearing forsterite. The results suggest that CO 2 can significantly affect the width of the olivine-wadsleyite transition, i.e., the 410-km seismic discontinuity, which is a function of the water content of olivine and wadsleyite.

Fine tuning the surface acid/base properties of single step flame-made Pt/alumina

The acid/base properties of Pt/Al 2O 3 were modified by doping the support with SiO 2 or Cs 2O and the catalysts were prepared by flame spray pyrolysis in a rapid, single step process with excellent reproducibility. The catalysts contained 4.7 wt.% Pt and the support composition varied in the range 0–100 wt.% SiO 2 or 0–10 wt.% Cs 2O. Up to 50 wt.% SiO 2 the BET surface areas could be kept in a narrow range (141 ± 15 m 2 g −1), while higher SiO 2 content increased the surface area and diminished the crystallinity of the support. CO chemisorption and XRD showed a positive correlation between the SiO 2 content and the size of the dominantly spherical Pt particles. The catalysts containing 22–30 wt.% SiO 2 were the most acidic as evidenced by TPD of NH 3. In the hydrogenation of acetophenone the SiO 2-doped catalysts were highly active and the selectivity to the hydrogenolysis byproducts increased with increasing SiO 2 content by more than 20-fold. This is a confirmation of the effect of support acidity on the properties of the Pt–H system. On the other hand, addition of Cs 2O barely affected the Pt particle size and TPD of CO 2 confirmed the increase of surface basicity with increasing Cs 2O content. Cs 2O was probably enriched at the Al 2O 3 surface and strongly influenced the acid/base properties and thus the catalytic performance already at very low concentrations. Only 0.25 wt.% Cs 2O blocked the hydrogenolysis of 1-phenylethanol to ethylbenzene.

The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ∼ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ∼ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions. The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H 2O, several hundred ppm of CO 2, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ∼ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H 2O, 8700 ppm Cl, 6000 ppm F, and CO 2 below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ∼ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H 2O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.

Stretched exponential relaxation and dispersive conductivity behavior in lithium bismuth silicate glasses

Glasses having compositions xLi 2O∙(85 − x)Bi 2O 3∙15SiO 2 ( x = 35, 40, and 45 mol%) were prepared by normal melt quenching technique. Electrical relaxation and conductivity in these glasses were studied using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 453 to 603 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency were extracted from the impedance spectra. The dc conductivity increases with increase in Li 2O content providing modified glass structure and large number of mobile lithium ions. Similar values of activation energy for dc conduction and for conductivity relaxation time indicate that the ions overcome the same energy barrier while conducting and relaxing. The non-exponential character of relaxation processes increases with decrease in stretched exponential parameter ‘ β’ as the composition parameter ‘ x’ increases. The observed conductivity spectra follow a power law with exponent ‘ s’ which increases regularly with frequency and approaches unity at higher frequencies. Nearly constant losses (NCL) characterize this linearly dependent region of the conductivity spectra. A deviation from the ‘master curve’ for various isotherms of conductivity spectra was also observed in the high-frequency region and at low temperatures, which supports the existence of different dynamic processes like NCL in addition to the ion hopping processes in the investigated glass system.

Study of optical band gap and FTIR spectroscopy of Li 2O·Bi 2O 3·P 2O 5 glasses

Glasses having composition (25 + x)Li 2O·(65 − x)P 2O 5·10Bi 2O 3 (0 ≤ x ≤ 25 mol%) were prepared by normal melt quenching technique. The optical absorption spectra of the polished samples were recorded in spectral range from 200 to 3300 nm at room temperature. The optical absorption edge ( λ cutoff), optical band gap ( E g) and Urbach energy (Δ E) were determined from the absorption spectra. Variation in these optical parameters has been associated with the structural changes occurring in these glasses with increase in Li 2O:P 2O 5 ratio. Variation in density ( D), molar volume ( V M) and glass transition temperature ( T g) has also been correlated with structural changes in the glass samples. The FTIR spectra have been recorded in the spectral range from 400 to 1400 cm −1. The FTIR spectra show the rapid depolymerization of phosphate chains with increase in Li 2O content and P–O–Bi bonds are formed.

