Éditeur(s) : HAL CCSD Springer Verlag
Résumé : The activity of the Damavand volcano (Central Alborz, northern Iran) began 1.8 Ma ago and continued up to 7 ka BP. Although the volcanic suite is clearly of shoshonitic affinity, only two petrographic types can be distinguished in the studied lavas: (1) weakly differentiated absarokites (49 < %SiO2 < 51), scattered around the volcano but with a regional extension, (2) highly differentiated banakites (59 < %SiO2 < 63), which form the bulk of the 4,000 m thick volcanic pile. All lavas are alkalic (3.7 < %K2O < 5), REE and LILE-rich (e.g., 85 < La < 148 ppm; 9 < Th < 32 ppm) and show highly fractionated REE patterns (69 < La/Yb < 115) and pronounced Nb-Ta negative anomalies. The absarokites are characterised by Sr (0.7045-0.7046) and Nd (0.51266-0.51269) isotope compositions close to the Bulk Earth values, and distinct from those of the banakites (0.7047 < Sr-87/Sr-86 < 0.7049, 0.51258 < Nd-143/Nd-144 < 0.51262). The Pb isotope ratios are also slightly lower in the absarokites than in the banakites (18.71 < Pb-206/Pb-204 < 18.77, 15.62 < Pb-207/Pb-204 < 15.63, 38.85 < Pb-208/Pb-204 < 38.91, and 18.77 < Pb-206/Pb-204 < 18.84, 15.62 < Pb-207/Pb-204 < 15.64, 38.94 < Pb-208/Pb-204 < 39.06, respectively). Overall, there is a clear tendency towards higher Sr, Pb and lower Nd isotope ratios with increasing degree of differentiation. This study suggests that the absarokites result from a low degree of partial melting (similar to 5%) of a highly metasomatized mantle source, which inherited its characteristics from an old subduction setting. The initiation of volcanic activity 1.8 Ma ago results from variations in the lithospheric thermal regime, probably related to lithospheric delamination as proposed for Anatolia (Pearce et al. 1990). The banakites are mainly generated by extensive fractional crystallisation (similar to 70%) of the absarokitic magma, with a limited amount (a few percents) of assimilation of an old crustal component, in the form of bulk assimilation or AFC processes, which both can explain the Sr, Nd and Pb isotope data.
ISSN: 1437-3254

hal-00411553
https://hal.archives-ouvertes.fr/hal-00411553
DOI : 10.1007/s00531-006-0159-6

Éditeur(s) : Elsevier Science Bv
Résumé : Here we present a new geochemical study of Lopevi volcano, one the most active volcanoes in the Vanuatu island arc. We focus on the temporally well-defined sequence of lava flows emitted since 1960, and for the first time, on pre-1960 volcanic products, including high-MgO basalts and felsic andesites, the most evolved lavas sampled so far on this island. This work reports the first Pb and Hf isotopic study of lavas from Lopevi island. These lavas display correlations between differentiation indexes such as SiO2 content and isotopic ratios. The felsic andesites extend the known correlations with both the least (Sr-Pb) and the most (Nd-Hf) radiogenic isotopic compositions on the island. Our results confirm that the rising magma interacted with the sub-arc crust. Assimilation-Fractional Crystallization (AFC) quantitative modeling of trace element ratios and isotopic compositions requires 1% and 10% of assimilated partial melts of a mafic oceanic crust to account for the pre- and post-1960 lavas, respectively. The post-1960 lavas differ from the former lavas emitted similar to 20 years earlier by enrichments in fluid mobile elements (K, Ba, Rb ... ), Th, and Light Rare Earth Elements (LREE). We ascribe these features to slight variations in the metasomatic agent added to the sub-arc mantle and ultimately derived from the subducted lithosphere. However, the contrasting time scales involved in subducted lithosphere dehydration and magma genesis, relative to the time elapsed between eruptions of the two lava series, suggest that two different portions of mantle which have undergone slightly different metasomatism, gave birth to the Lopevi lavas. These distinct magmas are still present beneath the volcano.
Journal Of Volcanology And Geothermal Research (0377-0273) (Elsevier Science Bv), 2013-08 , Vol. 264 , P. 72-84

