# Compositional effects on the solubility of minor and trace elements in oxide spinel minerals: Insights from crystal-crystal partition coefficients in chromite exsolution

Vanessa Colas, Jose Alberto Padron Navarta, Jose M. Gonzalez-Jimenez, W.

L. Griffin, Isabel Fanlo, Suzanne Y. O'Reilly, Fernando Gervilla, Joaquin A.

Proenza, Norman J. Pearson, Monica P. Escayola

► To cite this version:

Vanessa Colas, Jose Alberto Padron Navarta, Jose M. Gonzalez-Jimenez, W. L. Griffin, Isabel Fanlo, et al.. Compositional effects on the solubility of minor and trace elements in oxide spinel minerals: Insights from crystal-crystal partition coefficients in chromite exsolution. The American Mineralogist, 2016, 101 (5-6), pp.1360-1372. 10.2138/am-2016-5611. hal-01689423

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Manuscript 5611 Revision 1

Compositional effects on the solubility of minor and trace
elements in oxide spinel minerals: insights from crystal-
crystal partition coefficients in chromite exsolution

Vanessa Colás(a), José Alberto Padrón-Navarta(b), José María González-Jiménez(c),
William L. Griffin(d), Isabel Fanlo(a), Suzanne Y. O'Reilly(d), Fernando Gervilla(e),
Joaquín A. Proenza(f), Norman J. Pearson(b), Monica P. Escayola(g)

a. Universidad de Zaragoza, Departamento de Ciencias de la Tierra, Pedro Cerbuna 12,
50009 Zaragoza, Spain.

b. Géosciences Montpellier, CNRS & Univ. Montpellier (UMR5243), 34095, Montpellier, France.

c. Department of Geology and Andean Geothermal Center of Excellence (CEGA), Universidad de Chile, Plaza Ercilla # 803, Santiago, Chile.

d. ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC
National Key Centre, Department of Earth and Planetary Sciences, Macquarie
University, Sydney, NSW 2109, Australia.

e. Departamento de Mineralogía y Petrología and Instituto Andaluz de Ciencias de la Tierra (Universidad de Granada-CSIC), Facultad de Ciencias, Avda. Fuentenueva s/n, 18002 Granada, Spain.

f. Departament de Cristal·lografià, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona. Martí i Franquès s/n, 08028 Barcelona, Spain.

g. IDEAN-CONICET, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Intendente Güiraldes 2160, Ciudad Universitaria - Pabellón II -1º EP - Of. 29, (1428) - Buenos Aires, Argentina

To be submitted to *American Mineralogist*

*Corresponding author: Vanessa Colás

Address: Universidad de Zaragoza, Departamento de Ciencias de la Tierra, Pedro
Cerbuna 12, 50009 Zaragoza, Spain.

Phone: +34 976 84 10 98

E-mail: vcolas@unizar.es
35 **ABSTRACT**

36 Chromite from Los Congos and Los Guanacos in the Eastern Pampean Ranges of
37 Córdoba (Argentinian Central Andes) shows homogenous and exsolution textures. The
38 composition of the exsolved phases in chromite approaches the end-members of spinel
39 (MgAl₂O₄; Spl) and magnetite (Fe²⁺Fe³⁺₂O₄; Mag) that define the corners of the spinel
40 prism at relatively constant Cr³⁺/R³⁺ ratio (where R³⁺ is Cr+Al+Fe³⁺). The exsolution of
41 these phases from the original chromite is estimated to have accounted at ≥ 600 °C on
42 the basis of the major-element compositions of chromite with homogenous and
43 exsolution textures that are in equilibrium with forsterite-rich olivine (Fo95). The
44 relatively large size of the exsolved phases in chromite (up to ca 200 µm) provided, for
45 the first time, the ability to conduct *in situ* analysis with laser ablation inductively
46 coupled plasma mass spectrometry for a suite of minor- and trace-elements to constrain
47 their crystal-crystal partition coefficient between the spinel-rich and magnetite-rich
48 phases (Dᵢ<sup>Spl/Mag</sup>). Minor- and trace-elements listed in increasing order of compatibility
49 with the spinel-rich phase are Ti, Sc, Ni, V, Ge, Mn, Cu, Sn, Co, Ga and Zn. Dᵢ<sup>Spl/Mag</sup>
50 values span more than an order of magnitude, from D<sub>Ti</sub><sup>Spl/Mag</sup> = 0.30 ± 0.06 to D<sub>Zn</sub><sup>Spl/Mag</sup> =
51 5.48 ± 0.63. Our results are in remarkable agreement with data available for exsolutions
52 of spinel-rich and magnetite-rich phases in other chromite from nature, despite their
53 different Cr³⁺/R³⁺ ratio. The estimated crystal-crystal partitioning coefficients reflect the
54 effect that crystal-chemistry of the exsolved phases from chromite imposes on all
55 investigated elements, excepting Cu and Sc (and only slightly for Mn). The observed
56 preferential partitioning of Ti and Sc into the magnetite-rich phase is consistent with
57 high-temperature chromite/melt experiments and suggests a significant dependence on
58 Fe³⁺ substitution in the spinel structure. A compositional effect of major-elements on
Ga, Co and Zn is observed in the exsolved phases from chromite but not in the experiments; this might be due to crystal-chemistry differences along the MgFe₁⁻Al₂Fe³⁺₋₂ exchange vector, which is poorly covered experimentally. This inference is supported by the strong covariance of Ga, Co and Zn observed only in chromite from layered intrusions where this exchange vector is important. A systematic increase of Zn and Co coupled with a net decrease in Ga during hydrous metamorphism of chromitite bodies cannot be explained exclusively by compositional changes of major elements in the chromite (which are enriched in the magnetite component). The most likely explanation is that the contents of minor- and trace-elements in chromite from metamorphosed chromitites are controlled by interactions with metamorphic fluids involved in the formation of chlorite.

