Origin Of The Oceanic Crust

A widely accepted model of the petrologic processes occurring at ocean ridges was proposed by Cann (1970, 1974). In this model hot asthenospheric material ascends buoyantly (Nicolas et al., 1994) sufficiently rapidly up a narrow zone to pass through the basalt melting curve and provides an interstitial melt of basaltic composition. The molten fraction increases in volume as the asthenosphere rises, and eventually departs the parental material to ascend independently and produce a magma chamber within the lower part of the oceanic crust at the level of layer 3. Part of this magma rises through fissures in the crust and erupts onto the ocean floor to produce pillowed lava flows. Beneath the flows is a zone of dikes formed by solidification of magma in the fissures that feed the flows. The lavas and dikes together make up layer 2 of the oceanic crust. Kidd (1977)

modeled these processes of extrusion and intrusion and compared them with observations of ophiolite complexes. Layer 2C was found to consist entirely of sheeted dikes, which were intruded through zones less than 50 m wide. The dikes show some 10% more chilled margins on one side than the other, showing that approximately 10% of the dikes are cut by later dikes, such that the margins of the original dikes ended up on opposite sides of the ridge crest. The symmetry of sea floor spreading about the ridge axis is explained because dike intrusion will proceed preferentially into the hot central axis where existing dikes are weakest. It was suggested that the lavas extruded above the dikes cool rapidly in contact with sea water and flow less than 2 km before solidification. Lavas and dikes are predicted to rotate towards the ridge crest as they move away from the zone of extrusion as a result of isostatic adjustment (Fig. 6.17). They also undergo metamorphism near the ridge axis as they equilibrate at high temperatures in the presence of seawater.

This model for the origin of layer 2 has received striking confirmation from studies of sections through the upper crust revealed by major fault scarps and drill core from DSDP/ODP drill hole 504B, all in fast-spreading Pacific crust (Karson, 2002) (Section 6.9). Furthermore the model predicts that beneath the axial high the extrusive layer should be very thin and the dikes correspondingly closer to the sea floor (Fig. 6.17). This is confirmed by seismic studies that reveal a narrow central band of high seismic velocities beneath the axial high (Toomey et al., 1990; Caress et al., 1992) and a thin extrusive layer that thickens rapidly off axis within 1-2 km (Detrick et al., 1993b; Kent et al., 1994).

In the model of Cann (1974) the crust at lower levels develops from the crystallization of the axial magma chamber. The first minerals to crystallize in the magma chamber, olivine and chrome spinel, fall through the magma and form a basal layer of dunite with occasional accumulations of chromite. With further cooling pyroxene crystallizes and cumulate peridotitic layers (i.e. of olivine and pyroxene) are produced, giving way upwards to pyroxenites as the crystallization of pyroxene begins to dominate. Ultimately, plagioclase also crystallizes and layered olivine gabbros form. Much of the residual liquid, still volumetrically quite large, then solidifies over a very small temperature range to form an upper, "isotropic" gabbro. A small volatile-rich residuum of this differentiation process, consisting essentially of plagioclase and quartz, is the last fraction to crystallize, sometimes intruding upwards to form veins and small km

(b)

Fig. 6.17 Geologic interpretation of the model of Kidd (1977) for the construction of Layer 2 at a fast-spreading ridge crest. Note the prediction of a rapid increase in the thickness of the extrusive layer away from the ridge axis and the presence of dikes at shallow depths near the ridge axis (redrawn with permission from Karson, et al., 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

Fig. 6.17 Geologic interpretation of the model of Kidd (1977) for the construction of Layer 2 at a fast-spreading ridge crest. Note the prediction of a rapid increase in the thickness of the extrusive layer away from the ridge axis and the presence of dikes at shallow depths near the ridge axis (redrawn with permission from Karson, et al., 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

pockets of "plagiogranite" within the overlying sheeted dike complex. The abundance of volatiles, notably water, in the uppermost part of the magma chamber may be due, at least in part, to interaction with seawater percolating downwards and/or stoping of the

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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