1,721,003 research outputs found
Heterogeneous mantle domains in a modern OCT: new insights from the west Iberian margin (ODP Legs 149 and 173)
Full text linkMagma-poor ocean-continent transition zones (OCTs) represent wide areas of tectonically uplifted mantle.
While an important body of literature has dealt with the investigation of fossil analogues (e.g. Picazo et. al.,
2016, and references therein), studies on mantle sequences exhumed in modern OCTs remain a few (e.g.
McCarthy et al., 2020), leaving our knowledge somehow fragmentary. In particular, how, where and when
lithospheric breakup occurs, the timing of melt production, and the nature of the mantle source involved
remain hotly debated issues.
Among the best-documented continental margins worldwide, the West Iberian margin represents a unique
site where mantle has been accessed through scientific drilling more than three decades ago (e.g. Boillot et
al., 1989). However, these peridotites have remained poorly characterized, partly because of their pervasive
serpentinization.
In this contribution, we present the results of an in situ petrological and geochemical study performed on
a set of mantle peridotites from ODP Holes 899B, 1068A, and 1070A. In Hole 899B, relatively fresh spineland
plagioclase-bearing harzburgites (cpx ≈ 2-8 vol.%) occur. In contrast, highly serpentinized plagioclase
lherzolites and spinel harzburgites were sampled in Holes 1068A and 1070A.
Hole 899B spinel harzburgites bear refractory compositions, as attested by the low clinopyroxene contents
(≈ 2-3 vol.%), coupled to moderate spinel Cr# (0.230-0.343) and low pyroxene YbN (≈ 1-2). Geochemical
modeling based on pyroxene trace element compositions points to significant degrees of melt extraction (up to ≈ 15%), starting in the garnet stability field, followed by post-melting metasomatism.
Holes 899B and 1068A plagioclase-bearing samples share remarkable similarities, i.e. frequent occurrence
of (altered) plagioclase rims mantling elongated dark spinel, Na2O- and Al2O3-rich clinopyroxene associated
with high TiO2 spinel (up to 0.66 wt%). Clinopyroxene shows convex-upward REE patterns, yielding negative
Eu anomalies and higher HREE contents compared to abyssal peridotites. Hence, these peridotites are not
simple partial melting residues but experienced melt-rock interaction processes and re-equilibration in the
plagioclase stability field.
Hole 1070A peridotites exhibit contrasting signatures, as highlighted by the presence of Na2O-rich,
Al2O3-poor clinopyroxene coupled to spinel with high Cr# (0.246-0.428) and TiO2 below the detection limit.
Orthopyroxene trace element investigation of these samples revealed hump-shaped patterns with variable
LREE-MREE fractionation (LaN/SmN= 0.003- 0.16) and low YbN (≈ 1-2).
Our new data attest a wide spectrum of petrological and geochemical characters shown by mantle rocks
exhumed in a modern OCT. This heterogeneous nature possibly reflects a complex interplay between riftingrelated processes and previous depletion history inherited from the last Wilson cycle
A novel approach for quantitative analysis of S-bearing volatile species generated from HP-HT experiments
No full textA novel approach for quantitative analysis of S-bearing volatile species generated from HP-HT experiments
Unveiling mantle heterogeneity in a modern OCT: new insights from the West Iberian margin (ODP Leg 149 and 173)
No full textMagma-poor ocean-continent transition zones (OCTs) encompass extensive regions of tectonically uplifted mantle. While substantial literature has explored fossil analogues (e.g., Picazo et al., 2016), recent studies on mantle sequences exhumed in modern OCTs are limited (e.g., McCarthy et al., 2020), resulting in a fragmented understanding. Key questions regarding the mechanisms, timing, and location of lithospheric breakup, melt production, and the nature of the mantle source remain debated.
The West Iberian margin (WIM) is one the best-documented continental margins worldwide, with scientific drilling initiated more than three decades ago (e.g., Boillot et al., 1989). However, peridotites from this region have remained poorly characterized due to their extensive serpentinization.
