The geochemistry of oceanic basalts in the vicinity of transform faults: observations and implications

Details The geochemistry of oceanic basalts in the vicinity of transform faults: observations and implications The geochemistry of oceanic basalts in the vicinity of transform faults: observations and implications

Jul 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..69..107l&link_type=abstract

Earth and Planetary Science Letters, Volume 69, Issue 1, p. 107-127.

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79

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Consideration of geochemical data from basalts formed near major Atlantic and Pacific transform faults reveals two significant sets of observations. First, compared to basalts formed far from the transform, basalts near the ridge/transform intersection have, for the same MgO contents, higher abundances of TiO2 and other incompatible elements, higher La/Sm and La/Yb ratios, and often higher FeO. These enrichments are distinct from and occur in addition to the more variable and fractionated compositions which have been frequently noted [10-13]. Modeling of this ``transform fault effect'' using data from the Tamayo/EPR intersection suggests the chemical systematics are caused by decreasing extents of melting as the transform is approached.
Second, there are chemical discontinuities in the major element, trace element and isotopic chemistry of basalts across many transforms. These ``transform discontinuities'' occur in normal ocean crust as well as around hot spots.
Consideration of the melting zone in the mantle suggests that the transform fault effect is a natural consequence of the ridge/transform plate boundary. The melting zone beneath a ridge segment must terminate across the transform, leading to lower extents of melting at the transform edge. The surface manifestation of the change in the melting zone may be affected by the age of the transform offset, the spreading rate, the transform spacing and the interaction of mantle flow with the local thermal structure; it may be obscured by episodic magma chamber processes and mantle heterogeneity.
The significance of transform discontinuities depends on whether they persist with age. If they do not, then temporally variable crust-forming processes may produce changes along a flow line similar to those at zero age across a transform. If, on the other hand, a discontinuity persists with age, then the transform may be related to a fundamental discontinuity in the underlying mantle. Long-lived transform discontinuities would have profound implications for the nature of plate motions, mantle convection and mantle heterogeneity.

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