Chlorine isotope behavior in subduction zone settings revealed by olivine-hosted melt inclusions from the Central America Volcanic Arc

Abstract

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 Fo(90-70), were measured for Cl isotope ratios and trace element concentrations. Melt inclusions from samples from Guatemala to northwest Nicaragua have a restricted range of delta Cl-37 values (range < 1%o 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 (delta Cl-37 value range > 1%o, up to 3.8%o) and do not show any systematic variation along the arc. For some samples, the delta Cl-37 in the melt inclusions is shifted by up to 2%o 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 delta Cl-37 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 delta Cl-37 value close to +0.6%o. 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 delta Cl-37, possibly down to -2.5%o. This component has lower delta Cl-37 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 delta Cl-37 values (up to +3.0%o). The melting of amphibole, a mineral concentrating 37Cl over 35Cl, could explain the high delta Cl-37 values. The difference between melt inclusions and bulk rock delta Cl-37 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.