Mid Ocean Ridges

The SIMS technique is presently used in the investigation of trace and volatile elements in the frame of the Mid Ocean Ridge Research (lead by E. Bonatti, CNR-ISMAR, Istituto di Geologia Marina, Bologna) in cooperation with the Lamont Doherty Earth Observatory of the Columbia University (New York), the Institute of Geology of the Russian Academy of Sciences (Moscow), the Lab. Géosciences Marines, CNRS-IPGP Paris 7 University (Paris) and IFREMER, Marine Geosciences Dept. (Brest).

The Mid Ocean Ridge research program has generally followed an interdisciplinary approach, where problems have been tackled in parallel from the geological/geophysical to petrological/geochemical viewpoints pursuing the following main objectives:

  • Understand the heterogeneities in the thermal structure and composition of the mantle below Mid Ocean Ridges.

  • Clarify processes of partial melting of the mantle, and geochemical coupling between mantle and crust in ridge portions with contrasting features, i.e., "hot" ridges (Southernmost Mid Atlantic Ridge) and "cold" ridges (equatorial Mid Atlantic Ridge).

  • Evaluate the importance of H2O content for partial melting of the suboceanic mantle and in the formation of the crust, particularly in areas influenced by "mantle plumes".

  • Identify temporal variations in the processes of lithosphere formation in a single segment of Ridge.

Petrological investigation focuses on peridotites, basalts and basaltic glasses from the studied regions. IGM expeditions within the Ridge project are shown in Fig. 1*.

Fig. 1-   Predicted bathymetry of the Central and Southern Atlantic. The three regions of interest for our studies are shown: 1) Bouvet Triple Junction area (Southern Atlantic); 2) Romanche Fracture zone (Equatorial Atlantic); 3) Vema Fracture Zone (Central Atlantic 11ºN).

This region is located at the southernmost tip of the MAR (50° to 60° S) where three plates converge (South America-Africa and Antarctica) divided by three Mid Ocean Ridge systems (South American-Antarctic Ridge: AAR; Mid Atlantic Ridge: MAR and South West Indian Ridge: SWIR). The three Mid Ocean Ridges converging in this area are affected by major topographic/melting anomalies that reflect probably temperature and/or compositional anomalies in the underlying mantle. The two major anomalies are the Spiess seamount and the Bouvet Island at the western and eastern termination of the Bouvet Fracture Zone; an abundant collection of basaltic, gabbroic and ultramafic rocks has been obtained at 66 stations in key areas in this region (Fig. 2)*.

Fig. 2-  Locations where rocks were obtained in the Bouvet Triple Junction region.

Equatorial Region of the Mid Atlantic Ridges. The Ridge, offset here by a set of major transforms, is affected in this area by a mantle thermal minimum. A large collection of peridotites, gabbros and basalts is available from 40 sites in this area (Fig. 3)*.

Fig. 3-  Sites where rock samples were recovered at the eastern intersection of the Mid Atlantic Ridge with the Romanche Transform, Equatorial Atlantic. Main lithologies are indicated.

Mid Atlantic Ridge at 11° N (near the Vema Fracture Zone). A sliver of oceanic lithosphere, explored also by the submersible Nautile has been flexured, uplifted and exposed due to transform related tectonics (Fig. 4)*. This sliver, sampled in great detail, exposes 300 km long lithospheric section, equivalent to 20 My of lithosphere creation at the Ridge. This occurrence gives a unique opportunity to study temporal variations in processes of formation of lithosphere at a Ridge segment.

Fig. 4-  3-D bathymetry of the southern wall of the Vema FZ. Close spaced (3-5 km) sampling of the base of the northern scarp was carried out during various cruises recovering a large set of mantle residual peridotites (harzburgites) in all sites deeper than 4000 m b.s.l.

* courtesy of E. Bonatti


Although water is only present in trace amounts in the suboceanic upper mantle, it is thought to play a significant role in affecting mantle viscosity, melting and the generation of crust at mid-ocean ridges.The concentration of water in oceanic basalts has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hot spots and generated by ‘wet’ mantle plumes.The SIMS technique was crucial in studying basaltic glasses from a cold region of the mid-ocean-ridge system in the equatorial Atlantic Ocean. The H2O (wt%) and REE (ppm wt) concentrations were investigated by our Cameca IMS 4f ion microprobe. These basalts are sodium-rich, having been generated by low degrees of melting of the mantle, and contain unusually high water content (Fig. 5a) as well as high ratios of light versus heavy rare-earth elements (Fig. 5b), implying the presence of garnet in the melting region. We inferred that water-rich basalts from such regions of thermal minima derive from low degrees of ‘wet’ melting greater than 60 km deep in the mantle, with minor dilution by melts produced by shallower ‘dry’ melting—a view supported by numerical modelling. We therefore conclude that oceanic basalts are water-rich not only near hot spots, but also at ‘cold spots’ (Ligi et al., 2005).

