ALBERT

Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 9 (2018): 2864, doi:10.1038/s41467-018-05313-2.

Description: The mechanisms of transfer of crustal material from the subducting slab to the overlying mantle wedge are still debated. Mélange rocks, formed by mixing of sediments, oceanic crust, and ultramafics along the slab-mantle interface, are predicted to ascend as diapirs from the slab-top and transfer their compositional signatures to the source region of arc magmas. However, the compositions of melts that result from the interaction of mélanges with a peridotite wedge remain unknown. Here we present experimental evidence that melting of peridotite hybridized by mélanges produces melts that carry the major and trace element abundances observed in natural arc magmas. We propose that differences in nature and relative contributions of mélanges hybridizing the mantle produce a range of primary arc magmas, from tholeiitic to calc-alkaline. Thus, assimilation of mélanges into the wedge may play a key role in transferring subduction signatures from the slab to the source of arc magmas.

Description: This project was supported by the WHOI Ocean Exploration Institute (OEI) 27071178 to V.L.R.; Previous related projects were supported by NSF EAR-1348063 and WHOI OEI to H.R.M.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2023.

Description: Subduction zones are important sites of material recycling on Earth, with volatiles playing key roles in mass transfer processes and magma formation. This thesis investigates outstanding questions associated with a continuum of interrelated processes that occur as oceanic plates descend in subduction zones by integrating petrological and geochemical constraints from exhumed high-pressure rocks and erupted arc magmas, high pressure-temperature laboratory experiments, and thermodynamic calculations. Chapters 2 and 3 investigate the fluid-mediated reactions between mafic and ultramafic rocks at conditions relevant to the slab-mantle interface and show that Mg-metasomatism of mafic rocks to form chlorite-rich assemblages is favored and is likely more pervasive in subduction zones than in oceanic settings. Contrary to common belief, talc is unlikely to form in high abundance in ultramafic rocks metasomatized by Si-rich slabderived fluids. This means that talc-rich assemblages formed via Si-metasomatism along the slabmantle interface are less likely to be playing prominent roles in volatile transport, in facilitating slow-slip events, and in controlling the decoupling-coupling transition of the plate interface. Chapter 4 experimentally investigates the phase equilibria, melting, and density evolution of mélange rocks that formed by mixing and fluid-rock interactions. Results show that melting of mélanges is unlikely to occur along slab-tops at pressures ≤ 2.5 GPa. Accordingly, diapirism into the hotter mantle wedge would be required to initiate melting. The density contrast between mélanges and the overlying mantle would allow for buoyancy-driven diapirism at relatively low pressures and melting could subsequently occur in the hotter mantle wedge during ascent. However, diapir buoyancy may be limited at higher pressures due to the formation of abundant garnet especially in mélange rocks with peraluminous composition. Chapter 5 experimentally investigates the compositions of melts and mineral residues from melting of a mantle wedge hybridized with small amounts of mélange rocks to simulate an end-member scenario where solid mélange diapirs dynamically interact with the mantle wedge. Results from laboratory experiments show that melting of a mélange-hybridized mantle wedge can produce melts that display compositional characteristics similar to arc magmas. Finally, Chapter 6 presents new interpretations on the evolution of slab-to-mantle transfer mechanisms from subduction initiation to arc maturity. Analyses of published magma compositions from global arcs reveal that melting of mélange plays an increasingly important role in magma formation as slab-tops cool and arcs mature over time. This trend is attributed to the deepening of the decoupled plate interface during subduction where mélange zones can form more extensively and contribute to the melting process more significantly. Taken together, this thesis highlights (i) the dynamic connection between mechanical mixing of different lithologies and fluid-rock interactions along the slab-mantle interface, (ii) how these processes modify the petrophysical and geochemical properties of subducted materials, and (iii) how these processes collectively influence the mechanisms of slabto-mantle transfer, elemental cycles, and the formation of arc magmas worldwide.

Description: This work has been supported financially by the National Science Foundation-Office of International Science & Engineering, Petrology & Geochemistry (NSF-OISE) PIRE, Award# 1545903 to F.K. and the Geoprisms program Collaborative Research Award# 1852610 to V.L.R. Research support through the WHOI Ocean Ventures Fund, US Science Support Program E-FIRE European Training Fund, WHOI Conference Fund are awarded to E.A.C.

Description: These data sets collected geophysical data: multi-beam bathymetry, gravity, magnetics, sub-bottom profile to investigate the relationships between faulting, magmatism, and sea level change.

Description: Gravity, magnetic, and bathymetry data collected along a continuous 1400-km-long spreading-parallel flow line across the Mid-Atlantic Ridge indicate significant tectonic and magmatic fluctuations in the formation of oceanic crust over a range of timescales. The transect spans from 28 Ma on the African Plate to 74 Ma on the North American plate, crossing the Mid-Atlantic Ridge at 35.8 ºN. Gravity-derived crustal thicknesses vary from 3–9 km with a standard deviation of 1 km. Spectral analysis of bathymetry and residual mantle Bouguer anomaly (RMBA) show diffuse power at 〉1 Myr and concurrent peaks at 390, 550, and 950 kyr. Large-scale (〉10-km) mantle thermal and compositional heterogeneities, variations in upper mantle flow, and detachment faulting likely generate the 〉1 Myr diffuse power. The 550- and 950-kyr peaks may reflect the presence of magma solitons and/or regularly spaced ~7.7 and 13.3 km short-wavelength mantle compositional heterogeneities. The 390-kyr spectral peak corresponds to the characteristic spacing of faults along the flow line. Fault spacing also varies over longer periods (〉10 Myr), which we interpret as reflecting long-lived changes in the fraction of tectonically- vs. magmatically- accommodated extensional strain. A newly discovered off-axis oceanic core complex (Kafka Dome) found at 8 Ma on the African plate further suggests extended time periods of tectonically dominated plate separation. Fault spacing negatively correlates with gravity-derived crustal thickness, supporting a strong link between magma input and fault style at mid-ocean ridges.

