Diamond Formation From Organic Carbon in a Subducting Slab at Depths Between 250 and 450 km

Diamonds from the Jagersfontein kimberlite in South Africa contain inclusions of majorite garnet with eclogitic major element composition, providing evidence for diamond formation in ``basaltic'' environments at depths of up to 450 km. All 12 majoritic garnets recovered show distinct negative europium anomalies linking their ``basaltic'' source rocks to subducted oceanic crust. The carbon isotope composition (δ ¹³C) of the host diamonds is restricted to values between -17 and -24 ‰ . This tight distribution is completely different to the carbon isotope composition of ``normal'' (non-majoritic) eclogitic diamonds from Jagersfontein, which exhibit a broad range from -1 to -24 ‰ with a pronounced mode at -4 ‰ . The very distinct isotopic distributions show that diamonds of majoritic and normal eclogitic paragenesis form from different carbon sources. The small range in carbon isotopic composition of diamonds with majorite inclusions suggests that they directly reflect the isotopic signature of their carbon source. The isotopically light carbon isotopic composition of these diamonds is, therefore, consistent with a derivation from organic matter within a subducting slab. The main mechanism for the formation of the majorite bearing diamonds probably is direct conversion from graphite, which is expected to occur deep within the diamond stability field due to very sluggish kinetics.