Formation and evolution of a metasomatized lithospheric root at the motionless Antarctic plate: the case of East Island, Crozet Archipelago (Indian Ocean)

Sitting atop the nearly stagnant Antarctic plate (ca. 6.46 mm/yr), the Crozet archipelago midway between Madagascar and Antarctica constitutes a region of unusually shallow (1543-1756 m below sea level) and thickened oceanic crust (10-16.5 km), high geoid height, and deep low-velocity zone, which may reflect the surface expression of a mantle plume. Here, we present new major and trace element data for Quaternary sub-aerial alkali basalts from East Island, the easterly and oldest island (ca. 9 Ma) of the Crozet archipelago. Crystallization at uppermost mantle depth and phenocryst accumulation have strongly affected their parental magma compositions. Their trace element patterns show a large negative K anomaly relative to Ta-La, moderate depletions in Rb and Ba with respect to Th-U, and heavy rare earth element (HREE) depletions relative to light REE. These characteristics allow limits to be placed upon the composition and mineralogy of their mantle source. The average trace element spectrum of East Island basalts can be matched by melting of about 2 % of a garnet-phlogopite-bearing peridotite source. The stability field of phlogopite restricts melting depth to lithospheric levels. The modelled source composition requires a multistage evolution, where the mantle has been depleted by melt extraction before having been metasomatized by alkali-rich plume melts. The depleted mantle component may be sourced by residual mantle plume remnants stagnated at the melting locus due to a weak lateral flow velocity inside the melting regime, whose accumulation progressively edifies a depleted lithospheric root above the plume core. Low-degree alkali-rich melts are likely derived from the plume source. Such a mantle source evolution may be general to both terrestrial and extraterrestrial environments where the lateral component velocity of the mantle flow field is extremely slow

Eclogitic metatrondhjemites from metaophiolites of the Western Alps

In the Urtier valley (southern Aosta Valley, Italy), the Piemonte metaophiolites mainly consist of serpentinized peridotites including pods and boudinaged layers of Fe-metagabbro and trondhjemite transposed in the main eclogitic foliation. The contact between serpentinized peridotites and Fe-metagabbro/trondhjemite is locally lined by chloriteschist and rodingite. The high pressure parageneses in the Fe-metagabbro are omphacite-garnet-rutile-glaucophane-phengite, and in the metatrondhjemite plagioclase-quartz-phengite-clinozoisite-epidote-garnet, respectively. Bulk-rock major and trace elements in addition to O isotope analyses were performed in both rock types. Fe-metagabbros are characterized by MgO wt% ranging between 6.11 and 9.63%, EREE= 20-101 ppm, (La/Yb)N = 0.22-0.91; trondhjemites have SiO2 43%, Al2O3 ranging between 21 and 24%, CaO ranging between 17 and 20%, EREE = 172 - 272 ppm, (La/Yb)N ranging between 7.78 and 13.70. The 18O is 5.9 per-mil in a Fe-metagabbro sample and 7.4 per-mil in a trondhjemite sample, suggesting that these rocks have been affected by a weak oceanic low temperature alteration. The high CaO content may indicate a metasomatic process which could have occurred during the oceanic stage or at high pressure conditions

Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges

International audienc

Pétrogenèse des MORB dans les zones froides du manteau supérieur indien : la ride sud-ouest indienne et la discordance australo-antarctique