Droits : 2013 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00167/27801/26223.pdf
DOI:10.1016/j.jvolgeores.2013.07.005
http://archimer.ifremer.fr/doc/00167/27801/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Jacupiranga Complex is one of several Meso-Cenozoic alkaline intrusive complexes along the margins of the intracratonic Paraná Basin in southern Brazil. The complex encompasses a wide range of rock-types, including dunites, wehrlites, clinopyroxenites, melteigites-ijolites, feldspar-bearing rocks (diorites, syenites, and monzonites), lamprophyres and apatite-rich carbonatites. While carbonatites have been extensively investigated over the last decades, little attention has been paid to the silicate rocks. This study presents new geochonological and geochemical data on the Jacupiranga Complex, with particular emphasis on the silicate lithotypes. 40Ar/39Ar ages for different lithotypes range from 133.7 ± 0.5 Ma to 131.4 ± 0.5 Ma, while monzonite zircon analyzed by SHRIMP yields a U-Pb concordia age of 134.9 ± 1.3 Ma. These ages indicate a narrow time frame for the Jacupiranga Complex emplacement, contemporaneous with the Paraná Magmatic Province. Most of the Jacupiranga rocks are SiO2-undersaturated, except for a quartz-normative monzonite. Based on geochemical compositions, the Jacupiranga silicate lithotypes may be separated into two magma-evolution trends: (1) a strongly silica-undersaturated series, comprising part of the clinopyroxenites and the ijolitic rocks, probably related to nephelinite melts and (2) a mildly silica-undersaturated series, related to basanite parental magmas and comprising the feldspar-bearing rocks, phonolites, lamprophyres, and part of the clinopyroxenites. Dunites and wehrlites are characterized by olivine compositionally restricted to the Fo83-84 interval and concentrations of CaO (0.13–0.54 wt%) and NiO (0.19–0.33 wt%) consistent with derivation by fractional crystallization, although it is not clear whether these rocks belong to the nephelinite or basanite series. Lamprophyre dikes within the complex are considered as good representatives of the basanite parental magma. Compositions of calculated melts in equilibrium with diopside cores from clinopyroxenites are quite similar to those of the lamprophyres, suggesting that at least a part of the clinopyroxenites is related to the basanite series. Some feldspar-bearing rocks (i.e. meladiorite and monzonite) show petrographic features and geochemical and isotope compositions indicative of crustal assimilation, although this may be relegated to a local process. Relatively high CaO/Al2O3 and La/Zr and low Ti/Eu ratios from the lamprophyres and calculated melts in equilibrium with cumulus clinopyroxene point to a lithospheric mantle metasomatized by CO2-rich fluids, suggesting vein-plus-wall-rock melting mechanisms. The chemical differences among those liquids are thought to reflect both variable contributions of melting resulting from veins and variable clinopyroxene/garnet proportions of the source.
ISSN: 0895-9811

hal-01583547
https://hal.archives-ouvertes.fr/hal-01583547
DOI : 10.1016/j.jsames.2017.05.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
IODP Site U1309 was drilled at Atlantis Massif, an oceanic core complex, at 30 degrees N on the Mid-Atlantic Ridge (MAR). We present the results of a bulk rock geochemical study (major and trace elements) carried out on 228 samples representative of the different lithologies sampled at this location. Over 96% of Hole U1309D is made up of gabbroic rocks. Diabases and basalts cross-cut the upper part of the section; they have depleted MORB compositions similar to basalts sampled at MAR 30 degrees N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic and geochemical evidence of extensive melt-rock interactions. Gabbroic rocks comprise: olivine-rich troctolites (>70% modal olivine) and troctolites having high Mg# (82-89), high Ni (up to 2300 ppm) and depleted trace element compositions (Yb 0.06-0.8 ppm); olivine gabbros and gabbros (including gabbronorites) with Mg# of 60-86 and low trace element contents (Yb 0.125-2.5 ppm); and oxide gabbros and leucocratic dykes with low Mg# (<50), low Ni (similar to 65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros are amongst the most primitive and depleted oceanic gabbroic rocks. The main geochemical characteristics of Site U1309 gabbroic rocks are consistent with a formation as a cumulate sequence after a common parental MORB melt, although (lack of systematic) downhole variations indicate that the gabbroic series were built by multiple magma injections. In detail, textural and geochemical variations in olivine-rich troctolites and gabbronorites suggest chemical interaction (assimilation?) between the parental melt and the intruded lithosphere. Site U1309 gabbroic rocks do not represent the complementary magmatic product of 30 degrees N volcanics, although they sample the same mantle source. The bulk trace element composition of Site U1309 gabbroic rocks is similar to primitive MORB melt compositions; this implies that there was no large scale removal of melts from this gabbro section. The occurrence of such a large magmatic sequence implies that a high magmatic activity is associated with the formation of Atlantis Massif. Our results suggest that almost all melts feeding this magmatic system stays trapped into the intruded lithosphere.
ISSN: 0012-821X

hal-00413562
https://hal.archives-ouvertes.fr/hal-00413562
DOI : 10.1016/j.epsl.2008.12.034

