Mantle heterogeneity from trace elements: MAR triple junction near 14°N

Bougault, Henri; Dmitriev, L. V.; Schilling, Jean-Guy; Sobolev, Alexander V.; Joron, Jean-Louis; Needham, H. D.

doi:10.1016/0012-821x(88)90043-x

Cited by 108 publications

(42 citation statements)

References 32 publications

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“…Such a heterogeneity may be indistinguishable from the surrounding mantle in terms of rheology and density, but may be enriched in volatiles and therefore have a lower solidus temperature than the surrounding mantle, leading to initiation of melting at greater depth and to greater melt production than for a normal mantle peridotite [e.g., Bonatti , 1990]. A similar mechanism has been suggested for other areas of the Mid‐Atlantic Ridge with elevated Pb isotopic and incompatible trace element ratios and anomalous bathymetry, at 33°S, 14°N and 35°N [ Michael et al , 1994; Bougault et al , 1988; Dosso et al , 1991; Niu et al , 2001]. The scale of heterogeneity required is likely to be smaller than the ∼200 km along‐axis length of significantly elevated topography, because of dispersion during melt transport, along‐axis channeling of asthenospheric flow, and along‐axis transport of melt.…”

Section: Discussionmentioning

confidence: 86%

Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

Bruguier¹,

Minshull

Brozena

2003

J. Geophys. Res.

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[1] We present swath bathymetric, gravity, and magnetic data from the Mid-Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge-hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow-spreading mid-ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since $1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least $1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by $4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis-parallel seismic line than a model that includes additional along-axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume.

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Section: Discussionmentioning

confidence: 86%

Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

Bruguier¹,

Minshull

Brozena

2003

J. Geophys. Res.

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“…Variation in basalt composition is not isolated to the SWIR [Melson et al, 1976], and although MORB is dominated by incompatible element depleted tholeiite, the occurrence of incompatible element enriched basalt (E-MORB) reflects a key component of ocean crust generation. In some cases E-MORB is attributed to nearby plume-influence, for example the Iceland plume on the northern MidAtlantic Ridge [Bougault et al, 1988; or Galapagos plume on the Galapagos Spreading Center [Christie and Sinton, 1981;Fisk et al, 1982;Schilling et al, 1976;Schilling et al, 1982]. However, for non-plume influenced ridge sections, most notably along the East Pacific Rise (EPR) [Batiza and Niu, 1992;Langmuir et al, 1986;Mahoney et al, 1994;, the occurrence is not well understood.…”

Section: Introductionmentioning

confidence: 99%

The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9º-25ºE) : basalt composition sensitivity to variations in source and process

Standish¹

2006

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Between 9º-25º E on the ultraslow-spreading Southwest Indian Ridge lie two sharply contrasting supersegments. One 630 km long supersegment erupts N-MORB that is progressively enriched in incompatible element concentrations from east to west. The second 400 km long supersegment contains three separate volcanic centers erupting E-MORB and connected by long amagmatic accretionary segments, where mantle is emplaced directly to the seafloor with only scattered N-MORB and E-MORB erupted. Rather than a major break in mantle composition at the discontinuity between the supersegments, this sharp contrast in geometry, physiography, and chemistry reflects "source" versus "process" dominated generation of basalt.Robust along-axis correlation of ridge characteristics (i.e. morphology, upwelling rate, lithospheric thickness), basalt chemistry, and crustal thickness (estimated from gravity) provides a unique opportunity to compare the influence of spreading geometry and rate on MORB generation. What had not been well established until now is the importance of melting processes rather than source at spreading rates < 20 mm/yr. Along the orthogonally spreading supersegment (14 mm/yr) moderate degrees of partial melting effectively sample the bulk mantle source, while on the obliquely spreading supersegment (7-14 mm/yr) suppression of mantle melting to low degrees means that the bulk source is not uniformly sampled, and thus "process" rather than "source" dominates melt chemistry. AcknowledgementsThere are so many people to thank when reaching a milestone such as this, so let me start with those that have had the most recent impact on this accomplishment, my thesis committee; Henry Dick, Stan Hart, Ken Sims, Fred Frey, Anton le Roex, Mark Kurz, Jian Lin, and my thesis defense chairperson, Hans Schouten. With both Hans and Jian I have gained a new appreciation for and understanding of real-time seagoing geophysics, and I think all four of us (including Henry) have witnessed some pretty amazing human efforts and as a result grown together as people and scientists during the many days bouncing around the South Atlantic. Mark's analytical expertise, continuing support and encouraging words, and attention to detail, has kept my path over the last 6 years straight and narrow, and in the right direction. Anton has been a great source of information both regarding trace elements and more importantly the SW Indian Ridge. As one of the first geochemists to study the South Atlantic spreading ridges in detail, his early work through the 80's and 90's set the bar (and quite a high bar it was) for which all of my work will be gauged. It is a shame that our schedules did not coincide so that he could be here at WHOI for the defense, but I know much discussion will come from my work. Learning trace element geochemistry from Fred Frey was special. Fred's ability to get back to the basics and stress the importance to understanding "why" in the most fundamental terms is important in geochemistry. I very much appreciated his willingness to ...

