1,721,128 research outputs found
Mantle-Crust Isotopic Relationships along Mid Ocean Ridges: Constraints from the Analysis of Time Series
No full textIsotopic 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
Pyroxenites sow discord between parent mantle and daughter MORB
No full textMantle rocks and MORBs sampled along the 26 Ma
record of the Vema Lithospheric Section or VLS (Mid
Atlantic 11°N) reveal a profound divergence between parent
and daughter rocks in their compositional evolution. Finding
the cause of this discordance requires comparing the
evolution through time of the extent of melting with the
produced crustal thickness and changes in the chemistry and
isotopic composition of the sampled rocks. Mantle rocks
show a progressive increase of their degree of melting during
time, as if the potential temperature had increased in the last
26 Ma. However, plate kinematics reveal a significant
decrease of the spreading rate that should sensibly lower the
average degree of melting. MORB inferred degree of melting
decreases through time while their isotopic fingerprint
becomes more depleted. The observed variations can be
reconciled by considering that a variable amount of lowmelting
lithologies entered the melting region. The observed
decoupling of the degree of melting of mantle and MORB is
attributed to the effect of undercooling exerted by lowmelting
heterogeneities due to heat diffusion before the host
mantle starts melting itself. The observed temporal evolution
is matched by a decreasing amount of low-melting,
isotopically enriched, lithologies (pyroxenites) dispersed in a
DMM-type host mantle. Approaching the present day setting,
the amount of pyroxenites has become negligible restoring
harmony between parent mantle and MORB daughters in
terms of degree of melting and integrated melt production.
This observation can be extended to the entire MOR
system revealing the sensitivity of the spreading system to the
amount of low-melting lithologies dispersed in the depleted
mantle host. At limit conditions of the mantle potential
temperature, heat diffusion into the low-solidus melting
lithology prevents the host mantle from reaching its solidus
Comment on: metals in bones of the middle-aged inhabitants of Sardinia island (Italy) to assess nutrition and environmental exposure [Bocca et al. (2018), Environ Sci Pollut Res]
No full textThis comment addresses several inferences made by Bocca et al. (2018) to assess the nutrition of middle-aged inhabitants from four Sardinian localities (Alghero, Bisarcio, Geridu, and Sassari). Bocca et al. analyzed 72 human bone specimens dating from the twelfth to the eighteenth century for metal contents (Ba, Ca, Cd, Cu, Hg, Pb, Sr, and Zn) by SC-ICP-MS and applied a univariate and multivariate approach, using element-Ca ratios, to examine the eating habits and environmental exposure of the individuals. While we agree with their interpretation of the Hg/Ca and Pb/Ca ratios, attributable to various forms of environmental exposure, we disagree with some of their paleodiet claims
Commentary on “Analyses of human dentine and tooth enamel by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to study the diet of medieval Muslim individuals from Tauste (Spain)” by Guede et al. 2017, Microchemical Journal 130, 287–294
No full textThe study of trace element (TE) distribution within human and animal teeth is crucial to decrypt information about their diet, ecology and behaviours in the past. Thanks to several efforts TE applications have spread also to the study of modern environments, with repercussions in medicine and forensic contexts. However, the use of TE analysis to infer eating habits of our ancestors has been used for decades without the proper theoretical basis. After the paper of Ezzo Ezzo (1994) that demonstrated the non-validity in the use of bioessential TE elements in bones and teeth as palaeodiet markers this trend has decreased. However, still some recent papers continue to ignore that portion of the literature that strongly contrasts the use of TE analysis for palaeodiet. With this commentary, we would like to share our remarks on the paper of Guede et al. Guede et al. (2017), where, in our opinion, there is a lack of literature review and thus a misinterpretation of the TE dataset
Mantle peridotites from the Bouvet Triple Junction Region, South Atlantic
No full textThe Bouvet Triple Junction (BTJ) region in the South Atlantic,where the African, South American and Antarctica plates meet,is affected by several topographic ⁄ melting anomalies. Causesof these anomalies were investigated through a study ofmantle-derived serpentinized peridotites sampled from threesites in the BTJ region: (1) the Inner Corner High at theintersection of the America Antarctic Ridge (AAR) with theConrad transform; (2) the south wall of the Bouvet transform(South West Indian Ridge, SWIR); and (3) the eastern BouvetSWIR Transform Intersection. The degree of melting undergone by these rocks was estimated from relic mineral major- and trace-element composition. Geochemical profiles from residual peridotites and associated basalts show a > 1000-km-wide melting anomaly centred on the Bouvet and Spiess topographic anomalies
From magma-engorged to magma starved “constipated” lithosphere: The Mid Atlantic Ridge from 40°N to the Equator
No full textabstrac
REE pattern rotation in an open system melting: case studies from slow and ultraslow spreading ridges
