The modern oceanic mantle (here, we use the term oceanic mantle as synonymous with convecting upper mantle, or depleted mid-ocean ridge basalt mantle, aka DMM) is chemically and isotopically heterogeneous. This is amply evidenced in the diverse compositions (chemical and isotopic) of derivative melts, such as those sampled by mid-ocean ridge basalts (MORB). The processes that resulted in the creation of heterogeneities in the oceanic mantle most likely were dominated by a combination of partial melting (and melt removal), metasomatism, and lithospheric recycling, yet the relative impacts of each of these processes, the timing of the processes, and the length scales of the resulting heterogeneities remain topics of considerable debate. Clarifying the causes, timing and spatial extent of chemical and isotopic heterogeneities in the oceanic mantle is, therefore, critical for refining our knowledge of the present bulk composition of this major terrestrial reservoir, for achieving a more complete understanding of how the chemical and isotopic composition of Earths mantle evolved through time, and for assessing the efficiency by which the oceanic mantle is convectively stirred.
To tackle these issues we primarily utilize siderophile elements, as well as major and other trace element compositions of various mantle lithologies. Our work is mainly focused on abyssal peridotites, which are tectonically exposed slivers of upper oceanic mantle present in the ocean basins, and the mantle sections of ophiolites. Most ophiolites are believed to be tectonically obducted pieces of oceanic lithosphere that have become incorporated into continental crust. Abyssal peridotites and the mantle sections of ophiolites can provide important direct information about the composition of the oceanic mantle. The mantle sections of ophiolites are also noteworthy in that ophiolites from around the world sample >2 billion years of Earth history, and in some instances, also provide excellent field control of depth beneath Moho.
Photo of sheeted dike complex in the 1.96 billion year old Jormua Ophiolite, Finland. The bands of basaltic rocks, that are oriented parallel to the hammer in the photo, served as discrete conduits of magma from the mantle to the surface.
Through our research, we are attempting to answer several fundamental questions about the present and past oceanic mantle by studying modern abyssal peridotites, and ophiolites of various ages, including:
- What is the chemical and isotopic range of rocks present in the oceanic mantle? If the diversity of compositions can be documented, as well as the proportions of the different represented lithologies, it should be possible to generate improved estimates for the bulk composition of this major terrestrial reservoir.
- What is the geometry of long-term, exceptionally depleted portions of the oceanic mantle that are commonly found in both ophiolite peridotites and abyssal peridotites? Determining the sizes and shapes of these domains is critical for understanding their modes of formation and preservation histories, as well as their importance to the overall mass balance of the oceanic mantle. This information, in turn, can be used to constrain the small to medium-scale mixing history of the oceanic mantle.
- Do common oceanic mantle lithologies, such as pyroxenites and dunites, have chemical compositions and sufficient volume to be implicated in the generation of picritic or basaltic lavas with unusual isotopic characteristics, (e.g., enrichments in 186Os), such as are found in some ocean island lavas?
- What is the difference in the 187Os/188Os of the oceanic mantle (reflecting the long-term history of 187Re/188Os of this major reservoir) compared to estimates for the bulk silicate Earth? The difference in isotopic composition (if there is one) can potentially be used to estimate the long-term proportion of oceanic crust that has been produced at mid-ocean ridge spreading centers, recycled (by plate tectonics), yet remained isolated from the oceanic mantle, and is hidden (somewhere) in the mantle today.
To address these questions, we are currently studying modern abyssal peridotites, and the mantle sections of a variety of ophiolites of diverse ages, including the Jormua Ophiolite (Finland), the Leka Ophiolite (Norway), the Shetlands Ophiolite (Scotland), the Albanian Ophiolite Complex, and the Taitao Ophiolite, Chile. This is part of a larger international effort that includes collaborators from elsewhere in the US, the UK, Ireland, Austria, Finland and Chile.
Photo at left shows chromitite seams from the Harolds Grave location in the ca. 492 Ma Shetlands (Unst) Ophiolite, Scotland. The photo at right shows our sampling party during a rare sunny day on Unst (from l to r: R. Walker, B. O'Driscoll, S. Daly and J. Day)
The ca. 497 Ma Leka Ophiolite (Norway). Photo at left shows the excellent exposure of mantle rocks at Leka. Careful examination of photo will reveal color differences in the rocks which reflect the different weathering characteristics of the diverse lithologies (harzburgites, dunites, pyroxenites) present. Photo at right demonstrates that the location of the Moho (boundary between the crust and the mantle) is well determined for this ophiolite.
