ARC Future Fellow at the School of Earth, Atmosphere and Environment, Monash University, Melbourne.
As a scientist, university teacher, mentor and passionate geologist, I am taking an active role in our society. If you have any questions about my research or geology in general, if you are looking for research opportunities or require a media expertise, don't hesitate to contact me.
The composition of Earth’s mantle
The ocean floor is constantly rejuvenating: it forms at mid-ocean ridges (MOR) and is destroyed at convergent margins in subduction zones. However, the destruction of ocean floor in subduction zones is imperfect: parts of the oceanic crust, namely volatiles and so-called mobile components preferentially leave the crust at the crust-mantle interface and trigger arc volcanism (see below), which is second only in volume to MOR volcanism. The remnants of the ocean floor descend into the deeper mantle – its fate is still uncertain. A commonly accepted hypothesis is the partial burial of slabs (fossil oceanic plates) at the top of the Earth’s core (in the so-called D", a slab graveyard) with various stages of transit through the mantle. However, even mid-ocean ridge basalt chemistry (sourced from the shallow mantle) indicates mixing of mantle with slab components. Other sides of oceanic volcanism at intra-plate oceanic islands further show chemical signatures akin to crustal components that formerly resided on the Earth’s surface. This volcanism (e.g., at Hawaii or Iceland) is spectacular in appearance but also opens a unique window into the deep Earth’s interior. Other sides of massive mantle melting are preserved in the geologic record - either exposed at the surface or buried deep within our continents: flood basalt. Flood basalt volcanism has occurred multiple times in Earth’s history. These important time markers are linked to super-continent break-up, which, in turn, may be related to our past climate evolution. Hence flood basalts may have triggered mass extinction, so that an understanding of their mantle sources and eruption dynamics may hold clues on biodiversity and climate systematics. Other mantle-derived melts are preserved as layered intrusions, massive igneous bodies,which are the major source for our noble metals and thus of immense economic importance.
In a number of projects, I am studying (i) the fate of oceanic crust in the mantle, (ii) the processes of slab recycling (iii) dynamics of the physical and chemical evolution of the mantle and (iv) the chemical nature of mineral deposits. An understanding of these processes will help to better understand ore forming processes related to magmatic activity, explosivity of volcanic systems, past and future climate evolution related to plate tectonics and more.
Schematic sketch (not to scale) illustrating the status-quo on deep mantle recycling and reservoirs and highlighting the areas of interest for this proposal: I: Oceanic Island basalts – OIB; Hot spots; surface expression of mantle plume tails; II: Subduction Zone – SZ; Convergent plate margins with flux melting of the upper mantle wedge, III: Mantle Transition Zone – MTZ; Crystal phase transition in the mantle between 410-660 km depth, IV: Mid Ocean Ridges – MOR; Divergent plate margins with passive, de-compressional melting of the upper upwelling mantle, V: Sub-Continental Lithospheric mantle – SCLM; Non-ductile and cool part of the upper mantle, compositionally different to asthenosphere and can be up to 4 billion years old, VI: Flood basalts; Surface expression of mantle plume head melting, komatiites in the Archean, flood basalts in recent times; VII: Layered intrusions; Massive plutonic expressions of mantle plumes preserved in ancient cratons, VIII: Thermal Boundary Layer (TBL) and D’’ (d-double prime); Area overlying the core mantle boundary with a phase transition and considered a slab graveyard, IX: Secondary plumes; Uprising plumes that stall in the MTZ, spread out and develop secondary plume fingers, X: Lower mantle; Volumetrically largest planetary silicate reservoir; of unknown composition. Entrained material into rising plumes originates in the lower quarter of the lower mantle.
The interaction of two important reservoirs on Earth, the crust and the mantle, can lead to a number of important geologic phenomena, including explosive volcanism, high-grade ore deposition or even pollution of the atmosphere. Crust-mantle interaction occurs at convergent plate margins in subduction zones (see picture below) and at the base of the crust between lithospheric and asthenospheric mantle.
My research in crust-mantle interaction mainly focusses on subduction zones. The subduction of oceanic crust and overlying sediment has strong influence of the chemical composition and physical properties of volcanism in subduction zones. This includes the oxygen fugacity of melts and the ambient wedge mantle, which is the focus of my studies in recent years. With the analyses or arc and back-arc lavas (by using redox-sentiive isotope tracers), I aim for a better understanding of subduction zone processes - in present and past subduction regimes.
A key aspect of subduction is further the return of crustal components that directly relate to the composition of the depleted, and in parts re-enriched mantle (see section above).
Figure 2. Schematic sketch of processes relevant to the study of magma genesis and redox controls in subduction zones. Samples recording five major processes (I-V) are eventually required in order to unravel the locations and systematics of changes in redox states in subduction zones.
Early Earth and extra-terrestrial geochemistry
Reconstruction, chemically or otherwise, of the earliest stages of our planet are difficult. This is related to the immense time – billions of years - between then and now. Rocks that survived over such timescales are very rare and if preserved, then only after substantial chemical and physical modifications. The chemistry of some elements and their isotope composition may preserve footprints of their original precursor rocks. Studying these isotope clues allows constraints on processes from the infant stages of the planet - and sometimes even the timing, locus and age of these processes.
For the study of early planetary evolution I use radiogenic and stable isotopes, mainly the Sm-Nd and Lu-Hf long-lived isotope systems. In recent years I further used a combination of Hf and Fe isotopes, applied to a series of Archean komatiites and layered intrusion rocks. These samples all date back to the Archean eon, 4.0-2.5 billion years before today.
A part of my research is dedicated to the study of extraterrestrial material – meteorites! This space debris is a rare witness of the earliest stages of our solar system and holds clues to what really happened during and shortly after the birth of our solar system. In recent projects, together with a team of international colleagues, I investigated a series of ordinary chondrites and Martian samples for their stable isotope geochemistry. WIth this, we aim to get a better idea of how planets evolved and if this relates to the unique state of our habitable planet.
Igneous and high-temperature petrochemical processes and geochronology
The present day geologic record, as we sample it, is only a still life of complex and often high-energetic processes that shape the Earth. Due to extreme time scales, the massive size and often remote nature of the geological processes that led (and still lead) to the face of the Earth, make reconstruction of past environments (e.g., continent reconstructions), the status quo (e.g., ore deposits) or the eminent future (climate change, volcanic eruptions) difficult, the least to say.
A key part of my work is to get a deeper understanding of these processes through the study of the chemistry of whole rocks, their minerals or tiny inclusions in these minerals. For this I use wet-chemical analyses (acid dissolution and chemical purification of elements) and in-situ techniques (e.g., laser ablation of electron guns). I mainly employ elemental abundances and radiogenic and stable isotope compositions.
With using radiogenic isotopes I further study timescales of geological processes and determine the absolute age of geologic samples, mainly in samples older than a few million years (required for some radiogenic clocks), but, on the other hand, some samples date back to the earliest stages of planetary evolution in our solar system.
Figure 3: Two highly magnified sections of a rock 8in a thin section) with a fluid inclusion (A) in a mineral and the size of a typical ion beam spot. Figure B is showing a sulphide with a laser baltion pit after the analyses for trace elements.