Poster abstracts

Poster session: Fri 7th July, 16:30 – 18:00
(Geopolis, UNIL)

D/H composition and hydrogen content in pyroxenes: on-going developments of new reference materials

Piani L.1, Thomassin D.1, Rigaudier T.1
1CRPG, CNRS-Université de Lorraine

Measuring the hydrogen concentration and isotope composition in nominally anhydrous silicates, such as olivine or pyroxene, is of interest for tracing the source of water on terrestrial planets[1] or its distribution and transport in the Earth mantle[2]. Nonetheless, quantifying the hydrogen contents and D/H ratios in hydrogen-poor minerals remains challenging due to the complex distribution of hydrogen in silicates and the risk of terrestrial contamination during the measurements.    

We have initiated a series of analyses by SIMS and EA-IrMS (Elemental Analysis coupled with an Isotope ratio Mass Spectrometry) at CRPG to develop new pyroxenes standards for H concentrations and D/H ratios. We first focused on a few enstatite-rich orthopyroxene phenocrysts (cm-sized) with no visible melt inclusions. The samples were mounted in indium, stored under vacuum and analyzed by SIMS with good vacuum conditions (< 2 x 10-9 mbar) in the analytical chamber. This allows the hydrogen background and detection limit to be <8 wt. ppm and <3 wt. ppm H2O, respectively. Water contents in pyroxenes were calibrated using a series of pyroxenes for which the H2O concentration was determined by Fourier Transform InfraRed (FTIR) spectrometry. We have performed recent improvements of the EA-IrMS analytical procedure and data processing for H-poor samples that allow a 15 wt. ppm H2O detection limit to be achieved and D/H ratios to be measured in samples with H2O concentration down to 200 wt. ppm. We will present and discuss the results obtained for pyroxenes with the two techniques and the possible need of external hydrogen quantifications for obtaining trustable pyroxene standards for future D/H measurements by SIMS.

[1] Piani, L. et al., 2020. Sci. 369, 1110–1113
[2] Demouchy, S., Bolfan-Casanova, N., 2016. Lithos 240–243, 402–425.

To establish volatile concentration reference materials for deep mantle minerals

Yang Y.-N.1, Du Z.1
1Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

Water can greatly alter chemical and physical properties of mantle minerals and exert significant control on Earth’s dynamics and evolution. While it is nearly impossible to directly sample the deep part of Earth’s interior, laser-heated diamond anvil cell (LH-DAC) offers the only method available to synthesize and recover analog specimens down to the Earth’s core. The samples produced by LH-DAC under conditions of Earth’s lower mantle are typical of micron size. Hence measuring their water abundance requires in situ techniques with very high spatial resolution such as nano-scale secondary ion mass spectrometry (NanoSIMS) [1]. Quantitative determination of water contents by NanoSIMS requires matrix-matched reference materials. However, reference materials for volatile abundance are still limited. We established two natural orthopyroxene water working standards that are suitable for calibrating water concentration in bridgmanite, the most abundant mineral in the lower mantle. The matrix effect among orthopyroxene, olivine, and glass during NanoSIMS analysis is evaluated. With appropriate reference materials, applying the unique analytical capability of NanoSIMS to minute samples recovered from LH-DAC opens a new window to probe water and other volatiles in Earth’s deep mantle[2].

[1] Badro, J. et al., 2007. Chemical imaging with NanoSIMS: A window into deep-Earth geochemistry. Earth and Planetary Science Letters, 262(3): 543-551.
[2] Yang, Y.-N., Du, Z., Lu, W., Qi, Y., Zhang, Y.-Q., Zhang, W.-F. and Zhang, P.-F. (2023). NanoSIMS analysis of water content in bridgmanite at the micron scale: An experimental approach to probe water in Earth’s deep mantle. Front. Chem. 11:1166593.