Texte intégral accessible uniquement aux membres de l'Université de Lorraine jusqu'au 2034-01-01Non disponible / Not availableLa ride sud-ouest indienne (SWIR) est une des rides les plus lentes (taux d'expansion : 1.6 cm.an(-1)) et les plus profondes du réseau mondial de rides médio-océaniques. La mission EDUL a permis d'échantillonner des verres basaltiques localisés entre 69°E, près du point triple de Rodriguez et de 49°E, à 300 km de la zone de fracture Gallieni. Les compositions géochimiques des verres basaltiques permettent de définir trois grandes provinces magmatiques, séparées par les zones de fracture Malville (ZFM) et Gallieni. A l'est de la ZFM (60°45'E), les verres ont des compositions géochimiques qui diffèrent par leurs Na8.0 élevés et leurs faibles Fe8.0, Ti8.0 et CaO/Al2O3 de celles des autres rides médio-océaniques. Les indicateurs de degré de fusion partielle tels que le Na8.0 et le (Sm/Yb)n y sont découplés. Les variations des teneurs en éléments en traces très incompatibles et modérément incompatibles, bien que le Na8.0 élevé et le Fe8.0 bas reflètent la présence d'une température potentielle du manteau faible, ne peuvent être expliquées par de faibles taux de fusion partielle. Quelques unes de ces caractéristiques ont aussi été identifiées sur une autre section " froide ", la discordance australo-antarctique. A l'ouest de la ZFM, les échantillons suivent majoritairement les tendances globales définies par les autres MORB. Les compositions anormales observées dans la section la plus orientale pourraient résulter d'une refusion du manteau, en relation avec la propagation, au cours du temps de la ride vers le nord-est, s'accompagnant d'un épisode de metasomatisme. Cette hypothèse est étayée par les variations de compositions isotoniques du Nd et à un moindre degré du Sr qui semblent indiquer l'échantillonnage durant la fusion partielle de volumes de manteau distincts en composition et aussi d'un stade de métabolisme

Pétrogenèse des MORB dans les zones froides du manteau supérieur indien (la ride sud-ouest indienne et la discordance australo-antarctique)

La ride sud-ouest indienne (SWIR) est une des rides les plus lentes (taux d'expansion : 1.6 cm.an(- )) et les plus profondes du réseau mondial de rides médio-océaniques. La mission EDUL a permis d'échantillonner des verres basaltiques localisés entre 69E, près du point triple de Rodriguez et de 49E, à 300 km de la zone de fracture Gallieni. Les compositions géochimiques des verres basaltiques permettent de définir trois grandes provinces magmatiques, séparées par les zones de fracture Malville (ZFM) et Gallieni. A l'est de la ZFM (6045'E), les verres ont des compositions géochimiques qui diffèrent par leurs Na . élevés et leurs faibles Fe . , Ti . et CaO/Al O de celles des autres rides médio-océaniques. Les indicateurs de degré de fusion partielle tels que le Na . et le (Sm/Yb)n y sont découplés. Les variations des teneurs en éléments en traces très incompatibles et modérément incompatibles, bien que le Na . élevé et le Fe . bas reflètent la présence d'une température potentielle du manteau faible, ne peuvent être expliquées par de faibles taux de fusion partielle. Quelques unes de ces caractéristiques ont aussi été identifiées sur une autre section " froide ", la discordance australo-antarctique. A l'ouest de la ZFM, les échantillons suivent majoritairement les tendances globales définies par les autres MORB. Les compositions anormales observées dans la section la plus orientale pourraient résulter d'une refusion du manteau, en relation avec la propagation, au cours du temps de la ride vers le nord-est, s'accompagnant d'un épisode de metasomatisme. Cette hypothèse est étayée par les variations de compositions isotoniques du Nd et à un moindre degré du Sr qui semblent indiquer l'échantillonnage durant la fusion partielle de volumes de manteau distincts en composition et aussi d'un stade de métabolisme.NANCY/VANDOEUVRE-INPL (545472102) / SudocSudocFranceF

Pétrogenèse des MORB dans les zones froides du manteau supérieur indien (la ride sud-ouest indienne et la discordance australo-antarctique)

