Daniele Brunelli

At slow- and ultraslow-spreading ridges, the presence of mantle rocks outcropping at the seafloor indicates that plate spreading is mainly accommodated by tectonic processes, with little or no magmatism. The Southwest Indian Ridge (SWIR) between 42 and 46°E is one of these magma-starved segments, with large exposures of mantle rocks (54%) and basaltic rocks (40%) on the seafloor. Gabbroic rocks constitute only 1% of the recovered material. Mantle rocks sampled at 42-46°E include some of the freshest peridotites ever sampled in an oceanic context, which provide the unique opportunity to identify deep deformation mechanisms not overprinted by low temperature hydration alteration. These samples show variable degrees of deformation ranging from weakly deformed (protogranular/porphyroclastic) to strongly deformed (i.e., mylonites). In both samples, olivine microstructure combined to its crystallographic preferred orientation (CPO) suggest deformation in presence of melt, at high temperature and low strain conditions, close to the solidus. In porphyroclastic samples, melt circulation, evidenced by polymineralic film-like trails along olivine grain and subgrain boundaries, seems controlled by olivine crystallographic network. In mylonites, hydrated phases replace similar interstitial polymineralic assemblages along olivine grain boundaries, revealing the involvment of hydrous fluid at lower temperature conditions (T < 800°C). By combining chemical maps and EBSD data, we show that a Si-rich melt was involved during high-temperature deformation, forming these film-like trails; we propose that these melt reaction zones at near-solidus conditions, are the initial stage of strain localization in the mantle lithosphere, leading to further grain size reduction and to the formation of mylonites through further strain focusing and fluid/melt channelization.This work is supported by PRIN2017KY5ZX8. This project also received funding from the European Union’s Horizon 2020 research and innovation program (EXCITE) under grant agreement No 101005611 for Transnational Access conducted at the EBSD CNRS-INSU national facility at Géosciences Montpellier (CNRS & Université de Montpellier).

The Marion Rise (MR) at the central Southwest Indian Ridge (SWIR) is an ultra‐slow spreading ridge with thin crust, shallow ridge depth, sparse basaltic coverage, and exposed peridotite. Clinopyroxenes from the MR peridotites have highly variable Hf‐Nd isotopic composition extending to extreme ε_Nd of 94 and ε_Hf of 417, which requires extensive melting and evolution with high Lu/Hf for more than 1 Ga. The Yb content of clinopyroxenes is negatively correlated with the Cr# (molar Cr/Cr + Al) of spinel, but not with ε_Hf, indicating a multi‐stage evolution of depletion and melt‐rock reaction. The highly variable Hf‐Nd isotopic compositions of the MR basalts are not systematically correlated and range from ε_Nd −8 to 9.1 and ε_Hf −10 to 32. Therefore, the basalts are probably a mixture of melts from several lithologies, for example, a recycled crustal component with exceptionally low Hf‐Nd isotope ratios, in addition to melts from the volumetrically predominant, isotopically highly variable peridotites. The ancient melt‐depletion of the MR peridotites with high Hf isotope ratios also reduced their density. A peridotitic mantle melted to <10% can support the Marion Rise without the need of increased mantle temperature. Ultra‐depleted peridotites like those from the MR ones have been documented at multiple localities, indicating that they are ubiquitous in the sub‐ridge mantle. Hence, melts from such ultra depleted peridotite influences mid‐ocean ridge basalt (MORB) compositions and variably melt depleted sub‐ridge peridotites should be considered when evaluating ridge depth variations.

Mid-ocean ridges serve as key sites for understanding the composition of the mantle, but extensive melting usually masks its lithological diversity. This study explores how cold mid-ocean ridge segments, such as the eastern Romanche ridge-transform intersection (ERRTI), provide unique insights into mantle heterogeneity. Here, a thick cold lithosphere faces the warm ridge segment efficiently cooling the ridge tip, thus reducing melting and mixing, and allowing distinct short-scale lithologies to be sampled. Our findings reveal a mosaic of mantle components with diverse geochemical and isotopic signatures, reflecting dynamic mantle processes over time. By examining these cold regimes, this research sheds light on the mantle’s compositional complexity and its evolution, offering a fresh perspective on lithospheric dynamics and melt generation in settings independent of hotspot influences.