Patterns for the abundances of the highly siderophile elements (HSE) normalized to bulk silicate Earth concentrations are shown below for harzburgites from the Taitao (6 Ma), Leka (497 Ma) and Jormua (1.96 Ga) ophiolites. The patterns indicate that the range of absolute and relative HSE concentrations present in these ophiolites are similar to the range found in modern abyssal peridotites (Day et al., 2017). This is an important preliminary observation because most abyssal peridotites indisputably sample the oceanic mantle, and the good match between ophiolite peridotite and abyssal peridotite geochemistry suggests the processes leading to obduction of the ophiolite lithologies into continental crust (making them easily available for our direct study) did not significantly modify the abundances of the HSE. In comparison to abyssal peridotites, however, these ophiolites allow detailed mapping and sampling of mantle structures and also allow us to observe the field relations between different lithologies.
Bulk silicate Earth normalized HSE patterns for peridotites from: a. the 6 Ma Taitao ophiolite, Chile, b. the 497 Ma Leka ophiolite, Norway, and c. the 1.96 Ga Jormua ophiolite, Finland. The gray areas in each panel, show the range of compositions that have been reported for modern abyssal peridotites from Day et al. (2017). Most of these ophiolite peridotites have HSE patterns that are indistinguishable from patterns for abyssal peridotites, regardless of their age. Data are from Schulte et al. (2009), O'Driscoll et al. (2015) and unpublished.
Osmium isotopes in peridotites from the oceanic mantle allow an assessment of the timing of prior melting events. Age normalized 187Os/188Os ratios (converted to γOs values, which are the percent deviation of the Os isotopic composition of a sample from a chondritic reference at the time of formation) indicate the Re-Os isotopic evolution of abyssal peridotites and ophiolite peridotites from Taitao, Leka and Unst have all been quite similar, and also similar to the chondritic reference (γOs = 0). The tail of data down to γOs values of <-5 suggest that all portions of the oceanic mantle contain domains that evidently were melted more than 1 Ga prior to the present, in the case of abyssal peridotites, or prior to the formation of the ophiolite.
Histogram plot of bulk ophiolite data (from Leka, Unst and Taitao ophiolites) versus data for modern abyssal peridotites (compilation from Walker, 2016).
How are chemical and isotopic heterogeneities manifested in the mantle? One place to assess this is the mantle section of the Taitao ophiolite. The Taitao ophiolite, Chile, is located on the Taitao peninsula, 50 km southeast of the Chile triple junction (CTJ) and only 17 km from the Chile trench. This ophiolite is likely related to ridge subduction and collision, and lies close to the CTJ where the Nazca, Antarctic and South American plates are juxtaposed.
Photo of the Taitao Ophiolite provided by collaborator Prof. R. Anma (University of Tsukuba).
The chemical affinities of at least some of the ophiolite-related mafic rocks are of normal-type and enriched-type MORB. Dating of minerals contained within gabbros has revealed that the ophiolite is only approximately 6 million years old. As a consequence of the youth of the ophiolite, it is generally quite well preserved (see photo below).
Microphotograph of a Taitao peridotite with some well-preserved olivine and pyroxene. The olivine is highly fractured and individual grains range in size from about 1 to 4 mm across. Roughly 30% of this rock consists of olivine crystals. Serpentine and pyroxene comprise the bulk remainder of the rock. Photo was taken with crossed polarizers, and the field of view is approximately 3 mm across.
The Taitao ophiolite has proven important in the sense that the full range of HSE abundances and Os isotopic compositions observed in abyssal peridotites and some other ophiolites are also found here. A major contribution of our work is the correlation between these geochemical characteristics and field relations made possible by the careful mapping accomplished by the sampling party. See for example, the variations in Os isotopic composition over the exposure scale of only several kilometers in the figure below.
Of note, the low 187Os/188Os of 0.1169 (γOs = -8) for sample TPE 024 (see map) most likely required a melting event approximately 1.6 billion years ago. How this rock remained isotopically isolated from surrounding materials, despite the fact it was part of the convecting oceanic mantle until about 6 million years ago remains a mystery. We also dont yet know the nature of the event that caused the original melt depletion during the Proterozoic.
(left) Geological map of the transition between peridotite and gabbroic units in the Taitao ophiolite, showing the Os isotope ratios of the studied rocks (peridotites and gabbros in white and grey boxes respectively). Also shown are the different units recognized in Taitao peninsula and the geotectonic setting of the area. Isotopic compositions of peridotitic rocks range considerably, and include some quite depleted isotopic compositions consistent with ancient (Proterozoic) melt depletion events. Figure is from Schulte et al. (2009).
