The Lamont Earth Science Colloquium presents:
The sulfur isotope compositions of the marble cake-mantle
Dr. Jabrane Labidi, Researches, CNRS, Institut de Physique de Globe de Paris, France
Mantle outcrops show pyroxenite layers embedded within peridotites, with pyroxenites possibly linked with recycled oceanic crust. This observation underpins the "marble cake" mantle analogy (Allègre and Turcotte, 1986), which describes the upper mantle as dominantly peridotitic with “elongated strips of subducted oceanic lithosphere”. While pyroxenite origins are diverse, their presence as recycled crust in the mantle may explain the radiogenic isotope heterogeneity observed in mid-ocean ridge basalts (MORB).The marble cake model also provides a framework for understanding mantle volatile budgets and stable isotope compositions, distinguishing juvenile from recycled components. Sulfur isotope data from MORB reveal significant ³⁴S/³²S variations, consistent with the incorporation of subducted materials. These variations are accompanied by near-invariant ³³S signatures (average Δ³³S = +0.004 ± 0.012‰), ruling out substantial Archean contributions and suggesting that the marble cake ingredients are predominantly post-Archean.
In contrast to MORB data, plume-related basalts show variable Δ³³S, over ~ 0.10‰. This is much smaller than the 15‰ range exhibited by Archean rocks, but providing a clear constraint on the age of recycled slabs is more challenging. The most striking example of inherent slab heterogeneity is perhaps the Mangaia island, where samples investigated with isotope ratio mass spectrometry exhibit the most variable Δ³³S signatures, with values up to +0.049 ± 0.016‰. Strikingly, similar S isotope variability is observed in graphite-bearing garnet pyroxenites from the Beni Bousera Massif (Morocco), which share key geochemical features with the Mangaia plume source. This parallel provides direct link between the deep mantle sampled by plumes to the pyroxenite-rich lithologies exposed at the surface.
The diversity of sulfur isotope signatures observed in basalts illustrate how recycled oceanic crust contributes to the chemical and isotopic heterogeneity of Earth's mantle. Originally sketched to explain radiogenic isotope variability, the marble cake mantle analogy is helpful to explain the sulfur cycle. This analogy offers insights into the long-term recycling processes that shape the composition and dynamics of Earth's interior, bridging the gap between mantle geochemistry and geodynamics.
Host: Dr. Yves Moussallam, Assistant Professor, DEES.
The Earth Science Colloquium Series, sponsored by Lamont-Doherty Earth Observatory and Columbia University Department of Earth and Environmental Sciences (DEES), provides a lively forum for discussing a wide variety of topics within the Earth sciences and related fields. Colloquia are attended by the full range of scientific and technical staff at LDEO. Colloquium attendance is required of all pre-orals DEES graduate students. The Colloquium Series supports the Lamont Seminar Diversity Initiative.
