The Geochemical Earth Reference Model (GERM) initiative is a grass-root effort with the goals of establishing a community consensus on a chemical characterization of the Earth, its major reservoirs, and the fluxes between them. The GERM initiative will provide a review of available scientific constraints for: (1) the composition of all major chemical reservoirs of the present-day Earth, from core to atmosphere; (2) present-day fluxes between reservoirs; (3) the Earth's chemical and isotopic evolution since accretion; and (4) the chemical and isotopic evolution of seawater as a record of global tectonics and climate. Even though most of the constraints for the GERM will be drawn from chemical data sets, some data will have to come from other disciplines, such as geophysics, nuclear physics, and cosmochemistry. GERM also includes a diverse chemical and physical data base and computer codes that are useful for our understanding of how the Earth works as a dynamic chemical and physical system. The GERM initiative is developed in an open community discussion on the World Wide Web (http://www-ep.es.llnl.gov/germ/germ-home.html) that is moderated by editors with responsibilities for different reservoirs, fluxes, data bases, and other scientific or technical aspects. These editors have agreed to lay out an initial, strawman GERM for their respective sections and to moderate community discussions leading to a first, preliminary consensus. The development of the GERM began with an initial workshop in Lyon, France in March, 1996. Since then, the GERM has continued to be developed on the Internet, punctuated by workshops and special sessions at professional meetings. A second GERM workshop will be held in La Jolla, CA USA on March 10–13, 1998.
In order to undertake a preliminary assessment of the extent of symmetry in source composition and melting dynamics in the Azores plume we present new ICP‐MS trace element data along with Sr, Nd, Pb, and U‐Th‐Ra isotope data for samples from the islands Flores and Corvo west of the Mid‐Atlantic Ridge. We also present data from a picrite from Faial and new ICP‐MS trace element data for 28 basaltic lavas from the eastern Azores Plateau to augment data published previously from these samples. Rare earth element data for primitive lavas (MgO ≥ 5% and Mg # ≥ 60) have La/Yb N ∼ 10 and variable Ce and/or Eu anomalies. Multi‐incompatible trace element patterns normalized to primitive mantle are convex upward with small negative Th and K ± Pb anomalies. While lavas to the east are characterized by low Nb/Zr and generally lower La/Yb ratios (with the notable exception of eastern São Miguel), lavas from the western islands have slightly higher Nb/Zr and La/Yb inferred to reflect smaller degrees of partial melting. The Sr‐Nd‐Pb isotope systematics imply that Corvo and Flores sample components which range from an isotopic source commonly found in the Azores (e.g., at the eastern island of Graciosa) to a more depleted, MORB‐like mantle sampled at the Mid‐Atlantic Ridge. However, in common with uncontaminated samples from São Miguel, the Corvo and Flores samples appear to have slightly lower 230 Th excesses and higher La/Yb, Tb/Yb than the other Azores islands or the Mid‐Atlantic Ridge samples. The trace element and isotope data indicate a relatively symmetric pattern with distance across the MAR, while U‐Th disequilibria, and thus inferred melting dynamics, appear less symmetric. Nevertheless, the data suggest that heterogeneities in source composition do not have a large effect on melting dynamics, at least within the Azores islands.