La ride sud-ouest indienne (SWIR) est une des rides les plus lentes (taux d'expansion : 1.6 cm.an(- )) et les plus profondes du réseau mondial de rides médio-océaniques. La mission EDUL a permis d'échantillonner des verres basaltiques localisés entre 69E, près du point triple de Rodriguez et de 49E, à 300 km de la zone de fracture Gallieni. Les compositions géochimiques des verres basaltiques permettent de définir trois grandes provinces magmatiques, séparées par les zones de fracture Malville (ZFM) et Gallieni. A l'est de la ZFM (6045'E), les verres ont des compositions géochimiques qui diffèrent par leurs Na . élevés et leurs faibles Fe . , Ti . et CaO/Al O de celles des autres rides médio-océaniques. Les indicateurs de degré de fusion partielle tels que le Na . et le (Sm/Yb)n y sont découplés. Les variations des teneurs en éléments en traces très incompatibles et modérément incompatibles, bien que le Na . élevé et le Fe . bas reflètent la présence d'une température potentielle du manteau faible, ne peuvent être expliquées par de faibles taux de fusion partielle. Quelques unes de ces caractéristiques ont aussi été identifiées sur une autre section " froide ", la discordance australo-antarctique. A l'ouest de la ZFM, les échantillons suivent majoritairement les tendances globales définies par les autres MORB. Les compositions anormales observées dans la section la plus orientale pourraient résulter d'une refusion du manteau, en relation avec la propagation, au cours du temps de la ride vers le nord-est, s'accompagnant d'un épisode de metasomatisme. Cette hypothèse est étayée par les variations de compositions isotoniques du Nd et à un moindre degré du Sr qui semblent indiquer l'échantillonnage durant la fusion partielle de volumes de manteau distincts en composition et aussi d'un stade de métabolisme.NANCY/VANDOEUVRE-INPL (545472102) / SudocSudocFranceF

The complex Rodrigues triple junction migration since ca. 8 Ma: A response to episodic Amsterdam-St. Paul hotspot tail capture by the Southeast Indian Ridge?

The mutual intersection of three plate boundaries, so-called triple junctions, has long been recognized as crucial boundaries for unraveling the spatiotemporal motion of tectonic plates. Yet, the dynamic and tectonic processes ruling their migration remain enigmatic. At the Rodrigues triple junction, the Southwest Indian Ridge lengthens northeastward in response to the unsteady linearity of the Southeast Indian Ridge–Central Indian Ridge dueling ridge system. This mechanism is episodically compensated by the southeastward propagation of the Central Indian Ridge and alternating stages of recession and progression of the Southeast Indian Ridge. This has led to an apparent length constancy of the first Southeast Indian Ridge segment over the past ca. 8 Ma. These multiple ridge propagation episodes, from northwest, southwest, and southeast, result in a net northeastward migration of the Rodrigues triple junction. Here, we suggest that its migration since ca. 8 Ma is a consequence of short-term readjustments of its plate boundaries induced by transitory motion changes of the Capricorn plate, driven by episodic push forces exerted from ephemeral captures of the Amsterdam-St. Paul plume tail by the Southeast Indian Ridge at the southeastern part of the Capricorn plate (77.3°E–78.6°E)

An assessment of the record in compositional variations from mantle source to magmatism at East Island, Crozet archipelago

The Crozet archipelago, located midway between Madagascar and Antarctica, constitutes the emerged part of the easternmost bank of the Crozet plateau, which lies upon upper Cretaceous oceanic seafloor derived from the Southeast Indian Ridge. It forms an elongated chain of five islands and islets, divided into two groups: an older eastern island group (< 9 Ma) composed by large-scale volcanic landmasses (i.e. East and Possession islands) and a younger western one (< 5.5 Ma) with pint-sized islands. The whole region exhibits some of the most typical gravimetric, seismic and bathymetric characteristics associated with upwelling hotter than average mantle including: a geoid high, a topographic swell, a deep low-velocity zone (up to 2350 km), an anomalous heat flow and a thickened crust (10\u201316.5 km). Most of these features are exacerbated by the near stationary absolute motion of the Antarctic plate. However, since thirty years, the chemical composition of Crozet archipelago magmas has beneficiated from little interest compared to that of other Earth\u2019s hotspots. Because of the occurrence of both a thick and old lithosphere and of a near stagnant absolute plate motion, new data from the Crozet archipelago magmatic record will provide new critical perspective on oceanic island building processes. The data presented here are based on a basaltic suite of ~ 25 samples collected by a \u201cTerres Australes et Antarctiques Francaises\u201d expedition in 1969 from the northern part of East Island. Our alkali basalts from the Crozet archipelago are distinct from other oceanic within-plate magmatic rocks in showing ubiquitous large depletions in LILE with respect to other incompatible elements, although these rocks constitute one of the most incompatible-element-enriched suites among Earth\u2019s oceanic island basalts (OIB). The similarity of their trace element ratios and parallelism of their rare earth element patterns indicate: (1) a mantle source homogeneity over at least 1 Ma; (2) an uniformity of the melting conditions (i.e. degree of melting and residual mineralogy) during most of the sub-aerial eruptive history involving very small melting degrees of a garnet-phlogopite bearing, enriched mantle source, as to be expected to occur beneath an old tectonic plate where the bottom of the lithosphere is likely near the garnet to spinel transition. We will present new geodynamical and geochemical constraints on the mechanisms of formation for such garnet-phlogopite-bearing sources in oceanic island environments