At mid‐ocean ridges, the mantle temperature and composition, and the lithosphere thickness control the melting conditions, which influence the feeding and structure of the neo‐volcanic areas. We present a study of the eastern intersection between the Romanche transform fault (TF) and the adjacent southward segments of the Mid‐Atlantic Ridge (MAR). A strong thermal gradient is expected along the axis. We focus on the construction of the axial domain and on the spatial and temporal volcano‐tectonic interplays. The geological mapping of the intersection shows a progressive variation in the orientation of the normal faults from about 30° to 90° oblique to the spreading direction with increasing distance from the TF. This attests to the decrease of the transform‐related shear stress component and the thinning of the axial lithosphere southward. Contextually, the coupled thermal edge effect influences the effusion rate and magma supply which in this area are low compared to other MAR segments. However, recent volcanic activity along the axis starts about 10 km south of the ridge‐transform intersection. Rather than a progressive increase of the magma supply with the distance from the TF, the volcanic activity seems to increase abruptly about 30 km south of the intersection. The segment RC2, from ∼30 to 55 km from the Romanche TF, displays more numerous and larger seamounts than the segment RC3 located ∼80 km from the TF, an observation which we interpret to result from the increased thickness of the lithosphere under RC2, due to its proximity with the ridge‐transform intersection.

Mantle processes control plate tectonics and exert an influence on biogeochemical cycles. However, the proportion of mantle sampled in-situ is minimal, as it is buried beneath igneous crust and sediments. Here we report the lithological characteristics of two mantle sections from an embryonic ocean drilled by the International Ocean Discovery Program (IODP) in the Tyrrhenian Sea. Contrary to the mantle drilled at Mid Ocean Ridges (MORs) and hyperextended passive margins, our findings reveal exceptionally heterogeneous and fertile mantle lithologies, ranging from fertile lherzolites to depleted harzburgites and dunites, interlayered with pyroxenites. Plagioclase- and clinopyroxene-rich layers, hydrous potassic magmatic veins, and mafic intrusions indicate substantial mantle refertilization and delayed inception of magmatic crust. We propose that magma-poor rifts do not require a chemically depleted mantle, too refractory to melt. Deep lithospheric processes such as mantle refertilization and prolonged lithospheric thinning delayed melt focusing and the formation of a steady-state spreading center.

Volatiles (CO₂, H₂O) play a fundamental role in mantle melting beneath ocean spreading centers, but what role they play during the melt migration remains unknown. Using seismological data recorded by ocean-bottom seismometers, here we report the presence of deep earthquakes at 10–20 km depth in the mantle along the Mid-Atlantic Ridge axis, much below the brittle-ductile boundary. Syntheses of regional basaltic rock samples and their geochemical analyses indicate the presence of an abnormally high quantity of CO₂ (~0.4–3.0 wt%) in the primary melts. As the degassing of a high concentration of dissolved CO₂ produces volume change, we suggest that deep earthquakes in the mantle result from the degassing of CO₂. The large concentration of CO₂ in the primitive melt will influence the presence of melt beneath the lithosphere-asthenosphere boundary at sub-solidus temperatures.

Seafloor anomalies along mid-ocean ridges with exceptionally thick and compositionally distinct basaltic crust, for example, at Iceland, suggest that the underlying mantle is hotter and chemically different from the adjacent subridge mantle. Here we present hafnium and neodymium isotope ratios of peridotites from the Charlie Gibbs Transform Zone, which is located at the southern end of the Reykjanes Ridge south-west of Iceland. These peridotites are strongly depleted in incompatible elements with extremely high hafnium isotope ratios, suggesting that they had already melted to a large extent before being incorporated into the plume, at least 1 billion years ago, and thereby also became less dense. We argue that seismic velocity anomalies, geodynamic models and geochemical affinities of ridge basalts connect the peridotites from the Charlie Gibbs Transform Zone to the ‘Iceland plume’. The thermochemical buoyancy of the moderately hot Iceland plume, but also that of other plumes worldwide, may therefore be strongly influenced by composition. Variable peridotite depletion along the rising Iceland plume could also cause the transient, density-driven pulses in plume flux, which have formed the V-shaped Reykjanes Ridge south of Iceland. Overall, expansion of a ridge-centred plume along adjacent ridges and melting of heterogeneous plume material explains the topographic swell, the seismic anomaly and the formation of V-shaped ridges, as well as the regional distribution of basalts with Icelandic affinity.