Monique Seyler

Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: A coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK Area, Atlantic Ocean)

Nineteen samples from the Kane Fracture Zone have been studied for sulfide mineralogy and analyzed for S, Se, platinum-group elements (PGE), and Au to assess the effect of refertilization processes on the PGE systematics of abyssal peridotites. The lherzolites show broadly chondritic PGE ratios and sulfide modal abundances (0.01 to 0.03 wt%) consistent with partial melting models, although the few pyroxene-hosted sulfide inclusions and in situ LAM-ICPMS analyses provide evidence for in situ mobilization of a Cu-Ni–rich sulfide partial melt. The most refractory harzburgites (spinel Cr# > 29) are almost devoid of magmatic sulfides and show uniformly low PdN/IrN (<0.5) for variable PtN/IrN (0.8 to 1.2). The compatible behavior of Os, Ir, Ru, Rh, and Pt reflects the presence of primary Os-Ru alloys. Some harzburgites displaying petrographic evidence for refertilization by incremental melts en route to the surface are enriched in sulfides (up to 0.1 wt%). Some of these sulfides are concentrated in small veinlets of clinopyroxene and spinel crystallized from these melts. These S-rich harzburgites display superchondritic PdN/IrN (up to 2.04) positively correlated with sulfide modal contents. It is concluded that refertilization processes resulting in precipitation of metasomatic sulfides may significantly enhance Pd concentrations of abyssal peridotites while marginally affecting Pt (PtN/IrN ≤ 1.24) and Rh (RhN/IrN ≤ 1.23) as well. When the effects of such processes are screened out, our database suggests PGE relative abundances in the DMM (Depleted MORB Mantle; MORB: Mid-Ocean Ridge) within the uncertainty range of chondritic meteorites, without evidence of superchondritic Pt/Ir and/or Rh/Ir ratios.

Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20′N: ODP Hole 1274A

ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (∼0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt–rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites.

Regional-scale melt-rock interaction in lherzolitic mantle in the Romanche Fracture Zone (Atlantic Ocean)

Abstract Mantle-derived peridotites recovered from the Romanche Fracture Zone fall in two compositional groups. Group 1 nearly undepleted lherzolites, indicating a very low degree of melting, are prevalent in the western part of the transform. Group 2 peridotites, with higher Mg/Fe ratio but enriched in Ti and incompatible trace elements and generally containing plagioclase, are common in the eastern part of the transform, where undepleted samples are also present. Group 1 undepleted lherzolites are consistent with a relatively low upper mantle temperature below the Romanche region. Group 2 peridotites can be explained by reaction between melt and lherzolite, at a temperature close to the peridotite solidus, in a still upwelling upper mantle.

Clinopyroxene microtextures reveal incompletely extracted melts in abyssal peridotites

Textural evidence is interpreted to suggest that in regions where upwelling rates of the mantle are slow to very slow, a small amount (;2%) of melt was present when plagioclasefree abyssal peridotites entered the conductive regime at the base of the oceanic lithosphere. Upon crystallization, this melt appears to have been undersaturated in orthopyroxene, but precipitated clinopyroxene, Al-rich and Ti-poor spinel, and sulfides. Furthermore, the primary clinopyroxene grains have rare earth element patterns typical of residues of fractional melting, suggesting that the interstitial liquids were incremental partial melts rather than having mid-oceanic-ridge basalt compositions.

A Cold Suboceanic Mantle Belt at the Earth's Equator

An exceptionally low degree of melting of the upper mantle in the equatorial part of the mid-Atlantic Ridge is indicated by the chemical composition of mantle-derived mid-ocean ridge peridotites and basalts. These data imply that mantle temperatures below the equatorial Atlantic are at least ∼150�C cooler than those below the normal mid-Atlantic Ridge, suggesting that isotherms are depressed and the mantle is downwelling in the equatorial Atlantic. An equatorial minimum of the zero-age crustal elevation of the East Pacific Rise suggests a similar situation in the Pacific. If so, an oceanic upper mantle cold equatorial belt separates hotter mantle regimes and perhaps distinct chemical and isotopic domains in the Northern and Southern hemispheres. Gravity data suggest the presence of high density material in the oceanic equatorial upper mantle, which is consistent with its inferred low temperature and undepleted composition. The equatorial distribution of cold, dense upper mantle may be ultimately an effect of the Earths rotation.

Transform fault effect on mantle melting in the MARK area (Mid-Atlantic Ridge south of the Kane transform)

Mineral compositions of residual peridotites collected at various locations in the Mid-Atlantic Ridge south of the Kane transform (MARK area) are consistent with generally smaller degrees of melting in the mantle near the large offset Kane transform than near the other, small offset, axial discontinuities in the area. We propose that this transform fault effect is due to along-axis variations in the final depth of melting in the subaxial mantle, reflecting the colder thermal regime of the ridge near the Kane transform. Calculations made with a passive mantle flow regime suggest that these along-axis variations in the final depth of melting would not produce the full range of crustal thickness variations observed in the MARK area seismic record. It is therefore likely that the transform fault effect in the MARK area is combined with other mechanisms capable of producing crustal thickness variations, such as along-axis melt migration, the trapping of part of the magma in a cold mantle root beneath the ridge, or active mantle upwelling.

