Plume-derived Rocks Research Articles

Global mantle isotopic anomalies linked to supercontinent-superocean evolution for the past 700 million years

Earth’s mantle isotopic composition is highly heterogeneous, with enriched and depleted regions1–3 shaped by a complex history of depletion and re-enrichment over 4.5 billion years. The spatial configuration of large-scale heterogeneity in the mantle is not random4,5, but the timing and mechanisms that caused such heterogeneity are still debated2,6,7. We compiled radiogenic isotope data of mid-ocean ridge and plume-induced basalts from both present-day and past oceans for the past 900 million years of the relatively isotopically enriched African mantle domain. Our analysis shows that oceanic rocks of the African mantle domain exhibit a relatively homogenous composition similar to the prevalent mantle estimates (PREMA8) until ca. 350 million years ago when plume-derived rocks started becoming isotopically enriched with maximum enrichment reached in the mid-Cretaceous. From that point in time, plume-derived rocks shift toward depletion, getting closer to the prevalent mantle value. The enrichment trend is interpreted to have resulted from the recycling of crustal material during the assembly and the early stage of a break-up of Gondwana and Pangea, whereas the subsequent depletion trend is due to the outward retreat of the subduction girdle during the break-up of Pangea, thus both are legacies of geodynamic processes related to the supercontinent cycle.

Temporal evolution of primordial tungsten-182 and 3He/4He signatures in the Iceland mantle plume

Studies of short-lived radiogenic isotope systems and noble gas isotopic compositions of plume-derived rocks suggest the existence of primordial domains in Earth's present-day mantle. Tungsten-182 anomalies together with high 3He/4He in Phanerozoic rocks from large igneous provinces and ocean island basalts demonstrate the preservation of early-formed (within the first 60 Ma of solar system history) mantle domains tapped by modern mantle plumes. It has proven difficult to link the evidence for primordial domains with geochemical evidence for more recent processes, such as recycling. The Greenland-Iceland plume system, starting with eruptions of the Paleocene North Atlantic Igneous Province, is later manifested in the mid-Miocene to modern volcanic products of Iceland. Here, we report Pb isotopic compositions, μ182W (deviations in 182W/184W of a sample from a laboratory reference standard in parts per million), and 3He/4He, as well as highly siderophile element concentrations and Re-Os isotopic systematics of basaltic samples erupted at different times during the ~60 Ma history of the Greenland-Iceland plume. Paleocene samples from Greenland, representing the early stage of the mantle plume, are characterized by variable 3He/4He ranging from 7 to 48 R/RA (measured 3He/4He normalized to the atmospheric ratio) and an average μ182W of −4.0 ± 3.6 (2SD), within modern upper mantle-like values of 0 ± 4.5. The basalts from Iceland can be divided into two groups based on their Pb isotope compositions. One group, consisting mostly of Miocene basalts, is characterized by 206Pb/204Pb ranging from ~18.4 to 18.5, 3He/4He ranging from 17.8 to 40.2 R/RA, and μ182W values ranging from +1.7 to −9.1 ± 4.5. The other group, consisting mainly of Pleistocene and Holocene basalts, is characterized by higher 206Pb/204Pb, ranging from ~18.7 to 19.2, 3He/4He ranging from 7.9 to 25.7 R/RA, and μ182W values ranging from −0.6 to −11.7 ± 4.5. Collectively, the Greenland-Iceland suite examined requires mixing between a minimum of three mantle source domains characterized by distinct Pb-He-W isotopic compositions, in order to account for this range of isotopic data. The temporal changes in the isotopic data for these rocks appear to track the dominant contributing plume components as the system evolved. One of the domains is indistinguishable from the ambient upper oceanic mantle and contributed substantial material throughout the time progression. The other two domains are most likely primordial reservoirs that underwent limited de-gassing. Given the negative μ182W values in some rocks, one of these domains likely formed within the first 60 Ma of solar system history and is a major contributor to the youngest basalts. The isotopic characteristics of Greenland-Iceland plume-derived rocks reveal episodic changes in the source component proportions.

