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Laubier, Muriel

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Laubier

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Muriel

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Laubier, Muriel
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    Constraints on melting processes and plume-ridge interaction from comprehensive study of the FAMOUS and North Famous segments, Mid-Atlantic Ridge
    (Elsevier BV, 2013) Gale, Allison; Laubier, Muriel; Escrig, S.; Langmuir, Charles
    Detailed major element, trace element and isotopic study of the FAMOUS and North Famous segments within the geochemical gradient south of the Azores platform provides new constraints on controls on chemical variations at the segment scale and the origin of plume geochemical gradients. A comprehensive investigation of 110 samples along the entire length of the FAMOUS segment, coupled with a recent extensive melt inclusion study by Laubier et al. (2012), shows large trace element diversity within a single segment and substantial isotopic variability that largely correlates with trace element variations. Substantial variations are also present along the short (18 km) North Famous segment despite the presence of an axial volcanic ridge. These results confirm multiple supply of magmas along the length of these segments, the lack of a centrally supplied magma chamber, and the ability of melting processes to deliver highly diverse melts over short distances and times. With the exception of one group of high Al2O3, low SiO2 magmas (HiAl–LoSi) largely recovered in the original small FAMOUS area, the data can be simply explained by a two-component mixing model coupled with melting variations. The HiAl–LoSi magmas reflect assimilation and mixing in the crust, an interpretation supported by the diverse melt inclusions in these lavas.Since the mantle heterogeneity reflects two-component mixing, the end members can be constrained. Surprisingly, source mixing between the Azores plume and depleted mantle cannot produce the observations. This is evident regionally from the fact that nearly all basalts have highly incompatible trace element ratios (e.g., Th/La, Nb/La) as high or higher than the most plume-influenced MORB near the Azores hotspot, despite being over 300 km farther south and much less enriched isotopically. To account for the elevated highly incompatible trace element ratios, a metasomatic component formed by adding deep, low-degree melts of Azores plume material to a depleted mantle is required. The regional gradient south of the Azores then requires different processes along its length. Close to the Azores, plume material mixes with depleted mantle. The pure plume influence is spatially restricted, and enrichment farther to the south is caused by shallow mantle metasomatized by low-degree melts from deep plume flow. North Famous lavas are spatially closer to the Azores and yet are more depleted in trace elements and isotopes than FAMOUS lavas, suggesting delivery of the enriched component to individual segments is influenced by additional factors such as segment size and offset. The extent to which these processes operate in other regions of plume–ridge interaction remains to be investigated.
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    Melting and Crustal Processes at the FAMOUS Segment (Mid-Atlantic Ridge): New Insights from Olivine-hosted Melt Inclusions from Multiple Samples
    (Oxford University Press, 2012) Laubier, Muriel; Gale, Allison; Langmuir, Charles
    Most published studies of olivine-hosted melt inclusions from mid-ocean ridges have been based on a single sample. Here we present a comprehensive melt inclusion study of major and trace elements from a single ocean ridge segment, the FAMOUS segment of the Mid-Atlantic Ridge. The melt inclusion dataset includes 312 olivine-hosted (Mg-number 85–92) melt inclusions from 14 samples distributed along the segment. This permits a more comprehensive assessment of the variability within melt inclusions from a single region, and of the relationship between melt inclusion and lava compositions. One recent question has been the extent to which melt inclusions truly preserve the original melt compositions, or instead are modified by late-stage processes occurring at shallow levels. In the FAMOUS inclusions, major elements have been affected by post-entrapment processes, but trace elements show no evidence of such processes, suggesting that diffusion coefficients for incompatible elements are small. Melt inclusions can be divided into three groups. (1) High-Mg inclusions are the most primitive and may potentially constrain the composition of the parental magmas that contribute to other melt inclusion and lava compositions. Although their trace element contents range from highly depleted to almost as enriched as the FAMOUS segment lavas, they are on average more depleted and the melts appear to be derived by greater extents of melting than the lavas. (2) Low-Al inclusions occur in the lower Mg-number olivines, and their major and trace element characteristics reflect mixing between high-Mg melt inclusion and lava compositions. (3) High-Al melt inclusions display Al\(_{2}\)O\(_{3}\) contents as high as 18·4 wt %, SiO\(_{2}\) as low as 46·6 wt %, a strong depletion in the most incompatible elements and distinctively low middle to heavy rare earth element (MREE/HREE) ratios. The high Al\(_{2}\)O\(_{3}\) and low SiO\(_{2}\) contents, as well as positive Sr anomalies in some of the high-Al melt inclusions, are best explained by assimilation of plagioclase-bearing cumulates. The trace element variability in the high-Mg melt inclusions is not consistent with a simple continuous melting column and requires pooling of near-fractional melts within the melting regime and a variable mantle source composition. Because the mean composition of these melt inclusions reflects greater extents of melting than the lavas, we propose that the melt inclusions come from the upper portions of the melting regime. Lavas, in contrast, sample the entire melting regime, including low-degree melts from the wings of the regime that are transported more directly to the surface along high-porosity channels. The high-Al, trace element ultra-depleted, low MREE/HREE melt inclusions derive from melting of a residual mantle source formed by previous melt extraction in the garnet stability field. There is a marked lack of correspondence between major and trace element variations in the melt inclusions. This may reflect a combination of processes, such as cumulate assimilation and re-equilibration of the magmas during ascent, which can reset major elements while having little effect on the trace element variations. The melt inclusions are not simply representative unpooled melts from the melting regime and they do not fully reflect the range of melt compositions contributing to the lavas. Their compositions reflect source heterogeneity as well as melting processes, and major and trace element indicators of depth of origin do not correspond. Combined comprehensive studies of lavas and melt inclusions have much more to reveal than studies based on either data source alone.