Controls on the explosivity of scoria cone eruptions: Magma segregation at conduit junctions

Violent strombolian (transitional) eruptions are common in mafic arc settings and are characterized by simultaneous explosive activity from scoria cone vents and lava effusion from lateral vents. This dual activity requires magma from the feeder conduit to split into vertical and lateral branches somewhere near the base of the scoria cone. Additionally, if the flow is separated, gas and liquid (+ crystals) components of the magma may be partitioned unevenly between the two branches. Because flow separation requires bubbles to move independently of the liquid over time scales of magma ascent separation is promoted by low magma viscosities and by high magma H 2O content (i.e. sufficiently deep bubble nucleation to allow organization of the gas and liquid phases during magma ascent). Numerical modeling shows that magma and gas distribution between vertical and horizontal branches of a T-junction is controlled by the mass flow rate and the geometry of the system, as well as by magma viscosity. Specifically, we find that mass eruption rates (MERs) between 10 3 and 10 5 kg/s allow the gas phase to concentrate within the central conduit, significantly increasing explosivity of the eruption. Lower MERs produce either strombolian or effusive eruption styles, while MER > 10 5 kg/s prohibit both gas segregation and lateral magma transport, creating explosive eruptions that are not accompanied by effusive activity. These bracketing MER constraints on eruptive transitions are consistent with field observations from recent eruptions of hydrous mafic magmas.

Mineral and geochemical characterization of a leptic aluandic soil and a thapto aluandic-ferralsol developed on trachytes in Mount Bambouto (Cameroon volcanic line)

Mineral and geochemical investigations were carried out on soil samples and fresh rock (trachytes) from two selected soil profiles (TM profile on leptic aluandic soils and TL profile on thapto aluandic-ferralsols) from Mount Bambouto to better understand geochemical processes and mineral paragenesis involved in the development of soils in this environment. In TM profile, the hydrated halloysites and goethite occur in the weathered saprolite boulders of BC horizon while dehydrated halloysite, gibbsite and goethite dominate the soils matrices of BC and A horizons. In TL profile, the dehydrated halloysites and goethite are the most abundant secondary minerals in the weathered saprolites of C and BC horizons while gibbsite, hematite and kaolinite occur in the soil matrices of BC, B and A horizons. The highest gibbsite content is in the platy nodules of B horizon. In both soil profiles, organo-metal complexes (most likely of Al and Fe) are present in the surface A horizon. Geochemically, between the fresh rock and the weathered saprolites in both soils, SiO 2, K 2O, CaO, Na 2O and MgO contents decrease strongly while Fe 2O 3 and Al 2O 3 tend to accumulate. The molar ratio of SiO 2/Al 2O 3 (Ki) and the sum of Ca, Mg, K and Na ions (TRB) also decreases abruptly between fresh rocks and the weathered saprolites, but increases significantly at the soil surface. The TM profile shows intense Al enrichment whereas the TL profile highlights enrichment in both Al and Fe as the weathering progresses upwards. Both soil profiles are enriched in Ni, Cu, Ba and Co and depleted in U, Th, Ta, Hf, Y, Sr, Pb, Zr and Zn relative to fresh rock. They also show a relatively low fractionation of the rare earth elements (REE: La, Nd, Sm, Eu, Tb, Yb and Lu), except for Ce which tends to be enriched in soils compared to CI chondrite. All these results give evidence of intense hydrolysis at soil deep in Mount Bambouto resulting in the formation of halloysite which progressively transforms into gibbsite and/or dehydrated halloysite. At the soil surface, the prominent pedogenetic process refers to andosolization with formation of organo-metal complexes. In TL profile, the presence of kaolinite in soil matrices BC and B horizons is consistent with ferralitization at soil deep. In conclusion, soil forming processes in Mount Bambouto are strongly influenced by local climate: (i) in the upper mountain (> 2000 m), the fresh, misty and humid climate favors andosolization; whereas (ii) in the middle lands (1700–2000 m) with a relatively dry climate, both andosolization at the soil surface and ferralitization at soil deep act together.

A model for Ischia hydrothermal system: Evidences from the chemistry of thermal groundwaters

Ischia volcano, in Central Italy, has long been known for its copious surface hydrothermal manifestations, signs of a pervasive circulation of hot fluids in the subsurface. Because of the significant chemical heterogeneity of fumarolic gas discharges and hot spring discharges, evidences of a complex hydrothermal setting, a definite model of fluid circulation at depth is currently unavailable, in spite of the several previous efforts. Here, we report on the chemical and isotopic composition of 120 groundwater samples, collected during several sampling surveys from 2002 to 2007. The acquired data suggest that the composition of surface manifestations reflect contributions from meteoric water, sea water, and thermal fluids rising from two distinct hydrothermal reservoir, with equilibrium temperatures of respectively ~ 150 °C and ~ 270 °C, and depths of 150–300 m and > 300 m (but possibly > 1000 m). We also make use of an isotopic characterization of the dissolved gas phase in thermal waters to demonstrate that the Ischia hydrothermal system is currently supplied by a deep-rising gas component (DGC), characterized by CO 2 ~ 97.7 ± 1.2 vol.% (on a water-free basis), δ 13C CO2 = − 3.51 ± 0.9‰, and helium isotopic ratio of about 3.5 Ra ( 3He/ 4He ratio normalized to the air ratio, R a), likely magmatic in origin. An assessment of the thermal budget for Ischia hydrothermal system is also presented, in the attempt to derive a first estimate of the size and rate of degassing of the magmatic reservoir feeding the gas emissions. We calculate that a heat flow of about 153–222 MW presently drives hydrothermal circulation on the island, which we suggest is supplied in convective form (e.g., by the ascent of a high- T magmatic vapour phase) by complete degassing of 2.2–3.3 · 10 7 m 3 yr − 1 of trachytic magma (with ~ 2.1 wt.% dissolved H 2O content). If extrapolated to entire period of quiescence lasting since the Arso eruption in 1302 A.D., this volume corresponds to 1.6–2.3·10 10 m 3 of magma degassed in about 700 years of quiescent activity.