Explore Earth’s deep volatiles with ultrahigh pressure experiments and NanoSIMS

Du Z.1, Yang  Y.-n.1, Lu W.1, Yang X.1
1Guangzhou Institute of Geochemistry

The origin and nature of volatiles in Earth’s deep mantle remains an outstanding issue. Recent geochemical observations and planetary formation models indicate such volatiles were formed early and likely primordial. It raises an important, but largely unexplored question: how were early volatiles survived and retained in the mantle throughout Earth’s history? Our goal is to understand how volatiles would behave in mantle constituents. We simulate deep Earth conditions using ultrahigh pressure experiments, coupled with volatile analysis at the micron scale and trace level using secondary ion mass spectrometry [Yang et al., 2023, Du et al., 2013]. With our experimental data, we are hoping to better understand how Earth’s volatiles are retained, distributed and re-mobilized from the early magma ocean stage to the present day. The origin and nature of volatiles in Earth’s deep mantle remains an outstanding issue.   Recent geochemical observations and planetary formation models indicate such volatiles were formed early and likely primordial. It raises an important, but largely unexplored question: how were early volatiles survived and retained in the mantle throughout Earth’s history? Our goal is to understand how volatiles would behave in mantle constituents. We simulate deep Earth conditions using ultrahigh pressure experiments, coupled with volatile analysis at the micron scale and trace level using secondary ion mass spectrometry [Yang et al., 2023, Du et al., 2013]. With our experimental data, we are hoping to better understand how Earth’s volatiles are retained, distributed and re-mobilized from the early magma ocean stage to the present day.

Determining time, pressure and direction of petroleum migration with Nano-SIMS analysis of trace elements in walls of oil inclusions

Zhang L.2,  Li Z.1, Hao J.2, Wang Z.3
1Institute of Disaster Prevention
2Institute of Geology and Geophysics, Chinese Academy of Sciences
3School of Earth and Space Sciences, Peking University

Time, pressure and direction of petroleum migration are important information in investigating distribution patterns of oil accumulations. The existing method for these three kinds of information depends on formation temperatures of co-existing aqueous and oil inclusions. As there are not sufficient co-existing aqueous and oil inclusions in most reservoirs, it is often difficult to obtain time, pressure and direction of petroleum migration. In this paper, from the partition equation of trace elements between phases, it is was found that trace elements in walls of oil inclusions are related to formation temperatures of oil inclusions if these elements were saturated in paleo-water. To break through the limitation of co-existing aqueous and oil inclusions, a novel approach for formation temperatures of oil inclusions was established on the basis of Nano-SIMS analysis of trace elements in walls of oil inclusions. This approach was applied to the sandstone reservoir in the Sha-3 Member of the Eocene Shahejie Formation in the Dongying Depression, Eastern China. In this reservoir, Al, Ca and Mg were saturated in paleo-water when the oil inclusions formed, as feldspar grains and carbonate cements already existed before. The formation temperatures of dozens of oil inclusions were calculated from the Nano-SIMS analysis results of 27Al, 40Ca and 24Mg. These formation temperatures are closely correlated with those from co-existing oil and aqueous inclusions (R=0.97). The time and pressure of the petroleum migration were obtained by comprehensively using the formation temperatures and isochores of oil inclusions and burial history maps. The results indicate that the petroleum migrated from south to north at about 5 Ma, which coincides with the configuration of the source kitchen, sand-bodies and faults in this area. The case studies serve to support that the new approach provide a valid solution for determining time, pressure and direction of petroleum migration.

Apatite as a tool to quantify the volatile budget of crystallising plutonic rocks

