The Mid-Atlantic Ridge (MAR) at 26°S is an asymmetrical slow spreading ridge (Niu et al., 1994). The ridge segment between 25°40′ and 26°35′S is about 100 km long, and it is bounded by the Rio Grande Transform to the north, and the Moore Fracture Zone to the south. There is a topographic high near the centre of the ridge segment, in where the 26°S hydrothermal area is located. To the both sides of the ridge segment distribute welldeveloped axial rift valley of typical slow spreading ridge and numerous axially arranged topographic high.
Geochemistry of BasaltSamples collected at 26°S MAR are mainly composed of a cluster of microlitic basalt, porphyritic basalt, variolite and tachylite which belong to sodic calc-alkaline basalt. A summary of the major element composition of samples along segment of the MAR ridge at 26°S explained that magmatic differentiation was not complete, with Solidification Index (SI): 25.9 to 31.21, m/f: 0.81 to 1.04 and Cottine Index (τ): 4.54 to 7.68. Samples contain high concentrations of high-fieldstrength elements (HFSE), and low concentrations of large ion lithophile elements (LILE). Relative to N-MORB, they have the same model curves of trace element ratio, in addition, curves show well-pronounced Ba, Nd, P, Gd, and Sr negative anomalies, K, U, Ta and Pb positive anomalies. The samples are exclusively normal mid-ocean ridge basalt (N-MORB), their (La/Sm) N decrease from 0.91 to 0.49, the mean is 0.60, (Gd/Lu) N from 0.97 to 0.85, the mean is 0.92. Thus, their REE distribution patterns show weak positive Eu anomalies and nearly flat patterns, with only limited to moderate enrichment in heavy REE relative to the light REE. The REE patterns of basalts at 26°S MAR are similar to those from normal mid-ocean ridge and MARK area (Reynolds et al., 1997). The tectonic geochemical background observes in 26°S MAR is also consistent with N-MORB.