A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr-1 (refs 1–3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting3 or incomplete melt extraction2, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the ∼1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.
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We thank the Lamont Marine Office and the captain and crew of the RV Maurice Ewing for their efforts during cruise EW-0106. The efforts of J. DiBernardo, J. Stennet and J. Diebold are appreciated. This work was supported by the US National Science Foundation.
The authors declare that they have no competing financial interests.
Fig 1: Gravity model, seismically constrained crust. Fig 2: Gravity model, isostatically balanced crust. Fig 3: Gravity model, retained melt to 30-km depth. Fig 4: Gravity model, retained melt to 60-km depth. Fig 5: Summary of gravity modeling results. Fig 6: Melt extraction/retention cartoon. Fig 7: Amplitude analysis for FAIM profile Tecate. Fig 8: Portion of FAIM profile 420. Fig 9: Portion of FAIM profile cass. Fig 10: Portion of FAIM profile Tecate. Fig 11: Model parameters and fit statistics for the crustal thickness measurements. (PDF 4258 kb)
a) Descriptions of crustal-thickness averages, FAIM Line 1 gravity profile and modeling, and a conceptual model for melt retention. b) Supplemental figure captions, Figures S1-S10. (DOC 36 kb)
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Lizarralde, D., Gaherty, J., Collins, J. et al. Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data. Nature 432, 744–747 (2004). https://doi.org/10.1038/nature03140
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