(right) Plot of Mg# of whole rock (WR) and olivine versus initial 187Os/188Os for Taitao peridotites. The generally negative trends are consistent with variable ancient melt depletion and indicate little change in bulk composition of the rocks since the original melt depletion event(s). Low 187Os/188Os ratios (<0.118) indicate melt depletion occurred in spatially limited areas a minimum of about 1.6 billion years ago. Figure is from Schulte et al. (2009).
Recent work on Leka (Norway) and Jormua (Finland) ophiolites has been focused on sampling on the outcrop scale to enable us to map relatively small scale chemical and isotopic heterogeneity. The M.S. thesis work of Mitchell Haller reveals moderate Os isotopic heterogeneity on the outcrop scale of the Leka ophiolite (Haller et al., 2021).
Grid sampling of Leka LK15-10 area reveals moderate heterogeneity in the initial Os isotopic composition of peridotitic rocks. Photo shows γOs values (percent deviation of 187Os/188Os from that of a chondritic reference at 497 Ma) superimposed on an image of the outcrop. Samples were collected approximately 1 m apart. Xs show location from which samples were collected.
For more information about our research on this topic, please refer to the following papers:
Schulte R.F., Schilling M., Horan M.F., Anma R., Komiya T., Farquhar J., Piccoli P.M., Pitcher L. and Walker R.J. (2009) Chemical and chronologic complexity in the convecting upper mantle: evidence from the Taitao Ophiolite, southern Chile. Geochim. Cosmochim. Acta 73, 5793-5819.
Van Acken D., Becker H. and Walker R.J., McDonough W.F., Wombacher F., Ash R.D. and Piccoli P.M. (2010) Formation of pyroxenite layers in the Totalp ultramafic massif (Swiss Alps) insights from highly siderophile elements and Os isotopes. Geochim. Cosmochim. Acta 74, 661-683.
Van Acken D., Becker H., Hammerschmidt K., Walker R.J. and Wombacher F. (2010) Highly siderophile elements and Sr-Nd isotopes in refertilized mantle peridotites a case study from the Totalp ultramafic massif, Swiss Alps. Chem. Geol. 276, 257-268.
O'Driscoll B., Day J.M.D., Walker R.J., Daly S., McDonough W.F. and Piccoli P.M. (2012) Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from the Shetland Ophiolite Complex, Scotland. Earth Planet. Sci. Lett. 333-334, 226-237.
González-Jiménez J.M., Barra F., Walker R.J., Reich M. and Gervilla F. (2014) Geodynamic implications of ophiolitic chromitites in the La Cabaña ultramafic bodies, Central Chile. Internat. Geology Rev. 56, 1466-1483, DOI:10.1080/00206814.2014.947334.
O'Driscoll B., Walker R.J., Day J.M.D., Ash R.D. and Daly S.J. (2015) Generations of melt extraction, melt-rock interaction and high-temperature metasomatism preserved in peridotites of the ~497 Ma Leka Ophiolite Complex, Norway. Journ. Petrol. 56, 1797-1828.
Day J.M.D., O'Driscoll B., Strachan R.A., Daly J.S. and Walker R.J. (2017) Identification of mantle peridotite as a possible Iapetan ophiolite sliver in south Shetland, Scottish Caledonides. The Journal of the Geological Society of London, Special 174, 88-92, doi:10.1144/jgs2016-074.
Day J.M.D., Walker R.J. and Warren J.M. (2017) 186Os-187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys. Geochim. Cosmochim. Acta 200, 232-254.
O'Driscoll B., Walker R.J., Clay P.L., Day J.M. and Daly S.J. (2018) Length-scales of chemical and isotopic heterogeneity in the mantle section of the Shetland Ophiolite Complex, Scotland. Earth and Planet. Sci. Lett. 488, 144-154.
Haller M. B., O'Driscoll B., Day J.M.D., Daly J.S., Piccoli P.M. and Walker R.J. (2021) Meter-scale chemical and isotopic heterogeneities in the oceanic mantle, Leka Ophiolite Complex, Norway. Journal of Petrology 62, 1-29.
Finlayson V.A., Haller M., Day J.M.D., Ginley S., O’Driscoll B., Kontinen A., Hanski E., and Walker R.J. (2023) Oceanic and continental lithospheric mantle in the 1.95 Ga Jormua Ophiolite Complex, Finland: implications for mantle and crustal evolution. Journal of Petrology 64, 1-27.
Last Updated June 2024