Fig. 5-   a- Models of water content in the aggregated melt and observed H2O concentrations in basaltic glasses. Error bars: 1 s.d.; FZ = fracture zone; b- Chondrite-normalized Sm/Yb ratio, (Sm/Yb)n. The increasing influence of garnet as the ridge-transform intersection is approached is reflected by the increase of (Sm/Yb)n relative to the source (Ligi et al., 2005).

Melting processes beneath the Mid-Atlantic Ridge were studied in residual mantle peridotites sampled from a lithospheric section exposed near the Vema Fracture Zone at 11°N along the Mid-Atlantic Ridge. Fractional and dynamic melting models were tested based on clinopyroxene rare earth element and high field strength element data. Pure fractional melting (non-modal) cannot account for the observed trends, whereas dynamic melting with critical mass porosity <0.01 fits better the measured values. Observed microtextures suggest weak refertilization with 0.1–1% quasi-instantaneous or partially aggregated melts trapped during percolation. The temporal evolution of the melting process along the exposed section shows a steady increase of mantle temperature from 20 Ma to present (Brunelli et al., 2005).

Fig. 6-  Trace element distribution obtained by SIMS in residual clinopyroxenes from the Vema Lithospheric Section (Brunelli et al., 2005). (a) Chondrite-normalized clinopyroxene REE patterns for selected samples, normalized to the chondritic values of Anders & Grevesse (1989); the abyssal peridotite field is plotted for comparison. (b) Variation of Dy/Yb(n) (!; dark grey: abyssal peridotite field), Sm/Yb(n) (*; medium grey: abyssal peridotite field) and Ce/Yb(n) ('; light grey: abyssal peridotite field) vs Yb(n) in the clinopyroxene of the Vema Lithospheric Section peridotites; abyssal peridotite field is plotted for comparison. (c) Clinopyroxene Ti vs Zr contents (ppm) in the Vema Lithospheric Section samples and the abyssal peridotite field (grey).

A >300 km long lithospheric section (Vema Lithospheric Section or VLS) is exposed south of the Vema transform at 11°N in the Atlantic. It is oriented along a seafloor spreading flow line and represents ~26 Ma of accretion at a single 80 km long segment (EMAR) of the Mid-Atlantic Ridge. The basal part of the VLS exposes a mantle unit made mostly of relatively undeformed coarse-grained/porphyroclastic peridotites that were sampled at close intervals. Strongly deformed mylonitic peridotites were found at 14 contiguous sites within a ~80 km stretch (~4.7 Ma interval); they are dominant in a time interval of 1.4 Ma, from crustal ages of 16.8 to 18.2 Ma (mylonitic stretch). Some of the mylonites are ‘‘dry,’’ showing anhydrous high-T deformation, but most contain amphibole. The mylonitic peridotites tend to be less depleted than the porphyroclastic peridotites on the basis of mineral major and trace elements composition, suggesting that the mylonites parent was a subridge mantle that underwent a relatively low degree of melting (Fig. 7).

Fig. 7-  Chondrite-normalized REE distributions in VLS formations. (top) Clinopyroxenes from porphyroclastic peridotites (Brunelli et al., 2006), (middle) basaltic glasses (Cipriani et al., in preparation), and (bottom) amphiboles from ultramafic mylonites (Cipriani et al., 2009). Normalizing values are from Anders and Grevesse (1989).

 

 
The presence in the Earth's mantle of even small amounts of water and ather volatiles has major effects: first, it lowers drastically mantle's viscosity, thereby facilitating convection and plate tectonics; second, itlowers the melting temperature of the rising mantle affecting the formation of the oceanic crust. Basalts sampled in the Equatorial Atlantic close to the Romanche trasform , a thermal minimum in the Ridge system, have a H2O content that increases as the ridge is cooled approaching the transform offset. H2O enrichment is due not to an unusually H2O- rich mantle soiurce, but to a low extent of meltingof the upwelling manntle, confined to a deep wet melting region.Numerical models predict that this wet melting process takes place mostly in the mantle zone of stability of garnet. his prediction is verified by the geochemistry of our basalts showing that garnet must indeed have been present in their mantle source ( Ligi et al., 2011).
 
The article is a part of collection edited by the Departement of Earth and Environment - National Research Council (DTA-CNR) of Italy, which represents the different research activities and technologies developments carried out by CNR in the field of marine environment and is resources. Marine Research is a challenging field whose issues are tackled by expertse coming from different reseaech sectors and with an overarching perspective. Marine Research at CNR shows the potential of the DTA-CNR's research network with activities ranging from marine biology to observational and operational oceanography, from Mediterranean Sea ti the Artic and Antarctic Seas, from technology engineering to policy support.
 

