Marcel Bohn

Origin of anomalous rare-earth element and yttrium enrichments in subaerially exposed basalts: Evidence from French Polynesia

Abstract Basalts from French Polynesian islands occasionally display extremely high abundances and anomalous distributions of rare-earth elements (REE) and yttrium, whereas other incompatible element concentrations and O, Sr, Nd and Pb isotopic ratios do not differ from those of “normal” basalts from the same area. The REE- and Y-enriched basalts contribute up to 15% of the sample set, suggesting that this feature is more widespread than previously thought. REE-Y enrichment is related to the presence of rhabdophane-type REE-Y-phosphate identified through electron microprobe analyses in the most enriched sample and inferred from leaching experiments in the others. This phenomenon is confined to subaerially exposed basaltic sequences, indicating a close relationship to supergene processes. This is supported by negative Ce anomalies in these basalts, since decoupling of Ce from the other REE is restricted to oxidizing, low-temperature, aqueous environments. Similar Nd isotopic ratios for enriched and normal basalts allow us to exclude the possibility that additional REE and Y are derived from marine sediments or guano, but rather suggest an origin from the local basalts. Moreover, light REE enrichment in the REE-Y-phosphates suggests short migration distances of the fluids, supporting the conclusion that additional REE and Y were mobilized from weathered basalts and transported by descending meteoric waters.

Influence of ontogenetic and environmental changes in the otolith microchemistry of juvenile sole (Solea solea)

We analysed otolith composition, specifically Sr/Ca ratios, with the aim of determining whether metamorphosis and the transition to benthic life of Bay of Biscay sole occur in marine or estuarine conditions. Otoliths of wild juveniles (0-group) collected in two estuarine nurseries showed characteristic Sr/Ca chronologies, with a significant decrease of the Sr/Ca ratio from the core to the accessory growth centres. As the otolith Sr/Ca ratio decreases in respect of the ambient salinity, this could suggest a relative synchronism in the timing of estuarine nursery entrance. In order to validate this hypothesis, hatchery-produced larvae were reared in a tidal marsh mesocosm until they completed metamorphosis (around 1 month post-hatching) and settled on the bottom. The otoliths from these juveniles exhibited a similar Sr/Ca pattern, which could not be explained as a result of habitat changes, and especially salinity variations, but rather reflected an ontogenetic signal. Since both wild and reared soles achieved high growth rates during metamorphosis, this suggests a link between a high metabolic rate and the observed Sr/Ca drop. In addition, ontogenetic changes during this transitional phase could involve the setting of functional mechanisms, responsible for the regulation of free Sr 2+ in either plasma or endolymph or both. Nevertheless, this common trend appeared to be modulated by environmental cues: Sr/Ca ratios were significantly higher for reared fish compared to wild fish during the larval period. A possible explanation is that experienced temperatures were much higher in the incubator than in the field. Furthermore, Sr/Ca values obtained in the otolith juvenile area, with the lowest ratios for soles from the Loire nursery, suggest that the salinity regime of estuarine nurseries could exert an environmental control on otolith Sr/Ca ratios, thus providing ecological records of up-estuary migration after metamorphosis.

Geodiversity of Hydrothermal Processes Along the Mid‐Atlantic Ridge and Ultramafic‐Hosted Mineralization: a New Type Of Oceanic Cu‐Zn‐Co‐Au Volcanogenic Massive Sulfide Deposit