One of the most significant recent advances in understanding Earth's structure, dynamics, and evolution has been the recognition of two Large Low S-wave Velocity Provinces, or LLSVPs, at the base of the mantle: one beneath Africa, the other beneath the South-Central Pacific. Each has an aerial extent of thousands of km2 and rise several hundreds of km above the core-mantle boundary. Their nature is unclear, but several lines of evidence suggest they are both hotter and denser than surrounding mantle (see review by McNamara, 2019). Importantly, most mantle plumes appear to be generated from these regions, particularly along the margins, while subduction appears to be focused on regions surrounding the LLSVPs. Mantle plumes come in a limited variety of enriched geochemical flavors derived from recycling of oceanic and continental crust into the mantle. In a new paper in AGU Advances, Jackson and Macdonald (2022) argue that the EMI and EMII mantle flavors result from deep subduction of ancient rifted passive margins during the assembly of Gondwana in the Neoproterozoic. Jackson and Mcdonald build their case on a number of observations. First, the oldest high pressure metamorphic belts are of Neoproterozoic age (Brown & Johnson, 2019), implying wholesale deep subduction of continental crust only began around that time (Condie, 2021). Second, EM plumes are rooted within or on the margins of the two LLSVPs in the southern hemisphere. Third, assembly of Gondwana occurred in the southern hemisphere in the Neoproterozoic. They argue that while sediment subduction and subduction erosion may have occurred throughout Earth's history, “sediments and the products of subduction erosion are not attached to a down-going slab, so they are buoyant and relaminate” to the base of continents (Hacker et al., 2011) rather than being carried into the deep mantle. They also argue that lower crust foundering can be ruled out as a source because lower continental crust does not have the appropriate isotopic composition to produce EM geochemical flavors. This provocative proposal ties into two vigorous debates about crust and mantle evolution. The first of these is the question of when did modern plate tectonics begin and how much continental crust has been destroyed through subduction and related processes through time? On the one hand, continental crust bears the geochemical fingerprint of subduction in relative depletions of elements such as Ta and Nb and enrichment in others such as Pb, suggesting modern style plate tectonics has persisted through much of Earth's history. On the other hand, most Archean terranes lack modern analogs and there are clear geochemical distinctions between Archean and post-Archean crust and magmatism. These, and other observations, have led some (e.g., Davaille et al., 2017) to argue that mantle convection in the early Earth was fundamentally different, lacking lateral lithospheric motion (“stagnant-lid tectonics”). At one extreme, Stern (2008) argued that modern-style plate tectonics only began in the Neoproterozoic (<1 Ga). At the other, Aarons et al. (2020) argue that modern style subduction began in the Archean. Many other estimates of when modern plate tectonics began fall somewhere in between. Despite the debate over when modern plate tectonics began, most recognize that substantial volumes of continental crust have been lost since the early Archean. Thus Jackson and Mcdonald's argument that continental material carried into the mantle by sediment subduction and subduction erosion is “relaminated” to the base of continents is a key part of their hypothesis. The relamination model (Hacker et al., 2011) is based on a variety of observations, most notably that metasediments are surprisingly abundant among lower crustal lithologies and that silica-rich material, including most sediments and upper crustal lithologies, will become buoyant when carried into the upper mantle. However, in the Hacker et al. (2011) model, only two-thirds of the crust-to-mantle flux is relaminated, with the remaining third continuing into the deep mantle, a flux of about 3 × 1012 Tg/yr. This is a substantial flux of continental material into the mantle, comprising, for example, ∼1/3 of the long-term average crustal growth rate of 9 × 1012 Tg/yr estimated by Dhuime et al. (2017), although substantially lower than some estimates of present crustal recycling rates. Furthermore, the relamination rate estimated by Hacker et al. (2011) is much greater than needed to account for the abundance of metasediments in lower crustal granulites. That said, even if crustal recycling has occurred throughout Earth's history and has thoroughly polluted the mantle, it is nonetheless possible that creation of the EMI and EMII reservoirs is a direct result of a specific tectonic event such as the assembly of Gondwana. Indeed, 87Sr/86Sr ratios in some Samoan lavas are well above typical continental crust values and so extreme (0.7126; Jackson et al., 2007) that they require very substantial amounts of continental material in their source. The second debate is over the evolution of the LLSVPs and whether their location controls plate tectonic processes, particularly assembly and breakup of supercontinents, or vice versa. Dziewonski et al. (2010) suggested that deep mantle structure “may have formed early in the history of the convecting mantle, remained locked in place with respect to the Earth's rotation axis ever since, and is currently imposing the planform of flow in the mantle and of plate tectonics at the surface.” Indeed, eruption of large igneous provinces and kimberlites appear also to have been associated with these features in their current locations over the last 300 Ma and perhaps the entire Phanerozoic (Torsvik et al., 2021). Variations in xenon and tungsten isotope ratios produced by extinct radionuclides 129I, 244Pu, and 182Hf indicate that some plumes sample material that has been largely isolated since very early in Earth history (Jackson et al., 2020; Mukhopadhyay & Parai, 2019), which favors the long-term persistence of LLSVPs. On the other hand, others, Li and Zhong (2009), for example, have argued that circum-supercontinent subduction leads to the formation of LLSVPs. Regardless of whether LLSVPs control mantle flow and the supercontinent cycle or vice versa, processes at the base of the mantle and at the Earth's surface are intimately connected (Torsvik et al., 2021). The Neoproterozoic was a time of great changes at the surface of the Earth. In addition to assembly of Gondwana, it is marked by extreme glaciations, a rise in atmospheric oxygen, and the first appearance of metazoans. If, as Jackson and Mcdonald propose, it also saw the initiation of deep crustal subduction and consequent formation of highly incompatible element enriched reservoirs within LLSVPs, then the Neoproterozoic Revolution would have encompassed the entire silicate portion of the Earth. At the least, Jackson and Mcdonald proposal should generate some quite interesting future debate. The authors declare no conflicts of interest relevant to this study.
