Crustal recycling by subduction erosion in the central Mexican Volcanic Belt

Recycling of upper plate crust in subduction zones, or ‘subduction erosion’, is a major mechanism of crustal destruction at convergent margins. However, assessing the impact of eroded crust on arc magmas is difficult owing to the compositional similarity between the eroded crust, trench sediment and arc crustal basement that may all contribute to arc magma formation. Here we compare Sr–Nd–Pb–Hf and trace element data of crustal input material to Sr–Nd–Pb–Hf–He–O isotope chemistry of a well-characterized series of olivine-phyric, high-Mg# basalts to dacites in the central Mexican Volcanic Belt (MVB). Basaltic to andesitic magmas crystallize high-Ni olivines that have high mantle-like 3He/4He = 7–8 Ra and high crustal δ18Omelt = +6.3–8.5‰ implying their host magmas to be near-primary melts from a mantle infiltrated by slab-derived crustal components. Remarkably, their Hf–Nd isotope and Nd/Hf trace element systematics rule out the trench sediment as the recycled crust end member, and imply that the coastal and offshore granodiorites are the dominant recycled crust component. Sr–Nd–Pb–Hf isotope modeling shows that the granodiorites control the highly to moderately incompatible elements in the calc-alkaline arc magmas, together with lesser additions of Pb- and Sr-rich fluids from subducted mid-oceanic ridge basalt (MORB)-type altered oceanic crust (AOC). Nd–Hf mass balance suggests that the granodiorite exceeds the flux of the trench sediment by at least 9–10 times, corresponding to a flux of ⩾79–88 km3/km/Myr into the subduction zone. At an estimated thickness of 1500–1700 m, the granodiorite may buoyantly rise as bulk ‘slab diapirs’ into the mantle melt region and impose its trace element signature (e.g., Th/La, Nb/Ta) on the prevalent calc-alkaline arc magmas. Deep slab melting and local recycling of other slab components such as oceanic seamounts further diversify the MVB magmas by producing rare, strongly fractionated high-La magmas and a minor population of high-Nb magmas, respectively. Overall, the central MVB magmas inherit their striking geochemical diversity principally from the slab, thus emphasizing the importance of continental crust recycling in modern solid Earth relative to its new formation in modern subduction zones

Depth profiles of seawater dissolved 143Nd/144Nd from RRS James Cook JC-57 in the southwest Atlantic, Punta Arenas (Chile) to Las Palmas (Spain), March 2011 (GEOTRACES-SWAT project)

Dataset: GA02 Leg 3 - Dissolved 143Nd/144NdThis dataset includes depth profiles of seawater dissolved 143Nd/144Nd from samples collected on RRS James Cook cruise JC-57 in the southwest Atlantic, Punta Arenas (Chile) to Las Palmas (Spain) in March 2011. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/672203NSF Division of Ocean Sciences (NSF OCE) OCE-12605142022-10-3

Little change in ice age water mass structure from Cape Basin benthic neodymium and carbon isotopes

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hines, S. K. V., Bolge, L., Goldstein, S. L., Charles, C. D., Hall, I. R., & Hemming, S. R. Little change in ice age water mass structure from Cape Basin benthic neodymium and carbon isotopes. Paleoceanography and Paleoclimatology, 36(11), (2021): e2021PA004281, https://doi.org/10.1029/2021PA004281.A common conception of the deep ocean during ice age episodes is that the upper circulation cell in the Atlantic was shoaled at the Last Glacial Maximum compared to today, and that this configuration facilitated enhanced carbon storage in the deep ocean, contributing to glacial CO2 draw-down. Here, we test this notion in the far South Atlantic, investigating changes in glacial circulation structure using paired neodymium and benthic carbon isotope measurements from International Ocean Discovery Program Site U1479, at 2,615 m water depth in the Cape Basin. We infer changes in circulation structure across the last glacial cycle by aligning our site with other existing carbon and neodymium isotope records from the Cape Basin, examining vertical isotope gradients, while determining the relative timing of inferred circulation changes at different depths. We find that Site U1479 had the most negative neodymium isotopic composition across the last glacial cycle among the analyzed sites, indicating that this depth was most strongly influenced by North Atlantic Deep Water (NADW) in both interglacial and glacial intervals. This observation precludes a hypothesized dramatic shoaling of NADW above ∼2,000 m. Our evidence, however, indicates greater stratification between mid-depth and abyssal sites throughout the last glacial cycle, conditions that developed in Marine Isotope Stage 5. These conditions still may have contributed to glacial carbon storage in the deep ocean, despite little change in the mid-depth ocean structure.This work was supported by NSF grant OCE-1831415 (S. K. V. Hines, S. L. Goldstein., S. R. Hemming.).2022-04-2

Lithological information, strontium, rubidium and carbon concentrations and Sr and C isotope data of samples from OmanDP Hole BT1B (Semail ophiolite; ICDP Oman drilling project)

