Mantle partial melting

The easternmost part of the Southwest Indian Ridge (618–678E) is an end-member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase-bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors (''ultramafic seafloor basalts''); and basalts dredged from the intervening, more volcanically active domains (''volcanic seafloor basalts''). Ultramafic seafloor basalts have significantly lower CaO and Al 2 O 3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow-spreading MOR system.

A 20-Myr record of creation of oceanic lithosphere at a segment of the central Mid-Atlantic-Ridge is exposed along an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of ,3–4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the solid mantle is upwelling at an average rate of ,25mmyr21, but this appears to vary through time.
Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north–south mantle flow.

The Bouvet Triple Junction (BTJ) region in the South Atlantic,
where the African, South American and Antarctica plates meet,
is affected by several topographic ⁄ melting anomalies. Causes
of these anomalies were investigated through a study of
mantle-derived serpentinized peridotites sampled from three
sites in the BTJ region: (1) the Inner Corner High at the
intersection of the America Antarctic Ridge (AAR) with the
Conrad transform; (2) the south wall of the Bouvet transform
(South West Indian Ridge, SWIR); and (3) the eastern BouvetSWIR Transform Intersection. The degree of melting undergone
by these rocks was estimated from relic mineral major- and
trace-element composition. Geochemical profiles from residual
peridotites and associated basalts show a > 1000-km-wide
melting anomaly centred on the Bouvet and Spiess topographic
anomalies.

Keywords: mid ocean ridge oceanic lithosphere abyssal peridotite partial melting crustal thickness mantle upwelling velocity Temporal variations of temperature and composition of the mantle upwelling below a 80-km long segment of the Mid Atlantic Ridge were reconstructed from 20 to 4 Ma ago from peridotites sampled along a N 300-km long section of oceanic lithosphere (Vema Lithospheric Section or VLS) exposed south of the Vema transform at 11°, 2003. Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variation in the formation of oceanic lithosphere, Nature, 423, 499–505]. We extended this time interval from 26 to 2 Ma by sampling mantle ultramafics at 18 new sites along the VLS. Peridotite orthopyroxene, clinopyroxene and spinel chemistry suggest a weak trend of decreasing extent of melting of the mantle from 26 to 18.5 Ma ago with superimposed short-wavelength (~ 4 Ma) oscillations followed by a steady increase of degree of melting from 18.5 to 2 Ma ago, with superimposed 3–4 Ma oscillations. Temporal variations of crustal thickness inferred from the Residual Mantle Bouguer Anomaly calculated from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2 Ma) mantle suites differ in cpx Na 2 O content and CaO/Al 2 O 3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might have been different in the two suites. The two trends are separated by a ~ 1.4 Ma-long stretch (from 18.2 to 16.8 Ma) where deformed ultramafic mylonites prevail, indicating probably an interval of nearly a-magmatic lithospheric emplacement at ridge axis, corresponding to a thermal minimum. Spatially offset correlation along the VLS of crustal thickness (i.e., quantity of basaltic melt released by the mantle) and mantle peridotite degree of melting led to an estimate of ~ 16.1 mm/a for the solid mantle average velocity of upwelling, a value close to the average half spreading rate for the 26 Ma interval covered by the VLS. However, peridotite clinopyroxene geothermometry shows oscillations superimposed on a decreasing trend of calculated equilibration temperature from 26 to 18.5 Ma ago, followed by a steady increase from 16 to 4 Ma ago, suggesting that the solid mantle upwelling velocity varied through time. These results hint at the existence of ~ 10–20 Ma cycles in the activity of the northern Mid Atlantic Ridge.