Effects of variable magma supply on mid-ocean ridge eruptions : constraints from mapped lava flow fields along the Galápagos Spreading Center

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q08014, doi:10.1029/2012GC004163.Mapping and sampling of 18 eruptive units in two study areas along the Galápagos Spreading Center (GSC) provide insight into how magma supply affects mid-ocean ridge (MOR) volcanic eruptions. The two study areas have similar spreading rates (53 versus 55 mm/yr), but differ by 30% in the time-averaged rate of magma supply (0.3 × 106 versus 0.4 × 106 m3/yr/km). Detailed geologic maps of each study area incorporate observations of flow contacts and sediment thickness, in addition to sample petrology, geomagnetic paleointensity, and inferences from high-resolution bathymetry data. At the lower-magma-supply study area, eruptions typically produce irregularly shaped clusters of pillow mounds with total eruptive volumes ranging from 0.09 to 1.3 km3. At the higher-magma-supply study area, lava morphologies characteristic of higher effusion rates are more common, eruptions typically occur along elongated fissures, and eruptive volumes are an order of magnitude smaller (0.002–0.13 km3). At this site, glass MgO contents (2.7–8.4 wt. %) and corresponding liquidus temperatures are lower on average, and more variable, than those at the lower-magma-supply study area (6.2–9.1 wt. % MgO). The differences in eruptive volume, lava temperature, morphology, and inferred eruption rates observed between the two areas along the GSC are similar to those that have previously been related to variable spreading rates on the global MOR system. Importantly, the documentation of multiple sequences of eruptions at each study area, representing hundreds to thousands of years, provides constraints on the variability in eruptive style at a given magma supply and spreading rate.This work was supported by the National Science Foundation grants OCE08–49813, OCE08–50052, and OCE08– 49711.2013-02-2

Plume-ridge interaction along the Galapagos spreading center, 90 30'W to 98 W: a hydrous melting model to explain variations in observed glass compositions

vii, 74 leavesThe Galapagos Spreading Center (GSC) between 90°30'W and 98°W shows manifestations of its interaction with the nearby Galapagos plume by way of variations in lava geochemistry, crustal thickness, and morphology along the ridge axis. Rock samples with an average spacing of ~9 km were analyzed for major elements and dissolved H2O. Samples were classified as E-MORBs, T-MORBs, or N-MORBs based on K/Ti ratios. E-MORBs dominate the GSC east of 92.6°W. T-MORBs are mainly found between 92.6°W and 95.5°W. West of the propagating rift tip at 95.5°W, N-MORBs dominate. High K/Ti EMORBs are also characterized by higher H2O, Al2O3, and Na2O, and lower FeO*, SiO2, and CaO/Al2O3 relative to N-MORB at similar values of MgO. These compositional characteristics are consistent with lower mean extents of partial melting relative to NMORB. We developed a melting equation to assess the change in lava composition and mean fraction of partial melting (F) produced by contributions from the zone of hydrous melting whose presence is caused by the depression of the mantle solidus by H2O. We use our hydrous melting equation to model the source composition, the depth of the additional hydrous melting zone, the productivity in the hydrous region, and the upwelling rate that may combine to match our measured crustal thickness values and concentrations of K, Na2O, H2O, and Ti in lavas from the GSC. Model results indicate that GSC N-MORBs were created by F ~0.06 from a source with ~34±1 ppm K, 133±3 ppm H2O, 2250±50 ppm Na2O, and 1050±25 ppm Ti. E-MORB values can be predicted in a number of ways. Higher upwelling rates in the E-MORB region require less source enrichment than low upwelling rates. Upwelling rate has the strongest effect on F . The crustal thickness and glass compositional variations in the "enriched" region of the GSC can best be explained by only a slight increase in the temperature of the mantle (11±11°C), coupled with a moderately enriched mantle source and upwelling of 1.5-3.5 times passive upwelling rates. The transitional region requires only slight upwelling (UwUo =1.5) and a source enriched only in K

Glass Cplume‐ridge Interaction, and Hydrous Melting along the Galápagos Spreading Center, 90.5°W to 98°W