 

In Ligi et al. (2012), we obtained areal variations of crustal thickness, magnetic intensity, and degree of melting of the subaxial upwelling mantle at Thetis and Nereus Deeps, the two northernmost axial segments of initial oceanic crustal accretion in the Red Sea, where Arabia is separating from Africa. Basaltic glasses major and trace element (REE, HFSE, …) composition -including H- these latter quantified by means of our ion microprobe in Pavia, suggests a rift-to-drift transition marked by magmatic activity with typical MORB signature, with no contamination by continental lithosphere, but with slight differences in mantle source composition and/or potential temperature between Thetis and Nereus.

Fig. 8-  (a) Distribution of Na8 and (H2O)8 in MORB glasses along the axis of Thetis and Nereus. Compositions at a common 8% MgO were calculated using for Na2O the equation of Plank and Langmuir [1992], and for H2O that of Taylor and Martinez [2003]. Blue diamonds represent the modeled water content of the aggregated melt for a given mean degree of melting (Fv) with mantle source of N-MORB assumed to contain 0.01 and 0.02 wt% H2O. (c) Observed and predicted chondrite normalized (Sm/Yb)n ratios vs Na8. Blue diamonds and orange squares indicate predicted values at a given mean degree of melting from plate-thickening passive flow and dynamic mantle flow, respectively (see Ligi et al. 2012, for details).

SIMS references

Cipriani A., Santo A.P., Vaggelli G., Bonatti E., Brueckner H., Ottolini L.: Basaltic glasses from the Bouvet triple junction region (South Atlantic): Inferences on mantle heterogeneity, Fall Meeting, Dec. 8-12 (1997), San Francisco, Supplement to EOS, Transactions AGU, vol. 78 (46), Nov. 18, (1997), V41B-7.

Ligi M., Bonatti E., Brueckner H., Brunelli D., Cipriani A., Fabretti P., Ottolini L.: Contrasting Ultra-Slow Ridges near the Bouvet Triple Junction in the South Atlantic, Fall Meeting, Dec. 13-18 (1998), San Francisco, EOS, Vol.79, no.45, 877, 1998/Supplement.

Brunelli D., Cipriani A., Ottolini L., Bonatti E.: Vema Fracture Zone (Central Atlantic): temporal variations of mantle ultramafic composition, European Union of Geosciences, EUG X, Strasbourg (France), March 28-April 1 (1999), Blackwell Scientific Publ., TERRA abstracts, F02/2B, 387.

Bonatti E., Brunelli D., Cipriani A., Ottolini L., Seyler M.: Spatial and temporal heterogeneity of the oceanic mantle in the Central Atlantic, 3rd Int. Workshop on Orogenic Lherzolites and Mantle Processes, Pavia, Sept. 12-15 (1999), Ofioliti, (1999), 24, pag.72.

Tartarotti P., Susini S., Nimis P., Ottolini L.: Melt migration in the upper mantle along the Romanche Fracture Zone (Equatorial Atlantic), Lithos, 63, (2002), 125-149.

Brunelli D., Cipriani A., Ottolini L., Peyve A., Bonatti E.: Mantle Peridotites from the Bouvet Triple Junction Region, South Atlantic, Terra Nova, 15 (No 3), (2003), 194-203.

Bonatti E., Ligi M., Brunelli D., Cipriani A., Fabretti P., Ferrante V., Gasperini L., Ottolini L.: Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere, Nature, 423, (2003), 29 May Issue, 499-505.

Ligi M., Bonatti E., Cipriani A., Ottolini L.: Water-rich basalts at mid-ocean-ridge cold spots, Nature, 434, (2005), 3 March Issue, 66-69.

Brunelli D., Seyler M., Cipriani A., Ottolini L., Bonatti E. :  Discontinuous/episodic partial melt extraction from the melting region beneath the Mid Atlantic Ridge, J. Petrol., 47, (2006), 745-771. 

Cipriani C., Bonatti E., Seyler M., Brueckner H. K., Brunelli D., Dallai L., Hemming S. R., Ligi M., Ottolini L., Turrin B.D.: A 19 to 17 Ma amagmatic extension event at the Mid-Atlantic Ridge: Ultramafic mylonites from the Vema Lithospheric Section, Geochemistry, Geophysics, Geosystems (G3), 10, 23 October 2009, Q10011, doi:10.1029/2009GC002534.

Ligi M., Bonatti E., Brunelli D., Cipriani A., Ottolini L.: Water in Mid Ocean Ridge Basalts: Some Like it Hot, Some Like it Cold, in MARINE RESEARCH AT CNR, Volume DTA/06-2011, November 2011, pp. 671-690, ISSN 2239-5172.

Ligi M., Bonatti E., Bortoluzzi G., Cipriani A., Cocchi L., Caratori Tontini F., Carminati E., Ottolini L., Schettino A.: Birth of an ocean in the Red Sea: Initial pangs, Geochemistry, Geophysics, Geosystems (G3), 13 (8), 18 August 2012, Q08009, doi:10.1029/2012GC004155.