Mantle temperature anomalies along the past and paleoaxes of the Galápagos spreading center as inferred from gravity analyses
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To better understand the effects of hot spots on mid-ocean ridge thermal structure, we investigate the subsurface density structure of the Galapagos spreading center and nearby lithosphere. Using shipboard gravity and bathymetry data, we obtain maps of mantle Bouguer anomalies (MBA) by removing from the free-air gravity the attractions of seafloor topography and a 6-km-thick model crust. Comparison of observed and theoretical MBA profiles along isochrons for ages 0.0-7.7 Ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle. This result is consistent with the notion that seafloor topography along the Galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. Along the ridge axis, the MBA decreases from the east and west toward the Galapagos hot spot by similar to 90 mGal, reaching a minimum nearest the hot spot at 91 degrees W. Seafloor topography mirrors the MBA along axis, increasing by similar to 1.1 Km toward the hot spot. These variations in MBA and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50+/-25 degrees C toward the hot spot. The predicted crustal thickening of 2-4 km nearest the hot spot accounts for 70-75% of the along-axis MBA and bathymetry anomalies; mantle density variations account for the rest of the anomalies. From the crustal isochron of age 7.7 Ma to the present-day axis, the along-isochron amplitudes of MBA decrease from similar to 150 to similar to 90 mGal. The corresponding along-isochron bathymetry anomalies decrease from similar to 1.7 to similar to 1.1 km. These observations along the paleoaxes of the Galapagos spreading center indicate that the axial temperature anomaly was 70% hotter in the past (86+/-25 degrees C) and has steadily decreased to 50+/-25 degrees C as the ridge axis migrated away from the Galapagos hot spot. These along-isochron temperature anomalies, however, have remained well below that estimated for the hot spot itself (200 degrees C), indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10-20 times greater than that along the ridge axis over the past 7.7 m.y.