Modeling differentiation in igneous systems: On the importance of considering temperature & composition dependent partition coefficients

<p>       Studying magma reservoir processes is one of the keys to understand the evolution of igneous systems. One of the main processes, magma differentiation, governs the thermal evolution and chemical composition of the melt-crystal assemblage (magma or mush depending on the relative proportions), and therefore exerts a first order control over its physical properties (density, viscosity), and thus on reservoir dynamics. Various approaches have been implemented to model differentiation in an attempt to benchmark all the involved variables like initial and phase compositions, temperature, pressure, and oxygen fugacity (C<sub>0</sub>, X, T, P, <em>f</em>O<sub>2</sub>). Those approaches are among others mass balance calculations considering partition coefficients (D) values, experimental studies, thermodynamic models or a combination of those. In any of those cases, the evolution of trace elements is governed by the value of the D that is known to be dependent on (P, T, X, <em>f</em>O<sub>2</sub>). However, most of the present-day studies still use fixed values of D to provide first order estimates.</p><p>      Here, we present an approach combining thermodynamic modeling (relying on Rhyolite-MELTS, Gualda et al., 2012), that integrates X-T-P-<em>f</em>O<sub>2</sub>-dependent D for Rare Earth Elements (REE). We applied this new approach to a MORB system, with olivine, clinopyroxene and plagioclase as main mineral phases, and compared results to more classical approaches. D are derived from the models of Sun & Liang (2012, 2013, 2014) and Sun et al. (2017). The resulting model highlights that T & X effects on the D values can add or counterbalance each other depending on the mineral considered. In any cases our results emphasize the gain of using thermodynamic models along with both T- & X-dependent D values to properly model the evolution of igneous systems. Relying on our results, and on the corresponding thermodynamics constraints, we were also able to provide D for any mineral composition crystallized from this MORB system. Results bring to light that an error of ~1 order of magnitude of the D<sup>mineral-melt</sup> value could be introduced when considering a fixed value of D.</p><p> </p><p>Gualda et al. (2012) <em>Journal of Petrology</em>, 53-5, 875-890; Sun & Liang (2012) <em>Contrib Mineral Petrol</em> 163-5: 807-823; (2013) <em>Chem Geol</em> 358: 23-36; (2014) <em>Chem Geol</em> 372: 80-91; Sun et al. (2017) <em>Geochim Cosmochim Acta</em> 206: 273-295.</p><p> </p>

Weathering signals in Lake Baikal and its tributaries

<p>Lake Baikal is the world’s largest (by volume), deepest, and oldest (30-40 Ma) lake. In the catchment, climate varies from arid to semi-arid to arctic-boreal with extreme seasonal and spatial differences in temperature and precipitation<sup>1</sup>. Elevation ranges from 450-3000m, resulting in a large range of geomorphic settings. The catchment has also been affected by periodic Quaternary glaciations<sup>2</sup>. Although the geology of the catchment is diverse and contains igneous, metamorphic and sedimentary rocks of Archean to Cenozoic ages, the most prominent lithologies are granitoids and gneisses with only minor carbonate contributions<sup>1</sup>. Continuous lake sediment cores are available recording the Quaternary glacial cycles, and even dating back into the Miocene. Lake Baikal is therefore a promising site to study variation of silicate rock weathering in both space and time.</p><p>In preparation for paleo-studies, we constrain the present-day budget of the lake with respect to radiogenic weathering tracers (Nd, Pb, and Sr) and meteoric <sup>10</sup>Be/<sup>9</sup>Be isotope ratios.  Nd, Sr, Pb, and their radiogenic isotope systems show different behaviors in Lake Baikal. Sr concentrations in the lake are similar to riverine inputs, reflecting conservative behavior of Sr and resulting in a uniform isotopic composition that is slightly higher than the average of riverine inputs (possibly due to loess inputs<sup>3</sup>). Pb concentrations are higher in the lake than in the major tributaries. The isotopic composition of both lake and rivers point to anthropogenic sources of Pb. In contrast, Nd concentrations in the lake are much lower than in the rivers. Nd isotopic compositions are similar in the central and southern basin but less radiogenic in the northern basin. Both <sup>10</sup>Be and <sup>9</sup>Be concentrations are much lower in Lake Baikal than in its tributaries, possibly indicating removal due to pH induced changes in dissolved-particulate partitioning<sup>4</sup>. This may also explain the contrast in Nd concentrations between rivers and the lake. <sup>10</sup>Be/<sup>9</sup>Be ratios in the lake are slightly elevated compared to riverine  inputs, suggesting a potential role for dust and/or precipitation as a source for <sup>10</sup>Be<sup>5</sup>. We will also compare silicate weathering fluxes derived from meteoric Be isotope ratios with those derived from major element concentrations and riverine discharges.</p><p>Taken together, these results highlight the importance of assessing modern processes at sediment core locations prior to interpreting variation in the past, and the benefits of using a suite of weathering proxies rather than relying on one: while Sr isotopes at any core location record changes to the chemistry of the whole lake (and the processes in its catchment), Be and Nd isotopes are likely biased to the inputs of the nearest rivers.</p><ol><li>Zakharova et al. Chem. Geol. <strong>214</strong>, 223–248 (2005).</li> <li>Karabanov et al. Quat. Res. <strong>50</strong>, 46–55 (1998).</li> <li>Yokoo et al. Chem. Geol. <strong>204</strong>, 45–62 (2004).</li> <li>You et al. Chem. Geol. <strong>77</strong>, 105–118 (1989).</li> <li>Aldahan et al. Geophys. Res. Lett. <strong>26</strong>, 2885–2888 (1999).</li> </ol>

