Pia Antignani | Università degli Studi di Padova (original) (raw)

Papers by Pia Antignani

European Journal of Mineralogy

We have investigated a suite of natural diamonds from the kimberlite pipe of the Changma Kimberli... more We have investigated a suite of natural diamonds from the kimberlite pipe of the Changma Kimberlite Belt, Mengyin County, Shandong Province, China, with the aim of constraining pressures and temperatures of formation. Here we report the non-destructive investigation of an olivine inclusion still entrapped within a lithospheric diamond by single-crystal X-ray diffraction. We were able to refine anisotropically its crystal structure to R 1 = 1.42 % using ionized scattering curves; this allows estimation of the composition of the olivine as Mg 1.82 Fe 0.18 SiO 4. This composition corresponds to a calculated unit-cell volume equal to V = 292.70 Å 3 at room temperature and pressure. We have validated the above-calculated composition and unit-cell volume by releasing the inclusion from the diamond host, resulting in a consistent composition calculated using nondestructive methods of Mg 1.84 Fe 0.16 SiO 4 and V = 292.80 ± 0.07 Å 3. Considering that the unit-cell volume of the olivine still inside its diamond host is V = 289.7 ± 0.2 Å 3 , we calculated a residual pressure P inc = 1.4 ± 0.1 GPa with respect to the released crystal and P inc = 1.3 ± 0.2 GPa with respect to the volume calculated from the "composition" indirectly retrieved by the structure refinement under ambient conditions. The two values of P inc overlap within experimental uncertainty. We performed Fourier transform infrared (FTIR) analysis on the diamond host in order to calculate its mantle residence temperature, T res , which resulted in a value of 1189 • C (for an assumed diamond age of 3 Ga) and 1218 • C (for an age of 1 Ga), with an average T res equal to 1204 ± 15 • C. Using the most up-to-date pressure-volume-temperature equations of state for olivine and diamond, the residual pressure P inc = 1.4 ± 0.1 GPa and average residence temperature of the diamond host T res = 1204 • C, we retrieved a pressure of entrapment P trap = 6.3 ± 0.4 GPa. Using the non-destructive approach and relative P inc = 1.3 GPa, we obtained a perfectly overlapping P trap = 6.2 GPa, within experimental uncertainty. This entrapment pressure corresponds to depths of about 190 ± 12 km. These results demonstrate that for high-quality crystal structure data measured on inclusions still trapped within diamond hosts, even a non-destructive approach can be used to calculate the depth of formation of diamond-olivine pairs. In terms of geological implications, the results from this work show that Changma diamonds formed under a conductive geotherm lying between 35 and 40 mW m −2 , at a depth of about 190 km. This value lies within the recently reported upper limit of the average depth of formation of worldwide lithospheric diamonds, which is 175 ± 15 km and is in agreement with P-T data obtained in the literature from kimberlite xenoliths.

The present study was carried out for investigating the growth relationships between magnesiochro... more The present study was carried out for investigating the growth relationships between magnesiochromite inclusions and its diamond hosts, in order to determine its genetic nature, expressed in terms of singenesis (simultaneous growth between diamonds and inclusions) or protogenesis (pre-existing inclusions with respect to the diamonds). The aim is to understand if magnesiochromite and diamond grow in epitaxy, namely according to a specific crystallographic relationship, through the analysis of the crystallographic orientation relationship between magnesiochromite and diamond determined by single crystal X-ray diffraction on a total of 14 inclusions of magnesiochromite still trapped inside 8 lithospheric diamonds, coming from the major kimberlite districts (Udachnaya mine, Siberia; Damtshaa mine, Botswana; Panda mine, Canada).

Diamonds are the deepest “fragments” of the primordial Earth that reach the Earth’s surface after... more Diamonds are the deepest “fragments” of the primordial Earth that reach the Earth’s surface after traveling a long path through the interior of our planet. This makes diamonds small time capsules, which reveal crucial information about the geological processes that occurred within the inaccessible mantle regions throughout the Earth’s history. Since diamond is almost pure carbon and chemically inert mineral, its age and crystallization environment within the Earth’s mantle have been constrained over the last 50 years from the study of its inclusions. Most of the early studies on diamonds assumed that these inclusions and diamonds are syngenetic, i.e., they crystallized simultaneously and from the same genetic process. However, recent studies have challenged the paradigm of syngenesis, suggesting that the major portion of minerals included in lithospheric diamonds, instead, were formed before diamond, and are called protogenetic. This discovery has implications for all genetic aspect...

