SPECTROSCOPIC RESPONSES OF DIAMOND DURING LOW-TEMPERATURE ANNEALING: ANALOGUE FOR RESIDENCE IN NATURAL SEDIMENTARY SYSTEMS

Thermal annealing experiments on natural diamonds are typically conducted at ≥500 °C. These temperatures are in excess of those that prevail in sedimentary systems where diamonds may reside at 50-350 ⁰C and for billions of years. We present exciting experimental evidence of diamond annealing at low temperatures <500 ⁰C, atmospheric pressures, and durations >24 h manifested as changes in optical luminescence and photoluminescence spectroscopy. Our results have specific implications for diamond residence in sedimentary systems. We analyzed ten natural colorless diamonds by Fourier Transform Infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy, and optical UV luminescence (225 nm) to measure nitrogen (N) content and aggregation (%N_B), defect concentrations, and luminescence color responses, respectively. Diamond N contents range from ~ 0 – 1260 ppm and %N_B of 0 – 86%. The diamonds have visible blue to green luminescence and interstitial- and vacancy-related defects (GR1, NV centers, and others). We estimated defect concentrations from the integrated area beneath the zero phonon line in PL spectra. Diamonds were then heated in a convection oven at 220 – 500 ⁰C and ambient pressures for 24 – 336 h and subsequently re-measured by FTIR, PL and optical UV luminescence. Most diamonds showed no visible change in UV luminescence response color. Luminescence color in one diamond changed from blue to teal and preserved an increased area of green luminescence surrounding radiation stains. Following annealing, defect concentrations increased or decreased. GR1 concentrations decreased from no change to 67%, but one diamond measured directly on a green radiation stain recorded an increase of 25%. NV⁰ concentrations decreased from no change to 65% and NV^- concentrations increased from no change to 175%. Some interstitial defects show decreases in concentration. Our results provide empirical evidence of luminescence color, defect distribution and concentration changes at temperatures inferred for diamonds residing in sedimentary rocks. Our study suggests that mantle inherited diamond defects can be altered during residence in natural sedimentary systems and influence defect related color and luminescence distributions in placer diamond populations.