Abstract
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 ~ 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.
Highlights
The marked depletion of Nb in most reservoirs of the silicate Earth relative to Ta if compared to chondrites is well known, and its origin is matter of scientific debate (e.g., Rudnick et al 2000; Wade and Wood 2001; Cartier et al 2014; Münker et al 2003, 2017)
Island-arc lavas exhibit a characteristic depletion of the high-field strength elements (HFSE) such as Nb–Ta and occasionally Zr–Hf relative to the more fluid-mobile large-ion lithophile elements (LILE) or light rare earth elements (LREE; e.g., Pearce and Peate 1995) and sometimes compared to MORB
The studied samples can be divided into two groups based on their localities and K concentration: (1) a frontal arc suite with low to moderate K content, and (2) a rear-arc suite with high-K contents
Summary
The marked depletion of Nb in most reservoirs of the silicate Earth relative to Ta if compared to chondrites is well known, and its origin is matter of scientific debate (e.g., Rudnick et al 2000; Wade and Wood 2001; Cartier et al 2014; Münker et al 2003, 2017). Lavas from rear-arc (e.g., Whitford 1975) or post-collisional environments (Kirchenbaur and Münker 2015) frequently show an increasing enrichment of K that is often complemented by enrichments of Nb–Ta and other incompatible elements (LREE, LILE). This enrichment is readily interpreted as reflecting more fertile mantle sources that owe their characteristic HFSE budget to the breakdown of accessory HFSE-bearing minerals in subducted sediments or the subducted mafic portion of the slab with progressive subduction, facilitating their transport across the slab–mantle wedge interface by melts or fluids (Tatsumi 1989; Elliott et al 1997; Hermann and Rubatto 2009)
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