26 million years of mantle upwelling below a segment of the Mid Atlantic Ridge: The Vema Lithospheric Section revisited

Temporal variations of temperature and composition of the mantle upwelling below a 80-km long segment of the Mid Atlantic Ridge were reconstructed from 20 to 4 Ma ago from peridotites sampled along a > 300-km long section of oceanic lithosphere (Vema Lithospheric Section or VLS) exposed south of the Vema transform at 11° N [Bonatti, E., Ligi, M., Brunelli, D., Cipriani, A., Fabretti, P., Ferrante, V., Gasperini, L., Ottolini, L., 2003. Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variation in the formation of oceanic lithosphere, Nature, 423, 499–505]. We extended this time interval from 26 to 2 Ma by sampling mantle ultramafics at 18 new sites along the VLS. Peridotite orthopyroxene, clinopyroxene and spinel chemistry suggest a weak trend of decreasing extent of melting of the mantle from 26 to 18.5 Ma ago with superimposed short-wavelength (~ 4 Ma) oscillations followed by a steady increase of degree of melting from 18.5 to 2 Ma ago, with superimposed 3–4 Ma oscillations. Temporal variations of crustal thickness inferred from the Residual Mantle Bouguer Anomaly calculated from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2 Ma) mantle suites differ in cpx Na 2O content and CaO/Al 2O 3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might have been different in the two suites. The two trends are separated by a ~ 1.4 Ma-long stretch (from 18.2 to 16.8 Ma) where deformed ultramafic mylonites prevail, indicating probably an interval of nearly a-magmatic lithospheric emplacement at ridge axis, corresponding to a thermal minimum. Spatially offset correlation along the VLS of crustal thickness (i.e., quantity of basaltic melt released by the mantle) and mantle peridotite degree of melting led to an estimate of ~ 16.1 mm/a for the solid mantle average velocity of upwelling, a value close to the average half spreading rate for the 26 Ma interval covered by the VLS. However, peridotite clinopyroxene geothermometry shows oscillations superimposed on a decreasing trend of calculated equilibration temperature from 26 to 18.5 Ma ago, followed by a steady increase from 16 to 4 Ma ago, suggesting that the solid mantle upwelling velocity varied through time. These results hint at the existence of ~ 10–20 Ma cycles in the activity of the northern Mid Atlantic Ridge.

Fe-rich and Na-rich megacryst clinopyroxene and garnet from the Luxinga kimberlite cluster, Lunda Sul, Angola

The Luxinga kimberlite cluster forms part of a kimberlite field partially hosted by the Alto Cuilo exploration project in Lunda Sul province, northeast Angola. Compositional analysis of kimberlite indicator minerals recovered from drill core representing 18 of the Luxinga-area kimberlites has highlighted the presence of a megacryst mineral suite with significantly fractionated compositions, i.e. poorer in Mg/(Mg+Fe) than commonly described in existing literature. Garnet megacrysts are relatively common at Luxinga, clinopyroxene megacrysts are rare, and low-Cr ilmenite megacrysts are recorded present, but are not quantified in this work. The garnet and clinopyroxene megacrysts show elevated Na 2O content and Na/Ti ratios across a range of Mg/(Mg+Fe), and specifically at low Mg/(Mg+Fe) of ~ 0.58 and ~ 0.81 respectively. The unusual high-Fe, high-Na megacryst garnet compositions overlap with sodic garnet compositions typically considered characteristic of diamond-facies eclogite, websterite or pyroxenite, resulting in high “eclogitic” diamond potential assessments for several kimberlites in the Luxinga cluster. However, our exploration work has demonstrated low, or no diamond content in kimberlites from Luxinga. We have drawn on data for many different low-Cr garnet types at Luxinga and from a global database to design simple screens that isolate unusual Fe- and Na-rich garnet compositions. We document that they are related by fractionation to common garnet megacrysts (G1 garnets) and hence institute a “reclassified megacryst garnet” (G1R garnet) as an additional category to a recently-published garnet classification scheme. G1R garnets constitute 31% of the analysed megacryst population at Luxinga and would clearly impact the diamond potential perceived for the kimberlites. Our global database of garnet compositions suggests that such Fe-rich and Na-rich megacrysts are comparatively rare, however, and general implementation of the new G1R classification by diamond explorers is not necessarily indicated.