The Galápagos Spreading Center (GSC) between 90.5°W and 98°W manifests its interaction with the nearby Galápagos plume by way of variations in lava geochemistry, crustal thickness, and morphology along the ridge axis. Natural glasses from stations with ∼9 km average spacing were analyzed for major and minor elements, H2O, and CO2. Samples can be classified as enriched mid‐ocean ridge basalts (E‐MORB), transitional MORB (T‐MORB), or normal MORB (N‐MORB) on the basis of K/Ti ratios. E‐MORB dominate the GSC east of 92.6°W. T‐MORB are mainly found between 92.6°W and 95.5°W. West of the propagating rift tip at 95.5°W, N‐MORB dominate. High K/Ti E‐MORB also have higher H2O, Al2O3, and Na2O and lower FeO*, SiO2, and CaO/Al2O3 relative to N‐MORB at similar values of MgO, characteristics consistent with lower mean extents of partial melting relative to N‐MORB. We examine the melting process along this section of the GSC with a set of equations that simulate a deep zone of hydrous melting related to the depression of the mantle solidus by H2O. This model constrains the range of mantle source compositions, the depth of the additional hydrous melting zone, the melt productivity in the hydrous region, and the ratio of mantle flow rate through the hydrous zone relative to the anhydrous zone (Uw/U0) that can explain the measured crustal thickness as well as the fractionation‐corrected concentrations of K, Na2O, H2O, and Ti along the GSC. Far from the hot spot, the measured crustal thickness and N‐MORB compositions are explained by passive mantle upwelling (Uw/U0 = 1), mean melt fraction () ∼ 0.06, and a source with ∼35 ppm K, 130 ppm H2O, 2300 ppm Na2O, and 1050 ppm Ti. The transitional zone has a source enriched in K and could have a slight excess plume‐driven flow through the hydrous melting zone (Uw/U0 ≤ 1.5). The crustal thickness and glass compositions in the “enriched” region of the GSC nearest the hot spot are best explained by only a slight increase in the temperature of the mantle (\u3c∼20°C), coupled with a mantle source moderately enriched (relative to N‐MORB source) and plume‐driven flow through the hydrous zone of Uw/U0 = 1.5–3.5

Glass Cplume‐ridge Interaction, and Hydrous Melting along the Galápagos Spreading Center, 90.5°W to 98°W

The Galápagos Spreading Center (GSC) between 90.5°W and 98°W manifests its interaction with the nearby Galápagos plume by way of variations in lava geochemistry, crustal thickness, and morphology along the ridge axis. Natural glasses from stations with ∼9 km average spacing were analyzed for major and minor elements, H2O, and CO2. Samples can be classified as enriched mid‐ocean ridge basalts (E‐MORB), transitional MORB (T‐MORB), or normal MORB (N‐MORB) on the basis of K/Ti ratios. E‐MORB dominate the GSC east of 92.6°W. T‐MORB are mainly found between 92.6°W and 95.5°W. West of the propagating rift tip at 95.5°W, N‐MORB dominate. High K/Ti E‐MORB also have higher H2O, Al2O3, and Na2O and lower FeO*, SiO2, and CaO/Al2O3 relative to N‐MORB at similar values of MgO, characteristics consistent with lower mean extents of partial melting relative to N‐MORB. We examine the melting process along this section of the GSC with a set of equations that simulate a deep zone of hydrous melting related to the depression of the mantle solidus by H2O. This model constrains the range of mantle source compositions, the depth of the additional hydrous melting zone, the melt productivity in the hydrous region, and the ratio of mantle flow rate through the hydrous zone relative to the anhydrous zone (Uw/U0) that can explain the measured crustal thickness as well as the fractionation‐corrected concentrations of K, Na2O, H2O, and Ti along the GSC. Far from the hot spot, the measured crustal thickness and N‐MORB compositions are explained by passive mantle upwelling (Uw/U0 = 1), mean melt fraction () ∼ 0.06, and a source with ∼35 ppm K, 130 ppm H2O, 2300 ppm Na2O, and 1050 ppm Ti. The transitional zone has a source enriched in K and could have a slight excess plume‐driven flow through the hydrous melting zone (Uw/U0 ≤ 1.5). The crustal thickness and glass compositions in the “enriched” region of the GSC nearest the hot spot are best explained by only a slight increase in the temperature of the mantle (\u3c∼20°C), coupled with a mantle source moderately enriched (relative to N‐MORB source) and plume‐driven flow through the hydrous zone of Uw/U0 = 1.5–3.5