In this study, we revisit the peridotites from the OCT of the WIM, presenting new in situ data for a set of variably serpentinized samples. To capture a comprehensive view of the petrological and geochemical characteristics, we analyzed samples from three boreholes along an E-W transect, from the most proximal to the most oceanward domains: ODP Hole 1068A, 899B, and 1070A.
Hole 1068A samples are serpentinized plagioclase lherzolites (cpx ~ 10 vol.%) with Na2O- (0.61-0.87 wt%) and Al2O3-rich (5.83-7.10 wt%) clinopyroxene, high spinel Cr# (0.255-0.322) and TiO2 (0.21-0.30 wt%). These characteristics, along with clinopyroxene convex-upward REE patterns yielding negative Eu anomalies and high YbN= 15-17, resemble those of refertilized domains in the sub-continental lithospheric mantle (SCLM, e.g. Müntener et al., 2010). Geochemical modelling indicates an origin involving MORB-type melt impregnation followed by re-equilibration in the plagioclase stability field.
Hole 899B exhibits the higher degree of lithological and chemical heterogeneity. The investigated rock-types are coarse-grained peridotites, varying from spinel to plagioclase harzburgites (cpx ~ 2-8 vol.%). They show evidence of melt-rock interaction, highlighted by olivine-forming, pyroxene-dissolving microstructures. Plagioclase ± secondary orthopyroxene aggregates were also observed in sample 899-1. Mineral compositions (i.e. clinopyroxene and spinel) have moderately depleted to slightly enriched signatures (Cpx: Al2O3= 4.05-6.90 wt%, Na2O= 0.23-0.63 wt%; Sp: Cr#= 0.230-0.463, TiO2= 0.58-0.61 wt% for sample 899-1). Clinopyroxene in spinel harzburgites displays uncommon chondrite-normalized V-shaped REE patterns (LaN/SmN= 2.7-3.4, DyN/YbN= 0.5-0.6) previously recorded in mantle xenoliths from central and eastern Europe (e.g. Downes et al., 2003). In contrast, clinopyroxene in plagioclase-bearing sample shows concave-downward REE patterns, with depleted LREE segments (LaN/SmN= 0.02-0.03) and negative Eu anomalies, suggesting melt depletion followed by re-equilibration in the plagioclase stability field.
Hole 1070A peridotites are coarse-grained spinel harzburgites (cpx ~ 2-5 vol.%) displaying pyroxene-dissolving and olivine-precipitating microtextures. Positive correlations among melting indexes in clinopyroxene (i.e. Cr#, Al2O3 and Yb) suggest a residual origin for these lithologies. However, unusual Cr-Na enrichments and hump-shaped clinopyroxene REE patterns indicate open system melting in the spinel stability field, accompanied by percolation of an enriched melt. These features, though rare in modern oceans, were previously attested in some peridotites from slow- to ultraslow-spreading settings (Hellebrand & Snow, 2003; Seyler et al., 2011).
Calculated equilibrium temperatures for the WIM peridotites range within those of fossil OCTs (TCa-in-Opx= 921-1029 °C). They reflect a history of melt-rock interaction at high temperature (TREE-Y= 1098-1244 °C), followed by thermal re-equilibration at lower temperatures (TOl-Sp= 770-813 °C).
These new data provide compelling evidence of highly heterogeneous mantle domains exposed in the OCT of the WIM. The documented heterogeneity, observed among peridotites from the same borehole and among boreholes, challenges the traditional view of OCTs as regions with simple and fixed distribution of mantle domains with well-defined major and trace element compositions. We posit that melt-rock interaction plays a key role in generating petrological and chemical heterogeneity in OCTs and the upper mantle.