Éditeur(s) : HAL CCSD
Résumé : This PhD work is based on field, petrographic, and geochemical observations of rocks originated at the base of the sheeted dike complex, in the Oman ophiolite and at IODP Site 1256, coupled with an experimental study. It provides new constrains on processes that occur at the magma / hydrothermal system transition in oceanic crust formed at fast spreading ridges. The base of the sheeted dike complex is truncated by intrusive isotropic gabbros, and therefore reheated and recrystallized to the "granoblastic dikes" under temperatures up to 1030°C. Xenoliths of granoblastic microgabbros and microgabbronorites derived from the granoblastic dikes are commonly observed in the about 100 meters thick horizon of isotropic gabbro that underlies the sheeted dike complex. These features can be explained by upward migrations of the melt lens that is present at fast spreading centers. The occurrence of several assimilation features (xenoliths and granoblastic patches) in the isotropic gabbro horizon supports the hypothesis that this horizon represents the fossilization of the upper melt lens. The experimental study was designed to simulate experimentally the effect of partial melting of hydrothermally altered sheeted dikes. The results show that melting starts at 850°C, confirm the residual origin of granoblastic dikes and xenoliths, and attest to the anatectic origin of the oceanic plagiogranites that are commonly present close to the base of the sheeted dike complex. The major and trace element composition of the experimental anatectic melt that represents the main contaminant for primitive MORBs at fast spreading ridges has been determined. The upper axial melt lens at fast spreading mid-ocean ridges is herein described as a dynamic system that can migrate vertically, and which fossilizes when moving off-axis.
Ce travail de thèse est basé sur des observations de terrain, sur une étude pétrographique et géochimique des roches formées à la base du complexe filonien dans l'ophiolite d'Oman et au niveau du Site IODP 1256, ainsi que sur une étude expérimentale. De nouvelles contraintes sont apportées sur les processus se produisant à la transition magma / système hydrothermal dans la croute océanique formée au niveau des dorsales à expansion rapide. L'intrusion de gabbros isotropes dans la base du complexe filonien a provoqué son réchauffement et sa recrystallization en « dikes granoblastiques » jusqu'à des températures de 1030°C. Des xénolites de microgabbro à orthopyroxene dérivées des dikes granoblastiques sont souvent observées dans le niveau de gabbros isotropes épais de 100 mètres environ qui est présent à la base du complexe filonien. Ces différentes caractéristiques sont à relier à des migrations verticales vers le haut du sommet de la lentille magmatique supérieure qui est observée aux dorsales rapides. Les nombreuses évidences d'assimilation (xénolites et patchs granoblastiques) dans le niveau des gabbros isotropes appuient l'hypothèse que ce niveau représente la fossilisation de la lentille magmatique supérieure. L'étude expérimentale a consisté à tester l'effet de la fusion partielle du complexe filonien préalablement hydrothermalisé. Les résultats montrent que la fusion commence à 850°C, confirment l'origine résiduelle des dikes granoblastiques et des xénolites associées, et attestent de l'origine anatectique des plagiogranites océaniques qui sont couramment observés à proximité de la base du complexe filonien. La composition en éléments majeurs et traces du liquide anatectique a été déterminée. Ce liquide représente le principal contaminant pour les MORBs primitifs émis au niveau des dorsales rapides. La lentille magmatique supérieure présente au niveau des dorsales médio-océaniques à expansion rapide est ici décrite comme un système dynamique qui peut migrer verticalement, et qui est fossilisée lorsqu'elle se déplace hors axe.
https://tel.archives-ouvertes.fr/tel-00448699