This database provides measurements on 87Sr/86Sr, d13C from samples of Oman Drilling Project Hole BT1B. The database includes listvenites (n=50), serpentinites (n=14), metamorphic sole rocks (n=11). The sample names and grouping by Units were determined on-board D/V Chikyu from macroscopic observations (Visual Core Description; Kelemen et al. [2020]). Rb and Sr concentrations were determined using a Quadrupole Inductively-Coupled-Plasma-Mass Spectrometer (Q-ICP-MS) at the University of Montpellier (France) and were originally reported by Godard et al. (2021). 87Sr/86Sr were analyzed for interspersed with US National Institute of Standards and Technology (NIST) SRM 987 on a Thermo Scientific Neptune multi-collector ICP-MS at Lamont Doherty Earth Observatory (United States). Total Carbon (TC) was measured from the same bulk rock powder splits as for Strontium isotopes. Total Organic Carbon (TOC, or reduced carbon) was measured from the residual rock powder after the removal of Inorganic Carbon (carbonate carbon) through reaction with dilute (3 N) HCl for at least 3 days, followed by washing with Millipore® water. Concentrations and d13C ratios of Total Carbon (TC) and Total Organic Carbon (TOC), were determined using a Costech element analyzer coupled with a Thermo Scientific Delta V plus mass spectrometer at Lamont Doherty Earth Observatory (United States). References: Reference : Kelemen, P. B., J. M. Matter, D. A. H. Teagle, J. A. Coggon, and the Oman Drilling Project Science Team (2020), Proceedings of the Oman Drilling Project, College Station, TX. and Godard, Marguerite; Carter, Elliot; Decrausaz, Thierry; Lafay, Romain; Bennett, Emma; Kourim, Fatma; de Obeso, Juan-Carlos; Michibayashi, Katsuyoshi; Harris, Michelle; Coggon, Jude; Teagle, Damon A H; Kelemen, Peter B; The Oman Drilling Project Phase 1 Science Party (2021): Lithology, major, volatile and trace element composition of Hole BT1B samples (Semail ophiolite; ICDP Oman drilling project). PANGAEA, https://doi.org/10.1594/PANGAEA.93749

Late Holocene dust provenance at Siple Dome, Antarctica

International audienceCompositions of mineral dust in ice cores serve as tracers of paleo-atmospheric circulation patterns, providing linkages between sources and sinks. Here we document the geochemical makeup of dust reaching continental West Antarctica, on late Holocene samples from the Siple Dome A ice core (spanning ∼1030-1800 C.E). The Nd-Sr isotope signature is unusual for Antarctic ice core dust samples. Siple Dome data are characterized by low Nd isotope ratios (as low as εNd = -16.3) along with low Sr isotope ratios (highest 87Sr/86Sr = 0.7102) compared with other Antarctic dust signatures. A well-defined inverse correlation between Sr-Nd isotope ratios indicates two primary mixing sources. The low εNd-values indicate involvement of ancient (Archean-to-early Proterozoic) continental crust, as either the direct source or as a precursor of the source, and the low Sr-values require low Rb/Sr ratios that often reflect high-grade metamorphism. The known Antarctic terrane with these characteristics is parts of Enderby Land, nearly at the opposite end of Antarctica. The isotopic signature of the second end-member is compatible with West Antarctic volcanoes or Patagonia in South America. The Sr-Nd isotopes and trace element abundances are also chemically compatible with mixing between volcanic material from Gaussberg, a small lamproite volcano in Kaiser Wilhelm II Land in coastal East Antarctica whose source is ancient lithospheric mantle, with dust from Patagonia or material from West Antarctic volcanoes. We assess these potential mixing scenarios and conclude that Siple Dome's unusual geochemical signature can best be explained by a mixture of Patagonian dust and a Gaussberg-like source, with additional minor contributions from old eroded Archean-to-early Proterozoic bedrock sources such as those in Enderby Land. Moreover, Siple Dome dust compositions are distinct from dust deposited on Taylor and Clark Glaciers in the McMurdo Dry Valleys of the western Ross Sea, precluding the Dry Valleys as a late Holocene dust source to this region of the eastern Ross Sea

Assessing neodymium isotopes as an ocean circulation tracer in the Southwest Atlantic

The global overturning ocean circulation plays a key role in global climate by distributing heat around the Earth and by triggering or amplifying major climate changes. Neodymium (Nd) isotopes are widely used to trace present-day and past ocean circulation changes; however, their value as a circulation tracer has been increasingly challenged by studies that have focused on processes that can modify seawater Nd isotope ratios (for example, seawater interaction with particles, pore water fluxes from sediments). The Southwest Atlantic represents an excellent test bed for investigating the integrity of Nd isotopes as an ocean circulation tracer, as it includes the major Atlantic northern and southern hemisphere-sourced end-member water masses associated with the global overturning ocean circulation, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottom Water (AAIW, NADW, and AABW, respectively) and potential regional sources of Nd that could impact that integrity. This study reports Nd isotope data on the GEOTRACES GA02 Southwest Atlantic Meridional Transect, spanning the equator to ∼50°S. Below the pycnocline, substantial non-conservative behavior is observed only in samples dominated by AAIW in the northern portion of the transect (∼25°S to the equator); this appears to reflect addition of Nd from the cratons of South America or Africa from above the pycnocline. This effect is not observed at depths directly below dominated by NADW. Otherwise, Nd isotopes behave as a near-conservative water mass tracer along the transect, with 48% of samples within analytical error of the predicted value from water mass mixing, and 84% within 0.9 εNd-units (∼3 times the analytical error), thus confirming its potential at most depths and locations in the Southwest Atlantic to reconstruct past ocean circulation changes