An equation-based model of liquid metal segregation in Fe-bearing chondrite meteorites is presented. Textural data from natural samples provide the input conditions. Initial results confirm porous flow of Fe-Ni-S liquid alloy as an important metal segregation mechanism in planetary interiors

Lansdown, G.

Petford, Nick

Rushmer, Tracy

English

Bournemouth University Research Online

NUMERICAL MODELLING OF LIQUID METAL TRANSPORT IN PARTIALLY MOLTEN H5ORDINARY CHONDRITE. N. Petford1, T. Rushmer2 and G. Lansdown1, 1Centre for Earth & EnvironmentalScience Research, Kingston University, KT1 2EE London, UK (n.petford@kingston.ac.uk), 2Dept. of Geology,University of Vermont, Burlington, VT 05405, USA.Introduction: Although liquid iron-rich metal seg-regating from a molten silicate mantle (magma ocean) isa widely cited mechanism for segregating core formingmaterial [1-2], the short times (< 30 Ma) required forcore formation based on Hf-W isotope systematics [3]have reopened the debate on the role of magma oceansin core formation. In particular, the W-Hf data providesupporting evidence that the terrestrial planets and lar-ger asteroids formed by rapid accretion of planetesimalsthat had already undergone early differentiation andcontained proto-planetary cores. Recent experimentalstudies have investigated the physical efficiency of Fe-rich liquid segregation from a silicate matrix in bothstatic and in actively deforming (dynamic) environ-ments [4-6]. As shown by [4], under non-hydrostaticconditions, deformation mechanisms appear capable ofproviding high permeability pathways for metal segre-gation independent of surface tension effects. As a firststep towards modelling Fe metal segregation in deform-ing planetesimals, we have devised a computationmethodology that allows us to use actual rock texture asa basis for physics-based modelling of the segregationprocess.Fig. 1. Photomontage image of Kernouve H5 mete-orite showing textural distribution of Fe-Ni-S metal.Silicate mineralogy is mostly pyroxene and olivine.Image scale approx 1 cm by 0.5 cm.Experiments: Dynamic melting and deformation ex-periments were conducted on a partially molten ordi-nary H6 chondrite (Kernouve) using a solid-media rockdeformation apparatus to investigate the mechanisms ofmetal-silicate under applied stress. Textural and chemi-cal analyses confirm the mobility of Fe-Ni-S liquidsunder conditions where the silicate matrix remains sub-solidus. Fig. 1. shows the distribution of metal in Ker-nouve H6 in the sample (to scale) after partial melting.(T = 900-1000ºC, P = 1 GPa). Subsequent experimentsto investigate melt segregation during shear are de-scribed in [6].Image analysis: In order to perform accurate (geo-metrically constrained) numerical modelling of liquidFe-metal flow it was necessary to capture the textureand distribution of the metal phase in the sample. Thiswas achieved using images obtained from optical mi-croscopy (Fig. 1) and SEM backscatter (Fig. 2).Fig. 2. SEM backscatter image of Kernouve H5meteorite showing region used as basis for finite ele-ment (mesh) geometry and numerical flow modelling.Fe-Ni-S metal pockets shown as light grey.The more detailed SEM image shown in Fig 2 wasthen image processed so that the regions of interestsize (shape and distribution of the metal phase) couldbe segmented from the background matrix. The result-ing greyscale image transform is shown in Fig. 3. Thefinal stage in the imaging processes involved convert-ing the greyscale texture image (Fig. 3) into a geome-try object such that important elements of the originaltexture are preserved. The resultant geometry was thenmeshed ready to perform the finite element analysisand fluid flow modelling. The final pre-processingimage is shown in Fig. 4. The finite element mesh con-sists of 32000 grid points and can be further refined formore detailed analysis. Fe-Ni-S melt pockets areshown in outline. Mesh density is highest around themelt pockets as these regions display the most complexflow behavior.Fe-Ni–S MetalLunar and Planetary Science XXXVII (2006) 1603.pdfFig. 3. Thresholded and negative (inverted) grey-scale image of that shown in Fig. 2. Here the Fe-Ni-Smetal phase is shown in black. Scale as in Fig. 2.Fig. 4. Finite element mesh geometry derived fromFig 3. The mesh consists of 32000 grid pints. Fe-Ni-Smelt pockets shown in outline. Mesh density is highestaround the melt pockets as these regions develop com-plex flow fields.Results: Fig 5 shows the results of a numerical so-lution applied to the digitized Kernouve H5 textureshown in Fig. 4. The analysis was performed usingComsol Multiphysics v.3.2 and solved assuming aDarcyan flux. The plot shows the 2D surface pressuregradient, fluid flow streamlines (red) and velocity field(yellow arrows). Fe-metal fluid flow is towards theexisting pockets of Fe-Ni-S metal. In this simplemodel, metal fluid flow is driven in the porous matrix(blue) with fixed permeability of 10-11 m2 [5] by apressure gradient of c 103 Pa. Yellow arrows show thecorresponding direction and magnitude of the meltvelocity field. A key result is that porous flow of smallmelt fractions of Fe liquid metal takes place in the sili-cate matrix, where it is drawn to pre-existing 2D pools(in 3D these structures might be channels) which aresites of reduced pressure. In this way, small liquidfractions collect over a scale of several or more grainsand concentrate in preferred sites (sinks) of liquidmetal. Modelling microscale segregation may helpunderstand macroscale core formation. The modelsalso have potential to shed light on liquid metal trans-port across the CMB [e.g. 7].Fig 5. Numerical solution showing Fe metal flowvelocity field (arrows) in Kernouve H5. Melt is at-tracted towards low pressure Fe-Ni-S melt pockets(while regions).Discussion: The procedure outlined here offers anew way of modelling in detail grain-scale liquid metalsegregation in natural samples. In the numericalmodel, constrained by true (2D) textural geometry, thepressure field, matrix permeability and Fe-Ni melt vis-cosity and density can be changed at will, allowingdetailed sensitivity analysis of the transport process tobe made. It is also possible to investigate thermal ef-fects (e.g. temperature-dependent viscosity of the liq-uid metal), and in principle the electromagnetic effectsof metallic liquid pore-scale flow. Future work willconcentrate on highly detailed investigation of the mi-croscale physics of Fe metal-silicate melt segregationas a precursor to planetary core formation throughcombined experimental and numerical study.References: [1] Newsom, H. E. (1990) in Originof the Earth, 273–288 Oxford Univ. Press, UK. [2]Stevenson, D., (1990) in: Origin of the Earth, 231-249,Oxford Univ. Press, UK. [3] Halliday, A. (2004) Na-ture 427, 505–509. [4] Rushmer et al., (2000), in Ori-gin of the Earth and Moon, LPI Arizona, 227-245. [5]Yoshino et al., (2003), Nature, 422 154-157. [6]Rushmer et al., (2005), EPSL, 239, 185-202. [7]. Pet-ford et al., (2005), EPS 57, 459-464.Lunar and Planetary Science XXXVII (2006) 1603.pdf

Numerical modelling of liquid metal transport in partially molten H5 ordinary chondrite

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