Geochemical evidence for low magma supply and inactive propagation at the Galápagos 93.25°W overlapping spreading center

Abstract

[1] Major and trace element concentrations; Sr, Nd, and Pb isotopic ratios; and plagioclase and olivine compositions of samples from 92°35 0 W to 94°10 0 W on the Galápagos Spreading Center (GSC) constrain magmatic processes associated with the 93.25°W overlapping spreading center (OSC). Significant variations in parental magma compositions and extent of low-pressure fractionation over short along-axis distances suggest small, discontinuous, or poorly mixed magma chambers. Mineralogical, petrological, and geochemical data indicate this OSC is characterized by low overall melt supply. Regional along-axis geochemical gradients along the western GSC (e.g., in 206 Pb/ 204 Pb, Nb/Zr, and La (8) /Sm (8) ) are interrupted by increased variability within the OSC, consistent with less efficient mixing of melts in this region. Despite a long-term history of westward propagation, bathymetric and geochemical data are inconsistent with recent propagation of this offset; the eastern limb appears to have recently retreated and left behind a series of abandoned ridges within the overlap zone. The eastern limb shows two progressive westward trends of decreasing ratios of highly to moderately incompatible elements, one outside and one within the overlap zone. The overlapping portion of the western limb also has low ratios of these elements. These patterns are consistent with previously depleted mantle being preferentially melted beneath this offset.

Multiple expressions of plume‐ridge interaction in the Galápagos: Volcanic lineaments and ridge jumps

“…Modeled full spreading rates are between 47.3 km Myr À1 and 54.2 km Myr À1 with nearly symmetric spreading from 2 Ma to present and asymmetric spreading 25% faster to the north before 2 Ma. The predicted Rotella et al, 2009]. Before this jump, the half-spreading rate was slightly slower at 32 km Myr À1 up to 3.3 Ma.…”

“…The ridge jumped ∼10 km to the south at ∼2.5 Ma, creating a pseudofault that is manifest as a change in faulting pattern traversing our study area. This pseudofault marks the passage of the 93.3°W overlapping spreading center that ceased propagation at ∼0.4 Ma [ Wilson and Hey , 1995; Rotella et al , 2009]. Before this jump, the half‐ spreading rate was slightly slower at 32 km Myr −1 up to 3.3 Ma.…”

Mechanisms of geochemical and geophysical variations along the western Galápagos Spreading Center

“… Cushman et al [2004] published major element and volatile (H 2 O and CO 2 ) data for a detailed sampling of the WGSC between ∼91°W and 98°W, and Christie et al [2005] published major and trace element data for the EGSC (85.5°W–91°W). Rotella et al [2009] used elemental and isotopic data to investigate magmatic processes near the 93.3°W overlapping spreading center. Clear geographic trends emerge from these studies.…”

Morphotectonic and petrological variations along the southern Central Indian Ridge

“…A comparable OSC along the Galapagos Spreading Centre in the Pacific Ocean displays significant variations in parental magma compositions and extent of low-pressure fractionation, small, discontinuous, or poorly mixed magma chambers. In contrast, the petrological and geochemical data indicate that the OSC along the SCIR is characterized by a low melt supply (Rotella et al 2009). …”

“…In fact, the petrology and geochemistry of erupted lavas can be useful in understanding the magma chamber dynamics and melting conditions beneath the MOR (Rotella et al 2009). Several features distinguish the slowspreading ridge (SSR, spreading rate 20-30 mm/year, e.g.…”