tel-00448699
https://tel.archives-ouvertes.fr/tel-00448699
https://tel.archives-ouvertes.fr/tel-00448699/document
https://tel.archives-ouvertes.fr/tel-00448699/file/France_2009_PhD.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present new Sr-Nd-Pb isotope data of the western Alpujárride metamorphic basement and the pre-Miocene Flysch sediments of the Betic Cordillera (southern Spain). Nd model ages are consistent with an increasing detrital input from the Alborán domain to the Flysch Trough in the western Mediterranean during the late Oligocene. The Alpujárride metamorphic crustal rocks derived from Archean-Paleoproterozoic terranes located along the northern margin of Gondwana in the Neoproterozoic. The heterogeneous isotopic signatures of the Alpujárride units indicate that they have different sedimentary protoliths and underwent contrasted Variscan and pre-Variscan tectono-magmatic evolutions. Melts/fluids derived from the western Alpujárride gneisses contaminated the mantle source of the Ronda high-Mg pyroxenite dykes, implying that the Alpujárride lower crust underthrusted the subcontinental lithospheric mantle of the Alborán domain generating subduction-like magmatism in the late Oligocene. The western Alpujárride upper crust is involved in the Neogene volcanism of the Alborán Sea basin, but only contaminated some LREE-enriched calc-alkaline lavas erupted along the continental margins. On the other hand, tholeiitic lavas in the center of the basin show no isotopic evidence of crustal assimilation. This indicates that most of the crust in the central Alborán Sea accreted by Miocene tholeiitic magmatism and that Alpujárride lower crust is absent and likely foundered close to the continental margins of the basin.
ISSN: 0024-4937

hal-01526912
https://hal.archives-ouvertes.fr/hal-01526912
DOI : 10.1016/j.lithos.2016.10.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The decay of 187Re to 187Os provides a unique tracer of recycled lithosphere in the Earth's upper mantle, and the radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) have been taken to indicate the presence of recycled oceanic crust in the mantle source. However, others have attributed the data to contamination by seawater or an age effect (due to 187Re decay following igneous crystallisation). This study presents new Re Os, U Th Ra and B isotope data for normal-MORB (N-MORB) glass (including 18 samples analysed previously). The scale of the variation in 187Os/188Os observed here in N-MORB glass ranges from 0.126 to 0.148, with a mean value of 0.133 ± 0.009 (2?). Previously published Os isotope data for MORB is shown to have been affected by an analytical artefact, and in particular the very radiogenic Os isotope compositions reported earlier are now shown not to exist. Nevertheless, some radiogenic Os isotope compositions are still seen in the data obtained here. Coupled Th and Ra excesses appear to rule out an age effect, at least, for those samples studied, however, ?11B covariations with 187Os/188Os are consistent with the assimilation of older oceanic crust (affected by seawater contamination). Individual sulphides (less likely to be affected by such contamination) are always less radiogenic than their glass host, with 187Os/188Os values ranging from 0.123 to 0.131. This indicates that sulphide precipitation occurred prior to contamination of the surrounding melt, but the data also suggest that low-Os sulphides have been affected by contamination through partial equilibration with that melt. Taken together, these indicate that dominant process responsible for the radiogenic Os isotope compositions of N-MORB glass is contamination by older oceanic crust. Although the influence of age effects and the presence of recycled material cannot be completely ruled out, Os-rich sulphides yield sub-chondritic Os isotope compositions which point rather to a long-term depletion in Re in the MORB source likely to have resulted from formation of the continental crust over much of Earth's history.
ISSN: 0012-821X

