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Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data


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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Figure 1: FAIM experiment location and instrument layout.
Figure 2: Profiles from FAIM line 1 plotted at reduced travel time T (in seconds), using a reduction velocity V of 8.4 km s-1.
Figure 3: Velocity depth profiles at the line 1/line 2 crossing based on isotropic ray tracing of FAIM travel times.
Figure 4: Basement depth, spreading rate and crustal thickness.


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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.

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Correspondence to Daniel Lizarralde.

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Supplementary information

Supplementary Figures 1 - 11

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)

Supplementary Information

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).

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