T282 Gemological Research in the Twenty-First Century—Characterization, Exploration, and Geological Significance of Diamonds and Other Gem Minerals

Wednesday, 7 November 2018
1:30 PM - 5:30 PM
Room 145

EVIDENCE OF SUBDUCTED ALTERED OCEANIC CRUST INTO DEEP MANTLE FROM INCLUSIONS OF IAB DIAMONDS

Paper number: T282-9
Program: Technical Sessions
Time: 3:20 PM – 3:35 PM

Authors

Abstract

Nitrogen is one of the most common impurities in diamonds, and its aggregation styles in diamonds have been used as criteria for diamond classification. Pure IaB diamonds (with 100% nitrogen in B aggregation) are rather rare among natural diamonds. The occurrence of B center is generally associated with high temperature and long resident time of the host diamond, which would potentially provide information of Earth’s deep interior. Seawater circulation is the unique process that shapes the surface of our planet and potentially has profound effect on Earth’s interior due to slab subduction. Here, from 50 IaB diamonds with detectable micro inclusions submitted to the Gemological Institute of America (GIA) for screening, we found more than 70% of them content typical mineral assemblage from sublithosphere. Jeffbenite (TAPP), majorite garnet, enstatite pyroxene, ferropericlase have been observed which could be retrograde products of former bridgemanite. CaSiO3 Walstromite with larnite, and titanite is the dominate phase present in ~40% of all diamond samples. Direct evidence from oxygen isotope ratios measured by SIMS (δ18OVSMOW in the range +11.1 to +12.5 ‰) of CaSiO3 walstromite with coexisting larnite and titanite that retrograde from CaSiO3 perovskite suggest that hydrothermally altered oceanic basalt can subduct to depths of > 410 km in the transition zone. Incorporation of materials from subducted altered oceanic crust into deep mantle produced diamond inclusions that have both lower mantle and subduction signatures. Ca(Si,Al)O3 perovskite was observed with high concentration of REE elements (> 1.7 wt.%) that could be enriched under lower mantle PT conditions. Evidence from ringwoodite with hydroxide bond, precipitates of NH3 phase, and cohenite with trace amounts of Cl implies that the subducted brines can potentially introduce hydrous fluid to the bottom of transition zone. Increasing trends of carbon isotope ratio from core to rim region detected by SIMS (δ 13C from -5.5 ‰ to -4 ‰ ) of the diamonds with subducted materials suggests that an oxidized carbonate-dominated fluid was associated with recycling of the subducted hydrous material and played an important role in balancing redox exchange with the reduced lower mantle indicated by precipitated iron nano particles and coexisting hydrocarbons and carbonate phases.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-320390

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.