Paterno R. Castillo

Many petrological, geodynamical, and geochemical perspectives have offered circumstantial evidence for either an early onset of plate tectonics in the first 10% of Earth’s history or a late onset after the great oxidation event (2.5 Ga ago). This calls into question over what timescales plate tectonics have influenced terrestrial geological and geochemical processes. We present geochemical data from the products of ancient crustal subduction, which were recycled into the deep mantle and then tapped by the modern Marquesas volcanic hotspot. We demonstrate that these materials have distinct short-lived radiogenic ( ¹⁴⁶ Sm- ¹⁴² Nd, t _1/2 = 103 Ma) isotopic compositions (average μ ¹⁴² Nd = +2.3 ±1.3, n = 3) compared to other Marquesas lavas (average μ ¹⁴² Nd = −0.8 ±1.2, n = 7). These results require that the history of these subduction products diverged from those of other Marquesas magmas more than four billion years ago. Quantitative modeling suggests that the most geochemically enriched Marquesas samples represent up to ~0.6% recycled crustal sediments, which may have been subducted at any time in Earth’s history but were most likely subducted in the late Hadean Eon to early Eoarchean Era. The inferred felsic composition of such materials further requires that both crustal melting and sedimentation processes were active in some form on the early Earth. Further, the preservation of evidence for foundational planetary events in geologically young rocks reveals that Earth’s volcanic hotspots could provide a defining perspective on the early planetary-scale processes that build Earth-like planets.

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high ²⁰⁶Pb/²⁰⁴Pb coming from one of the end-components with a long time-integrated high ²³⁸U/²⁰⁴Pb or μ (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high ²⁰⁶Pb/²⁰⁴Pb, indicating a HIMU-like source. Moreover, measured ²³²Th/²³⁸U (κ) for most MORB are lower than those deduced from their ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb. Such high μ and low κ features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1^st and 2^nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.

Meter‐scale AUV bathymetric mapping and ROV sampling of the entire 47 km‐long Alarcon Rise between the Pescadero and Tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest, hottest, and highest effusion rate lava flows. This shallowest inflated part is located ∼1/3 of the way between the Tamayo and Pescadero transforms and is paved by a 16 km² channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the East Pacific Rise or on the Gorda and Juan de Fuca Ridges. Starting ∼5 km south of the Pescadero transform, 6.5 km of the Alarcon Rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77.3 wt % SiO₂. These are the first known rhyolites from the submarine global mid‐ocean ridge system. Silicic lavas range from >11.7 ka, to as young as 1.1 ka. A basalt‐to‐basaltic andesite sequence and an andesite‐to‐dacite‐to‐rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite. Magmatism occurred along the bounding Tamayo and Pescadero transforms as extensive channelized flows. The flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills. The transforms are transtensional to accommodate magma migration from the adjacent Alarcon Rise.