Depth of Formation of Ferropericlase Included in Super-Deep Diamonds

Super-deep diamonds are believed to have formed at depths of at least 300 km depth (Harte, 2010). A common mineral inclusion in these diamonds is ferropericlase, (Mg,Fe)O (see Kaminsky, 2012 and references therein). Ferropericlase (fPer) is the second most abundant mineral in the lower mantle, comprising approximately 16\u201320 wt% (660 to 2900 km depth), and inclusions of fPer in diamond are often considered to indicate a lower-mantle origin (Harte et al., 1999). Samples from S\ue3o Luiz/Juina, Brazil, are noteworthy for containing nanometer-sized magnesioferrite (Harte et al., 1999; Wirth et al., 2014; Kaminsky et al., 2015; Palot et al., 2016). Based upon a phase diagram valid for 1 atm, such exsolutions would place the origin of this assemblage in the uppermost part of the lower mantle. However, a newly reported phase diagram for magnesioferrite demonstrates that the latter is not stable at such pressures and, thus, it cannot exsolve directly from fPer at lower-mantle conditions (Uenver-Thiele et al., 2017). Here we report the investigation of two fPer inclusions, extracted from a single S\ue3o Luiz diamond, by single-crystal X-ray diffraction and field emission scanning electron microscopy. Both techniques showed micrometer-sized exsolutions of magnesioferrite within the two fPers. We also completed elastic geobarometry (see Angel et al., 2015), which determined an estimate for the depth of entrapment of the two ferropericlase \u2013 diamond pairs. In the temperature range between 1273 and 1773 K, pressures varied between 9.88 and 12.34 GPa (325-410 km depth) for one inclusion and between 10.69 and 13.16 GPa (350-440 km depth) for the other one. These results strengthen the hypothesis that solitary fPer inclusions might not be reliable markers for a lower-mantle provenance

Sulfide breccias from the Semenov-3 hydrothermal field, Mid-Atlantic Ridge: authigenic mineral formation and trace element pattern

The aim of this paper is the investigation of the role of diagenesis in the transformation of clastic sulfide sediments such as sulfide breccias from the Semenov-3 hydrothermal field (Mid-Atlantic Ridge). The breccias are composed of marcasite\u2013pyrite clasts enclosed in a barite\u2013sulfide\u2013quartz matrix. Primary hydrothermal sulfides occur as colloform, fine-crystalline, porous and radial marcasite\u2013pyrite clasts with inclusions or individual clasts of chalcopyrite, sphalerite, pyrrhotite, bornite, barite and rock-forming minerals. Diagenetic processes are responsible for the formation of more diverse authigenic mineralization including framboidal, ovoidal and nodular pyrite, coarse-crystalline pyrite and marcasite, anhedral and reniform chalcopyrite, inclusions of HgS phase and pyrrhotite\u2013sphalerite\u2013chalcopyrite aggregates in coarse-crystalline pyrite, zoned bornite\u2013chalcopyrite grains, specular and globular hematite, tabular barite and quartz. The early diagenetic ovoid pyrite is enriched in most trace elements in contrast to late diagenetic varieties. Authigenic lower-temperature chalcopyrite is depleted in trace elements relative to high-temperature hydrothermal ones. Trace elements have different modes of occurrence: Se is hosted in pyrite and chalcopyrite; Tl is related to sphalerite and galena nanoinclusions; Au is associated with galena; As in pyrite is lattice-bound, whereas in chalcopyrite it is related to tetrahedrite\u2013tennantite nanoinclusions; Cd in pyrite is hosted in sphalerite inclusions; Cd in chalcopyrite forms its own mineral; Co and Ni are hosted in chalcopyrite

Crystallographic orientations of magnesiochromite inclusions in diamonds: what do they tell us?