Grocolas T.1
1University of Lausanne

Volatile elements play a key role in the differentiation of magmas by controlling their chemical and physical properties. Volatile concentrations within the melt are commonly inferred from melt inclusions which, however, may have experienced post-entrapment modification. Alternatively, hydrous minerals such as apatite represent an important archive to reconstruct the volatile history of their parental melt. Here we investigate the volatile evolution of differentiating magmas within the upper crustal Adamello batholith (Italy). The investigated Western Adamello tonalite (WAT) and Listino ring complex (LRC) are tonalitic bodies displaying in situ crystal-melt segregations. Apatite is ubiquitous in these lithologies and exhibits a compositional trend ranging from intermediate F (≤1.5 wt.%) to high F (2.5-3.0 wt.%) contents. Preliminary analyses by secondary ion mass spectrometry reveal that LRC apatite contains 0.59-0.90 wt.% H2O, 200-480 ppm S, and 40-700 ppm CO2 with the highest values probably reflecting inherited crystals. By combining existing apatite partition coefficients with mineral geothermometers and hygrometers, we find that the calculated melts from both locations are fluid-saturated (~5.5 wt.% H2O) upon the onset of apatite crystallisation (~925°C) and exhibit a trend of decreasing Cl (WAT: 1900-100 ppm; LRC: 1000-200 ppm) at roughly constant F (WAT: 400-800 ppm; LRC: 300-600 ppm) concentrations with differentiation. We interpret this evolution as the result of Cl incorporation into an exsolving fluid, whereas F is incompatible in the fluid phase and partitions equally between crystals and melt. Simple modelling of such a continuous exsolution process using published distribution coefficients and mass balance suggests that, at 85% crystallisation, 60-80% H2O has exsolved while at least 75% of the bulk Cl is partitioned into the fluid phase. Fluid exsolution, recorded by miarolitic cavities, is potentially linked to compaction and shearing which might connect the exsolved products and eventually form fluid pathways probably enhancing the crystal-melt segregation process.

Correlation between oxygen and boron isotope systematics in oceanic lizardite: an in situ study via SIMS

Vesin C.1, Rubatto D.1,2, Pettke T.1
1University of Bern,
2University of Lausanne

The hydration of the oceanic lithosphere under the seafloor is mainly due to the serpentinisation of mantle peridotites. Serpentine is the major product of the reaction and replaces mantle minerals in distinct textural domains: mesh rim and mesh centre after olivine, and bastite after orthopyroxene. The main loci of oceanic serpentinisation are the mid-ocean ridges (MOR) and extended passive margins (PaMa). The samples presented in this study are from oceanic drill cores: Mid-Atlantic Ridges (Site 1274A), Hess Deep (Site 895D) and the Iberian passive margin (Site 1070A).

The interaction between seawater and ultramafic rocks occurs under variable temperature and fluid composition. The oxygen isotope systematics in the serpentine in the different textural domains show the temperature variation and can also be related to different water-rock interactions and fluid compositions. On the other hand, the boron isotope composition reveals not only temperature variation but also pH variability.Our oxygen isotope dataset shows that there is a decrease in temperature from the early stage (i.e., mesh rim serpentinisation) to the late stage of serpentinisation (i.e., bastite serpentinisation), covering a temperature range from 290°C to 120°C. These temperature decreases are consistent with boron isotope data, which decreases from the seawater composition (δ11B = 39.5‰) down to 16‰. However, the boron isotope fractionation between the fluid and serpentine shows that the temperature variation is not enough to justify the significant variation in the boron isotope composition of the serpentine. Another parameter must be affecting the system, and a decrease in pH during the serpentinisation of orthopyroxene is most likely to result in the low δ11B of the bastite. A sample from PaMa shows low δ11B of 8–11‰ with no variation between the different textures, which may indicate peculiar serpentinisation conditions in PaMa settings compared to MOR.

Measurement Bias of Li Isotopes by SIMS in Standard Glasses

Denny A.1, Zimmer M.1, Cunningham H.1, Sievers N.1
1Pacific Northwest National Laboratory

