Geochemical and isotope geochemical modeling of fluid-rock interaction and wall-rock assimilation building on bulk processes (which allows for the use of simple mixing equations) is well-established. Such bulk modeling yield robust results for major elements and those trace elements that are bound to major minerals. In contrast, modeling may yield highly arbitrary results for trace elements, whose budget is dominated by accessory minerals. Elements sequestered in accessory minerals may uncouple from major elements.

(i) Elemental and isotopic uncoupling during fluid-rock interaction

(ii) Elemental and isotopic uncoupling during metasomatism

(iii) Fractionationation of Li and B during fluid rock interaction

(iv) Material cycles in collisional orogens

(i) Elemental and isotopic uncoupling during fluid-rock interaction

In fluid-rock interaction, O and C represents major elements that allow for bulk modeling. In contrast, Pb, Nd, Sr, B, and Li are trace-elements that behave entirely different during fluid-rock interaction, depending on their solubility, the presence or absence of minerals that have a high compatibility of these elements, and the reaction history of the fluid. Actually, depending on the evolution of the system, the fluid may impose systematically changing trace element signatures on the rock, and vice versa. As a consequence the trace element signature of the rock may be entirely unrelated to its major element composition and the fluid may have lost the geochemical signature of its source.

Associated publications:

Romer, R.L., Heinrich W., Schröder-Smeibidl, B., Meixner, A., Fischer, C.-O., and Schulz, C. (2005) Elemental dispersion and stable isotope fractionation during reactive fluid-flow and fluid immiscibility in the Bufa del Diente aureole, NE-Mexico: Evidence from radiographies and Li, B, Sr, Nd, and Pb isotope systematics. Contrib. Mineral. Petrol., 149: 400-429.

Romer, R.L., Wawrzenitz, N., and Oberhänsli, R. (2003) Anomalous unradiogenic ⁸⁷Sr/⁸⁶Sr ratios in UHP crustal carbonates – evidence for fluid infiltration during deep subduction? Terra Nova, 15: 330-336.

Romer, R.L. and Heinrich, W., (1998). Transport of Pb and Sr in leaky aquifers of the Bufa del Diente contact metamorphic aureole, North-East Mexico. Contrib. Mineral. Petrol., 131: 155-170.

(ii) Elemental and isotopic uncoupling during metasomatism

Partial melting of rocks will yield geochemical and isotopic signatures in the melt that differ from the one in the residue. The major element composition is controlled by the phases forming the eutectic. The trace-element inventory is controlled by these phases and the solubility of accessory minerals in the melt. Finally, the isotopic composition of elements with radiogenic growth in melt and residue will depend on the distribution of these elements in minerals participating in melting and staying in the residue, the parent-to-daughter relation in these minerals, and the age of the material being molten… The uncoupling of trace-elements is important in migmatites and rocks affected by wall-rock assimilation. It provides a possibility to uncouple geochemical and isotopic fingerprints from the bulk-rock geochemistry.

Potassic and ultrapotassic rocks such as orogenic lamproites show a distinct uncoupling of major elements and trace elements, although for these rocks, the uncoupling is due to strongly contrasting trace element contents in the two contributing members. The rocks are derived from a mantle source (as seen from the composition of xenocrysts) and show trace element signatures and Sr-Nd-Hf-Pb isotopic compositions that reflect the subducted lithologies eventually giving rise to metasomatism and refertilization of the mantle. Regional variation of the metasomatic component of Mediterranean lamproites demonstrates that metasomatism is related to young subduction. Furthermore, the isotopic character of the metasomatic component does not provide age constraints on metasomatism, but represents a fingerprint of the crustal source of metasomatism.

Associated publications:

Soder, C., Altherr, R., Romer, R.L. (2016) Mantle metasomatism at the edge of a retreating subduction zone: Late Miocene lamprophyres from the island of Kos, Greece. J. Petrol., 57: 1705-1728.

Prelević, D., Akal, C., Foley, S.F., Romer, R.L., Stracke, A., and van den Bogaard P. (2012) Ultrapotassic mafic rocks as geochemical proxies for post-collisional dynamics of orogenic lithospheric mantle: the case of southwestern Anatolia, Turkey. J. Petrol., 53: 1019-1055.