hal-00320075
https://hal.archives-ouvertes.fr/hal-00320075
DOI : 10.1016/J.EPSL.2007.05.014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Kimberlites of the Udachnaya-East pipe (Siberia) include a uniquely dry and serpentine-free rock type with anomalously high contents of chlorine (Cl ≤ 6.1 wt%), alkalies (Na2O + K2O ≤ 10 wt%) and sulfur (S ≤ 0.50 wt%), referred to as a “salty” kimberlite. The straightforward interpretation is that the Na-, K-, Cl- and S-rich components originate directly from a carbonate-chloride kimberlitic magma that is anhydrous and alkali-rich. However, because brines and evaporites are present on the Siberian craton, previous studies proposed that the kimberlitic magma was contaminated by the assimilation of salt-rich crustal rocks. To clarify the origin of high Cl, alkalies and S in this unusual kimberlite, here we determine its sulfur speciation and isotopic composition and compare it to that of non-salty kimberlites and kimberlitic breccia from the same pipe, as well as potential contamination sources (hydrothermal sulfides and sulfates, country-rock sediment and brine collected in the area). The average δ34S of sulfides is − 1.4 ± 2.2‰ in the salty kimberlite, 2.1 ± 2.7‰ in the non-salty kimberlites and 14.2 ± 5.8‰ in the breccia. The average δ34S of sulfates in the salty kimberlites is 11.1 ± 1.8‰ and 27.3 ± 1.6‰ in the breccia. In contrast, the δ34S of potential contaminants range from 20 to 42‰ for hydrothermal sulfides, from 16 to 34‰ for hydrothermal sulfates, 34‰ for a country-rock sediment (Chukuck suite) and the regional brine aquifer. Our isotope analyses show that (1) in the salty kimberlites, neither sulfates nor sulfides can be simply explained by brine infiltration, hydrothermal alteration or the assimilation of known salt-rich country rocks and instead, we propose that they are late magmatic phases; (2) in the non-salty kimberlite and breccia, brine infiltration lead to sulfate reduction and the formation of secondary sulfides – this explains the removal of salts, alkali-carbonates and sulfates, as well as the minor olivine serpentinization; (3) hydrothermal sulfur was added to the kimberlitic breccia, but not to the massive kimberlites. In situ measurements of sulfides confirm this scenario, clearly showing the addition of two sulfide populations in the breccia (pyrite-pyrrhotites with average δ34S of 7.9 ± 3.4‰ and chalcopyrites with average δ34S of 38.0 ± 0.4‰) whereas the salty and non-salty kimberlites preserve a unique population of djerfisherites (Cl- and K-rich sulfides) with δ34S values within the mantle range. This study provides the first direct evidence of alkaline igneous rocks in which magmatic sulfate is more abundant than sulfide. Although sulfates have been rarely reported in mantle materials, sulfate-rich melts may be more common in the mantle than previously thought and could balance the sulfur isotope budget of Earth's mantle.
ISSN: 0009-2541

hal-01571982
https://hal.archives-ouvertes.fr/hal-01571982
DOI : 10.1016/j.chemgeo.2016.10.037

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts: (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high-temperature end-member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of similar to 80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256: relatively primitive MORB melts under near-liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens-sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well-connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near-solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to "granoblastic dikes" by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so-called "varitextured gabbro" unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.
ISSN: 1525-2027

hal-00644798
https://hal.archives-ouvertes.fr/hal-00644798
DOI : 10.1029/2011GC003655

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Whole-rock concentrations of Se and Te were determined in 53 peridotite samples collected in orogenic bodies from the Eastern Pyrenees, including the Lherz massif (33 samples), the type-locality of Iherzolite. Orogenic Iherzolites tectonically emplaced into the continental crust escaped devolatilisation that may strongly alter concentrations of those volatile trace elements in basalt-hosted peridotite xenoliths. The Se and Te concentration ranges (<3.5-100.7 ppb; <1-11 ppb) are consistent with the few analyses of mantle samples yet available, yet extending to higher Se concentrations. In-situ analyses performed by LAM-ICPMS on base metal sulfides indicate that the Se budget is controlled by sulfides whereas a large fraction of Te (50 +/- 30%) does not reside in sulfides. The missing fraction is accounted for by fractional crystallization of Pt-Pd tellurides from Cu-Ni sulfide melts. A maximum of one third of the S budget of Eastern Pyrenean peridotites was added by assimilation of crustal sulfides, which crystallized as pyrite. Disregarding those contaminated samples, some features of the Se and Te systematics (e.g. bivariate plots vs. fertility index) cannot be explained by partial melting processes. The best fit for 33 peridotites (mostly from Lherz) yields higher than chondritic S/Se (3.1213 +/- 500; r(2) = 0.96) for near chondritic Se/Te (8.6 +/- 2; r(2) = 0.9). Our study suggest that such superchondritic S/Se ratios are better explained by petrogenetic processes involving refertilization of harzburgitic lithosphere by metal (Cu,Ni)-rich sulfide melts precipitated along with clinopyroxene and Al-spinel from silicate melts en-route to the surface.
ISSN: 0009-2541

hal-00544577
https://hal.archives-ouvertes.fr/hal-00544577
DOI : 10.1016/j.chemgeo.2010.09.007