OS21C-08. Eberhart, G. L., P. A. Rona, and J. Honnorez (1989), Geologic controls of hydrothermal activity in the Mid-Atlantic Ridge rift valley; tectonics and volcanics, Mar. Geophys. Res., 10(3-4), 233–259. Edmond, J. M., A. C. Campbell, M. R. Palmer, G. P. Klinkhammer, C. R. German, H. N. Edmonds, H. Elderfield, G. Thompson, and P. Rona (1995), Time series studies of vent fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge: a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide orebodies, in Hydrothemal Vents and Processes, edited by L. M. Parson et al., pp. 77–86, Geol. Soc., London. Edmonds, H. N., P. J. Michael, E. T. Baker, D. P. Connelly, J. E. Snow, C. H. Langmuir, H. J. B. Dick, R. Muhe, C. R. German, and D. W. Graham (2003), Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic, Nature, 421(6920), 252–256. Elderfield, H., et al. (1993), Preliminary geochemical results from the Broken Spur hydrothermal field, 29° N, Mid-Atlantic Ridge, Eos Trans. AGU, 74(43), Fall Meet. Suppl., 99. Escartín, J., and M. Cannat (1999), Ultramafic exposures and the gravity signature of the lithosphere near the Fifteen-Twenty fracture zone (Mid-Atlantic Ridge, 14°–16.5° N), Earth Planet. Sci. Lett., 171(3), 411–424. Escartín, J., and J. Lin (1998), Tectonic modification of axial crustal structure; evidence from spectral analyses of residual gravity and bathymetry of the Mid-Atlantic Ridge flanks, Earth Planet. Sci. Lett., 154(1-4), 279–293. Escartín, J., D. K. Smith, J. Cann, H. Schouten, C. H. Langmuir, and S. Escrig (2008), Central role of detachment faults in accretion of slow-spreading oceanic lithosphere, Nature, 455(7214), 790–794. Fouquet, y. (1997), Where are the large hydrothermal sulphide deposits in the oceans?, Philos. Trans. R. Soc. London, Ser. A, 355(1723), 427–440. Fouquet, y., et al. (1993a), Sulfide mineralizations associated with ultramafic rocks on the MAR near 15° 20′N, Terra Nova Abstr., 5, suppl. 1, 444–445. Fouquet, y., U. von Stackelberg, J. L. Charlou, J. Erzinger, P. M. Herzig, R. Muehe, and M. Wiedicke (1993b), Metallogenesis in back-arc environments; the Lau Basin example, Econ. Geol., 88(8), 2150–2177. Fouquet, y., A. Wafik, P. Cambon, C. Mevel, G. Meyer, and P. Gente (1993c), Tectonic setting and mineralogical and geochemical zonation in the Snake Pit sulfide deposit (Mid-Atlantic Ridge at 23° N), Econ. Geol., 88(8), 2014–2032. Fouquet, Y., J. L. Charlou, I. Costa, J. P. Donval, J. Radford-Knoery, H. Pellé, H. Ondréas, N. Lourenço, M. Ségonsac, and M. KingstonTivey (1994), A detailed study of the Lucky Strike hydrothermal site and discovery of a new hydrothermal site: Menez Gwen. Preliminary results of the DIVA1 cruise (5–29 May, 1994), InterRidge News, 3(2), 14–18. Fouquet, Y., H. Ondréas, J. L. Charlou, J. P. Donval, J. RadfordKnoery, I. Costa, N. Lourenço, and M. K. Tivey (1995), Atlantic lava lakes and hot vents, Nature, 377, 201. Fouquet, Y., R. Knott, P. Cambon, A. Fallick, D. Rickard, and D. Desbruyeres (1996), Formation of large sulfide mineral deposits along fast spreading ridges; example from off-axial deposits at 12° 43′N on the East Pacific Rise, Earth Planet. Sci. Lett., 144(1-2), 147–162. Fouquet, y., et al. (1997), Discovery and first submersible investigations on the Rainbow