Prelevič, D., Akal, C., Romer, R.L., and Foley, S.F. (2010) Lamproites as indicators of accretion and/or shallow subduction in the assembly of south-western Anatolia, Turkey. Terra Nova, 22: 443-452.

Franz, L. and Romer, R.L. (2010) Different styles of metasomatic veining in ultramafic xenoliths from TUBAF Seamount (Bismarck Microplate, Papua New Guinea). Lithos, 114: 30-53.

Prelevič, D., Stracke, A., Foley, S.F., Romer, R.L., and Conticelli, S. (2010) Hf isotope compositions of Mediterranean lamproites: mixing of melts from asthenosphere and crustally contaminated lithosphere. Lithos, 119: 297-312.

Preleviç, D. Foley, S.F., Romer R., Conticelli, S. (2008) Mediterranean Tertiary lamproites derived from multiple source components in postcollisional geodynamics. Geochim. Cosmochim. Acta, 72: 2125-2156.

(iii) Fractionationation of Li and B during fluid rock interaction

The loss of fluids during metamorphism does not only represent a process to selectively remove material from a subducting rock and to transpose the geochemical signature of one rock onto another one exposed to the escaping fluid. Instead, the partitioning of elements into the fluid also will result in isotopic fractionation. The partitioning and isotopic fractionation of the light elements has been quantified using hydrothermal experiments and major B and Li carrier in different lithologies.

Associated publications:

Wunder, B., Deschamps, F., Watenphul, A., Guillot, S., Meixner, A., Romer, R.L., Wirth, R. (2010) The effect of chrysotile-nanotubes on the serpentine-fluid Li-isotopic fractionation. Contrib. Mineral. Petrol., 159: 781-790.

Meyer, C., Wunder, B., Meixner, A., Romer, R.L., Heinrich, W. (2008) The boron-isotope partitioning between tourmaline and fluid: An experimental re-investigation. Contrib. Mineral. Petrol., 156: 259-267.

Wunder, B., Meixner, A., Romer, R.L., Feenstra, A., Schettler, G., Heinrich, W. (2007) Lithium isotope fractionation between Li-bearing staurolite, Li-mica and aqueous fluids: an experimental study. Chem. Geol, 238: 277-290.

Wunder, B., Meixner, A., Romer, R.L., and Heinrich, W. (2006) Temperature-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids. Contrib. Mineral. Petrol., 151: 112-120.

(iv) Material cycles in collisional orogens

Modeling material loss during progressive metamorphism is a highly speculative task as the composition of the input material often is not well known. Especially the behavior of the fluid-mobile elements Li and B has been subject to debate as field-based and experimental studies seem to be in conflict. Lithologies of distinctive chemical composition provide an option to trace a particular unit within different tectonic units of an orogen. We studied the Li and B isotopic composition of such a unit exposed to different metamorphic peak conditions. Our major results are:

* The initial Li and B isotopic variability within chemically similar lithologies may be larger than the variation typically assigned to metamorphic material loss.

* Loss and gain during metamorphism depend on the stability of mineral sequestering the elements of interest. Changes in the isotopic composition in metamorphic rocks are not properly described by models for continuous loss and continuous fractionation.

Associated publications:

Romer, R.L., Meixner, A., and Hahne, K. (2014) Lithium and boron isotopic composition of sedimentary rocks – the role of source history and depositional environment: a 250 Ma record from the Cadomian orogeny to the Variscan orogeny. Gondwana Research, 26: 1093-1110.

Romer, R.L. and Meixner, A. (2014) Lithium and boron isotopic fractionation in sedimentary rocks during metamorphism – the role of rock composition and protolith mineralogy. Geochim. Cosmochim. Acta, 128: 158-177.

Romer, R.L., Meixner, A., and Förster, H.-J. (2014) Lithium and boron in late-orogenic granites – Isotopic fingerprints for the source of crustal melts? Geochim. Cosmochim. Acta, 131: 98-114.