**Keywords:** Chromite exsolution, spinel-magnetite, partition coefficient, minor and trace elements, hydrous metamorphism.

## INTRODUCTION

A large body of experimental results show that the partitioning of certain minor and trace elements between chromite and melt (± olivine) is sensitive to oxygen fugacity, temperature (Horn et al. 1994; Nielsen et al. 1994; Nielsen and Beard 2000; Canil 1999, 2002; Connolly and Burnett 2003; Righter et al. 2004, 2006; Wijbrans et al. 2015) and a lesser extent to pressure (Canil 1999, 2002; Mallmann and O'Neill 2009). The effect of composition of major- and minor-elements in chromite has also been experimentally investigated, although less systematically (Horn et al. 1994; Nielsen et al. 1994; Nielsen and Beard 2000; Righter et al. 2006; Mallmann and O'Neill 2009; Wijbrans et al. 2015). These findings have been used to estimate the oxygen fugacity of the mantle source (Lee et al. 2003, 2005; Mallmann and O'Neill, 2009), and more
83 generally the petrogenesis of chromite-bearing rocks (Paktunc and Cabri 1995; Barnes
84 1998; Barnes and Roeder 2001; Kamenetsky et al. 2001; Mondal et al. 2006; Righter et
85 al. 2006; Li et al. 2008; Pagé and Barnes 2009; Dare et al. 2009; González-Jiménez et
86 al. 2011, 2015; Perinelli et al. 2014; Zhou et al. 2014).

87 Several works have suggested that subsolidus modifications such as fluid-rock
88 interactions can significantly disturb magmatic signatures in chromite (e.g., Evans and
89 Frost 1975; Burkhard 1993; Barnes 2000; Barnes and Roeder 2001; Mukherjee et al.
90 2010; Gervilla et al. 2012; Prabhakar and Bhattacharya 2013; Singh and Singh 2013;
91 Gargiulo et al. 2013) including their pattern of minor- and trace-element abundances
92 (Colás et al. 2014; González-Jiménez et al. 2015; Mukherjee et al. 2015). Fluid-rock
93 interactions below 600-650 °C result in (1) the crystallization of hydrous phases such as
94 chlorite and antigorite, (2) the enrichment of chromium and ferric iron in the chromite
95 and, eventually, (3) a coeval change in apparent oxygen fugacity (e.g., Evans and Frost
96 1975; Candia and Gaspar 1997; Barnes 2000; Barnes and Roeder 2001; Gervilla et al.
97 2012). It is therefore challenging to unravel as to that ultimately contributes to the
98 disturbance of magmatic signatures in chromite. This hampers the interpretation of the
99 complex behavior of minor- and trace-elements observed in chromite from
100 metamorphosed chromitites (e.g., Colás et al. 2014; González-Jiménez et al. 2015) or
101 the evaluation of other possible factors such open-system behavior (i.e. element
102 solubility in the fluid phase) that usually predominate during metamorphism of
103 chromite-bearing rocks (e.g., Barnes 2000; Mukherjee et al. 2015).

104 To date, experimental investigations (at relevant conditions) of hydrous
105 metamorphism in which chromite became modified both texturally and compositionally
106 are lacking. Therefore the evaluation of the compositional factors controlling the
107 solubility of minor- and trace-elements in these settings relies on explorations from
high-temperature experiments, which may suffer additional complications such as bulk element loss (particularly important for Zn; Wijbrans et al. 2015). An alternative approach to investigating the effect of composition, which has not been explored so far, is to use crystal-crystal element partitioning data from exsolved phases in chromite.