Geochemistry of noble gas and CO2 in fluid inclusions from lithospheric mantle beneath Eifel and Siebengebirge (Germany)

<p>The study of fluid inclusions (FI) composition (He, Ne, Ar, CO<sub>2</sub>) integrated with the petrography and mineral chemistry of mantle xenoliths representative of the Sub Continental Lithospheric Mantle (SCLM) is a unique opportunity for constraining its geochemical features and evaluating the processes and the evolution that modified its original composition. An additional benefit of this type of studies is the possibility of better constraining the composition of fluids rising through the crust and used for volcanic or seismic monitoring.  </p><p>In this respect, the volcanic areas of Eifel and Siebengebirge in Germany represent a great opportunity to test this scientific approach for three main reasons. First, these volcanic centers developed in the core of the Central European Volcanic Province where it is debated whether the continental rift was triggered by a plume (Ritter, 2007 and references therein). Second, Eifel and Siebengebirge formed in Quaternary (0.5-0.01 Ma) and Tertiary (30-6 Ma), respectively, thus spanning a wide range of age. Third, Eifel is characterized by the presence of CO<sub>2</sub>-dominated gas emissions and weak earthquakes that testify that local magmatic activity is nowadays dormant, but not ended (e.g., Bräuer et al., 2013). It is thus important to better constrain the noble gas signature expected in surface gases in case of magmatic unrest.</p><p>This work focuses on the petrological and geochemical study of mantle xenoliths sampled in the West Eifel and Siebengebirge volcanic areas (Germany) and aims at enlarging the knowledge of the local SCLM. Gautheron et al. (2005) carried out the first characterization of noble gases in FI of crystals analyzed by crushing technique (as in our study) but limited to olivines and to West Eifel eruptive centers. Here, we integrate that study by analyzing olivines, orthopyroxenes and clinopyroxenes from a new suite of samples and by including two eruptive centers from Siebengebirge volcanic field (Siebengebirge and Eulenberg quarries).</p><p>Xenoliths from the Siebengebirge localities are characterized by the highest Mg# for olivine, clinopyroxene and Cr# for spinel, together with the lowest Al<sub>2</sub>O<sub>3</sub> contents for both pyroxenes, suggesting  that the mantle beneath Siebengebirge experienced high degree of melt extraction (up to 30%) while metasomatic/refertilization events were more efficient in the mantle beneath West Eifel.</p><p>In terms of CO<sub>2</sub> and noble gas concentration, clinopyroxene and most of the orthopyroxene show the highest gas content, while olivine are gas-poor. The <sup>3</sup>He/<sup>4</sup>He varies between 5.5 and 6.9 Ra. These values are comparable to previous measurements in West Eifel, mostly within the range proposed for European SCLM (6.3±0.4 Ra), and slightly below that of MORB (Mid-Ocean Ridge Basalts; 8±1Ra). The Ne and Ar isotope ratios fall along a binary mixing trend between air and MORB-like mantle. He/Ar* in FI and Mg# and Al<sub>2</sub>O<sub>3</sub> content in minerals confirm that the mantle beneath Siebengebirge experienced the highest degree of melting, while the metasomatic/refertilization events largely affected the Eifel area.</p><p>References</p><p>Bräuer, K., et al. 2013. Chem. Geol. 356, 193–208.</p><p>Gautheron, C., et al. 2005. Chem. Geol. 217, 97–112.</p><p>Ritter, J.R.R., 2007. In: Ritter, J.R.R., Christensen, U.R. (Eds.), Mantle Plumes: A Multidisciplinary Approach. Springer-Verlag, Berlin Heidelberg, pp. 379–404.</p>

Isotopomer approaches to the detection of anaerobic oxidation of natural gas hydrocarbons