European Journal of Mineralogy

We have investigated a suite of natural diamonds from the kimberlite pipe of the Changma Kimberli... more We have investigated a suite of natural diamonds from the kimberlite pipe of the Changma Kimberlite Belt, Mengyin County, Shandong Province, China, with the aim of constraining pressures and temperatures of formation. Here we report the non-destructive investigation of an olivine inclusion still entrapped within a lithospheric diamond by single-crystal X-ray diffraction. We were able to refine anisotropically its crystal structure to R 1 = 1.42 % using ionized scattering curves; this allows estimation of the composition of the olivine as Mg 1.82 Fe 0.18 SiO 4. This composition corresponds to a calculated unit-cell volume equal to V = 292.70 Å 3 at room temperature and pressure. We have validated the above-calculated composition and unit-cell volume by releasing the inclusion from the diamond host, resulting in a consistent composition calculated using nondestructive methods of Mg 1.84 Fe 0.16 SiO 4 and V = 292.80 ± 0.07 Å 3. Considering that the unit-cell volume of the olivine still inside its diamond host is V = 289.7 ± 0.2 Å 3 , we calculated a residual pressure P inc = 1.4 ± 0.1 GPa with respect to the released crystal and P inc = 1.3 ± 0.2 GPa with respect to the volume calculated from the "composition" indirectly retrieved by the structure refinement under ambient conditions. The two values of P inc overlap within experimental uncertainty. We performed Fourier transform infrared (FTIR) analysis on the diamond host in order to calculate its mantle residence temperature, T res , which resulted in a value of 1189 • C (for an assumed diamond age of 3 Ga) and 1218 • C (for an age of 1 Ga), with an average T res equal to 1204 ± 15 • C. Using the most up-to-date pressure-volume-temperature equations of state for olivine and diamond, the residual pressure P inc = 1.4 ± 0.1 GPa and average residence temperature of the diamond host T res = 1204 • C, we retrieved a pressure of entrapment P trap = 6.3 ± 0.4 GPa. Using the non-destructive approach and relative P inc = 1.3 GPa, we obtained a perfectly overlapping P trap = 6.2 GPa, within experimental uncertainty. This entrapment pressure corresponds to depths of about 190 ± 12 km. These results demonstrate that for high-quality crystal structure data measured on inclusions still trapped within diamond hosts, even a non-destructive approach can be used to calculate the depth of formation of diamond-olivine pairs. In terms of geological implications, the results from this work show that Changma diamonds formed under a conductive geotherm lying between 35 and 40 mW m −2 , at a depth of about 190 km. This value lies within the recently reported upper limit of the average depth of formation of worldwide lithospheric diamonds, which is 175 ± 15 km and is in agreement with P-T data obtained in the literature from kimberlite xenoliths.

The present study was carried out for investigating the growth relationships between magnesiochro... more The present study was carried out for investigating the growth relationships between magnesiochromite inclusions and its diamond hosts, in order to determine its genetic nature, expressed in terms of singenesis (simultaneous growth between diamonds and inclusions) or protogenesis (pre-existing inclusions with respect to the diamonds). The aim is to understand if magnesiochromite and diamond grow in epitaxy, namely according to a specific crystallographic relationship, through the analysis of the crystallographic orientation relationship between magnesiochromite and diamond determined by single crystal X-ray diffraction on a total of 14 inclusions of magnesiochromite still trapped inside 8 lithospheric diamonds, coming from the major kimberlite districts (Udachnaya mine, Siberia; Damtshaa mine, Botswana; Panda mine, Canada).

Diamonds are the deepest “fragments” of the primordial Earth that reach the Earth’s surface after... more Diamonds are the deepest “fragments” of the primordial Earth that reach the Earth’s surface after traveling a long path through the interior of our planet. This makes diamonds small time capsules, which reveal crucial information about the geological processes that occurred within the inaccessible mantle regions throughout the Earth’s history. Since diamond is almost pure carbon and chemically inert mineral, its age and crystallization environment within the Earth’s mantle have been constrained over the last 50 years from the study of its inclusions. Most of the early studies on diamonds assumed that these inclusions and diamonds are syngenetic, i.e., they crystallized simultaneously and from the same genetic process. However, recent studies have challenged the paradigm of syngenesis, suggesting that the major portion of minerals included in lithospheric diamonds, instead, were formed before diamond, and are called protogenetic. This discovery has implications for all genetic aspect...