Fibrous diamonds through time-insights into mantle fluids and metasomatism

Lead PI: Yaakov Weiss , Steven L. Goldstein

Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)

January 2015 - December 2017
Project Type: Research

DESCRIPTION: Carbon plays a major role in many fundamental aspects of our planet, for example, it is the basis of life, and regulates climate. The surface carbon cycle is intimately linked to the carbon cycle of the Earth's deep interior, where ~90% of global carbon is stored. Our knowledge about deep carbon is limited, mainly relying on mantle xenoliths and xenocrysts sampled by magmas ascending from the Earths' deep interior. Diamonds, the high-pressure polymorph of elemental carbon, are brought to the surface from 150-200 km or even deeper and are formed from carbon-bearing fluids or melts; they hold key information of how carbon is transported and stored in the deep mantle. Fibrous diamonds are a fast growing form containing micrometer-scale inclusions of the medium from which diamond grew. Due to the diamond's physical and chemical strength, they remain pristine from the time of their encapsulation deep in the mantle until they are released in the laboratory. Micro-analytical and ultraclean laboratory techniques make it now possible to determine the chemical nature of these trapped fluids/melts. This study will measure major and trace element and isotopic compositions in the fluids in order to determine the origin of deep carbon-bearing mantle fluids and characterize the deep carbon cycle in our planet through time. The high-density fluids (HDFs) trapped by fibrous diamonds vary in composition between silicic, saline and high- and low-Mg carbonatitic end-members, and display two principle trace-element patterns, irrespective of the provenance of their host diamonds. Data on the Sr and Nd isotopic compositions of HDFs are currently very sparse, but indicate that Archean diamonds sample depleted mantle, whereas late Phanerozoic diamonds sample enriched mantle and continental crust. In this study, the characteristics of the trapped fluids from previously unstudied locations, with emplacement ages from Cenozoic to Paleo-Proterozoic, will be explored through comprehensive chemical and isotopic characterizations of the diamond-forming fluids. Nd-Sr isotopic signatures of HDFs "fingerprint" the fluid's sources, and systematic relationships with major and trace element compositions can distinguish between different possible mantle sources of carbon-bearing fluids/melts, such as oceanic-type mantle, lithospheric mantle, or subduction-modified sources. The study will provide new insights on fluids in the deep earth through time, and their role in transporting volatiles and trace elements between the mantle and crust. Since diamonds represent the deepest direct samples of the Earths interior, the study will provide new perspectives on the Earth's deep carbon cycle through time.