Contrasting volcanic-tectonic processes during the past 2 Ma on the Mid-Atlantic Ridge: submersible mapping, petrological and magnetic results at lat. 34 degrees 52 ' N and 33 degrees 55 ' N

In August-September of 1995, 20 Nautile dives and detailed magnetic surveys (spaced every 1.8 km) were undertaken on two segments of the Mid-Atlantic Ridge between the Oceanographer and Hayes fractures zones. These two segmen ts are only 65 km apart and show strong morphology and gravity contrasts. O H1 is shallower and has a large mantle Bouguer anomaly (MBA) bull's eye, wh ereas OH3 is deeper and has a smaller MBA bull's eye. Thirteen dives were devoted to segment OH1. The Median Ridge (MR) located o n the central high (1700 m deep) is topped by 100 to 300 m high circular vo lcanoes. The volcanics consists mainly of porphyritic and/or vesicular pill ows and volcaniclastics. The NVZ (2200 m deep), located in the valley floor east of the MR, consists of near aphyric fluid lava flows. A chain of off- axis volcanoes, displaying a magnetic continuity with surroundings, extends on both sides of the axis. Three volcanoes on the east side and one on the west side of the axis were explored and sampled by submersible. The off-ax is increase of weathering, Fe-Mn coating and magnetic signature suggest tha t the volcanoes were built at or near the ridge axis. The spacing of NS elo ngated hills bearing circular volcanoes and separated large magnetic signat ure (2 to 4 km) depressions suggests that several similar volcanic events o ccured during the past 2 Ma. The last 1 Ma episode involves (1) the constru ction of an axial ridge (MR) by fissure eruptions and the formation of circ ular summit volcanoes by focused volcanism, and (2) the extrusion of fluid magma in the depressions formed by further fissuring and faulting of the MR . The rift valley floor of segment OH3 is smoother than in OH1 and devoid of axial ridge, or other noticeable edifices. The results of 7 dives show that the deep (>3000 m) valley floor is dominated by fissuring and faulting. Th e NVZ is restricted to 'young-looking' pillow haystacks concentrated in the eastern side of the southern valley floor. Serpentinized ultramafic massif s exposed at both 'inside corners' of the segment ends confirm that magma s upply on segment OH3 was much lower than on OH1, and tectonic stretching do minant at both extremities. The average enrichment in alkalis and K/Ti ratios of the segment OH1 basalt ic glasses, with respect to the systematic depletion in OH3, indicates that the chemical influence of the Azores regional mantle anomaly southward bey ond 34 degrees N is negligible in terms of major element compostion. The pr eserved glasses of segment OH1 range from normal mid-ocean ridge basalts (N -MORBs, K/Ti < 0.2) to transitional (T-MORBS, 0.2 < K/Ti < 0.4) and enriche d (E-MORBS, K/Ti > 0.4) compositions. Crystal fractionation cannot explain entirely the observed variations of K/Ti and Na2O+K2O. Several processes ca n provide these composition: (1) Variation of the mantle source composition through time or melting of a heterogeneous mantle with limited extent of m ixing, and (2) progressive melting of a composite mantle with multi-stage s egregation. In the last case, the most depleted melts are produced at the e nd of the melting process and the fluid lava flows of the NVZ, which are N- MORBs, should represent the end of a magmatic episode. On the other hand, t he segment OH3 glasses are N-MORBs to low K/Ti T-MORBs, depleted in alkalis , and dominantly fluid lava flows. The largest range of composition is obse rved in the pillow haystacks of the NVZ, which is consistent with the model of volcanic-tectonic cycle deduced from segment OH1, if the viscous, less depleted pillows represent the very beginning of a new cycle after a period of magma-starved spreading.