Frequently Shifting Magma Sources at Endeavour Segment, Juan de Fuca Ridge
Abstract
Different mantle sources and magma batches contribute to crustal growth over short temporal and spatial scales at the 10-km-long summit of the Endeavour segment of the intermediate spreading rate Juan de Fuca Ridge. Based on analyses of >275 basalts collected by submersible, about twenty "chemo-stratigraphic units" have been identified and mapped. Each reflects a different combination of mantle sources, differentiation path, and mixing history. Each represents a different filling of a magma chamber and most include a range in differentiation. Most occur within the <1 km-wide axial valley that is thought to be <10 Ka in age. The maximum along-strike length of any one unit is ~2 km. The flanking ridges differ from each other and from most of the axis in their uppermost basalts. The maximum extent of fractional crystallization within any single chemostratigraphic unit is about 30%; more fractionated magmas do not erupt, or mix in the chamber with subsequent magma batches. Two types of enriched basalts are recognized. E-MORB is enriched in K, LREE, and Nb, and depleted in Y+HREE, but is similar isotopically to axial N-MORB. We attribute it to low degree deep melting of damp peridotite. In contrast, although T-MORB has K/Ti ratios intermediate between N- and E-MORB, this is because of preferential enrichment in HFSE. Its Pb, Sr, Nd, and Hf isotope ratios are maxima, approaching those of FOZO. It is attributed to low degree melting of a chemically distinct component, perhaps pyroxenitic. Both enriched components lie on the same Pb isotope chords so are thought to be similar in age and origin.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.V21B0602W
- Keywords:
-
- 1021 Composition of the oceanic crust;
- 3618 Magma chamber processes (1036);
- 3619 Magma genesis and partial melting (1037);
- 8416 Mid-oceanic ridge processes (1032;
- 3614)