Synthesis of large mordenite crystals with different aspect ratios

Large mordenite crystals hold interest for fundamental research on separation and catalysis as well as emerging applications such as functional devices. A number of methods are now available to obtain large mordenite crystals. However, it still remains a challenge to synthesize large and uniform crystals with a desired aspect ratio. Moreover, a detailed description of a reproducible synthesis method is still lacking. We report here large and uniform mordenite crystals (27 × 36 × 30 μm 3) with an accessible micropore volume of 0.17 cm 3/g that can be reproducibly synthesized. The synthesis procedure is described in detail. The effects of synthesis parameters on mordenite morphology (size and aspect ratio of the crystals), such as silicon precursors, H 2O content, Na 2O/SiO 2/Al 2O 3 ratios, alcohol additives, have been systematically studied. Large mordenite crystals (up to 85 μm) with prismatic shape were formed from a dense synthesis gel with high SiO 2/Al 2O 3 and Na 2O/SiO 2 ratios, or by adding alcohol additives. Moreover, the aspect ratio of the large crystals was controlled by finely altering the above synthesis parameters. In the presence of an excess amount of alcohol, hollow and core–shell mordenite crystals were formed.

Chlorine variations in the magma of Soufrière Hills Volcano, Montserrat: Insights from Cl in hornblende and melt inclusions

The Soufrière Hills Volcano in Montserrat erupts a Cl-rich, porphyritic andesite. HCl degassing accompanies eruption and is dependent on the growth rate of the lava dome. The magma contains hornblende phenocrysts that show repetitive zoning in most elements, including Cl. On the basis of the zoning data, (Cl/OH) ratios in the melt, calculated from partitioning data, increase rimward through each zone, indicating that the phenocrysts formed under conditions of varying (Cl/OH) m. An empirical relationship between A-site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature. Each zone ends at a resorption horizon, and crystallisation recommences at lower temperature and (Cl/OH) m. Melt inclusion H 2O and Cl contents for the 8th January 2007 explosive eruption can be explained by closed-system degassing with D Cl fl-m between 5 and 30, or by open-system degassing accompanied by a small amount of crystallisation. However, neither simple closed-system degassing nor convective circulation of magma can explain the positive correlation of (Cl/OH) m with temperature. We suggest that the zoning can be caused by accumulation of CO 2-rich vapour in the andesite, probably as a result of mafic magma injection into the chamber. Decreasing H 2O fugacity and/or increasing Cl m result in increasing (Cl/OH) m while heat transferred with the volatiles causes the rise in temperature. Intermittently, the accumulated fluid is lost to the surface, possibly as a result of renewed eruptive activity. This model requires the CO 2-rich fluid to be decoupled from the magma, consistent with previous observations of continuous CO 2 emissions at the surface.

Hydrous komatiites from Commondale, South Africa: An experimental study

This study examines the emplacement conditions of komatiites in the 3.33 Ga Commondale Ultramafic Suite in South Africa. The komatiites of Commondale are unlike any other komatiites in both their physical structure and chemical nature. Komatiite unit chill margins preserve original komatiite liquid compositions with an Mg# of 0.91, MgO = 31.9 wt.%, Al 2O 3 wt.%/TiO 2 = 80 (wt.%), and SiO 2 content of 49.7 wt.%. A common feature throughout the komatiite sequence is the presence of orthopyroxene spinifex, where original orthopyroxene crystals are still preserved. The compositional information preserved in the most primitive of the natural pyroxenes present in these spinifex zones (Mg# = 0.92), provides insight into the original emplacement conditions of the komatiites. This study used anhydrous and hydrous equilibrium experiments, along with disequilibrium cooling-rate experiments, to quantify the crystallization conditions of the Commondale komatiites. The anhydrous, 1-atm liquidus was found at 1550 °C, with Fo97 olivine being the initial crystallizing phase, followed by spinel and then by protoenstatite, Mg# 0.95, at 1335 °C. The phase relations were also examined at 200 MPa under H 2O saturated conditions. The addition of ~ 4 wt.% H 2O lowers the appearance temperature of the initial pyroxene by 210 °C, thereby producing orthopyroxene with a Mg# closer to that of the most primitive preserved orthopyroxenes found in the komatiites. Additionally, dynamic cooling-rate experiments show that the natural pyroxenes preserve a chemical signature indicative of crystallization and cooling within an inflated flow complex. Estimates of the pre-eruptive H 2O content for the Commondale komatiites are between ~ 2 and 4.3 wt.% H 2O in the liquid. This range is similar to that estimated for 3.5 Ga komatiites of the Barberton Mountainland and may indicate formation of both suites in similar tectonic environments.