We have studied by X-ray diffractometry the crystallographic orientation relationships (CORs) between magnesiochromite (mchr) inclusions and their diamond hosts in gem-quality stones from the mines Udachnaya (Siberian Russia), Damtshaa (Botswana) and Panda (Canada); in total 36 inclusions in 23 diamonds. In nearly half of the cases (n = 17), [111]mchr is parallel within error to [111]diamond, but the angular misorientation for other crystallographic directions is generally significant. This relationship can be described as a case of rotational statistical COR, in which inclusion and host share a single axis (1 df). The remaining mchr\u2013diamond pairs (n = 19) have a random COR (2 df). The presence of a rotational statistical COR indicates that the inclusions have physically interacted with the diamond before their final incorporation. Of all possible physical processes that may have influenced mchr orientation, those driven by surface interactions are not considered likely because of the presence of fluid films around the inclusions. Mechanical interaction between euhedral crystals in a fluid-rich environment is therefore proposed as the most likely mechanism to produce the observed rotational COR. In this scenario, neither a rotational nor a random COR can provide information on the relative timing of growth of mchr and diamond. Some multiple, iso-oriented inclusions within single diamonds, however, indicate that mchr was partially dissolved during diamond growth, suggesting a protogenetic origin of these inclusions

Fe-rich ferropericlase and magnesiow\ufcstite inclusions reflecting diamond formation rather than ambient mantle

At the core of many Earth-scale processes is the question of what the deep mantle is made of. The only direct samples from such extreme depths are diamonds and their inclusions. It is commonly assumed that these inclusions reflect ambient mantle or are syngenetic with diamond, but these assumptions are rarely tested. We have studied inclusion\u2013host growth relationships in two potentially superdeep diamonds from Juina (Brazil) containing nine inclusions of Fe-rich (XFe 480.33 to 650.64) ferropericlase-magnesiow\ufcstite (FM) by X-ray diffractometry, X-ray tomography, cathodoluminescence, electron backscatter diffraction, and electron microprobe analysis. The inclusions share a common [112] zone axis with their diamonds and have their major crystallographic axes within 3\ub0\u20138\ub0 of those of their hosts. This suggests a specific crystallographic orientation relationship (COR) resulting from interfacial energy minimization, disturbed by minor post-entrapment rotation around [112] due to plastic deformation. The observed COR and the relationships between inclusions and diamond growth zones imply that FM nucleated during the growth history of the diamond. Therefore, these inclusions may not provide direct information on the ambient mantle prior to diamond formation. Consequently, a \u201cnon-pyrolitic\u201d composition of the lower mantle is not required to explain the occurrence of Fe-rich FM inclusions in diamonds. By identifying examples of mineral inclusions that reflect the local environment of diamond formation and not ambient mantle, we provide both a cautionary tale and a means to test diamond-inclusion time relationships for proper application of inclusion studies to whole-mantle questions

Crystallographic relationships between diamond and its inclusions

The study of the crystallographic orientations of minerals included in diamonds can provide an insight into the mechanisms of their incorporation and the timing of their formation relative to the host diamond. The reported occurrence of non-trivial orientations for some minerals in some diamonds, suggesting an epitactic relationship, has long been considered to reflect contemporaneous growth of the diamond and the inclusion (= syngenesis). Correct interpretation of such orientations requires (i) a statistically significant data set, i.e. crystallographic data for single and multiple inclusions in a large number of diamonds, and (ii) a robust data-processing method, capable of removing ambiguities derived from the high symmetry of the diamond and the inclusion. We have developed software which performs such processing, starting from crystallographic orientation matrixes obtained by X-ray diffractometry. Preliminary studies indicate a wide variety of trends in the orientations of different inclusion phases in diamonds. In contrast to previous claims, olivine inclusions in lithospheric diamonds from Udachnaya do not show any preferred orientations with respect to their diamond hosts, but multiple inclusions in a single diamond often show very similar orientations within a few degrees (Nestola et al. 2014). Chromite (spinel) inclusions exhibit a strong tendency for a single (111) plane of each inclusion to be parallel to a (111) plane of their diamond host, but without any statistically significant orientation of the crystallographic axes a, b, and c. By contrast, 7 inclusions of ferropericlase studied in 2 different super deep diamonds (four inclusions in one diamond and three inclusions in the second diamond) from Brazil all exhibit the same orientation with their axes practically coincident with those of diamonds regardless of the position and the shape of the inclusions. The implications of these observations for the mechanisms of diamond growth will be explored

