Midocean Ridge Geochemistry and Petrology

The most volcanically active regions of our planet are concentrated along the axes of the globe, encircling midocean ridges. These undersea mountain ranges, and most of the oceanic crust, result from the complex interplay between magmatic (i.e., eruptions of lavas on the surface and intrusion of magma at depth) and tectonic (i.e., faulting, thrusting, and rifting of the solid portions of the outer layer of the earth) processes. Magmatic and tectonic processes are directly related to the driving forces that cause plate tectonics and seaSoor spreading. Exploration of midocean ridges by submersible, remotely operated vehicles (ROV), deep-sea cameras, and other remote sensing devices has provided clear evidence of the effects of recent magmatic activity (e.g., young lavas, hot springs, hydrothermal vents and plumes) along these divergent plate boundaries. Eruptions are rarely observed because of their great depths and remote locations. However, over 60% of Earth’s magma Sux (approximately 21 km year~) currently occurs along divergent plate margins. Geophysical imaging, detailed mapping, and sampling of midocean ridges and fracture zones between ridge segments followed by laboratory petrologic and geochemical analyses of recovered rocks provide us with a great deal of information about the composition and evolution of the oceanic crust and the processes that generate midocean ridge basalts (MORB). Midocean ridges are not continuous but rather broken up into various scale segments reSecting breaks in the volcanic plumbing systems that feed the axial zone of magmatism. Recent hypotheses suggest that the shallowest and widest portions of ridge segments correspond to robust areas of magmatism, whereas deep, narrow zones are relatively magma-starved. The unusually elevated segments of some ridges (e.g., south of Iceland, central portion of the Galapagos Rift, Mid-Atlantic Ridge near the Azores) are directly related to the inSuence of nearby mantle plumes or hot spots that result in voluminous magmatism. Major differences in the morphology, structure, and scales of magmatism along midocean ridges vary with the rate of spreading. Slowly diverging plate boundaries, which have low volcanic output, are dominated by faulting and tectonism whereas fast-spreading boundaries are controlled more by volcanism. The region along the plate boundary within which volcanic eruptions and high-temperature hydrothermal activity are concentrated is called the neovolcanic zone. The width of the neovolcanic zone, its structure, and the style of volcanism within it, vary considerably with spreading rate. In all cases, the neovolcanic zone on midocean ridges is marked by a roughly linear depression or trough (axial summit collapse trough, ASCT), similar to rift zones in some subaerial volcanoes, but quite different from the circular craters and calderas associated with typical central-vent volcanoes. Not all midocean ridge volcanism occurs along the neovolcanic zone. Relatively small ( 1 km high), near-axis seamounts are common within a few tens of kilometers of fast and intermediate spreading ridges. Recent evidence also suggests that signiRcant amounts of volcanism may occur up to 4 km from the axis as off-axis mounds and ridges, or associated with faulting and the formation of abyssal hills. Lava morphology on slow spreading ridges is dominantly bulbous, pillow lava, which tends to construct hummocks ( 50 m high, (500 m diameter), hummocky ridges (1}2 km long), or small circular seamounts (10s}100s of meters high and 100s}1000s of meters in diameter) that commonly coalesce to form axial volcanic ridges (AVR) along the valley Soor of the axial rift zone. On fast spreading ridges, lavas are dominantly oblong, lobate Sows and Suid sheet Sows that vary from remarkably Sat and thin ( 4 cm) to ropy and jumbled varieties. Although the data are somewhat limited, calculated volumes of individual Sow units that have been documented on midocean ridges show an inverse exponential relationship to spreading rate, contrary to what might be expected. The largest eruptive units are mounds and cones in the axis of the northern Mid-Atlantic Ridge whereas the smallest units are thin sheet/lobate Sows on the East PaciRc Rise. Morphologic, petrologic, and structural studies of many ridge segments suggest they evolve 0001