314 research outputs found
Mantle-Crust Isotopic Relationships along Mid Ocean Ridges: Constraints from the Analysis of Time Series
Isotopic relationships between parent mantle and
daughter MORBs have been used to reveal the composite
nature of the source tracking the missing components in their
isotopic fingerprints. An opportunity to address this issue is
given by temporal sections of single ridge segments where
MORB and residual parent mantle peridotites can be assessed
together. The possibility has been offered by the flexured and
uplifted lithospheric slab that exposes, on the sea floor along
a seafloor spreading flow line, a zero to 26 Ma relatively
undisturbed lithospheric section (Vema Lithospheric Section
or VLS) generated at the 80 km long Mid Atlantic ridge
segment (EMAR) at 11°N.
Temporal variations of the Nd isotopic composition of
crustal basalts and parental mantle along the VLS reveal a
large dispersion of residual isotopic composition with respect
to the melt products. Equilibration with partially mixed melts
can account for the observed relationships in the mantle
rocks. The mean MORB isotopic composition and the
average composition of the residues do not match because of
the preferential extraction of the low-melting component. The
compositional difference in both isotopic and elemental
distribution is a function of the average degree of melting of
the mantle. This observation can only be justified by
progressive melting of composite lithologies where mantle
potential temperature and amount of dispersed low-melting
lithologies control the relative extent of melting of the mantle
host and that of the dispersed heterogeneities resulting in
differential fractions of mixed melts in the final products.
This observation is confirmed by global correlations in Nd
isotopes and chemical indicators of degree of melting from
other portions of the mid ocean ridge system
A Software-based Low-Jitter Servo Clock for Inexpensive Phasor Measurement Units
This paper presents the design and the implementation of a servo-clock (SC)
for low-cost Phasor Measurement Units (PMUs). The SC relies on a classic
Proportional Integral (PI) controller, which has been properly tuned to
minimize the synchronization error due to the local oscillator triggering the
on-board timer. The SC has been implemented into a PMU prototype developed
within the OpenPMU project using a BeagleBone Black (BBB) board. The
distinctive feature of the proposed solution is its ability to track an input
Pulse-Per-Second (PPS) reference with good long-term stability and with no need
for specific on-board synchronization circuitry. Indeed, the SC implementation
relies only on one co-processor for real-time application and requires just an
input PPS signal that could be distributed from a single substation clock
Cooperative UAVs Gas Monitoring using Distributed Consensus
This paper addresses the problem of target detection and localisation in a
limited area using multiple coordinated agents. The swarm of Unmanned Aerial
Vehicles (UAVs) determines the position of the dispersion of stack effluents to
a gas plume in a certain production area as fast as possible, that makes the
problem challenging to model and solve, because of the time variability of the
target. Three different exploration algorithms are designed and compared.
Besides the exploration strategies, the paper reports a solution for quick
convergence towards the actual stack position once detected by one member of
the team. Both the navigation and localisation algorithms are fully distributed
and based on the consensus theory. Simulations on realistic case studies are
reported.Comment: 7 pages, 6 figure
Post-Mesozoic Rapid Increase of Seawater Mg/Ca due to Enhanced Mantle-Seawater Interaction
The seawater Mg/Ca ratio increased significantly from , 80 Ma to present, as suggested by studies of
carbonate veins in oceanic basalts and of fluid inclusions in halite. We show here that reactions of
mantle-derived peridotites with seawater along slow spreading mid-ocean ridges contributed to the
post-Cretaceous Mg/Ca increase. These reactions can release to modern seawater up to 20% of the yearly Mg
river input. However, no significant peridotite-seawater interaction and Mg-release to the ocean occur in
fast spreading, East Pacific Rise-type ridges. The Mesozoic Pangean superocean implies a hot fast spreading
ridge system. This prevented peridotite-seawater interaction and Mg release to the Mesozoic ocean, but
favored hydrothermal Mg capture and Ca release by the basaltic crust, resulting in a low seawater Mg/Ca
ratio. Continent dispersal and development of slow spreading ridges allowed Mg release to the ocean by
peridotite-seawater reactions, contributing to the increase of the Mg/Ca ratio of post-Mesozoic seawater
Archaeometric characterization of prehistoric pottery from Baħrija, Malta
The end of prehistory in the Maltese archipelago is characterized by the production of a problematic class of pottery, until now attested just at the site of Baħrija, on the western coast of Malta. Such a production represents a break with the tradition in terms of repertoire of shapes, style and technology and it has been interpreted as the result of contact between locals and foreign immigrants. The recent overall reappraisal of the unpublished ceramic assemblage collected during the excavations carried out at Baħrija, represents a unique opportunity to focus on the technological aspects of the production, trying to shed light on the issue of the break with the tradition and the impact of external influxes. Petrographic analysis on thin sections and chemical analyses via X-ray fluorescence spectroscopy (XRF) and laser ablation-inductively coupled plasma mass spectroscopy (LA-ICP-MS) have been carried out to characterize the Baħrija pottery production in order to interpret from a different angle the issue of the possible arrival of newcomers and establishment of a foreign enclave in Malta, which until now has been hypothesized only on the basis of the sudden emergence of the Baħrija pottery
Mineralogical and Chemical Investigations of the Amguid Crater (Algeria): Is there Evidence on an Impact Origin?