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“…Trace element and to some extent major element data are subject to regional mantle heterogeneities 22 , in addition to local variability 23 . We thus restrict our investigation to pairs of adjacent segments, one of which has asymmetrical and the other symmetrical accretion, and from both of which multiple chemical analyses are available.…”

mentioning

confidence: 99%

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

Escartı́n

Smith

Cann

et al. 2008

Nature

439

374

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Enhanced seismicity is probably generated along detachment faults accommodating a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which concentrates primarily at their ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. The large fields of detachment surfaces recently identified in oceanic crust formed along the slow spreading MAR and ultra-slow spreading South-West Indian Ridge (SWIR) 3,6 demonstrate the involvement of these structures in the accretion of a larger portion of the oceanic lithosphere than previously inferred from seafloor corrugated planes alone 7 . The resulting seafloor morphology and lithospheric structure on the flanks of the ridge axis are strongly asymmetrical 3 and differ from the more regular and roughly symmetrical axis-parallel abyssal hill fabric believed to characterize 'normal' slow-spreading seafloor. The abyssal hill morphology is caused by ridge-parallel, highangle faulting of volcanic seafloor 8 (Figures 1a-c). In contrast, detachment-related terrain is caused by long-lived steep, normal faults initiated beneath the rift valley floor that rotate to low angles as their footwalls are exposed 7,8 . Distinctive narrow ridges with steep outward-facing slopes that are often curved in plan view develop near exposed detachments at the seafloor, and bound deep swales 7 (Figures 1d-e), producing blocky and chaotic terrain 7,9 . The asymmetric nature of accretion in the presence of detachments is also observed in the overall lithospheric structure, composition and geophysical character wherever data are available 3,4,10 . The MAR lacks the broad ridges only found along the melt-poor SWIR, likely a manifestation of detachment faulting that is different from striated fault planes and associated structure 3 . There is an excellent correlation between mode of accretion and seismicity at the ridge axis. This section of the MAR was hydroacoustically monitored between January 1999 and September 2003 11 . The hydroacoustic catalogue is complementary to the >30 year teleseismic catalogue, as it records smaller magnitude events (magnitude of completeness of 3 and 5, respectively 12 ), over a shorter period of time (<5 vs. >30 years). Both seismic catalogues show that detachment-dominated, asymmetrical ridge sections host ~75% more hydroacoustic events and ~65% more teleseismic events than 4 symmetrical segments (Figure 2b and c). The concentration of seismicity at segments shown to have active detachment faults (Figures 1d-e), such as the Logachev massif south of the Fifteen-Twenty Fracture zone and the TAG detachment fault near 26°N 6,7,13 , is thus a general pattern. Active detachments also control the zones of sustained seismicity, which lack shock-aftershock sequences that were previously identified along the northern MAR 14 . Differences between the hyd...

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Mantle heterogeneity from trace elements: MAR triple junction near 14°N

Cited by 108 publications

References 32 publications

Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9º-25ºE) : basalt composition sensitivity to variations in source and process

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

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