Lithium isotopes are used as geochemical tracers for studies involving weathering fluxes, crustal recycling in the mantle, and the fate of fluids in subduction zones and hydrothermal systems, as well as in situ measurements in mineral grains to evaluate heterogeneity and diffusion at the micron scale. Secondary ion mass spectrometry (SIMS) is used for spatially-resolved, high precision Li isotope measurements, but the technique is subject to analytical mass biases and material-specific matrix effects that must corrected to achieve accurate results. Fourteen USGS, MPI-DING, and NIST reference materials (RMs) were analyzed by LG-SIMS to assess δ7Li matrix effects and lithium yields in silicate rock glasses. The analyzed RMs cover a range of SiO2 contents from 45.5 to 75.6%, spanning a compositional range found in natural volcanic glasses of komatiite to rhyolite. This is the largest published compositional range yet to be evaluated for δ7Li matrix-induced bias (matrix effect). We observe a matrix effect of up to 18‰ over the studied range of RM compositions that linearly correlates with SiO2 content, conclusively demonstrating that SiO2 content must be considered when using silicate glasses to standardize the δ7Li of unknowns. The δ7Li homogeneity of glass RMs was also evaluated and found to be isotopically homogeneous at the precision of measurements made, indicating that these RMs could be used to aid in generating SiO2 matrix effect corrections. Lithium yields estimated from SIMS pit geometry measurements are very high, exceeding 5% in all fused-rock RMs and reaching >15% specifically in low SiO2 fused-rock RMs measured at high beam intensities. Lithium ionizes several times more efficiently in fused rock RMs than synthetic glass RMs. The oxidized nature of the fused rock glasses relative to their synthetic counterparts indicates that different positioning or bonding of lithium atoms in the glass structure may influence this behavior.

The utility of chlorine isotope measurements in melt inclusions: application to six different volcanic arcs

Anne-Sophie Bouvier1, Estelle F. Rose-Koga2, Maxim Portnyagin3,4, Alexander R.L. Nichols5, Stamastis Flemetakis6and Timm John7
1ISTE, University of Lausanne, Lausanne, Switzerland
2ISTO, CNRS UMR 7327, Orléans, France
3GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany
4Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Russia
5Department of Geological Sciences, University of Canterbury, New Zealand         
6Institut für Mineralogie, Universität Münster, Germany
7Institute of Geological Sciences, Freie Universität Berlin, Germany

Chlorine is a highly hydrophile and incompatible element which may provide insights into the transfer of elements from the slab to the surface in subduction zone settings. We studied Cl isotopes in olivine-hosted melt inclusions (OHMIs) from six different volcanic arcs [1-4] in order to improve our understanding of the behavior of Cl and d37Cl during degassing and fluid/melt transport.

The isotope data were obtained with an ion probe at the University of Lausanne. The reference materials reproducibility was ~0.3 – 0.4‰ (2SD). The δ37Cl values of OHMIs vary from -3.4 to +3.1‰, a range comparable to that measured in bulk rock for volcanic arc [e.g., 5]. Within a single sample δ37Cl in OHMIs vary by more than 2‰. Combined with either other stable isotopes systems or trace elements within the same OHMIs, it is possible to trace the signature of the different Cl sources beneath the different subduction zones. For example, for the Aeolian arc or northern Izu-Bonin arc, OHMIs have the lowest d37Cl of the dataset, reflecting the imprint of subducted sediments. On the contrary, beneath the Central America Volcanic Arc, the mantle wedge has a high d37Cl, up to +3‰, significantly higher than depleted MORB mantle and higher than any subducted material, which might reflect the presence of amphibole in the mantle source. Compared to bulk rocks from the same arcs, OHMIs display either statistically higher, lower or similar weighted average d37Cl. The difference between bulk rocks and OHMIs suggests that the latter preserve undegassed signatures which might be lost in bulk rocks. OHMIs can thus be very useful to: (i) better constrain the behavior of Cl and δ37Cl in subduction zone settings, in particular during fluid-rock interaction within the mantle wedge; and (ii) track the influence of amphibole in the context of arc magma genesis and differentiation.

[1] Bouvier A.-S. et al. (2022) Front. Earth Sci., doi:10.3389/feart.2021.793259
[2] Bouvier A.-S. et al (2022) EPSL 581, doi:10.1016/j.epsl.2022.117414
[3] Bouvier A.-S. et al. (2019) EPSL 507, doi:10.1016/j.epsl.2018.11.036
[4] Manzini M. et al. (2017) Chem.Geol. 449, doi:10.1016/j.chemgeo.2016.12.002
[5] Barnes et al., (2009), G3, doi:10.1029/2009GC002587