Cryptic variations in abyssal peridotite compositions : evidence for shallow-level melt infiltration in the oceanic lithosphere
Cryptic variations in abyssal peridotite compositions : evidence for shallow-level melt infiltration in the oceanic lithosphere
Date
2009-12-13
Authors
Warren, Jessica M.
Shimizu, Nobumichi
Shimizu, Nobumichi
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Abstract
Ranges in clinopyroxene trace elements of 2-3 orders of magnitude occur over <2 cm distance
in peridotite samples from the Atlantis II Fracture Zone on the Southwest Indian Ridge. This represents the smallest length-scale at which clinopyroxene trace element concentrations have been
observed to vary in abyssal peridotites. Due to the absence of any accompanying veins or other
macroscopic features of melt-rock interaction, these peridotites are interpreted as being the result of cryptic metasomatism by a low volume melt. The small length-scale of the variations,
including porphyroclastic clinopyroxene grains of 2 mm diameter with an order of magnitude variation in light rare earth elements, precludes an ancient origin for these anomalies. Calculation
of diffusive homogenization timescales suggests that for the trace element variations to be preserved, metasomatism occurred in the oceanic lithospheric mantle at 1000-1200°C and 10-20 km
depth. This observation provides constraints for the on-axis thickness of the lithospheric mantle
at an ultra-slow spreading ridge. Trace amounts of plagioclase are present in at least two of the
metasomatized samples. Textural and trace element observations indicate that it formed following
the trace element metasomatism, indicating that the mantle can be infiltrated multiple times by
melt during the final stages of uplift at the ridge axis. The peridotites in this study are from two
oceanic core complexes on the Atlantis II Fracture Zone. Our observations of multiple late-stage
metasomatic events in the lithospheric mantle agree with current models and observations of melt
intrusion into the mantle during oceanic core complex formation. These observations also indicate
that heterogeneous lithospheric mantle can be created at ultra-slow spreading ridges.
Description
Author Posting. © The Authors, 2009. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Journal of Petrology
51 (2010): 395-423, doi:10.1093/petrology/egp096.