Jérôme Ganne

Statistical petrology reveals a link between supercontinents cycle and mantle global climate

Abstract The breakup of supercontinents is accompanied by the emplacement of continental flood basalts and dike swarms, the origin of which is often attributed to mantle plumes. However, convection modeling has showed that the formation of supercontinents result in the warming of the sub-continental asthenospheric mantle (SCAM), which could also explain syn-breakup volcanism. Temperature variations during the formation then breakup of supercontinents are therefore fundamental to understand volcanism related to supercontinent cycles. Magmatic minerals record the thermal state of their magmatic sources. Here we present a data mining analysis on the first global compilation of chemical information on magmatic rocks and minerals formed over the past 600 million years: a time period spanning the aggregation and breakup of Pangea, the last supercontinent. We show that following a period of increasingly hotter Mgrich magmatism with dominant tholeiitic affinity during the aggregation of Pangea, lower-temperature minerals crystallized within Mg-poorer magma with a dominant calc-alkaline affinity during Pangea disassembly. These trends reflect temporal changes in global mantle climate and global plate tectonics in response to continental masses assembly and dispersal. We also show that the final amalgamation of Pangea at ~300 Myr led to a long period of lithospheric collapse and cooling until the major step of Pangea disassembly started at ~125 Myr. The geological control on the geosphere magma budget has implications on the oxidation state and temperature of the Earth’s outer envelopes in the Phanerozoic and may have exerted indirect influence on the evolution of climate and life on Earth.

Primary magmas and mantle temperatures through time

Chemical composition of mafic magmas is a critical indicator of physicochemical conditions, such as pressure, temperature, and fluid availability, accompanying melt production in the mantle and its evolution in the continental or oceanic lithosphere. Recovering this information has fundamental implications in constraining the thermal state of the mantle and the physics of mantle convection throughout the Earths history. Here a statistical approach is applied to a geochemical database of about 22,000 samples from the mafic magma record. Potential temperatures (Tps) of the mantle derived from this database, assuming melting by adiabatic decompression and a Ti-dependent (Fe2O3/TiO2 = 0.5) or constant redox condition (Fe2+/∑Fe = 0.9 or 0.8) in the magmatic source, are thought to be representative of different thermal “horizons” (or thermal heterogeneities) in the ambient mantle, ranging in depth from a shallow sublithospheric mantle (Tp minima) to a lower thermal boundary layer (Tp maxima). The difference of temperature (ΔTp) observed between Tp maxima and minima did not change significantly with time (∼170°C). Conversely, a progressive but limited cooling of ∼150°C is proposed since ∼2.5 Gyr for the Earths ambient mantle, which falls in the lower limit proposed by Herzberg et al. [2010] (∼150–250°C hotter than today). Cooling of the ambient mantle after 2.5 Ga is preceded by a high-temperature plateau evolution and a transition from dominant plumes to a plate tectonics geodynamic regime, suggesting that subductions stabilized temperatures in the Archaean mantle that was in warming mode at that time.

Time and mode of exhumation of the Cordillera Blanca batholith (Peruvian Andes)

The Cordillera Blanca batholith (12–5 Myr) forms the highest Peruvian summits and builds the footwall of the Cordillera Blanca normal fault (CBNF). Even if several models have been proposed, the processes driving both the exhumation of the Cordillera Blanca and extensional deformation along the CBNF are still debated. Here we quantify the emplacement depth and exhumation of the batholith of the northern Peru arc from the late Miocene to present. Based on a compilation of crystallization ages and new thermobarometry data in the Cordillera Blanca batholith, we propose that the batholith was emplaced at a depth of ~3 km in successive sills from 14 to 5 Ma. By contrast, the younger rocks exposed at the surface were emplaced the deepest (i.e., ~6 km) and are located close to the CBNF, suggesting post 5 Ma tilting. Furthermore, a formal inversion of the thermochronologic data indicates an increase of the exhumation rates in the Cordillera Blanca during the Quaternary. The higher predicted exhumation rates correlate with areas of high relief, both in the northern and central part of the Cordillera Blanca, suggesting that Quaternary valley carving by glaciations have a significant impact on the latest stage of the Cordillera Blanca exhumation (2–0 Ma).

Retrieving past geodynamic events by unlocking rock archives with μ-XRF and μ-spectroscopy

Rocks are commonly polycrystalline systems presenting multi-scale chemical and structural heterogeneities inherited from crystallization processes or successive metamorphic events. This work illustrates how spatially resolved analytical techniques coupled with thermodynamic approaches allow rock compositional variations to be related to large-scale geodynamic processes. Emphasis is placed on the contribution of quantitative chemical imaging to the study of 2.2-2.0 Gy old metamorphic rocks from the West African Craton. A thorough analysis of elemental chemical maps acquired on rock thin sections enabled high pressure relic minerals to be located and re-analyzed later with precise point analyses. The pressure-temperature conditions of crystallization calculated from these analyses are typical of modern subduction zones. These results push back the onset of modern-style plate tectonics to 2.15 Gy, i.e. more than one billion years earlier than was consensually accepted. The second part of the paper describes the imaging capabilities offered by the new ultra-bright diffraction limited synchrotron sources. Experimental data acquired with the Maia detector at beamline P06 at Petra III as well as simulations of μ-XRF spectra that will be generated at the SRX beamline at NSLS-II are presented. These results demonstrate that cm2 large chemical maps can be acquired with submicron spatial resolution and a precision suitable for thermobarometric estimates, with dwell time smaller than 1 millisecond. The last part of the paper discusses the relevance of utilizing recent X-ray fluorescence nanoprobes for diagenetic to low grade metamorphism applications.

Strike-slip metamorphic core complexes: Gneiss domes emplaced in releasing bends

Thermobarometric calculations for paragneisses (samples MI11 and MI44) were performed using the version 6.6.8 (11_2013) of Perple_X (Connolly, 2009) and the internally consistent thermodynamic dataset “solution_model.dat” in the CaTiNKFMMnASH system for H2O content corresponding to the loss on ignition values obtained by ICP‐MS. Endmembers used for Mi‐11 pseudosection calculation are given in Table 1 (see documentation in Perple_X for references on solid solution models — http://www.perplex.ethz.ch). The reliability of our thermobarometric estimate is supported by a set of petrological data on metamorphic minerals (garnet, biotite, plagioclase, muscovite and oxides) provided in Table 2. Figure DR1 yields detail on the Perple_X‐elaborated P–T pseudosection built for sample Mi‐11. The pseudosection shows the invariant points (solid dots), univariant reactions (heavy solid lines), divariant fields (unshaded), trivariant fields (lighter‐grey shaded), to the extreme (n=6) variant fields (black). Bulk composition (weight percent) obtained by ICPMS of the metapelite Mi‐11 and details on the metamorphic phase assemblages present in all the fields are given at the top and bottom of the diagram, respectively. Red star: estimated peak P–T conditions; green star: retrograde P–T conditions.