Hydrothermal Field on the MAR (36°14N), Eos Trans. AGU, 78(46), Fall Meet. Suppl., F832. Fouquet, y., et al. (1998a), FLORES diving cruise with the Nautile near the Azores. First dives on the Rainbow field: Hydrothermal seawater/mantle interaction, InterRidge News, 7(1), 24–28. Fouquet, y., K. Henry, R. Knott, and P. Cambon (1998b), Geochemical section of the TAG hydrothermal mound, in TAG: Drilling an Active Hydrothermal System on a Sediment-Free 362 ULTRAMAFIC-HOSTED SULFIDE MINERALIZATION ALONG THE MAR Slow-Spreading Ridge, edited by P. M. Herzig et al., Proc. Ocean Drill. Program Sci. Results, 158, 363–388. Fouquet, y., et al. (2000), Hydrothermal processes in oceanic ultramafic environments; the Rainbow hydrothermal sulfide deposit, paper presented at 31st International Geological Congress, Int. Union of Geol. Sci., Rio de Janeiro, Brazil. Fouquet, Y., G. Cherkashov, J. L. Charlou, H. Ondréas, M. Cannat, N. Bortnikov, S. Silantyev, J. Etoubleau, and P. Serpentine (2007), Diversity of ultramafic hosted hydrothermal deposits on the Mid Atlantic Ridge; first submersible studies on Ashadze, Logatchev 2 and Krasnov vent fields during the Serpentine cruise, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract T51F-03. Fouquet, y., et al. (2008), Serpentine cruise–ultramafic hosted hydrothermal deposits on the Mid Atlantic Ridge: First submersible studies on Ashadze 1 and 2, Logatchev 2 and Krasnov vent fields, InterRidge News, 18, 15–19. Fournier, R. O., R. J. Rosenbauer, and J. L. Bischoff (1982), The Solubility of quartz in aqueous sodium chloride solution, Geochim. Cosmochim. Acta, 46, 1975–1978. Francheteau, J., et al. (1979), Massive deep-sea sulfide ore deposits discovered on the East Pacific Rise, Nature, 277, 523–528. Fruh Green, G. L., D. S. Kelley, S. M. Bernasconi, J. A. Karson, K. A. Ludwig, D. A. Butterfield, C. Boschi, and G. Proskurowski (2003), 30,000 years of hydrothermal activity at the Lost City vent field, Science, 301(5632), 495–498. Gaal, G., and J. Parkkinen (1993), Early Proterozoic ophiolitehosted copper-zinc-cobalt deposits of the Outokumpu type, in Mineral Deposit Modeling, edited by R. V. Kirkham et al., pp. 335–341, Geol. Assoc. of Canada, Toronto, Ont., Canada. Gablina, I. F., N. N. Mozgova, y. S. Borodaev, T. V. Stepanova, G. A. Cherkashev, and M. I. Il’in (2000), Copper sulfide associations in recent oceanic ores of the Logatchev hydrothermal field (Mid-Atlantic Ridge, 14° 45′ N), Geol. Ore Deposits, 42(4), 296–316. Gallant, R. M., and K. L. Von Damm (2006), Geochemcial controls on hydrothermal fluids from the Kairei and Edmond vent fields, 23°–25° S, Central Indian Ridge, Geochem., Geophys., Geosyst., 7, Q06018, doi:10.1029/2005GC001067. German, C. R., and J. Lin (2004), The thermal structure of the oceanic crust, ridge-spreading and hydrothermal circulation: How well do we understand their inter-connections?, in Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans, Geophys. Monogr. Ser., vol. 148, edited by C. R. German, J. Lin, and L. M. Parson, pp. 1–18, AGU, Washington, D. C. German, C. R., and L. M. Parson (1998), Distributions of hydrothermal activity along the Mid-Atlantic Ridge; interplay of magmatic and tectonic controls, Earth Planet. Sci. Lett., 160(3-4), 