Research Article| December 01, 1999 Illegitimate magmas of the Galápagos: Insights into mantle mixing and magma transport Dennis Geist; Dennis Geist 1Department of Geology and Geological Engineering, University of Idaho, Moscow, Idaho 83844, USA Search for other works by this author on: GSW Google Scholar William White; William White 2Department of Geological Sciences, Cornell University, Ithaca, New York 14850, USA Search for other works by this author on: GSW Google Scholar Terry Naumann; Terry Naumann 3Geology Department, University of Alaska, Anchorage, Alaska 99508, USA Search for other works by this author on: GSW Google Scholar Robert Reynolds Robert Reynolds 4Department of Science, Central Oregon Community College, Bend, Oregon 97701, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Dennis Geist 1Department of Geology and Geological Engineering, University of Idaho, Moscow, Idaho 83844, USA William White 2Department of Geological Sciences, Cornell University, Ithaca, New York 14850, USA Terry Naumann 3Geology Department, University of Alaska, Anchorage, Alaska 99508, USA Robert Reynolds 4Department of Science, Central Oregon Community College, Bend, Oregon 97701, USA Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1999) 27 (12): 1103–1106. https://doi.org/10.1130/0091-7613(1999)027<1103:IMOTGP>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Dennis Geist, William White, Terry Naumann, Robert Reynolds; Illegitimate magmas of the Galápagos: Insights into mantle mixing and magma transport. Geology 1999;; 27 (12): 1103–1106. doi: https://doi.org/10.1130/0091-7613(1999)027<1103:IMOTGP>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Roughly 1–2% of the flows erupted from flank vents of the western Galápagos shield volcanoes have anomalous compositions. We call these illegitimate magmas because of their uncertain parentage. Because some illegitimate magmas are compositionally indistinguishable from lavas of an adjacent volcano and erupt from the flank facing the adjacent volcano, such magmas apparently result from lateral intrusion of magma from the adjacent volcano. Other illegitimate magmas come from parts of the Galápagos plume that have incompletely mixed or result from unusually advanced melting of part of the mantle. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The mantle can be sampled directly only very rarely. Geochemists have thus come to rely heavily on mantle‐derived magmas to study the composition and evolution of the mantle. Only those compositional features that are unaffected by magmatic processes are useful as tracers of mantle processes. These include radiogenic isotope ratios such as those of He, Sr, Hf, and Os, stable isotope ratios, and ratios of highly incompatible elements or elements of similar incompatibility, such as Ba/Nb or Pb/Ce. The term “incompatible” denotes a preference of the element for a melt over mantle minerals. Highly incompatible elements will partition entirely into the melt under most circumstances, so that the ratio of two such elements in a basalt will be virtually identical to that ratio in its source. This is also true to a lesser degree of ratios such as La/Sm and Zr/Nb, as Zr and Sm are not highly incompatible elements.