DISCOVERY OF FERROPICRITES AND HIGH-MAGNESIAN ANDESITES FROM THE ERDENETSOGT FORMATION, CENTRAL MONGOLIA

New geochemical and petrological results are presented for greenstones from the Erdenetsogt Formation hosted by the Tsetserleg accretionary terrane in the Hangay region, with particular emphasis on newly found picritic and andesitic rocks. These rocks occur mostly in the lower portion of the Erdenetsogt Formation as massive lavas, sills, and dykes closely associated with varicolored bedded ribbon cherts and siltstones. The protoliths of the studied greenstones comprise (1) plume-derived tholeiitic greenstones with oceanic plateau basalt affinity, (2) arc-derived, calc-alkaline andesites. The plume-derived rocks are characterized by chemical signatures such as slight LREE enrichment similar to that of tholeiitic OIB and the existence of ferropicrite with high FeO* (>14 wt%) and MgO (12–22 wt%), which is characteristic of large igneous provinces (LIPs), including oceanic plateaus. Therefore, their tholeiitic composition and high-Fe and -Ti contents require melting of the source mantle peridotite with addition of some recycled Fe- and Ti-rich basaltic material. The andesites are characterized by glassy texture, high MgO content (up to 7 wt%), and significant LREE enrichment with depletion in Nb and resemble sanukite type of high-magnesian andesite (HMAs). We infer that the Hangay tholeiitic greenstones probably represent an accreted upper section of an oceanic plateau that developed in the deep-water region of the Hangay-Henteypaleo-ocean in the Upper Silurian-Lower Devonian. The Hangay HMAs may have been produced by subduction of young oceanic plate after an oceanward back-stepping of the subduction zone that was a result of the collision during the Carboniferous of the oceanic plateau and the active continental margin of the Central Mongolian Massif.

Isotopic (Sr, Nd, Pb, and Os) composition of highly magnesian dikes of Vestfjella, western Dronning Maud Land, Antarctica: A key to the origins of the Jurassic Karoo large igneous province?

There is a substantial debate on the mantle sources and ultimate origins of continental flood basalts (CFBs), e.g., whether they are related to deep-seated thermal upwellings (i.e. mantle plumes) in the upper mantle or not. Here we present high-precision isotopic (Sr, Nd, Pb, and Os) whole-rock data on some primitive dike rocks (ferropicrites, meimechites, picrobasalts, and basalts) associated with the Antarctic extension of the Jurassic (~ 180 Ma) Karoo CFB province. The isotopic data together with previously published trace element data show that the parental melts of the studied rocks sampled two distinctive geochemical reservoirs in the deep sub-Gondwanan mantle. The isotopic signatures of the relatively depleted types show evidence of extensive melt extraction in the past and are indistinguishable from those of mid-ocean ridge basalts (MORBs) of the SW Indian Ridge. On the other hand, the relatively enriched type isotopically resembles modern oceanic island basalts (OIBs) and may sample pyroxenitic sources either formed by melt infiltration in the upper mantle or by reaction of peridotite with recycled oceanic crustal components (with up to 15% of sediment material). The depleted types have previously been associated with anomalously hot mantle sources (> 1600 °C) and mantle plumes, but their MORB-like isotope compositions are unusual for plume-derived rocks. Instead, our findings are more concordant with a recent model [Coltice, N., Bertrand, H., Rey, P., Jourdan, F., Phillips, B.R., Ricard, Y., 2009. Global warming of the mantle beneath continents back to the Archean. Gondwana Research 15, 254–266] that suggests the generation of the Karoo CFBs in an extensive melting episode caused by internal heating of the upper mantle beneath the Gondwana supercontinent.

Isotope systematics of noble gases in the Earth's mantle: possible sources of primordial isotopes and implications for mantle structure

The Earth's mantle contains a mixture of primordial noble gases, in particular solar-type helium and neon, and radiogenic rare gases from long-lived U, 232Th, 40K and short-lived 129I, 244Pu. Rocks derived from deep mantle plume magmatism like on Hawaii or Iceland contain a higher proportion of primordial nuclides than rocks from the shallow upper mantle, e.g. mid ocean ridge basalts (MORBs). This is widely regarded as the key evidence for survival of a less degassed and more “primitive” reservoir within the lower mantle. We present an evaluation of noble gas composition showing the shallow mantle to have about five times more radiogenic (relative to primordial) isotopes than Hawaii/Iceland-type plume reservoirs, no matter if short- or long-lived decay systems are considered. This fundamental property suggests that both MORB and plume-type noble gases are mixtures of: (1) a homogeneous radiogenic component present throughout most of the mantle and (2) a uniform primordial noble gas component with very minor radiogenic ingrowth. This conclusion depends crucially on the observed excess of radiogenic Xe in plume-derived rocks, and is only valid if this Xe excess is inherent to the plume sources. Possible sources of the primordial component of mantle plume reservoirs—and possibly also the MORB mantle—could be mantle reservoirs that remained relatively isolated over most of Earth's history (“blobs”, a deep abyssal layer, or the D” layer), but these need a considerable concentration of primordial gases to compensate U, Th, K decay over 4.5 Ga. Earth's core is evaluated as an alternative viable source feeding primordial nuclides into mantle reservoirs: even low metal–silicate partitioning coefficients allow sufficient primordial noble gases to be incorporated into the early forming core, as the undifferentiated proto-Earth was initially gas-rich. Massive mantle degassing soon after core formation then provides the opposite concentration gradient that allows primordial noble gases reentering the mantle at the core-mantle boundary, probably via partial mantle melts. Another possible source of primordial noble gases in Earth's mantle are subducted sediments containing extraterrestrial dust with solar He and Ne, but this supply mechanism crucially depends on largely unconstrained parameters. The latter two scenarios do not require the preservation of a “primitive” mantle reservoir over 4.5 Ga, and can potentially better reconcile increasing geochemical evidence of recycled lithospheric components in mantle plumes and seismic evidence for whole mantle convection.