Transform tectonics, metamorphic plagioclase and amphibolitization in ultramafic rocks of the Vema transform fault (Atlantic Ocean)

Abstract Based on microstructures and mineral chemistry, we show that ultramafic rocks sampled in the south wall of the Vema transform have recorded a suite of deformation events that had approximately the same geometry but occurred successively in spinel, plagioclase and amphibolite facies conditions. This tectonic and metamorphic evolution is inferred to have taken place at depth near the eastern intersection of the Vema transform with the Mid-Atlantic Ridge. We discuss the origin of plagioclase-bearing ultramafic mylonites and conclude that they resulted from dynamic recrystallization of moderately depleted spinel peridotites in the lower pressure plagioclase stability field. Low degrees of partial melting in these samples were likely due to the cold thermal regime of the transform region. Strain softening that accompanied recrystallization of plagioclase may be a common effect in the upper mantle below large-offset transforms, favouring strain localization and the initiation of ductile shear zones. Finally, amphibolite facies recrystallization followed the introduction into the ultramafic rocks of a metasomatic hydrous fluid enriched in titanium, sodium and calcium. We propose that this fluid was hydrothermal in origin, and had been chemically modified by interactions with crustal rocks, prior to its circulation in the ultramafics.

Multiscale chemical heterogeneities beneath the eastern Southwest Indian Ridge (52°E-68°E): Trace element compositions of along-axis dredged peridotites

The Southwest Indian Ridge is characterized by frequent outcrops of mantle rocks in a very slow spreading context. In situ measurements of trace element concentrations in pyroxenes of these rocks, and associated petrogenetic modeling, are reported. Overall, the measured compositions cover the whole range typically observed for abyssal peridotites. The greatest subkilometer-scale compositional variability is observed in the region east of the Melville fracture zone. The best explanation for the observed variability is given by concurrent melting and migration of melts strongly enriched in the most incompatible rare earth elements, such as those produced by a garnet-bearing source, or by refertilization with mixed garnet- and spinel-derived partially aggregated melts. Because the regionally associated basalts bear no “garnet signature” in their chemical compositions, we conclude that the residual mantle preserves the signature of a mantle source component that does not appear in the erupted magmas. Comparison between along-axis variations of basalt isotopic compositions and peridotite chemical compositions suggests that local isotopic enrichments displayed by some basalts can be associated with the “garnet signature” in the peridotite and that our sampling represents only a fraction of the global variability of the subaxial mantle. To the west of the Melville fracture zone, samples are more depleted and homogeneous at dredge scale. In addition to containing enriched components, petrologic modeling indicates that the peridotitic mantle beneath the entire section underwent (previous?) partial melting in the garnet stability field before melting at lower pressures.

A Pan African origin and uplift for the gneisses and peridotites of Zabargad Island, Red Sea: A Nd, Sr, Pb, and Os isotope study

A Sr, Nd, Pb, and Os isotopic study of peridotites and granulite-facies gneisses from Zabargad Island in the Red Sea suggests that the tectonothermal, petrogenetic, and geochemical evolution of these rocks occurred largely during the Pan African Orogeny rather than the recent opening of the Red Sea. Sm-Nd model ages and whole rock errorchrons indicate that spinel Iherzolites and gneisses differentiated from a common depleted mantle source about 700 Ma. The Iherzolites were mylonitized, metasomatized, and amphibolitized during a structural event that juxtaposed the peridotites with the gneiss complex and uplifted the gneiss/peridotite complex to relatively shallow crustal levels. Most radiometric dating schemes suggest a Pan African age for this event. The gneisses generally have lower 143Nd/144Nd, 87Sr/86Sr, 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios than the peridotites. They extend linear trends defined by the spinel and amphibole peridotites on Sr-Nd, Sm-Nd, and Pb-Pb diagrams, suggesting the gneisses were either the source or buffering medium for the Pan African metasomatism. Only one post-Pan African event had a significant effect on the geochemistry of the gneiss/peridotite complex: shallow level metasomatism by ultrahot (750–900°C) hypersaline solutions with high 87Sr/86Sr (≈0.710) ratio led to the development of gem-quality olivine crystals as well as low-pressure mineral assemblages in the peridotites, gneisses and younger rocks. Plagioclase-rich assemblages with apparent igneous textures (“troctolites”) that are most common in the southern peridotite body may have formed by interaction of these fluids with peridotite (i.e., are “pseudo-troctolites”). Metasomatism changed the 87Sr/86Sr, Sm/Nd, and Re/Os ratios of the plagioclase peridotites making them unsuitable representatives of the Pan African mantle.

Steady-state creation of crust-free lithosphere at cold spots in mid-ocean ridges

Mid-ocean ridges create oceanic lithosphere consisting normally of basaltic crust a few kilometers thick overlying a peridotitic mantle. However, lithosphere free of basaltic crust formed during the past ∼30 m.y. at an ∼50-km-long stretch of Mid-Atlantic Ridge south of the Romanche Fracture Zone, giving rise to a >500-km-long strip of ocean floor exposing mostly mantle peridotites that have undergone an unusually low (≤5%) degree of melting, mixed with peridotites that reacted with a small fraction of basaltic melt. This lithosphere contains <10% of scattered gabbroic pockets, representing melt frozen above 25 km depth within a relatively cold subaxial lithosphere. Numerical modeling excludes dry melting below this crust-free lithosphere, because of the cooling effect of the long- offset Romanche transform combined with a regional mantle thermal minimum; however, modeling allows a limited extent of hydrous melting. This unusual lithosphere, unable to expel the melt fraction, characterizes cold spots along mid-ocean ridges.