Subcretion of altered oceanic crust beneath the SW Sao Francisco Craton, Brazil - A stable isotope study on diamonds and their inclusions

The presence of diamonds of lherzolitic, eclogitic and websteritic paragenesis in proximal alluvial deposits on the southwestern edge of the S & atilde;o Francisco Craton documents the incorporation of subducted oceanic crust and associated metasomatism through slab dehydration fluids affecting the local SCLM. To better constrain the subduction-association of diamond substrates and metasomatic events, we conducted a combined study of the delta C-13-delta N-15-[N] characteristics of 81 diamonds and the delta O-18 values of four of their eclogitic garnet inclusions. Diamond carbon isotope compositions range from -25.5 to +0.5 parts per thousand, with C-13-depleted diamonds (<= -7 parts per thousand) being exclusively of eclogitic/websteritic paragenesis while the C-13-enriched (>= -2 parts per thousand) tail of the distribution is related to diamonds with lherzolitic inclusions. Nitrogen isotope values range from -14.2 to +25.5 parts per thousand, with about half of the values being positive. A general absence of coherent trends in delta C-13-delta N-15-[N] across growth zones implies that diamond formation did not occur under fluid-limited conditions. Instead, the observed heterogeneity in carbon and nitrogen isotope compositions reflects contributions of distinct source reservoirs hosted in both altered oceanic crust and Earth's mantle. Nitrogen contents peak around a delta N-15 value of -3.5 parts per thousand, indicating that more N-rich fluids, presumably representing a primitive endmember composition, have a mantle-like delta N-15 signature. While positive and negative delta N-15 values occur equally near the delta C-13 mantle value (-5 +/- 2 parts per thousand), C-13-depleted diamonds have nitrogen isotope compositions skewed towards positive values.C-13 depletion and N-15 enrichment is a signature of biogenic carbonates/organic matter and low-T clays in uppermost, basaltic sections of oceanic crust that experienced low-temperature seawater alteration prior to subduction. Correspondingly, the oxygen isotope compositions of eclogitic garnet inclusions fall in a restricted range between +5.5 parts per thousand to +7.0 parts per thousand. For three of the four samples, the stable isotope signatures of inclusions and host diamonds display perfect agreement, with the intensity of seawater alteration signatures, in the form of garnet inclusion O-18 enrichment and host diamond C-13 depletion and N-15 enrichment, increasing together. For the fourth sample, the delta O-18 signature of the garnet inclusion (+5.5 parts per thousand) and delta C-13-delta N-15 signatures of the diamond host (-25 and + 19 parts per thousand, respectively) are decoupled. While the mantle-like delta O-18 signature indicates a diamond substrate derived from deeper levels in oceanic crust (e.g., deep sheeted dikes), the diamond-forming fluids must have originated from sources that originally resided near the sea water interface. A viable mode of mixing such disparate isotopic signatures is the interaction of diamond-forming fluids derived from shallow oceanic crust altered at low-temperatures with eclogitized substrates originally formed in deeper levels of oceanic crust. This process likely occurred during diamond formation in a tectonic subduction m & eacute;lange, which juxtaposes deeper and shallower levels of oceanic crust. In the lithospheric mantle above the subcreted oceanic slab, the elevated carbon isotope and highly variable nitrogen isotope compositions of diamonds formed in lherzolitic substrates likely relate to devolatilization and/or melting of principally oceanic sediments containing marine carbonates and clays and subsequent mixing with mantle-derived volatile components. In combination, our diamond and inclusion stable isotope data provide insight into multiple processes that promote diamond formation both inside subducted slabs accreted to the S & atilde;o Francisco cratonic keel and in adjacent subcontinental lithospheric mantle.

Automated summary

The presence of diamonds in alluvial deposits on the southwestern edge of the S & atilde;o Francisco Craton indicates the influence of subducted oceanic crust on the local mantle. The study of carbon and nitrogen isotope compositions of diamonds, as well as oxygen isotope compositions of their inclusion, reveals the contribution of different sources in the formation of diamonds, including altered oceanic crust and Earth's mantle. The isotopic signatures suggest that diamond-forming fluids may have originated from shallow oceanic crust and interacted with eclogitized substrates in deeper levels of oceanic crust. The research provides insight into diamond formation processes in subducted slabs and the subcontinental lithospheric mantle.