Mineralogical and chemical investigations were carried out on intra-craterial bedrocks (Lower Devonian sandstone) and regolithic residual soil deposits present around the Amguid structure, to discuss the hypothesis of its formation through a relatively recent (about 0.1 Ma) impact event. Observations with an optical microscope on intra-craterial rocks do not unequivocally confirm the presence of impact correlated microscopic planar deformation features (PDFs) in quartz crystals. Field observations, and optical and instrumental analysis (Raman spectroscopy) on rocks and soils (including different granulometric fractions) do not provide any incontrovertible pieces of evidence of high energy impact effects or products of impact (e.g., high pressure—temperature phases, partially or totally melted materials, etc.) either in target rocks or in soils. A series of selected main and trace elements (Al, Fe, Mg, Ni, Co and Cu) were analysed on rocks and soils to evaluate the presence in these materials of extraterrestrial sources. Comparative chemical data on rocks and soils suggest that these last are significantly enriched in Fe-poor Mg-rich materials, and in Co, Ni and Cu, in the order. A large number of EDAX-SEM analyses on separated soil magnetic particles indicate an abnormally high presence of Al-free Mg-rich sub-spherical or drop-like silicate particles, showing very similar bulk chemistries compatible with forsterite olivine. Some particles were found associated with a Ni-rich iron metal phase, and this association suggests a specific extraterrestrial origin for them. Electron microscope analysis made on a large number of soil magnetic particles indicates that 98% of them are terrestrial phases (almandine garnet, tourmaline and Fe-oxides, in abundance order), whereas, only a few grains are of questionable origin. One of the Mg-rich silicate particles was found to be a forsterite (Mg = 0.86) Mn-rich (MnO: 0.23%) Cr-free olivine, almost surely of extraterrestrial sources. Electron microprobe analysis of three soil particles allowed identification of uncommon Cr-rich (Cr2O3 about 8%) spinels, poorly compatible with an origin from terrestrial sources, and in particular from local source rocks. We propose a specific extraterrestrial origin for sub-spherical olivine particles characterised by quite similar magnesian character. Excluding any derivation of these particles from interplanetary dust, two other possible extraterrestrial sources should be considered for them, i.e., either normal micrometeorite fluxes or strongly un-equilibrated, or the Vigarano type Carbonaceous (CV) chondrite meteorite material. In this case, further studies will confirm an impact origin for Amguid, as such magnesian olivine components found in soils might represent the only remnants of a vaporised projectile of ordinary non-equilibrated meteoritic composition
Multistage asthenospheric melt/rock reaction in the ultraslow eastern SWIR mantle
Very small amounts of melt are produced during mantle upwelling beneath the ultraslow spreading South West
Indian Ridge. Sectors of this Oceanic Ridge are characterized by nearly amagmatic spreading with rare limited
eruptions of basalts spotting a mantle-derived serpentinitic crust. A large peridotite dataset was recovered during the
Smoothseafloor French expedition leaded by D. Sauter and M. Cannat in 2005 (Sauter et al., 2013). Mantle-derived
rocks show a significant modal variability from the sample to the dredge scale with frequent occurrences of millimetric
to centimetric spinel-bearing pyroxenitic veins. Mantle residua record a multistage reactional history between small
amount of transient melts and variably depleted mantle parcels. Incomplete mineral replacements are widespread
showing that both pyroxenes are repeatedly dissolved and recrystallized leaving poekilitic pyroxene and spinel textures.
