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Frozen magma lenses below the oceanic crust


The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust1,2,3. Thermal modelling4,5,6, tomography7, compliance8 and wide-angle seismic studies9, supported by geological evidence3,10,11,12,13,14,15,16,17,18, suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate19 Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies.

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Figure 1: Study area and strength of the Moho reflection event for the 2002 Juan de Fuca ridge flank seismic profiles is plotted in colour over a sun-illuminated grey bathymetry map.
Figure 2: Seismic reflection structure of the oceanic crust characterized by thick and thin MTZs.
Figure 3: Modelled and measured reflection amplitude versus offset dependence for the MTZ events shown in Fig. 2b.
Figure 4: A series of subcrustal reflection events recorded on line 87-89-73-89a is displayed at the distance to depth ratio of about 1:1.

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We are grateful to P. B. Kelemen and W. R. Buck for their reviews. K. Vasudevan provided us with a stacked section of the Canadian line 1989-15, shown in Supplementary Information. The Doherty and National Science Foundations supported this work.

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Correspondence to Mladen R. Nedimović.

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Supplementary information

Supplementary Notes

This file contains the Supplementary Methods, Supplementary Discussions and additional references. (DOC 49 kb)

Supplementary Figure S1

(a) A close-up look on the most extensive series of subcrustal reflection events recorded during the EW0207 cruise and (b) stack of the same part of profile 17-3-1 shown in (a). (PDF 140 kb)

Supplementary Figure S2

Partial stacks of the line 17-3-1 area displayed in Fig. S1. (PDF 1374 kb)

Supplementary Figure S3

Shot-to-shot time intervals for segment 3 of line 17-3-1 (PDF 91 kb)

Supplementary Figure S4

Plan view of the area where the SW end of the Geological Survey of Canada profile 1989-15 meets the part of the EW0207 profile 17-3-1 (PDF 73 kb)

Supplementary Figure S5

Partially stacked super CMP gathers from Line 17-3-1 (a) and Line 34-32 (b). (PDF 66 kb)

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Nedimović, M., Carbotte, S., Harding, A. et al. Frozen magma lenses below the oceanic crust. Nature 436, 1149–1152 (2005).

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