References
Boillot, G., Féraud, G., Recq, M. & Girardeau, J., 1989. Undercrusting by serpentinite beneath rifted margins. Nature, 341, 523–525. https://doi.org/10.1038/341523a0
Downes, H., Reichow, M. K., Mason, P. R. D., Beard, A. D. & Thirlwall, M. F., 2003. Mantle domains in the lithosphere beneath the French Massif Central: Trace element and isotopic evidence from mantle clinopyroxenes. Chemical Geology, 200, 71–87. https://doi.org/10.1016/S0009-2541(03)00126-8
Hellebrand, E. & Snow, J. E., 2003. Deep melting and sodic metasomatism underneath the highly oblique-spreading Lena Trough (Arctic Ocean). Earth and Planetary Science Letters, 216, 283–299. https://doi.org/10.1016/S0012-821X(03)00508-9
McCarthy, A., Falloon, T. J., Sauermilch, I., Whittaker, J. M., Niida, K. & Green, D. H., 2020. Revisiting the Australian-Antarctic Ocean-Continent Transition Zone Using Petrological and Geophysical Characterization of Exhumed Subcontinental Mantle. Geochemistry, Geophysics, Geosystems, 21, 7, e2020GC009040. https://doi.org/10.1029/2020GC009040
Müntener, O., Manatschal, G., Desmurs, L. & Pettke, T., 2010. Plagioclase Peridotites in Ocean–Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere. Journal of Petrology, 51, 255–294. https://doi.org/10.1093/petrology/egp087
Picazo, S., Müntener, O., Manatschal, G., Bauville, A., Karner, G. & Johnson, C. 2016. Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle. Lithos, 266–267, 233–263. https://doi.org/10.1016/j.lithos.2016.08.029
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. Geochemistry, Geophysics, Geosystems, 12, 9. https://doi.org/10.1029/2011GC00358
Mantle Heterogeneity Generated by Melt-Rock Reaction: New Insights from the West Iberian Margin Peridotites (ODP Legs 149 and 173)
No full textAmong continental margins worldwide, the West Iberian Margin
(WIM) stands out as one of the best-documented ocean-continent
transition zone with oceanic scientific drilling sampling several
exhumed mantle outcrops (Boillot et al., 1989). However, due to their
high serpentinization, the petrogenesis of the WIM mantle
exposures was largely unexplored.
Here, we present new petrological and geochemical data obtained
on variably serpentinized peridotites from ODP Holes 899B, 1068A,
and 1070A.
Hole 899B spinel (spl)-harzburgites show pyroxene embayments
filled by olivine, anomalously high olivine content (up to ≈ 80 vol.%)
and small interstitial orthopyroxene (opx). These textures coupled to
depleted signatures (clinopyroxene (cpx)≈ 2-3 vol.%, Yb cpx ≈ 1-6 –N
= chondrite normalized) suggest moderate melt extraction followed
by reactive porous flow.
Holes 1068A and 899B pl-peridotites share notable similarities,
including the presence of plagioclase rims mantling deformed
spinel, Na O- and Al O -rich cpx, high spl Cr# and TiO . These
features, along with cpx convex-upward REE patterns yielding
negative Eu anomalies and HREE higher than abyssal peridotites,
indicate melt-rock reaction and re-equilibration in the plagioclase
field.
Hole 1070A peridotites show contrasting geochemical signatures,
with compositions ranging from abyssal-like (Al O -poor cpx; high
Cr# = 0.246-0.428) to refertilized sub-continental mantle (Na O rich cpx). Opx displays hump-shaped REE patterns with low HREE
(Yb ≈ 1-2). Our data reveal widespread heterogeneous mantle domains
exposed along a modern OCT. We interpret this heterogeneity as
resulting from different stages and styles of melt-rock reaction
operating on variably depleted sub-continental mantle lithosphere
protoliths. We posit that melt-rock interactions represent among the
most effective mechanisms to create petrological and chemical
heterogeneity in OCTs and the upper mantle in general
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
