Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations

In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites, we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calc-alkaline, and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic plagiogranites; however, the compositions of the coexisting experimental minerals significantly deviate from those of the granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the sheeted dike complex, starting at temperatures exceeding 850°C. The newly formed melt can either crystallize to form oceanic plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated.     

“Galileo Galilei” (GG) a small satellite to test the equivalence principle of Galileo, Newton and Einstein

“Galileo Galilei” (GG) is a small satellite designed to fly in low Earth orbit with the goal of testing the Equivalence Principle—which is at the basis of the General Theory of Relativity—to 1 part in 10¹⁷. If successful, it would improve current laboratory results by 4 orders of magnitude. A confirmation would strongly constrain theories; proof of violation is believed to lead to a scientific revolution. The experiment design allows it to be carried out at ambient temperature inside a small 1-axis stabilized satellite (250 kg total mass). GG is under investigation at Phase A-2 level by ASI (Agenzia Spaziale Italiana) at Thales Alenia Space in Torino, while a laboratory prototype (known as GGG) is operational at INFN laboratories in Pisa, supported by INFN (Istituto Nazionale di fisica Nucleare) and ASI. A final study report will be published in 2009.     

Methodology for studies of carbon dynamics between phytoplankton primary production and heterotrophic bacterial secondary production

The SCENTO-System was used to study the carbon dynamics between phytoplankton primary production and heterotrophic bacterial secondary production. Most of the methods used nowadays in situ for limnological synecology studies were applied. Primary production measurement showed an increasing tendency with increasing content of chlorophylla. It provided a true photosynthetic rate lying within the range of eutrophic lakes. Net EOC released from the algae ranged from 8.5 to 27.5 μg C l⁻¹(6h)⁻¹. Accompanying the algal products the number of bacteria increased from 1.475 ×10⁹ to 8.074×10⁹ cells l⁻¹. The bacterial mean cell volume was small, between 0.0315 and 0.0548μm³. Bacterial carbon production from direct growth estimates was compared with independent calculations of bacterial growth from EOC uptake and³H-thymidine incorporation. Direct estimates were 2.97–10.0 μg Cl⁻¹ (24h)⁻¹ with the exception of a zero-growth on the third day. EOC uptake was 123.5–191.0 μg Cl⁻¹ (6h)⁻¹. That calculated from³H-thymidine incorporation was 0.2–0.5 μg Cl⁻¹ (6h)⁻¹.¹⁴C-glucose dark uptake ran parallel to the increasing bacterial biomass. The respiration of glucose was 6.5% (avg.) of the gross uptake. Since the system operated without grazing pressure, a real carbon flow from primary production to bacterial secondary production could be observed.     

The formation of SiO<Subscript>2</Subscript>-rich melts within the deep oceanic crust by hydrous partial melting of gabbros

Small amounts of felsic, evolved plutonic rocks, often called oceanic plagiogranites, always occur as veins or small stocks within the gabbroic section of the oceanic crust. Four major models are under debate to explain the formation of these rocks: (1) late-stage differentiation of a parental MORB melt, (2) partial melting of gabbroic rocks, (3) immiscibility in an evolved tholeiitic liquid, and (4) assimilation and partial melting of previously altered dikes. Recent experimental data in hydrous MORB-type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust. Experiments show that TiO₂ is a key parameter for the discrimination between different processes: TiO₂ is relatively low in melts generated by anatexis of gabbros which is a consequence of the low TiO₂ contents of the protolith, due to the depleted nature of typical cumulate gabbros formed in the oceanic crust. On the other hand, TiO₂ is relatively high in those melts generated by MORB differentiation or liquid immiscibility. Since the TiO₂ content of many oceanic plagiogranites is far below that expected in case of a generation by simple MORB differentiation or immiscibility, these rocks may be regarded as products of anatexis. This may indicate that partial melting processes triggered by water-rich fluids are more common in the deep oceanic crust than believed up to now. At slow-spreading ridges, seawater may be transported via high-temperature shear zones deeply into the crust and thus made available for melting processes.     

Crystallization of quartz dioritic magmas at 2 and 1 kbar: experimental results

Crystallization experiments were performed on quartz diorite (~55 wt.% SiO₂, 3.1–8.4 wt.% MgO) from the G?siniec Intrusion (Bohemian Massif, SW Poland) at 1?2 kbar, 750–850°C, various mole fractions of water and with fO₂ buffered by the NNO buffer. The two natural quartz diorites (leucocratic poikilitic quartz diorite - ‘LPD’ and melanocratic quartz diorite - ‘MD’) differ in whole rock and mineral composition with MD being richer in MgO and poorer in CaO than LPD, probably due to accumulation of mafic minerals or melt removal in MD. LPD represents melt composition and is used to reconstruct crystallization conditions in the G?siniec Intrusion. The crystallization history of LPD magma, deduced from experimental and natural mineral compositions, includes a higher pressure stage probably followed by emplacement at ~2 kbar of partly crystallized magma at temperatures ~850?800°C and quick cooling. The mineral assemblage present in LPD requires water contents in the magma of at least 5 wt% and oxygen fugacity below that controlled by the NNO buffer. The compositions of mafic minerals in the MD composition were equilibrated at temperatures below 775°C and at subsolidus conditions. The equilibration was probably due to the reaction between water-rich, oxidizing residual melt and the cumulatic-restitic mineral assemblage. MD is characterized by occurrence of the euhedral cummingtonite and increasing anorthite content in the rims of plagioclase. A similar reaction was reproduced experimentally in both LPD and MD compositions indicating that cummingtonite may be a late magmatic phase in quartz dioritic systems, crystallizing very close to solidus and only from water saturated magma.     