<p>Hydrocarbons are the main constituents of natural gas. Their chemical and isotope abundance is a window to biogeochemical processes occurring in the subsurface. Stable isotopes of natural gas hydrocarbons are traditionally measured through compound-specific isotope analysis (CSIA) where each hydrocarbon is separated before its isotope ratio is determined.</p><p>Recently a variety of methods have been developed to determine position-specific isotope composition of propane, the first hydrocarbon with two distinct isotopomers: central and terminal [1][2][3][4]. The relative abundance of propane isotopomers (e.g. Δ<sup>13</sup>C<sub>central</sub> = δ<sup>13</sup>C<sub>central</sub> - δ<sup>13</sup>C<sub>terminal</sub>) is a promising tool for tracing sources and sinks of hydrocarbons in natural gas reservoirs. In particular, anaerobic oxidation of propane starts with a fumarate addition at the central position, which is expected to lead to a specific enrichment of the central <sup>13</sup>C-isotopomer of the remaining propane.</p><p>We measured Δ<sup>13</sup>C<sub>central</sub> values of propane throughout the course of its oxidation by bacteria BuS5 [5] and showed that the isotope fractionation is located mainly on the central position, which differs from the signature expected for thermogenic evolution [6]. The approach has been used to detect anaerobic oxidation of propane in several natural gas reservoirs: Southwest Ontario (Canada), Carnarvon Basin (Australia), Michigan (USA) [6], and more recently Tokamachi mud volcano in Japan [7]. In addition, isotopomers of n-butane and i-butane analysed using the same technique allows gaining insights into the mechanism of their microbial oxidation.</p><p>The isotopomer approach presented here can thus shed light on the fate of natural gas hydrocarbons. In combination with clumped isotope measurements of methane and ethane, the approach can provide unprecedented information regarding carbon cycling in the subsurface.</p><p> </p><p>[1] Gilbert et al., <strong>2016</strong> GCA v177, p205</p><p>[2] Piasecki et al., <strong>2016 </strong>GCA v188 p58</p><p>[3] Gao et al., <strong>2016</strong> Chem Geol. v435, p1</p><p>[4] Liu et al., <strong>2018</strong> Chem Geol. v491, p14</p><p>[5] Kniemeyer et al., <strong>2007</strong> Nature v449, p898</p><p>[6] Gilbert et al., <strong>2019</strong> PNAS v116, p6653</p><p>[7] Etiope et al., <strong>2011</strong> Appl. Geochem. v26, p348</p>

Investigating mantle melting temperatures on Earth, Mars and the Moon using Al-in-olivine thermometry

<p>Al-in-olivine thermometry, based upon the temperature-dependent solubility of Al in the olivine crystal structure [1], has become a widely adopted method to investigate the crystallisation temperatures of primitive mantle melts on Earth [2]. The thermometer is calibrated using the Al contents of co-existing olivine and spinel: these phases are on or near the liquidus of primitive magmas, so the thermometer should access liquidus temperatures of mantle melts, thereby constraining the minimum mantle melting temperature. CFB-associated primitive melts have average olivine crystallisation temperatures well in excess of MORB, and back-calculation to the potential temperature of their mantle source regions suggests mantle thermal anomalies of several hundred degrees [3].</p><p>Whilst mantle thermal anomalies are moderately well-understood on Earth, relatively little is known about the melting conditions in the mantles of the Moon and Mars that led to the production of Maria basalts and Martian surface basalts and associated volcanic activity. Several samples returned from the Moon and basaltic meteorites from Mars (shergottites) are primitive and rich in both olivine and spinel, so would appear ideal samples for the application of Al-in-olivine thermometry to unravel their respective mantle melting conditions and, more generally, the thermal structures of those planetary interiors. In this study, we present preliminary investigations into a) five Apollo 12 primitive lunar basalts, and b) two olivine-phyric shergottites. We find that pervasive shock features make the trace Al concentrations of shergottitic olivines difficult to use, because high Al concentrations are associated with a fine micron to sub-micron network of K-rich melt veins, suggestive of fluid-mediated melt transport. On the other hand, olivine phenocrysts in all five lunar samples yield clear trends in Al contents and are excellent targets for Al-in-olivine studies. We present preliminary thermal results, as well as a newly-calibrated set of relevant thermodynamic parameters needed for back-calculating lunar melting temperatures. A fully quantitative assessment of lunar maria liquidus temperatures is, however, currently hampered by the limited calibration range of the Al-in-olivine thermometer and the unconstrained effect of high spinel TiO<sub>2</sub> contents on the results.</p><p>[1] Coogan, L. A., Saunders, A. D. & Wilson, R. N. Chem. Geol. <strong>368</strong>, 1–10 (2014).</p><p>[2] Trela, J. et al. Nat. Geosci. <strong>10</strong>, 451–456 (2017).</p><p>[3] Jennings, E. S., Gibson, S. A. & Maclennan, J. Chem. Geol. <strong>529</strong>, 119287 (2019).</p>