Subduction-zone crust-mantle interaction is a common mechanism for the origin of oceanic arc and island basalts

Abstract:

We present a generalized model for the origin of oceanic arc basalts (OAB) and oceanic island basalts (OIB). This is realized by an integrated study of their major-trace element and stable-radiogenic isotope compositions. Many continental basalts are geochemically indistinguishable from common OIB, a fact that requires part of the upper mantle to have been a common reservoir beneath both oceans and continents. In addition, this reservoir must have been isolated from the convective asthenosphere for preservation of geochemical anomalies. Common OAB and OIB show consistent enrichment of LILE and LREE relative to normal MORB. On the primitive mantle-normalized spidergram, however, OAB are characterized by negative Nb and Ta anomalies but a positive Pb anomaly, whereas OIB show positive or no Nb and Ta anomalies but a negative Pb anomaly. Such differences are attributed to the difference in the property of metasomatic agents (aqueous solutions, hydrous melts and supercritical fluids) derived from subducting crustal rocks. The metasomatic agents are highly enriched in fluid/melt-mobile incompatible trace elements such as LILE and LREE, transferring enriched components from the crustal rocks to the mantle sources of OAB and OIB. The stability of rutile in the subducting crustal rocks dictates the abundance of Nb and Ta in the metasomatic agents. Lead is preferentially partitioned into the metasomatic agents when released at subarc depths, whereas dehydrated Pb-poor restites were subducted to greater depths. This explains the positive Pb anomaly in OAB but the negative Pb anomaly in OIB. We accept the assumption that normal MORB are derived from partial melting of the normal asthenospheric mantle, a common reservoir of isotopic depletion. We extend the chemical reaction at the slab-mantle interface in subduction channel from subarc depths to those above the mantle transition zone, generating metasomatic ultramafic rocks (metasomes) in the upper mantle. The reaction at subarc depths produces serpentinized to chloritized peridotites, serving as the source of OAB; the reaction at greater depths produces pyroxene-rich peridotites, pyroxenite and hornblendite, serving as the source of OIB. These metasomes have low solidii and thus are more susceptible to partial melting than the peridotite at the same P-T conditions.