Are magmatic processes at volcanoes from motionless plates analogues to those of the giant shield volcanoes on Mars?

The near \u201cone-plate\u201d planet evolution of Mars has led to the edification of long-lasting giant shied volcanoes. Unlike the Earth, Mars would have been a transient convecting planet, where plate tectonic would have possibly acted only during the first hundreds of million years of its history. On Earth, where plate tectonic is active, most plates are regenerated and recycled through convection. However, the Nubian and Antarctic plates could be considered as poorly mobile surfaces of various thicknesses that are acting as conductive lids on top of Earth\u2019s deeper convective system. In these environments, volcanoes do not show any linear age progression at least for the last 30 Ma, but constitute the sites of persistent, spatially focused long-lived magmatic activity. Here, the near stationary absolute plate motion probably exerts a primary control on volcanic processes, and more specifically, on the melting ones. Depleted mantle residues left behind by the melting processes are difficultly dragged away from the melting locus. The thickening of the near-stationary depleted layer progressively forces the termination of melting to higher depths, reducing melt production rate, extraction and increasing mantle lithospheric-melt interactions. With time, it might cause long term fluctuations of the volcanic activity, in generating long periods of quiescence. The pronounced topographic swells/bulges observed in these environments are thus probably both supported by large scale mantle upwelling and residual mantle roots. Most of these processes are likely similar to those observed on Martian giant shield volcanoes. The goal of this presentation will be to describe the essential characteristics of intra-oceanic magmatic processes on slow moving plates on the Earth and to point out their similarities with those of the large shield volcanoes from the Tharsis region

Are magmatic processes at volcanoes from motionless plates analogues to those of the giant shield volcanoes on Mars?

The near \u201cone-plate\u201d planet evolution of Mars has led to the edification of long-lasting giant shied volcanoes. Unlike the Earth, Mars would have been a transient convecting planet, where plate tectonic would have possibly acted only during the first hundreds of million years of its history. On Earth, where plate tectonic is active, most plates are regenerated and recycled through convection. However, the Nubian and Antarctic plates could be considered as poorly mobile surfaces of various thicknesses that are acting as conductive lids on top of Earth\u2019s deeper convective system. In these environments, volcanoes do not show any linear age progression at least for the last 30 Ma, but constitute the sites of persistent, spatially focused long-lived magmatic activity. Here, the near stationary absolute plate motion probably exerts a primary control on volcanic processes, and more specifically, on the melting ones. Depleted mantle residues left behind by the melting processes are difficultly dragged away from the melting locus. The thickening of the near stationary depleted layer progressively forces the termination of melting to higher depths, reducing melt production rate, extraction and increasing mantle lithospheric-melt interactions. With time, it might cause long-term fluctuations of the volcanic activity, in generating long periods of quiescence. The pronounced topographic swells/bulges observed in these environments are thus probably both supported by large scale mantle upwelling and residual mantle roots. Most of these processes are likely similar to those observed on Martian giant shield volcanoes. The goal of this presentation will be to describe the essential characteristics of intra-oceanic magmatic processes on slow moving plates on Earth and to point out their similarities with those of the large shield volcanoes from the Tharsis region (Meyzen et al., 2015)