Sub-arc mantle enrichment in the Sunda rear-arc inferred from HFSE systematics in high-K lavas from Java

Many terrestrial silicate reservoirs display a characteristic depletion in Nb, which has been explained in some studies by the presence of reservoirs on Earth with superchondritic Nb/Ta. As one classical example, K-rich lavas from the Sunda rear-arc, Indonesia, have been invoked to tap such a high-Nb/Ta reservoir. To elucidate the petrogenetic processes active beneath the Java rear-arc and the causes for the superchondritic Nb/Ta in some of these lavas, we studied samples from the somewhat enigmatic Javanese rear-arc volcano Muria, which allow conclusions regarding the across-arc variations in volcanic output, source mineralogy and subduction components. We additionally report some data for an along-arc sequence of lavas from the Indonesian part of the Sunda arc, extending from Krakatoa in the west to the islands of Bali and Lombok in the east. We present major and trace element concentrations, Sr-Nd-Hf-Pb isotope compositions, and high-field-strength element (HFSE: Nb, Ta, Zr, Hf, W) concentrations obtained via isotope dilution and MC-ICP-MS analyses. The geochemical data are complemented by melting models covering different source compositions with slab melts formed at variable P-T conditions. The radiogenic isotope compositions of the frontal arc lavas in combination with their trace element systematics confirm previously established regional variations of subduction components along the arc. Melting models show a clear contribution of a sediment-derived component to the HFSE budget of the frontal arc lavas, particularly affecting Zr-Hf and W. In contrast, the K-rich rear-arc lavas tap more hybrid and enriched mantle sources. The HFSE budget of the rear-arc lavas is in particular characterized by superchondritic Nb/Ta (up to 25) that are attributed to deep melting involving overprint by slab melts formed from an enriched garnet-rutile-bearing eclogitic residue. Sub-arc slab melting was potentially triggered along a slab tear beneath the Sunda arc, which is the result of the forced subduction of an oceanic basement relief similar to 8 Myr ago as confirmed by geophysical studies. The purported age of the slab tear coincides with a paucity in arc volcanism, widespread thrusting of the Javanese basement crust as well as the short-lived nature of the K-rich rear-arc volcanism at that time.

Automated summary

Many terrestrial silicate reservoirs show depleted Nb content, which is explained by the presence of reservoirs on Earth with superchondritic Nb/Ta. K-rich lavas from the Indonesian Sunda rear-arc have been used as an example to tap into these high-Nb/Ta reservoirs. In this study, samples from the Javanese rear-arc volcano Muria were analyzed to understand the petrogenetic processes and causes for the superchondritic Nb/Ta in these lavas. Data from an along-arc sequence of lavas from the Indonesian part of the Sunda arc were also included. Major and trace element concentrations, isotope compositions, and HFSE concentrations were analyzed. The geochemical data, combined with melting models, show regional variations in subduction components along the arc, with sediment-derived components affecting the HFSE budget of the frontal arc lavas and more hybrid and enriched mantle sources contributing to the K-rich rear-arc lavas. The superchondritic Nb/Ta in the rear-arc lavas are attributed to deep melting involving slab melts formed from an enriched garnet-rutile-bearing eclogitic residue. Sub-arc slab melting may have been triggered along a slab tear beneath the Sunda arc, coinciding with a paucity in arc volcanism and thrusting of the Javanese basement crust.