Reacting conditions are modelled assuming an incremental open-system melting model under variable critical
porosity/F ratios (Seyler et al., 2011; Brunelli et al., 2014). Incoming melts result to be generated by low degrees of
melting in the garnet field then reacting with the rock under near-batch conditions, i.e. at low rates of melt extraction
with respect to the actual rock porosity. As a consequence Na (and LREE) countertrends with melting indicators as
mineral Cr# and concentration of the moderately incompatible elements (HREE, HFSE). This results in rotation of the
REE patterns around a pivot element instead of showing progressive depletion as expected after suboceanic mantle
decompression.
Brunelli D., Paganelli E. & Seyler, M. 2014. Percolation of enriched melts during incremental open-system melting in
the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge. Geoch. et Cosmoch. Acta,
127, 190–203. doi:10.1016/j.gca.2013.11.040.
Sauter D., Cannat M., Searle R. 2013. Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge
for 11 million years. Nature Geosci., 6(4), 1–7. doi:10.1038/ngeo1771.
Seyler M., Brunelli D., Toplis M. J. & Mével C. (2011). Multiscale chemical heterogeneities beneath the eastern
Southwest Indian Ridge (52°E-68°E): Trace element compositions of along-axis dredged peridotites. Geochem.
Geophys. Geosyst., 12, Q0AC15. doi:10.1029/2011gc003585
Micro-scale investigation of carbonation process in partially serpentinized peridotites
The carbonation of ultramafic rocks is, theoretically, the most efficient reaction to trap CO2 irreversibly in
the form of solid carbonates, as predicted by equilibrium thermodynamic calculations. However, the success
of industrial or natural carbonation in large ultramafic aquifers or oceanic ultramafic exposures does not only
rely on the thermodynamic conditions of chemical reactions, but also on their feedback effects on the reactive
surface area and on the local porosity and permeability. In addition, side processes like serpentinization, redox
reactions, abiotic catalytic effects, and biological activity, can be expected in such complex natural system. Their
occurrence and implications on carbon speciation and carbon transfers during hydrothermal alteration of oceanic
peridotites have not been explored yet and requires detailed study of natural and/or experimental carbonation zones.
We have combined petrographic and electron microscopy with SIMS, Raman and FTIR microspectroscopy
on partially serpentinized peridotites drilled during the IODP leg 304 (30
N, MAR) in order to characterize
the mechanisms of peridotite carbonation at the fluid-mineral interface and identify the associated speciation of
carbon (inorganic and organic carbon occurrences). We present first results on zones located close to talc-tremolite
sheared veins in holes 1309B and D. Associations of carbonates, porous phyllosilicates and oxides are observed
in close vicinity of relict olivines that underwent a previous stage of serpentinization. The olivine-carbonate
interface is nanoporous which facilitates mass transfer between fluid and mineral. The phyllosilicate identified as
saponite results from the metasomatic replacement, during the carbonation stage, of previously formed serpentine.
These observations do not favour reaction-induced cracking but rather a transfer-controlled process in an open
system. Among the submicrometric dark clusters widely-distributed in saponite and in serpentine, vibrational
microspectroscopy reveals the presence of various types of organic compounds that tend to be located close to
micrometric sulphides grains.
Those results underline the microscale variability of carbon speciation within hydrothermally altered peridotites. The association of reduced carbon phases with the carbonation texture suggests that CO2 conversion
may not be limited to solid carbonate formation in natural systems and that biological activity and/or abiotic
CO2 reduction, possibly catalyzed by Ni-rich sulphides, can occur contemporaneously. This complex association
of reactions has to be unravelled further to determine the respective contribution of abiotic versus biological
processes and integrate them in carbon transfers modelling through the oceanic lithosphere
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