Petrogenesis of lavas from the AMAR Valley and Narrowgate region of the FAMOUS Valley, 36°–37°N on the Mid-Atlantic Ridge

Abstract

 Basaltic lavas from the AMAR Valley and the Narrowgate region of the FAMOUS Valley on the Mid-Atlantic Ridge (36° to 37° N) range in texture from aphyric to highly plagioclase phyric (>25% large plagioclase phenocrysts). Based on ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, most of these lavas can be subdivided into two distinct, isotopically homogeneous, groups: Group I has lower ⁸⁷Sr/⁸⁶Sr (0.70288±1) and higher ¹⁴³Nd/¹⁴⁴Nd (0.51312±1) ratios; Group II has higher ⁸⁷Sr/⁸⁶Sr (0.70296±1) and lower ¹⁴³Nd/¹⁴⁴Nd (0.51309±2) ratios. Most Group II lavas are aphyric, whereas Group I lavas are primarily plagioclase phyric. Lavas from both groups show a wide range in incompatible element abundance ratios (e.g., Zr/Nb =6–29; (La/Sm)_n=0.6–1.7). Aphyric lavas have relatively constant Sc (40±1.5 ppm) abundances and CaO/Al₂O₃ ratios (0.80±0.02). Group I lavas are confined primarily to the AMAR rift valley floor whereas Group II lavas are found along the east and west marginal highs. We interpret the isotopic differences between the two groups as reflecting a temporal change in the upwelling mantle beneath this region of the Mid-Atlantic Ridge which is south of the Azore Islands. For each group, a petrogenetic model consistent with the geochemical data is multi-stage decompression melting of an initially enriched, homogeneous, mantle source region. If the early derived, incompatible-element enriched, melt increments are not always pooled with subsequent increments, the erupted magma batches may have the major element characteristics of melts derived by 10 to 20% melting, but with incompatible element abundance ratios reflecting the change from an enriched to depleted source during the incremental melting process. In this process an initially homogeneous source can generate primary magmas with the required range in incompatible element abundance ratios shown by each group. The nearly constant CaO/Al₂O₃ ratios and Sc contents of the aphyric lavas with decreasing Mg ? reflects subsequent polybaric fractionation of clinopyroxene, plagioclase and olivine over the pressure interval 8–6 kbar (24–18 km), followed by rapid transport to the surface and eruption. There is no geochemical evidence for a crustal magma chamber beneath this section of the Mid-Atlantic Ridge.