Description: Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 20, (2019): 6123-6139, doi: 10.1029/2019GC008711.

Description: Gravity, magnetic, and bathymetry data collected along a continuous 1,400‐km‐long spreading‐parallel flow line across the Mid‐Atlantic Ridge indicate significant tectonic and magmatic fluctuations in the formation of oceanic crust over a range of time scales. The transect spans from 28 Ma on the African Plate to 74 Ma on the North American plate, crossing the Mid‐Atlantic Ridge at 35.8°N. Gravity‐derived crustal thicknesses vary from 3–9 km with a standard deviation of 1.0 km. Spectral analysis of bathymetry and residual mantle Bouguer anomaly show a diffuse power at 〉1 Myr and concurrent peaks at 390, 550, and 950 kyr. Large‐scale (〉10 km) mantle thermal and compositional heterogeneities, variations in upper mantle flow, and detachment faulting likely generate the 〉1 Myr diffuse power. The 550‐ and 950‐kyr peaks may reflect the presence of magma solitons and/or regularly spaced ~7.7 and 13.3 km short‐wavelength mantle compositional heterogeneities. The 390‐kyr spectral peak corresponds to the characteristic spacing of faults along the flow line. Fault spacing also varies over longer periods (〉10 Myr), which we interpret as reflecting long‐lived changes in the fraction of tectonically versus magmatically accommodated extensional strain. A newly discovered off‐axis oceanic core complex (Kafka Dome) found at 8 Ma on the African plate further suggests extended time periods of tectonically‐dominated plate separation. Fault spacing negatively correlates with gravity‐derived crustal thickness, supporting a strong link between magma input and fault style at mid‐ocean ridges.

Description: Data and supplemental materials are available at the Woods Hole Open Access Server (doi.org/10.26025/1912/24796). We would like to thank the Woods Hole Oceanographic Institution, National Science Foundation, Naval Oceanographic Office, and the captain and crew of R/V Neil Armstrong for making the SCARF cruise possible. We would also like to thank Eboné Pierce for her help during the cruise. We thank Meghan Jones for advice using MBSystem. We also thank Maurice Tivey, John Greene, and Masako Tominaga for advice on processing the magnetic data sets. We would like to thank Peter Huybers for sharing his spectral analysis codes. We would like to thank Rob Sohn for his help on interpreting the spectral analysis. We would like to thank Del Bohnenstiel, Milena Marjanović, one anonymous reviewer, and Editor Thorsten Becker for their very helpful comments that improved this manuscript. Funding was provided for this research by NSF OCE‐14‐58201.

Description: 2020-05-19

Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Codillo, E., Klein, F., Dragovic, B., Marschall, H., Baxter, E., Scambelluri, M., & Schwarzenbach‬, E. Fluid‐mediated mass transfer between mafic and ultramafic rocks in subduction zones. Geochemistry Geophysics Geosystems, 23, (2022): e2021GC010206, https://doi.org/10.1029/2021gc010206.

Description: Metasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass-transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high-pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints from bulk-rock geochemical compositions and petrophysical properties, mineral chemistry, and thermodynamic modeling to understand the formation of reaction zones between juxtaposed metagabbro and serpentinite as exemplified by the Voltri Massif (Ligurian Alps, Italy). Distinct zones of variably metasomatized metagabbro are dominated by chlorite, amphibole, clinopyroxene, epidote, rutile, ilmenite, and titanite between serpentinite and eclogitic metagabbro. Whereas the precursor serpentinite and oxide gabbro formed and were likely already in contact in an oceanic setting, the reaction zones formed by diffusional Mg-metasomatism between the two rocks from prograde to peak, to retrograde conditions in a subduction zone. Metasomatism of mafic rocks by Mg-rich fluids that previously equilibrated with serpentinite could be widespread along the subduction interface, within the subducted slab, and the mantle wedge. Furthermore, the models predict that talc formation by Si-metasomatism of serpentinite in subduction zones is limited by pressure-dependent increase in the silica activity buffered by the serpentine-talc equilibrium. Elevated activities of aqueous Ca and Al species would also favor the formation of chlorite and garnet. Accordingly, unusual conditions or processes would be required to stabilize abundant talc at high P-T conditions. Alternatively, a different set of mineral assemblages, such as serpentine- or chlorite-rich rocks, may be controlling the coupling-decoupling transition of the plate interface.

Description: M. Scambelluri acknowledges the Italian Ministry of Research MUR for granting the PRIN project n. 2017ZE49E7. This research was funded by NSF-OISE (Office of International Science & Engineering, Petrology & Geochemistry) PIRE, Award #1545903, and the WHOI Ocean Ventures Fund.