Exsolution is a process in which chromite solid solution becomes metaestable below critical temperature, and the initial homogeneous chromite separates into at least two different phases that are in equilibrium with each other. Thus, the effect of the major element composition in chromite will be reflected in those minor- and trace-elements with a crystal-crystal partition coefficient from exsolved phases that is significantly different from unity. Pairs of exsolved phases, with contrasting major element compositions, are relatively common and are developed during post-magmatic cooling (Purvis et al. 1972; Muir and Naldrett 1973; Zarkrzewski 1989; Jan et al. 1992; van der Veen and Maaskant 1995; Appel et al. 2002; Garuti et al. 2003; Tamura and Arai 2004, 2005; Krause et al. 2007; Ahmed et al. 2008) and/or high- to medium-grade metamorphism (Loferski and Lipin 1983; Eales et al. 1988; Candia and Gaspar 1997; Abzalov 1998; Proenza et al. 2008). The advantage of this approach is that the temperature (and potentially the oxygen fugacity) of exsolution can be reasonably constrained from the available experimental and thermodynamic data (Turnock and Eugster 1962; Cremer 1969; Muan 1975; Sack and Ghiorso 1991). However, the limitation of this approach for minor- and trace-elements is directly related with the size of the exsolved phases relative to the spatial resolution of modern laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

In this work we report an exceptional case where the size of the exsolved phases in chromite is large enough to be measured *in situ*, with precision, minor- and trace-elements using laser ablation instruments. Thus, we provide the first assessment of the
133 compositional effect on the solubility of minor- and some trace-elements in natural
134 chromite, specifically along the MgFe₁-Al₂Fe³⁺₂ exchange vector (spinel-magnetite
135 join) in the spinel prism. Note that the name "chromite" is used in this work as a general
136 term for Cr-spinel [(Mg, Fe²⁺)(Cr, Al, Fe³⁺)₂O₄] from mafic and ultramafic rocks, rather
137 than the *sensu stricto* definition of chromite as the Fe²⁺-Cr-rich spinel end-member
138 (Fe²⁺Cr₂O₄; Chr).

139

SAMPLES AND METHODS

140 The samples analyzed in this study were collected from outcrops of six
141 ophiolitic chromitite bodies from the metamorphosed ultramafic massifs of Los Congos
142 and Los Guanacos in the southern part of the Eastern Pampean Ranges of Córdoba
143 (Argentinian Central Andes; Proenza et al. 2008). These two massifs are separated by 6
144 km and mainly consist of highly serpentinised metaharzburgite that enclose bodies of
145 metadunites and metalherzolites, and sills of metagabbro. The location and detailed
146 geological description of the studied ultramafic massifs have been given elsewhere
147 (Proenza et al. 2008; Escayola et al. 2004). Escayola et al. (2004) suggested that these
148 metaultramafic rocks where subjected to four metamorphic events, including: (1) low-
149 grade sea-floor metamorphism, overprinted by (2) granulite-facies metamorphism (7.3-
150 9.4 kbar and 760-860 °C; Rapela et al. 1998; Martino et al. 2010), (3) retrograde
151 amphibolite-facies metamorphism (3.5-7.3 kbar and 590-730 °C; Rapela et al. 1998;
152 Martino et al. 2010) and (4) a low-temperature hydrothermal event.

153 The chromitite bodies show massive (>85 vol% chromite), semi-massive (60-85
154 vol% chromite) and disseminated (<60 vol% chromite) textures (Table 1). Chromite
155 grains are predominantly subhedral and less frequently anhedral with rounded shapes,
156 and have a fracture network of variable density (Fig. 1). Silicate minerals in the
chromitites are mainly antigorite, clinochlore and occasionally clinopyroxene (Fig. 1; Table 1). These minerals are also included in chromite grains. The primary olivine is not preserved (Proenza et al. 2008), whereas the fractures that usually cut across chromite grains are filled with serpentinite, chlorite and less frequently carbonates.

The careful study of chromite using transmitted and reflected optical microscopy and scanning electron microscope (JEOL SM 6400 SEM belonging to University of Zaragoza, Spain) reveals the presence of three textural varieties of chromite (Fig. 1; Table 1).

*Type I* chromite is optically and compositionally homogeneous and occurs in clinopyroxene-bearing massive, semi-massive and disseminated chromitites from Los Congos and Los Guanacos (Fig. 1a-b; Table 1).

*Type II* chromite shows an inhomogeneous (exsolved) texture and occurs in clinopyroxene-free massive, semi-massive and disseminated chromitite samples from Los Congos, but the disseminated chromitite sample was selected in this study. It is composed of a low-reflective phase that is complexly intergrown with a high-reflective phase (Fig. 1c-d; Table 1). The high-reflective phase occurs as coarse, sub-rounded, irregular and lobated blebs (from 5-10 to 150 µm) that are distributed randomly throughout the grains, concentrated in the rims or forming linear strings (Fig. 1c). The low-reflective phase itself contains another set of exsolved high-reflective phases forming very fine lamellae (ca 1 µm thick), which are arranged in dense networks with an apparent crystallographic control (Fig. 1d).

