Sierra Gorda 009: A new member of the metal‐rich G chondrites grouplet

Marina A. Ivanova, Cyril A. Lorenz, Munir Humayun, Catherine M. Corrigan, Thomas Ludwig, Mario Trieloff, Kevin Righter, Ian A. Franchi, Alexander B. Verchovsky, Ekaterina V. Korochantseva, Vladimir V. Kozlov, Svetlana N. Teplyakova, Alexander V. Korochantsev, Victor I. Grokhovsky

Meteoritics & Planetary Science
Version of Record online: 03 August 2020


“We investigated the metal‐rich chondrite Sierra Gorda (SG) 009, a member of the new G chondrite grouplet (also including NWA 5492, GRO 95551). G chondrites contain 23% metal, very reduced silicates, and rare oxidized mineral phases (Mg‐chromite, FeO‐rich pyroxene). G chondrites are not related to CH‐CB chondrites, based on bulk O, C, and N isotopic compositions, mineralogy, and geochemistry. G chondrites have no fine‐grained matrix or matrix lumps enclosing hydrated material typical for CH‐CB chondrites. G chondrites’ average metal compositions are similar to H chondrites. Siderophile and lithophile geochemistry indicates sulfidization and fractionation of the SG 009 metal and silicates, unlike NWA 5492 and GRO 95551. The G chondrites have average O isotopic compositions Δ17O>0‰ ranging between bulk enstatite (E) and ordinary (O) chondrites. An Al‐rich chondrule from SG 009 has Δ17O>0‰ indicating some heterogeneity in oxygen isotopic composition of G chondrite components. SG 009’s bulk carbon and nitrogen isotopic compositions correspond to E and O chondrites. Neon isotopic composition reflects a mixture of cosmogenic and solar components, and cosmic ray exposure age of SG 009 is typical for O, E, and R chondrites. G chondrites are closely related to O, E, and R chondrites and may represent a unique metal‐rich parent asteroid containing primitive and fractionated material from the inner solar system. Oxidizing and reducing conditions during SG 009 formation may be connected with a chemical microenvironment and possibly could indicate that G chondrites may have formed by a planetesimal collision resulting in the lack of matrix.”