Geomagnetic variations and their time derivatives during geomagnetic storms at different levels of intensity

The latitudinal distributions of horizontal geomagnetic variations, ΔH, and their time derivatives, ∂H/∂t, were analysed statistically over the three-year period 2003–2005. It appears that the amplitude distributions of horizontal geomagnetic variations and their time derivatives differ systematically between different geomagnetic latitudes and storm intensity levels. We show that the magnetic field variations observed at auroral and polar cap latitudes are under all geomagnetic storm levels comparable in amplitude (in a statistical sense) while they are smaller at subauroral latitudes. In contrast, their time derivatives are clearly the largest at auroral latitudes at all storm levels. These distributions determine in a general sense where and with which probability technological systems and operational procedures may be affected by geomagnetic storms. However, one observes in individual cases that the peak ∂H/∂t (the largest in all horizontal directions) is not necessarily the one which triggers a power system blackout.     

The effect of water activity on the oxidation and structural state of Fe in a ferro-basaltic melt

Experimental investigations have been performed at T = 1200°C, P = 200 MPa and fH₂ corresponding to H₂O-MnO-Mn₃O₄ and H₂O-QFM redox buffers to study the effect of H₂O activity on the oxidation and structural state of Fe in an iron-rich basaltic melt. The analysis of Mössbauer and Fe K-edge X-ray absorption nearedge structure (XANES) spectra of the quenched hydrous ferrobasaltic glasses shows that the Fe³⁺/ΣFe ratio of the glass is directly related to aH₂O in a H₂-buffered system and, consequently, to the prevailing oxygen fugacity (through the reaction of water dissociation H₂O ↔ H₂ + 1/2 O₂). However, water as a chemical component of the silicate melt has an indistinguishable effect on the redox state of iron at studied conditions. The experimentally obtained relationship between fO₂ and Fe³⁺/Fe²⁺ in the hydrous ferrobasaltic melt can be adequately predicted in the investigated range by the existing empiric and thermodynamic models. The ratio of ferric and ferrous Fe is proportional to the oxygen fugacity to the power of ∼0.25 which agrees with the theoretical value from the stoichiometry of the Fe redox reaction (FeO + ¼ O₂ = FeO_1.5). The mean centre shifts for Fe²⁺ and Fe³⁺ absorption doublets in Mössbauer spectra show little change with increasing Fe³⁺/ΣFe, suggesting no significant change in the type of iron coordination. Similarly, XANES preedge spectra indicate a mixed (C3h, Td, and Oh, i.e., 5-, 4-, and sixfold) coordination of Fe in hydrous basaltic glasses.     

Single crystal structure investigations under high-pressure of the mineral cordierite with an improved high-pressure cell

An improved single-crystal high-pressure anvil cell with beryllium-gaskets was used for the investigations of structure and lattice parameters of cordierite which had been heated in an Ar stream at about 1,000° C to remove natural water from its structural channels. The influences of pressure transmitting media were studied by using water as a pressure medium at pressures of 0.3, 0.9, 1.2, and 2.3 GPa and fluorcarbon, a liquid consisting of large molecules, at 2.2 GPa. Water, but not fluorcarbon, is able to enter the channels in the cordierite structure. Large variations in the lattice constants resulted from changing the pressure medium used. A previously supposed discontinuity of the b lattice constant at nearly 0.3 GPa could not be established by the measurements taken so that there is no evidence for a phase transition at this pressure. Possibly the observed tilting of two tetrahedra against each other in this structure could have led to this misinterpretion. When water, but not fluorcarbon, is used as a pressure medium at 2.3 GPa, an additional electron density peak, presumably a water oxygen atom, appears in the channels. The water prevents the channels from shrinking and fixes their width at a value comparable to that of a naturally hydrated cordierite. In one of the silicate-tetrahedra the Si-O bond lengths are compressed almost 1 percent (2.3 GPa). This process may initiate a phase transition at higher pressures.     

Plagioclase transfer from a host granodiorite to mafic microgranular enclaves: diverse records of magma mixing

Chemical and structural zoning in plagioclase can develop in response to a number of different magmatic processes. We examine plagioclase zonation formed during the transfer of plagioclase from a granodioritic host to a monzodioritic enclave to understand the development of different zonation patterns caused by this relatively simple magma mixing process. The transferred plagioclase records two stages of evolution: crystallization of oscillatory plagioclase in the host granodioritic magma and crystallization of high An zones and low An rims in the hybrid enclave magma. High An zones (up to An₇₂) are formed only in the hybrid enclaves after plagioclase transfer. Plagioclase from a primitive enclave, showing no or only minimal interaction with the host, is An_30–43. The implication is that high An zones crystallize only from the hybrid magma and not from the primitive one, probably because of an increase in water content in the hybrid magma. Complex interactions between the two magmas are also recorded in Sr content in plagioclase, which indicates an initial increase in Sr concentration in the melt upon transfer. This is contrary to what is expected from the mixing of low Sr enclave magma with a high Sr granodiorite one. Such Sr distribution in the plagioclase implies that the transfer of the plagioclase took place before the onset of plagioclase crystallization in the enclave magma. Therefore, the mixing between high Sr granodiorite magma and low Sr enclave magma was recorded only in plagioclase rims and not in the high An zones.