No full textThe REE compositional space provides a reliable means to recognize the degree of depletion and melt rock reactional events undergone by a parcel of mantle. We model residual clinopyroxene compositions from slow (MAR) and ultraslow (SWIR) sectors of the Mid Ocean Ridges. REE ratios (e.g. SmN/YbN vs. YbN) show compositional trends crosscutting the expected partial melting trends at the typical kilometer length scale. In the REE compositional space they appear as pattern rotations around a mid-point. Open-system melting modeling reveals the intensity of the rotation and the position of the pivot element, depending mainly on the ratio between input/output melt flux and on the enrichment of the percolating melt with respect to the depleted screen. Variations of the residual porosity of the system (Ø) with respect to the degree of melting F result in variations of the nature of the melting process. At low Ø/F the process behaves as near-fractional while at high Ø/F the process behaves as near batch. In an open melting system scenario the effect of an enriched melt fluxing a melting portion can be strongly enhanced by melt stagnation i.e. approaching a near-batch process with low melt output. Model trends present strong enrichment of SmN/YbN at decreasing YbN values. At high Ø/F even YbN increases along with SmN/YbN. These trends well fit measured countertrends at the km lengthscale, i.e. a typical dredge lengthscale. Our observations suggest that in some portions of the melting region the vertical porosity profile can vary resulting in a variability of the nature of the melting process from near-fractional (when extraction prevails) to near batch (when stagnation prevails). Percolation of enriched melts through a melting mantle parcel results in REE pattern rotation whose intensity and midpoint depends on Ø/F, mixing factors and obviously on the composition of the melt itself. Our preliminary results suggest that, first, melt batches generated deep in the mantle are transported out of equilibrium to shallower portions of the melting region and redistributed to the rock porosity. Second, that porosity barriers are present at depth resulting in melt accumulation and stagnation in the spinel facies of the melting region
Multi-stage impregnation of the lithospheric mantle at the Andrew Bain FZ (SWIR)
No full textThe Southern ridge-transform intersection of Andrew Bain Fracture Zone (ABFZ) is interpreted as a "cold spot" in the mid-ocean ridge system being characterized by a negative thermal anomaly in the oceanic upper mantle. The negative thermal anomaly is associated to the cold-edge effect due to the great age contrast of the active ridge segments. During the oceanic expedition AB06-S23, in 2006, (organized by ISMAR-CNR, Bologna, Italy, and co-financed by PRNA, Italy) with the russian R/V N. Strakhov, several samples of abyssal peridotites have been collected. Textures and modal distribution of the samples have been investigated revealing a multistage impregnation history. Deep spinel-field impregnation assemblages (sp+cpx‚±opx‚±ol) are followed by plagioclase-field patches and mineral trails (pl+cpx‚±ol) and late shallow gabbroic pockets and veins. The major elements mineral chemistry reveals compositional trends of low-P/T subsolidus partial- to-complete re-equilibration undergone by the upper mantle during the upwelling beneath the ridge. These samples have experienced variable degrees of melting and reacted with percolating melts of possible different composition. In particular, samples showing the lowest degrees of melting have interacted with MORB-like melts and pyroxenitic-derived melts in the spinel and plagioclase stability fields. The presence of these two kinds of melts might prove the presence of enriched portions scattered in a normal depleted mantle beneath ocean ridges. MELTS-based runs provide constraints to variable extents of pyroxenitic-derived melt interaction with the mantle source and crystallization at variable depth of the products of such an interaction. Supported by MIUR-PRIN Cofin project 200
Is the Mid-Atlantic Ridge becoming hotter with time?
No full textMore than 20 million years of oceanic lithosphere accretion history at a segment of the Mid-Atlantic Ridge are recorded in the Vema Lithospheric Section (VLS), a 300 km long flexured and uplifted sliver of lithosphere exposed near the Vema Fracture Zone in the Central Atlantic. Systematic sampling of the basal mantle ultramafic unit and of crustal basalts along the VLS together with geophysical surveys gave us the opportunity to study temporal changes in the processes of generation of the oceanic lithosphere at a ridge axis. The degree of melting of the mantle upwelling below the ridge axis, estimated from the chemistry of mantle-equilibrated mineral phases in the peridotites, as well as crustal thickness, inferred from shipboard and satellite gravity data, both show ~3-4 my long oscillations superimposed on long-range steady increases with time. Based on basaltic glasses elemental and isotopic chemistry, we assume the composition of the source stayed nearly constant. The steady increase with time of mantle degree of melting and of crustal thickness suggests that the mantle rising beneath the MAR became gradually hotter during the last 20 million years, even though the spreading half rate slowed significantly during this time. We offer two explanations for the increase in mantle temperature with time at the Mid-Atlantic Ridge. A first possibility, of local significance, calls for gradual lengthening of the eastern MAR segment where the VLS was created, leading to an increasing degree of melting below center of the segment as it lengthens, due to the a decreasing influence of the "cold edge effect" from the Vema transform. The second, of broader significance, calls for a gradual increase of mantle potential temperature along a significant portion of the northern MAR during the last 20 million years, resulting in an increase of melt production despite decreasing spreading rates. This second hypothesis is supported by an increase of crustal thickness towards ridge axis observed at several other locations in the northern MAR. The chemistry of basaltic glasses, collected along the VLS above the peridotites, suggests that no deep plume source is involved in the steady heating of the Ridge
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