Other| November 01, 1991 Large role of sediments in the genesis of some Lesser Antilles andesites and dacites (Soufriere, St. Lucia); isotopic constraints P. Vidal; P. Vidal Univ. Blaise Pascal, Clermont-Ferrand, France Search for other works by this author on: GSW Google Scholar M. Le Guen de Kerneizon; M. Le Guen de Kerneizon Search for other works by this author on: GSW Google Scholar R. C. Maury; R. C. Maury Search for other works by this author on: GSW Google Scholar B. Dupre; B. Dupre Search for other works by this author on: GSW Google Scholar W. M. White W. M. White Search for other works by this author on: GSW Google Scholar Author and Article Information P. Vidal Univ. Blaise Pascal, Clermont-Ferrand, France M. Le Guen de Kerneizon R. C. Maury B. Dupre W. M. White Publisher: Société Géologique de France First Online: 07 Mar 2017 Online Issn: 1777-5817 Print Issn: 0037-9409 GeoRef, Copyright 2004, American Geological Institute. Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique, Vandoeuvre-les-Nancy, France Bulletin de la Société Géologique de France (1991) 162 (6): 993–1002. https://doi.org/10.2113/gssgfbull.162.6.993 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P. Vidal, M. Le Guen de Kerneizon, R. C. Maury, B. Dupre, W. M. White; Large role of sediments in the genesis of some Lesser Antilles andesites and dacites (Soufriere, St. Lucia); isotopic constraints. Bulletin de la Société Géologique de France 1991;; 162 (6): 993–1002. doi: https://doi.org/10.2113/gssgfbull.162.6.993 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin de la Société Géologique de France Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Return of oceanic crust to the mantle through subduction was explicitly part of plate tectonic theory (Isacks et al. , 1968; Le Pichon, 1968), but it took longer for the idea that sediment, and therefore indirectly continental crust, could also be subducted into the mantle. Here again, Dick
Research Article| July 01, 1996 Beyond EM-1: Lavas from Afanasy-Nikitin Rise and the Crozet Archipelago, Indian Ocean J. J. Mahoney; J. J. Mahoney 1School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96826 Search for other works by this author on: GSW Google Scholar W. M. White; W. M. White 2Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar B. G. J. Upton; B. G. J. Upton 3Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom Search for other works by this author on: GSW Google Scholar C. R. Neal; C. R. Neal 4Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556 Search for other works by this author on: GSW Google Scholar R. A. Scrutton R. A. Scrutton 3Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information J. J. Mahoney 1School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96826 W. M. White 2Department of Geological Sciences, Cornell University, Ithaca, New York 14853 B. G. J. Upton 3Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom C. R. Neal 4Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556 R. A. Scrutton 3Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (7): 615–618. https://doi.org/10.1130/0091-7613(1996)024<0615:BELFAN>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. J. Mahoney, W. M. White, B. G. J. Upton, C. R. Neal, R. A. Scrutton; Beyond EM-1: Lavas from Afanasy-Nikitin Rise and the Crozet Archipelago, Indian Ocean. Geology 1996;; 24 (7): 615–618. doi: https://doi.org/10.1130/0091-7613(1996)024<0615:BELFAN>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Lavas from Afanasy-Nikitin Rise, possibly the Late Cretaceous product of the Crozet hotspot, cover a wide range of isotopic compositions that includes the lowest (206Pb204Pb)t (to 16.77) and ϵNd(t) (to −8) values yet found among oceanic islands or spreading centers worldwide, as well as high (87Sr/86Sr)t (to 0.7066). In contrast, young basalts from the Crozet Archipelago exhibit a narrow range of variation around ϵNd ∼ +4, 87Sr/86Sr ∼ 0.7040, and 206Pb/204Pb ∼ 19.0, closely resembling that of shield lavas of the Réunion hotspot. The Afanasy-Nikitin rocks also have much higher Ba/Nb, Ba/Th, and Pb/Ce than modern oceanic island or ridge lavas, as well as high La/Nb. The data do not obviously support the Crozet plume model but, assuming the model to be plate tectonically correct, would indicate that the plume-source composition either changed dramatically or that Afanasy-Nikitin magmatism involved significant amounts of nonplume mantle. The low 206Pb/204Pb, low ϵNd lavas provide the best evidence to date of the sort of material that, by variably contaminating much of the Indian mid-ocean-ridge basalt (MORB) source asthenosphere, may be responsible for the isotopic difference between most Indian MORB and Pacific or North Atlantic MORB. The combined isotopic and trace element results suggest an ultimate origin in the continental crust or mantle lithosphere for this material, although whether it was cycled through the deep mantle or resided at shallow levels in the convecting mantle cannot currently be determined. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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