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : Mid-ocean ridges magmatism is, by and large, considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and/or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast-spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg # up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration, resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid-ocean ridges or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast-spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid-ocean ridges lineage.
ISSN: 1525-2027

hal-00939400
https://hal.archives-ouvertes.fr/hal-00939400
DOI : 10.1002/ggge.20137

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Cobalt-bearing pyrite (0.5-2.0 wt.% Co) is abnormally abundant (up to 35 vol.% of the total volume of the sulfide phase) in some eastern Pyrenean peridotite massifs, compared to pieces of subcontinental mantle studied so far for sulfides. Pyrite occurs as vermicular intergrowths inside pentlandite and/or chalcopyrite or as coarser, blocky grains in the intergranular pores of host peridotites. Those different pyrites are characterized by different platinum-group element systematics (measured by laser ablation microprobe and ICP-MS). Vermicular pyrite intergrown with pentlandite displays Os-, Ir-, Ru- and Rh-enriched chondrite normalized PGE patterns of Monosulfide solid solution (Mss). In contrast, coarse-grained intergranular ("blocky") pyrites, are PGE-poor. Chalcophile trace elements (i.e. Zn, Pb, Ag, Au) that are not usually concentrated in mantle-derived sulfides were commonly detected. By contrast, selenium contents are generally low, yielding thus pyrite with high S/Se ratio (> 10(5)), consistent with a sedimentary sulfur source. Pyrite microtextures and chalcophile trace element contents support a process of assimilation of crustal sulfur from the metamorphosed sedimentary country rocks. These latter generated highly reactive CO2-S fluids, which were injected into structural discontinuities of the lherzolitic bodies. Sulfur has reacted at T = 300-550A degrees C with pre-existing, mantle-derived, metal-rich sulfide assemblages (pentlandite-chalcopyrite). Addition of crustal sulfur did produce Mss which, on cooling, exsolved the Os-rich pyrite in addition to pentlandite. The coarse-grained pyrite types have crystallized directly from S-rich fluids.
ISSN: 0010-7999

hal-00617425
https://hal.archives-ouvertes.fr/hal-00617425
DOI : 10.1007/s00710-010-0138-2

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30 degrees N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (->70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poilkilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82-88 in olivine-rich troctolites, to 78-83 in gabbros) and is in Mg-Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Yb-cpx similar to 5-11 x Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.
ISSN: 0009-2541

hal-00420916
https://hal.archives-ouvertes.fr/hal-00420916
DOI : 10.1016/j.chemgeo.2009.02.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Several recent studies have documented that reactions between melt and crystal mush in primitive gabbroic rocks (via reactive porous flow) have an important control in the formation of the lower oceanic crust and the evolution of MORBs. In this context, olivine-rich rocks can form either by fractional crystallization of primitive melts or by open system reactive percolation of pre-existing (possibly mantle-derived) olivine matrix. To address this question, we performed in-situ trace element analyses (by LA-ICP-MS) of olivine from the Erro-Tobbio ophiolite Unit (Ligurian Alps), where mantle peridotites show gradational contacts with an hectometer-scale body of troctolites and plagioclase wehrlites, and both are cut by later decameter-wide lenses and dykes of olivine gabbros. Previous studies inferred that troctolites and olivine gabbros represent variably differentiated crystallization products from primitive MORB-type melts. Olivines in the three rock types (mantle peridotites, troctolites, olivine gabbros) exhibit distinct geochemical signature and well-defined elemental correlations. As expected, compatible elements (e.g. Ni) show the highest concentrations in peridotites (2580–2730 ppm), intermediate in troctolites (2050–2230 ppm) and lowest in gabbros (1355–1420 ppm), whereas moderate incompatible elements (e.g. Mn, Zn) show the opposite behaviour. By contrast, highly incompatible elements like Zr, Hf, Ti, HREE are variably enriched in olivines of troctolites, and the enrichment in absolute concentrations is coupled to development of significant HFSE/REE fractionation (ZrN/NdN up to 80). AFC modelling shows that such large ZrN/NdN ratios in olivines are consistent with a process of olivine assimilation and plagioclase crystallization at decreasing melt mass, in agreement with textural observations. In-situ trace element geochemistry of olivine, combined with microstructural investigations, thus appears a powerful tool to investigate reactive percolation and the origin of olivine-rich rocks in the lower oceanic crust.
ISSN: 0016-7037

hal-01419938
https://hal.archives-ouvertes.fr/hal-01419938
DOI : 10.1016/j.gca.2016.06.029