327–341. German, C. R., et al. (1994), Hydrothermal activity on the Reykjanes Ridge: The Steinahóll vent-field at 63°06′N, Earth Planet. Sci. Lett., 121, 647–654. German, C. R., et al. (1999), A segment scale study of fluxes through the Rainbow hydrothermal plume, 36°N Mid-Atlantic Ridge, Eos Trans. AGU, 80(46), Fall Meet. Suppl., F957– F958. Gibson, H. L., R. L. Morton, and G. J. Hudak (1999), Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits, in Volcanic-Associated Massive Sulfide Deposits; Processes and Examples in Modern and Ancient Settings, edited by C. T. Barrie and M. D. Hannington, Rev. Econ. Geol., 8, 13–51. Goodfellow, W. D., and J. M. Franklin (1993), Geology, mineralogy, and chemistry of sediment-hosted clastic massive sulfides in shallow cores, Middle Valley, northern Juan de Fuca Ridge, Econ. Geol., 88(8), 2037–2068. Gracia, E., D. Bideau, R. Hekinian, Y. Lagabrielle, and L. M. Parson (1997), Along-axis magmatic oscillations and exposure of ultramafic rocks in a second-order segment of the Mid-Atlantic Ridge (33° 43′N to 34° 07′N), Geology, 25(12), 1059–1062. Gracia, E., J. L. Charlou, J. Radford Knoery, and L. M. Parson (2000), Non-transform offsets along the Mid-Atlantic Ridge south of the Azores (38° N–34° N): Ultramafic exposures and hosting of hydrothermal vents, Earth Planet. Sci. Lett., 177(1-2), 89–103. Haase, K. M., et al. (2007), young volcanism and related hydrothermal activity at 5° S on the slow-spreading southern Mid-Atlantic Ridge, Geochem., Geophys., Geosyst., 8, Q11002, doi:10.1029/ 2006GC001509. Halbach, P., et al. (1989), Probable modern analogue of Kurokotype massive sulphide deposits in the Okinawa Trough back-arc basin, Nature, 338(6215), 496–499. Halbach, P., B. Pracejus, and A. Maerten (1993), Geology and mineralogy of massive sulfide ores from the central Okinawa Trough, Japan, Econ. Geol., 88(8), 2210–2225. Halls, C., and R. Zhao (1995), Listvenite and related rocks: Perspectives on terminology and mineralogy with reference to an occurrence at Cregganbaun, Co. Mayo, Republic of Ireland, Mineral. Dep., 30, 303–313. Hannington, M., P. Herzig, S. Scott, G. Thompson, and P. Rona (1991), Comparative mineralogy and geochemistry of goldbearing sulfide deposits on the mid-ocean ridges, Mar. Geol., 101(1-4), 217–248. Hannington, M., et al. (2001), First observations of high-temperature submarine hydrothermal vents and massive anhydrite deposits off the north coast of Iceland, Mar. Geol., 177(3-4), 199–220. Hannington, M. D., and S. D. Scott (1988), Gold mineralisation in volcanogenic massive sulphides; modern and ancient, in Bicentennial Gold; 1988; Extended Abstracts; Oral Programme, edited by A. D. T. Goode and L. I. Bosma, pp. 353–358, Geol. Soc. of Aust., Sydney, N.S.W., Australia. Hannington, M. D., and S. D. Scott (1989), Sulfidation equilibria as guides to gold mineralization in volcanogenic massive sulfides; evidence from sulfide mineralogy and the composition of sphalerite, Econ. Geol., 84(7), 1978–1995. Hannington, M. D., G. Thompson, P. A. Rona, and S. D. Scott (1988), Gold and native copper in supergene sulphides from the Mid-Atlantic Ridge, Nature, 333(6168), 64–66. Hannington, M. D., I. R. Jonasson, P. M. Herzig, and S. Petersen (1995), Physical and chemical processes of se

Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA 1240): An HED parent body impact melt

NWA 1240 is an unusual eucrite recently recovered in Morocco as a single stone of 98 g. It is an unbrecciated greenish-brown rock nearly devoid of fusion crust. It displays porphyritic texture consisting of skeletal hollow low-Ca pyroxene phenocrysts set in a variolitic (fan-spherulitic) mesostasis of fine elongate pyroxene and plagioclase crystals. Minor phases are skeletal chromite, iron, silica, troilite, ilmenite and minute amounts of phosphate and fayalite. Pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a eucrite, from En(76.0)Wo(1.9)FS(22.1) to compositions nearly devoid of Mg (unusual ferrosilite and Fe-augite symplectites and possibly pyroxferroite). Plagioclase crystals contain significant amounts of Fe and Mg, which are possibly controlled by the Ca(Mg,Fe2+)Si3O8 plagioclase component.

Posteucritic magmatism on Vesta: Evidence from the petrology and thermal history of diogenites

We report on the petrology and the thermal histories of 13 diogenites in order to constrain the formation processes of the Vestan crust. We classify diogenites into unequilibrated and equilibrated diogenites in a scheme similar to that for basaltic eucrites. Pyroxenes in unequilibrated diogenites are chemically zoned, indicating that they crystallized rapidly from melts and escaped from global crustal metamorphism. The presence of unequilibrated diogenites casts doubt on the fact that all the diogenites formed at depth in the parent body, as commonly thought. Some diogenites probably crystallized in shallow intrusions or were extruded on the surface. These facts strengthen the geochemical evidence that diogenites and eucrites are not directly cogenetic and suggest that at least some diogenites have intruded the early formed eucritic crust. Thus, diogenites are certainly not the products of the crystallization of the magma ocean that triggered the differentiation of Vesta but are more likely cumulates associated with a later stage of magmatism. Furthermore, the intrusion of diogenites could have significantly thickened the early formed crust, making it difficult to excavate deep‐seated olivine mantle by moderate impact events.

Phase equilibria in the systems TiCN, ZrCN and HfCN

Abstract The phase equilibria at 1150 °C in the solid regions of the systems TiCN, ZrCN and HfCN were investigated by means of arc-melted as well as hot-pressed alloys which were analysed for nitrogen and carbon and investigated by X-ray diffraction, metallography and electron probe microanalysis. In the TiCN system the δ-Ti(CxN1−x)1−y phase has a broader homogeneity range with respect to the ( [C] + [N]) [Ti] ratio than do the f.c.c. boundary phases TiN1−y and TiC1−y. The subnitride phase ζ-Ti4N3−x does not perceptibly dissolve any carbon. In the ZrCN system the metal-rich phase boundary of the δ-Zr(CxN1−x)1−y phase follows a nearly straight line between ZrC1−x and ZrN1−x. Phase diagrams of the TiCN and ZrCN systems are presented. Owing to the low diffusion rates carbon and nitrogen and the resulting inhomogeneous samples, the HfCN system could be delineated but not clarified with respect to the phase equilibria where the hafnium subnitride phases are involved. Similarly to the δ-Ti(CxN1−x)1−y phase, the δ-Hf(CxN1−x)1−y phase has a broader homogeneity range than HfC1−y and HfN1−y have.

Chemical fluxes during hydrothermal alteration of a 1200-m long section of dikes in the oceanic crust, DSDP/ODP Hole 504B

Abstract Chemical interactions between seawater and the oceanic crust have been widely investigated during recent years. However, most of these studies concern the uppermost volcanic part of the crust. The contribution of the underlying sheeted dike complex to the global budget of the oceans is inferred solely from some ophiolite studies and from the 500-m high-level dike section of DSDP/ODP 504B which was drilled in 1981. Hole 504B is the only place where a continuous and long (1260 m) section in the sheeted dike complex has been cored, and it is now regarded as a reference section for the upper oceanic crust. Many petrological and chemical data from these dolerites are available, including the relative proportions of veins, extensively altered adjacent rocks, and less altered “host-rocks”. For these three reasons, considering the entire dike section penetrated by Hole 504B is a unique chance to study chemical fluxes related to hydrothermal alteration of this part of the oceanic crust. The calculation of any chemical flux implies knowledge of the chemical composition of the fresh precursor (protolith). Previously, mean compositions of glasses (=P1a) or basalts from the Hole 504B volcanics have been used as protoliths. In this paper, we calculate and discuss the use of various protoliths based on dolerites from Hole 504B. We show that the most adequate and realistic protolith is the mean of individual protoliths that we calculated from the acquisition, by automatic mode, of about 1000 microprobe analyses in each thin-section of dolerite from the Hole 504B lower dikes. Consequently, PFm is further used to calculate chemical fluxes in the dike section of Hole 504B. The chemical compositions of the host-rocks adjacent to alteration halos tend to converge to that of PFm with depth, except for Fe2O3t and TiO2. Because the volume percent of alteration halos increases with depth, the total fluxes related to these halos increase with depth. This explains why the mean flux (host-rocks+halos+veins) of the upper dikes is roughly similar to the mean flux of the lower dikes. During the alteration of the entire Hole 504B dike section, the dolerites gained relatively large quantities of Fe2O3t (+4.0 g/100 cm3) and released much SiO2 (−6.8 g/100 cm3), CaO (−5.8 g/100 cm3), and TiO2 (1.6 g/100 cm3), and minor Al2O3 (−0.7 g/100 cm3) and MgO (−0.7 g/100 cm3). We show the importance of the choice of the protolith in the calculation of chemical budget, particularly for elements showing low flux values. In Hole 504B, the Mg uptake by the volcanics during low temperature alteration added to the Mg release by the dikes gives a net flux of −0.07×1014 g/year. We propose that part of the Mg uptake by the oceanic crust, which is necessary to compensate the rivers input (−1.33×1014 g/year), occurs in the underlying gabbros and/or in sections which are altered such as Trinity and Troodos ophiolites. Compared with ophiolites, fluxes calculated for elements other than Mg for the entire crust are generally similar (in tendency, if not in absolute value) to that we obtained from Hole 504B.

Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low-temperature to hydrothermal alteration

Hydroschorlomite, a Ti-, Ca-, Fe-rich andraditic garnet present in the deepest cores of basalts (661–749 mbsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro-Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5–50 μm) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100°C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5–26.5 wt%), TiO2 (22.0–28.6 wt%), and FeOt (6.2–12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2–97.3 wt%) is confirmed by the OH vibration at 3557 cm−1 in the micro-Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny (<10 μm) skeletal titanomagnetite. The occurrence of hydroschorlomite in Hole 1256D basalts coincides with a general downward increase in temperatures and overall intensity of alteration manifest by the alteration of plagioclase and the occurrence of small amounts of mixed-layer chlorite-smectite. The titanium necessary to form hydroschorlomite is provided by the breakdown of primary tiny (<10 μm) titanomagnetite, while calcium is provided by the replacement of plagioclase by albite. Hydroschorlomite is thus an indicator of alteration of titanomagnetite under conditions transitional from low-temperature alteration to hydrothermal metamorphism with formation of titanite and may affect magnetic properties of the rocks.

Electron-probe microanalysis of light elements in multiphase diffusion couples

Wavelength-dispersive electron-probe microanalysis of transition metal carbide and nitride diffusion couples was investigated as a tool to quantitatively determine portions of phase diagrams. In crosssections, diffusion couples generally show a sequence of several phases composed of crystallites with different orientations and hardness values. Thus, the polishing procedure can adversely affect the accuracy of the analysis due to creation of an uneven surface (relief effect). In addition, as a result of the polishing process the edges of the samples are rounded so that the concentration at the original sample surface is difficult to measure. The most rapid and straightforward procedure to detect influences of the relief effect is a plot of the non-normalized analytical total vs. distance, even where the concentration profile is discontinuous at the interface between phase bands. It is shown that careful experimental preparation, as well as the application of restricted diffusion geometry, has several advantages for establishing phase diagrams.

High-temperature reactive phase formation in the Nb-N system

Abstract The formation of niobium nitrides was investigated in the temperature range 1400–1800°C by reaction of niobium sheet with nitrogen (1–30 bar N2). Equilibrated samples yielded the p−T−[N]/[Nb] relationship of δ-NbN1−x: [N]/[Nb]=1.585−4.148·10−4·T−0.05176·ln(pN2)+4.984·10−5·T·ln(pN2). In diffusion couples the formation of the phases δ-NbN1−x, γ-Nb4N3±x and β-Nb2N was observed. Between δ-NbN1−x and γ-Nb4N3±x a concentration discontinuity was absent, which is due to the fact that γ-Nb4N3±x is not stable in the investigated temperature range but forms upon cooling. This is visible by the typical hatched microstructure at the nitrogen-poor boundary of δ-NbN1−x. The c/a ratio of the tetragonal cell of γ-Nb4N3±x increases with increasing nitrogen content. The homogeneity ranges of phases were determined from the diffusion bands by wavelength-dispersive EPMA. A portion of the phase diagram is presented. The thickness ratio of the β-Nb2N phase band vs. the δ-NbN1−x/γ-Nb4N3±x phase band was found to be dependent on the applied nitrogen pressure. This is due to the different surface concentration of nitrogen in the δ-NbN1−x phase and the influence of the homogeneity range on the layer growth rate.