Melt inclusions in olivine and plagioclase phenocrysts from Antarctic-Phoenix Ridge basalts : Implications for origins of N- and E-type MORB parent magmas

2013

Article référencé dans le Web of Science WOS:000316534300006

Choi S. H., Schiano Pierre, Chen Y., Devidal J. L., Choo M. K., Lee J. I.

Journal of Volcanology and Geothermal Research, 2013, 253, p. 75-86 ISSN 0377-0273

The Antarctic-Phoenix Ridge (APR) is a fossil spreading center in the Drake Passage, Antarctic Ocean. Spreading ceased in chron C2A (ca. 3.3 Ma). Although the APR is a normal ridge that is not influenced by a hotspot, enriched (E-type) mid-ocean ridge basalt (MORB) coexists with normal (N-type) MORB in the ridge's axial region. The E-type APR basalt is relatively young (<3.1 Ma) compared to the N-type basalt (>3.5 Ma). The E-type basalt is characterized by elevated K2O/TiO2 (=0.4-0.8) and (La/Sm)(N) (=22-3.4) ratios, relative to the N-type basalt (K2O/TiO2=0.1-0.3; (La/Sm)(N)=0.7-0.8). To better understand the compositional variation in the APR basalts and their mantle source regions through time, silicate melt inclusions in primitive olivine (Fo(87-89)) and plagioclase (An(85-89)) phenocrysts from the N-type APR basalt were studied. Rehomogenized melt inclusions were analyzed by electron microprobe and LA-ICPMS for major and trace elements. The melt inclusions are more primitive than the host basalt, with Mgs from 67.5 to 74.1. All inclusions exhibit patterns that are depleted in the light rare earth elements. The inclusions have K2O/TiO2 from 0.1 to 0.3 and (La/Sm)N ratios from 0.4 to 0.9; these values overlap with those of the N-type APR basalt. Furthermore, the melt inclusions have elevated (Lu/Hf)(N) and (Sm/Nd)(N) ratios compared to the E-type basalts. The N-type APR basalts do not contain any melt inclusions that are enriched in incompatible elements. The E-type basalt was generated by a low degree of partial melting of a relatively incompatible-element-enriched mantle source. In contrast, chemistries of melt inclusions and N-type basalts are compatible with high degrees of partial melting of an increasingly depleted mantle source. Assuming a veined or otherwise heterogeneous mantle, the absence of E-type inclusions from the N-type host has implications for cyclic magmatic activity beneath the APR. Multi-stage mantle melting and melt extraction from a composite source with sequential extraction of melt fluids might give rise to the primary melt diversity documented in the APR axis. The mantle source of the N-type melts may have been the residue from an earlier phase of melting that removed the easily melted, enriched components. The N-type APR basalt studied represents melt at the end of single cycle, whereas the E-type basalt may represent the early stage of a new pulse that was dominated by highly enriched components.