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Qais Akbar Omar (GRS’16), a graduate student in the Creative Writing Program, has published a much-praised memoir, A Fort of Nine Towers: An Afghan Family Story. He recalls how the violence and tumult of civil war jolted his family, who, despite losing relatives, their home, and possessions, continued to nurture his wish to attend a university.

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Shedding Light on Earth’s Early Mantle

May 3rd, 2013

An international team led by CAS Department of Earth and Environment researchers has found evidence that material contained in young oceanic lava flows originated at the Earth’s surface in the Archean Eon, dating back to more than 2.45 billion years. The new finding helps constrain the timing of the initiation of plate tectonics, the origin of some of the chemical heterogeneity in the Earth’s mantle, and may shed light on how the chaotically convecting mantle could preserve such material for so long. The study appears in the April 25 issue of the journal Nature (doi:10.1038/nature12020).

Rita Cabral, a graduate student in BU’s Department of Earth and Environment, is the study’s primary author. Assistant Professor of Earth and Environment Matthew Jackson conceived of the project, secured the funding, and serves as Cabral’s thesis advisor.

Tectonic plates at the Earth’s surface move around and collide at areas called subduction zones. In these areas, one plate is forced beneath the other and is transported into the Earth’s mantle. It has long been suggested that this subducted material must be re-erupted at a later time. However, the residence time of the subducted material in the mantle is uncertain and convincing evidence of its return to the surface has been lacking.

Sulfur isotopes provide the key to the authors’ discovery.  According to the researchers, because mass-independently fractionated (MIF) sulfur isotope signatures were generated exclusively through atmospheric photochemical reactions until about 2.5 billion years ago, material containing such isotope signatures must have originated at the Earth’s surface in the Archean. In the new study, the researchers found MIF sulfur-isotope signatures in olivine-hosted sulfides from relatively young (20-million-year-old) ocean island basalts (OIB) from Mangaia, Cook Islands (Polynesia), providing evidence that material once at the Earth’s surface has been recycled through the mantle and re-erupted at a young ocean island.

“The discovery of MIF-S isotope in these young oceanic lavas suggests that sulfur—likely derived from the hydrothermally-altered oceanic crust—was subducted into the mantle more than 2.5 billion years ago and recycled into the mantle source of the Mangaia lavas,” says Cabral.

The data also complement evidence for sulfur recycling of ancient sedimentary materials to the subcontinental lithospheric mantle previously identified in diamond inclusions.

The following authors contributed to this study: Rita A. Cabral, Department of Earth and Environment, Boston University, 675 Commonwealth Ave., Boston, MA, 02215, USA; Matthew G. Jackson, Department of Earth and Environment, Boston University, 675 Commonwealth Ave., Boston, MA, 02215, USA; Estelle F. Rose-Koga, Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS UMR6524, IRD R163, 5 rue Kessler, 63038 Clermont-Ferrand, France; Kenneth T. Koga, Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS UMR6524, IRD R163, 5 rue Kessler, 63038 Clermont-Ferrand, France; Martin J. Whitehouse, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden and Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden; Michael A. Antonelli, Department of Geology and ESSIC, University of Maryland, College Park, MD 20742, USA; James Farquhar, Department of Geology and ESSIC, University of Maryland, College Park, MD 20742, USA; James M.D. Day, Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA, 92093-0244, USA; Erik H. Hauri, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.

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