ALBERT

Description: We present new measurements of 138Ce/142Ce and 143Nd/144Nd isotopic ratios in terrestrial and extra-terrestrial samples. The mean value obtained from nine chondrites defines the 138Ce/142Ce ratio of the chondritic uniform reservoir (CHUR) as (2sd). MORBs and OIBs define the mantle array in the εNd vs. εCe diagram to be . From MORB measurements, we derive the isotopic composition of the depleted MORB mantle (DMM) to be (2sd). Both CHUR and a modelled early-depleted mantle reservoir plot on the mantle array. Thus, the precise determination of the mantle array does not further constrain the La/Ce and Sm/Nd ratios of the bulk silicate Earth (BSE; i.e., primitive mantle). The composition of 1.8 Ga upper continental crust obtained from aeolian sediments is (2sd; ), and that of its 2.2 Ga equivalent is (2sd; ). Binary mixing models between depleted (DMM) and enriched (upper crust or mafic crust composition) components do not reproduce the linear Ce-Nd mantle array but plots close to the island arc basalt data. When the bulk Ce isotopic composition of the continental crust is calculated from the range of accepted Nd isotope values and a mass-balance budget of the BSE, the mixing curves are closer to the mantle array. However the calculated Ce isotopic composition for the bulk crust is always less radiogenic than measurements. Adjusting the Ce-Nd isotopic composition or the Ce/Nd ratio of the end-members to fully linearise the mixing curve leads to unrealistic values never measured in terrestrial samples. We propose a recycling model to reconstruct the mantle array with the participation of both oceanic crust and sediments in the mantle through time. Cerium is a redox sensitive element, making the La-Ce and Sm-Nd systematics an ideal combination to investigate sediment recycling through time. In this recycling model, the most extreme EM-like signatures require the involvement of oceanic sediments that formed under reduced conditions before the Great Oxygenation Event at 2.4 Ga, and which are devoid of Ce elemental anomalies.

Description: Highlights • Chlorine isotopes were measured in melt inclusions along CAVA. • Melt inclusions have on average higher Cl than bulk rocks. • Aqueous fluids, melt-like component and metasomatized mantle form three distinct signatures. • The high Cl of the metasomatized mantle wedge suggests the presence of amphibole. • The amphibole signature in bulk rocks is diluted by late-stage processes. The isotopic composition of Cl, a highly hydrophilic and incompatible element, can provide new insights into the processes of element recycling in subduction zone settings. Samples from 13 localities in Guatemala, El Salvador, Nicaragua and Costa Rica, representing a ca. 1000 km long NW-SE segment along the Central American Volcanic Arc (CAVA), were selected. Ninety-seven melt inclusions, hosted by olivine Fo90−70, were measured for Cl isotope ratios and trace element concentrations. Melt inclusions from samples from Guatemala to northwest Nicaragua have a restricted range of Cl values (range 〈 1‰ within a sample) with values decreasing from Santa Maria (Guatemala) to San Miguel (El Salvador), whereas melt inclusions from Nicaragua and Costa Rica display larger variation within a sample (Cl value range 〉1‰, up to 3.8‰) and do not show any systematic variation along the arc. For some samples, the Cl in the melt inclusions is shifted by up to 2‰ to higher values compared to bulk rock data from the same volcanic center, for which the extent of Cl degassing is not known. The combination of Cl values in melt inclusions with trace elements and the existing knowledge about the slab contributions along the arc allows us to elucidate the Cl isotope composition of different endmembers in this subduction zone. From Guatemala to northwest Nicaragua, a fluid component, originating from serpentinite, has a Cl value close to +0.6‰. This value, similar to lithospheric serpentinites, confirms that despite the aqueous fluid migration through the entire slab, Cl isotopes do not fractionate significantly during transport. A melt-like component, present in the southern part of the arc, has negative Cl, possibly down to −2.5‰. This component has lower Cl than values of the oceanic crust but similar to sediments currently subducting beneath CAVA. Finally, a common component, most likely amphibole-bearing metasomatized mantle, is identified in samples with the highest Cl values (up to +3.0‰). The melting of amphibole, a mineral concentrating 37Cl over 35Cl, could explain the high Cl values. The difference between melt inclusions and bulk rock Cl in some volcanic centers probably results from late-stage processes such as mixing of different batches of magma at shallower levels after melt inclusions entrapment. Melt inclusions thus give a more comprehensive picture of Cl isotope systematics along the CAVA and in primitive subduction-related magmas.