| Summary: | Two ultraslow mid ocean ridges, i.e., the westernmost portion of the Southwest Indian Ridge (SWIR) and the easternmost portion of the American-Antarctic Ridge (AAR), meet the southernmost portion of the Mid Atlantic Ridge (MAR) at the Bouvet Triple Junction. Although both the SWIR and the AAR have very slow spreading rates, i.e., 7.5 mm/y and 10 mm/y, respectively, they display strongly contrasting morphostructural and petrological characteristics. The AAR axial zone is deeper than normal (>4000m); some of its segments are oblique, being oriented roughly 45 degrees from the direction of spreading; basalt and peridotite chemistry suggests a relatively low degree of melting of the mantle below the AAR. In contrast, the SWIR axial zone is shallower than normal ($<$2500m); the last segment (Spiess Ridge) is particularly anomalous, reaching ~400 m below sea level. Basalt and peridotite chemistry of this portion of SWIR suggests high extents of melting of the upper mantle. The H2O content of the SWIR basaltic glasses ranges between 0.6% and 1.0% and is significantly higher than in normal MORB. The high extent of melting at the SWIR segments is probably related not only to a mantle thermal anomaly, but also to its high volatiles content. Mantle thermal structure and composition, rather than spreading rate, are the main factors determing the structure of these ridges.
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