Diamond-inclusion system recording old deep lithosphere conditions at Udachnaya (Siberia)

Diamonds and their inclusions are unique fragments of deep Earth, which provide rare samples from inaccessible portions of our planet. Inclusion-free diamonds cannot provide information on depth of formation, which could be crucial to understand how the carbon cycle operated in the past. Inclusions in diamonds, which remain uncorrupted over geological times, may instead provide direct records of deep Earth’s evolution. Here, we applied elastic geothermobarometry to a diamond-magnesiochromite (mchr) host-inclusion pair from the Udachnaya kimberlite (Siberia, Russia), one of the most important sources of natural diamonds. By combining X-ray diffraction and Fourier-transform infrared spectroscopy data with a new elastic model, we obtained entrapment conditions, Ptrap = 6.5(2) GPa and Ttrap = 1125(32)–1140(33) °C, for the mchr inclusion. These conditions fall on a ca. 35 mW/m2 geotherm and are colder than the great majority of mantle xenoliths from similar depth in the same kimberlite. Our results indicate that cold cratonic conditions persisted for billions of years to at least 200 km in the local lithosphere. The composition of the mchr also indicates that at this depth the lithosphere was, at least locally, ultra-depleted at the time of diamond formation, as opposed to the melt-metasomatized, enriched composition of most xenoliths

Notulae to the Italian flora of algae, bryophytes, fungi and lichens: 7

In this contribution, new data concerning algae, bryophytes, fungi, and lichens of the Italian flora are presented. It includes new records and confirmations for the algae genus Chara, the bryophyte genera Cephalozia, Conardia, Conocephalum, Didymodon, Sphagnum, Tetraplodon, and Tortula, the fungal genera Endophyllum, Gymnosporangium, Microbotryum, Phragmidium, and Pluteus, and the lichen genera Candelariella, Cladonia, Flavoplaca, Lichenothelia, Peltigera, Placolecis, Rinodina, Scytinium, and Solenopsora

Notulae to the Italian flora of algae, bryophytes, fungi and lichens: 13

In this contribution, new data concerning bryophytes, fungi and lichens of the Italian flora are presented. It includes new records and confirmations for the bryophyte genera Bryum, Cryphaea, Didymodon, and Grimmia; the fungal genera Bryostigma, Cercidospora, Conocybe, Cortinarius, Endococcus, Inocybe, Psathyrella, and Sphaerellothecium; the lichen genera Agonimia, Anisomeridium, Bilimbia, Diplotomma, Gyalecta, Huneckia, Lecidella, Lempholemma, Myriolecis, Nephroma, Pannaria, Pycnothelia, Pyrrhospora, Rinodina, Stereocaulon, Thalloidima, Trapelia, Usnea, Variospora, and Verrucaria

Notulae to the Italian alien vascular flora 6

In this contribution, new data concerning the distribution of vascular flora alien to Italy are presented. It includes new records, confirmations, exclusions, and status changes for Italy or for Italian administrative regions of taxa in the genera Acalypha, Acer, Canna, Cardamine, Cedrus, Chlorophytum, Citrus, Cyperus, Epilobium, Eucalyptus, Euphorbia, Gamochaeta, Hesperocyparis, Heteranthera, Lemna, Ligustrum, Lycium, Nassella, Nothoscordum, Oenothera, Osteospermum, Paspalum, Pontederia, Romulea, Rudbeckia, Salvia, Sesbania, Setaria, Sicyos, Styphnolobium, Symphyotrichum, and Tradescantia. Nomenclature and distribution updates, published elsewhere, and corrigenda are provided as supplementary material