Horst R. Marschall

On the occurrence and boron isotopic composition of tourmaline in (ultra)high-pressure metamorphic rocks

Abstract: The extensive P–T stability and the high chemical variability of tourmaline (Tur) together with its common occurrence in metasediments proves its high potential for petrological and (isotope) geochemical studies on fluid–rock interaction in subduction- and collision-related rocks. This paper reviews the occurrence, major element chemistry and boron isotopic composition of Tur in high- and ultrahigh-pressure metamorphic (UHPM) rocks. In addition, it presents a new discovery of coesite-bearing Tur (schorl) from the Erzgebirge (Germany), as well as Tur (dravite) related to the retrograde history of coesite- and diamond-bearing rocks from the Erzgebirge and the Kokchetav Massif (Kazakhstan). The scarce data on worldwide occurrences of (U)HPM Tur reveal a high occupation of the crystallographic X-site (dominated by Na) and the possible presence of excess B, with little further distinctiveness in its major element composition when compared with Tur from medium-grade rocks. High K2O contents in Tur are probably not related to UHP growth or equilibration. The B isotopic composition of (U)HPM Tur ranges in δ11B from −16 to +1‰, with many samples in or below the range of continental crust. In contrast, Tur formed during retrograde fluid influx typically shows high δ11B values (up to +28‰), suggesting heavy-B fluids infiltrating the exhuming (U)HPM units. Coesite inclusions in Tur, characterized by Raman spectroscopy, are regarded as the best indicator for its UHP stability. Supplementary material: Analytical methods, tourmaline compositions and boron isotope values are available at http://www.geolsoc.org.uk/SUP18354.

Early accretion of water in the inner solar system from a carbonaceous chondrite–like source

History recorded in asteroids water Astronomers know that interstellar water is abundantly available to young planetary systems—our blue planet collected (or accreted) plenty of it. Still, the details of waters movement in the inner solar system are elusive. Sarafian et al. measured water isotopes in meteorite samples from the asteroid Vesta for clues to the timing of water accretion. Their samples have the same isotopic fingerprint of volatiles as both Earth and carbonaceous chondrites, some of the most primitive meteorites. The findings suggest that Earth received most of its water relatively early from chondrite-like bodies. Science, this issue p. 623 Similar volatile isotopes of Earth and ancient meteorites point to an early accumulation of water for terrestrial bodies. Determining the origin of water and the timing of its accretion within the inner solar system is important for understanding the dynamics of planet formation. The timing of water accretion to the inner solar system also has implications for how and when life emerged on Earth. We report in situ measurements of the hydrogen isotopic composition of the mineral apatite in eucrite meteorites, whose parent body is the main-belt asteroid 4 Vesta. These measurements sample one of the oldest hydrogen reservoirs in the solar system and show that Vesta contains the same hydrogen isotopic composition as that of carbonaceous chondrites. Taking into account the old ages of eucrite meteorites and their similarity to Earth’s isotopic ratios of hydrogen, carbon, and nitrogen, we demonstrate that these volatiles could have been added early to Earth, rather than gained during a late accretion event.

Tetrahedrally coordinated boron in tourmalines from the liddicoatite-elbaite series from Madagascar: Structure, chemistry, and infrared spectroscopic studies

Abstract Four colorless tourmalines of the liddicoatite-elbaite series from pegmatites from Anjanabonoina, Madagascar, have been characterized by crystal-structure determination and by chemical analyses. Optimized formulae range from X(Ca0.57Na0.29□0.14) Y(Al1.41Li1.33Mn2+0.07□0.19) ZAl6T(Si5.86B0.14)O18 (BO3)3V(OH)3.00W[F0.76(OH)0.24] [a = 15.8322(3), c = 7.1034(3) Å] to X(Na0.46Ca0.30□0.24) Y(Al1.82Li0.89Fe2+0.01 Mn2+0.01□0.27) ZAl6T(Si5.56B0.44)O18 (BO3)3V(OH)3.00W[(OH)0.50F0.50] [a = 15.8095(9), c = 7.0941(8) Å] (R = 1.3.1.7%). There is a high negative correlation (r2 = 0.984) between the bond-lengths (~1.618.1.614 Å) and the amount of IVB (from the optimized formulae). Similar to the olenites (from Koralpe, Austria) the liddicoatite-elbaite samples show a positive correlation between the Al occupancy at the Y site and IVB (r2 = 0.988). Short-range order configurations show that the presence of IVB is coupled with the occupancy of (Al2Li) and (Al2□) at the Y site. The structural formulae of the Al-rich tourmalines from Anjanabonoina, Madagascar, show ~ ⃞0.2 (vacancies) on the Y site. We believe that short-range order configurations with Y(Al2□) are responsible for these vacancies. Hence, an oft-used calculation of the Li content by difference on the Y site may be problematic for Al-rich tourmalines (olenite, elbaite, rossmanite). Fourier transform infrared (FTIR) spectra were recorded from the most IVB-rich tourmaline sample. The bands around 5195 and 5380 cm-1 can be assigned to H2O. Because these bands still could be observed in FTIR spectra at temperatures from -150 to +600 °C, it seems unlikely that they result from H2O in fluid inclusions. Interestingly, another FTIR spectrum from a dravite in which the X site is filled completely with Na, does not show bands at ~5200 and ~5400 cm-1. Although not definitive, the resulting spectra are consistent with small amounts of H2O at the X site of the elbaite. The rare-earth element (REE) pattern of the B-rich elbaite (ΣREE: ~150 ppm) demonstrates that this sample is strongly enriched in LREEs compared to HREEs and exhibits a negative Eu anomaly. This sample shows the strongest enrichment of LREEs and a high LaN/YbN ratio of ~351, which seems to confirm an important role of the fractional crystallization process.

Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions

Abstract Tourmaline grains extracted from rocks within three ultrahigh-pressure (UHP) metamorphic localities have been subjected to a structurally and chemically detailed analysis to test for any systematic behavior related to temperature and pressure. Dravite from Parigi, Dora Maira, Western Alps (peak P-T conditions ~3.7 GPa, 750 °C), has a structural formula of X(Na0.90Ca0.05K0.01⃞0.04) Y(Mg1.78Al0.99Fe2+0.12Ti4+0.03⃞0.08)Z(Al5.10Mg0.90)(BO3)3TSi6.00O18V(OH)3W[(OH)0.72F0.28]. Dravite from Lago di Cignana, Western Alps, Italy (~2.7-2.9 GPa, 600-630 °C), has a formula of X(Na0.84Ca0.09K0.01⃞0.06)Y(Mg1.64Al0.79Fe2+0.48Mn2+0.06Ti4+0.02Ni0.02Zn0.01)Z(Al5.00Mg1.00)(BO3)3T(Si5.98Al0.02)O18V(OH)3W[(OH)0.65F0.35]. “Oxy-schorl” from the Saxonian Erzgebirge, Germany (≥4.5 GPa, 1000 °C), most likely formed during exhumation at >2.9 GPa, 870 °C, has a formula of X(Na0.86Ca0.02K0.02⃞0.10)Y(Al1.63Fe2+1.23Ti4+0.11Mg0.03Zn0.01) Z(Al5.05Mg0.95)(BO3)3T(Si5.96Al0.04)O18V(OH)3W[O0.81F0.10(OH)0.09]. There is no structural evidence for significant substitution of [4]Si by [4]Al or [4]B in the UHP tourmaline ( distances ~1.620 Å), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high-T-low-P tourmaline, which typically has significant amounts of [4]Al. There is an excellent positive correlation (r2 = 1.00) between total [6]Al (i.e., YAl + ZAl) and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a negative correlation (r2 = 0.97) between F content and the temperature conditions of UHP tourmaline formation and between F and YAl content (r2 = 1.00) that is best explained by the exchange vector YAlO(R2+F)-1. This is consistent with the W site (occupied either by F, O, or OH), being part of the YO6-polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature.

Metamorphic Na- and OH-rich disordered dravite with tetrahedral boron, associated with omphacite, from Syros, Greece: chemistry and structure

Metamorphic Fe-bearing dravite from a glaucophane schist from Syros, Greece, associated with omphacite, has been characterized by chemical analyses (EMPA, SIMS, Mossbauer study) and by crystal structure determination. The optimized formula, calculated using chemical and structural data (including from Mossbauer spectroscopy) is x (Na 0.96 Ca 0.02 □ 0.02 ) Y (Mg 1.29 Al 0.99 Fe 2+ 0.44 Fe 3+ 0.19 Ti 0.05□ 0.04 ) Z (Al 4.90 Mg 1.10 ) T (Si 5.83 B 0.17 ) B 3 O 27 [(OH) 3.93 F 0.07 ], with a = 15.9443(3), c = 7.2094(3) A, R = 0.017. The OH- content is nearly 4 apfu , thus there is no significant O 2− at the V and W site. The X site is nearly completely filled with Na, contrary to most natural tourmalines. We conclude that tourmaline samples from the dravite-schorl series where the O1 site ( W site) is mainly occupied by OH- and/or F, but not by O 2− , favour an X -site occupation with Na. Mg and Al are strongly disordered in this tourmaline sample. This Mg-rich tourmaline is an unusual example of Al-Mg disorder which is not driven by the short-range requirements of O 2− at the O 1 site. Surprisingly, we found small amounts of [4] B in this Mg-rich dravite. This is confirmed by the chemical analysis (including light elements), by the refinement (∼ 0.26 [4] B apfu ), as well as by the relatively small T -O> distance of 1.6179 A. This is the first known example of a Mg-rich tourmaline which contains significant amounts of [4] B. The formation of this dravite (at PT conditions of ∼ 6 to 7 kbar/∼ 400°C) took place in a subduction-exhumation environment at undersaturated SiO 2 -conditions.

Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so

Detrital zircon grains are employed to decipher sediment provenances and crustal evolution, and they provide unique evidence of Hadean crust-mantle differentiation processes. We demonstrate that mineral inclusions in zircon provide valuable information on the conditions under which zircon crystallized. Zircon grains from selected plutonic rocks from Dronning Maud Land, Antarctica, contain inclusions of apatite, biotite, amphibole, and pyroxenes that accurately reflect the chemical compositions of the equivalent phases in the host-rock matrix, and the compositions of the whole rocks. High concentrations of Y and low concentrations of Sr in apatite inclusions in zircon are diagnostic of evolved, felsic granitoid host rocks. In contrast, the relative abundances and compositions of plagioclase and alkali feldspar inclusions in zircon are decoupled from the composition of the whole rock, and they are generally indicative of granitic melts regardless of the bulk rock. This is best explained by the late crystallization of zircon relative to the bulk of the feldspars. We conclude that inclusions of apatite and mafic phases in zircon constrain the potential source rocks of detrital zircon, whereas feldspar inclusions do not.

Geochemical evidence for mélange melting in global arcs

Arc lavas form from melting of mélange rocks; sediment melts and slab-derived fluids are not major contributors. In subduction zones, sediments and hydrothermally altered oceanic crust, which together form part of the subducting slab, contribute to the chemical composition of lavas erupted at the surface to form volcanic arcs. Transport of this material from the slab to the overlying mantle wedge is thought to involve discreet melts and fluids that are released from various portions of the slab. We use a meta-analysis of geochemical data from eight globally representative arcs to show that melts and fluids from individual slab components cannot be responsible for the formation of arc lavas. Instead, the data are compatible with models that first invoke physical mixing of slab components and the mantle wedge, widely referred to as high-pressure mélange, before arc magmas are generated.

Development and evolution of detachment faulting along 50 km of the Mid‐Atlantic Ridge near 16.5°N

A multifaceted study of the slow spreading Mid-Atlantic Ridge (MAR) at 16.5°N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along ∼50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, noncorrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal a new detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multiscale corrugations are observed 2–3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low-angle (6°) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust >2 Ma, indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two-sided detachment faulting.

Fluorapatite-monazite relationships in granulite-facies metapelites, Schwarzwald, southwest Germany

Abstract Fluorapatite grains with monazite inclusions and/or rim grains are described in two of four samples from a set of granulite-facies metapelites collected from the Variscan Schwarzwald, southern Germany. Fluorapatite in all four samples appears to have experienced some dissolution in the partial granitic melt formed during granulite-facies metamorphism. Monazite inclusions and rim grains are highly deficient in Th and are presumed to have formed from fluorapatite in association with partial melting during granulite-facies metamorphism. Monazite inclusions range from very small (<1 μm) and very numerous to small (1-2 μm), sometimes elongated, and less numerous; both types are evenly distributed throughout the fluorapatite grain interior. Monazite rim grains tend to be 1 - 10 μm. The formation of monazite inclusions is proposed to be due to dissolution-reprecipitation of the fluorapatite by the aqueous fluids inherent in the granitic melt. We propose that an increase in inclusion size coupled with a decrease in inclusion number is due to Ostwald ripening (interfacial energy reduction), which is greatly facilitated by the presence of an interconnected, fluid-filled porosity in the metasomatized fluorapatite. We further propose that monazite rim grains formed principally during partial dissolution of the fluorapatite in the granitic melt and to a lesser extent by partial dissolution- reprecipitation of the fluorapatite grain rim area allowing for the partial removal of (Y+REE). We conclude that fluorapatite, with monazite inclusions and rim grains, experienced partial dissolution in a H2O-rich peraluminous granitic melt compared to fluorapatite with monazite rim grains and no inclusions which reacted with a similar, relatively less H2O-rich melt. In contrast, monazite-free fluorapatite experienced partial dissolution in a comparatively H2O-poor, subaluminous, possibly peralkaline melt.

A secondary ion mass spectrometry (SIMS) re-evaluation of B and Li isotopic compositions of Cu-bearing elbaite from three global localities

Abstract Cu-bearing elbaite from Paraíba (Brazil) is a highly-prized gem tourmaline. Specimens of similar quality from localities in Mozambique and Nigeria are being sold, and reliable provenance tools are required to distinguish specimens from the original locality from ‘Paraíba-type’ tourmaline from Africa. Here we present Li and B isotope analyses of Cu-bearing elbaite from all three localities and demonstrate the suitability of these isotope systems as a provenance tool. Isotopic profiles across chemically zoned grains revealed homogenous B and Li isotopic compositions, demonstrating a strong advantage of their application as a provenance tool as opposed to major, minor or trace element signatures. Li and B isotopes of all investigated samples of Cu-bearing elbaites from the three localities are within the range of previously published granitic and pegmatitic tourmaline. Anomalous isotope compositions published previously for these samples are corrected by our results.