*Type III* chromite is restricted to clinopyroxene-free massive chromitite samples from Los Congos and Los Guanacos, however only chromitite sample from Los Guanacos were studied (Table 1). This chromite shows an irregular coarse mottling or
symplectic texture composed of variable proportions of high-reflective and low-reflective phases (Fig. 1e-f). The exsolved phases in Type III chromites are coarser (up to *ca* 200 µm) than in the Type II ones (below *ca* 40 µm) (Fig. 1c-f).

The contents of major- and minor-elements in the different phases of these three types of chromite were determined quantitatively using both microprobe and LA-ICP-MS at the Geochemical Analysis Unit at CCFS/GEMOC, Macquarie University, Sydney (Australia). Electron-microprobe analyses (EMPA) were obtained by wavelength-dispersive spectrometer (WDS mode) with a Cameca SX-100 under the following working conditions: 20 kV acceleration voltage, 20 nA beam current and 2-3 µm beam size. Peak counting times were 10 s for Cr, Fe, Ti, V, Mn, Zn, and Ni, 20 s for Mg, and 30 s for Al. Standards used were a combination of natural and synthetic minerals and pure metals. Structural formulae of chromite were calculated assuming stoichiometry, following the procedure of Droop (1987).

LA-ICP-MS analyses were carried out using a New Wave UP 266 laser ablation unit coupled to an Agilent 7500cs ICP-MS. The chromite was analyzed for the following masses: <sup>45</sup>Sc, <sup>47</sup>Ti, <sup>51</sup>V, <sup>55</sup>Mn, <sup>59</sup>Co, <sup>60</sup>Ni, <sup>65</sup>Cu, <sup>66</sup>Zn, <sup>71</sup>Ga, <sup>72</sup>Ge and <sup>118</sup>Sn.

The isotopes <sup>29</sup>Si and <sup>42</sup>Ca were monitored to control the presence of silicate inclusions.

The analyses were conducted using a 30-50 µm beam diameter, 5 Hz frequency and 0.152 mJ/pulse power, during 180 s analysis (60 s for the gas blank and 120 s on the chromite). The smallest beam diameters (~30 µm) were used during the measurement of the small blebs in *Type II* chromite. The GLITTER software (Griffin et al. 2008) was used for data reduction. Count signal vs time diagrams were checked during single ablation runs to avoid the ablation of mixtures of low-reflective and high-reflective phases in chromite with exsolution textures (Fig. 1S, Supplementary material). The analyses were calibrated against the NIST 610 silicate glass (National Institute
206 Standards and Technology, Gaithersburg, USA) (Norman et al. 1996). Aluminum
207 values obtained by EPMA were used as internal standard. The basaltic glass BCR-2g
208 (Norman et al. 1998; Gao et al. 2002) and the in-house secondary chromite standard
209 LCR-1 (Lace mine, South Africa; Locmelis et al. 2011) were analyzed as unknowns
210 during each analytical run to check the accuracy and precision of the analyses. The
211 results obtained during the analyses of these two standards display very good
212 reproducibility (0.4%-11.4%) (Table 1S, Supplementary material).

213 # RESULTS

214 Major element composition

215 The composition of *Type I* chromite in massive chromitites from Los Congos
216 and Los Guanacos follows a distinctive linear trend defined by an increase of Cr#
217 [Cr/(Cr+Al) in atomic ratio] from 0.47 to 0.72 coeval with a decrease of Mg#
218 [Mg/(Mg+Fe²⁺) in atomic ratio] from 0.68 to 0.52 (Fig. 2a; Table 2). In Los Guanacos
219 the *Type I* chromite from semi-massive and disseminated chromitites show similar
220 trends but are displaced towards lower Mg# (0.48-0.55 in semi-massive chromitites;
221 0.42-0.52 in disseminated chromitites) (Fig. 2a; Table 2). The ratio of Fe³⁺ to other
222 trivalent cations [Fe³⁺/R³⁺ = Fe³⁺/(Fe³⁺+Al+Cr) in atomic ratio] increases from semi-
223 massive (Fe³⁺/R³⁺ = 0.08-0.10) to disseminated chromitites (Fe³⁺/R³⁺ = 0.18-0.26) (Fig.
224 2b; Table 2). It is noteworthy that the massive chromitites show Fe³⁺/R³⁺ ratio (0.13-
225 0.21) intermediate between these two extremes (Fig. 2b; Table 2). All textural types are
226 poor in TiO₂, consistent with the absence of ilmenite exsolution in chromite.

227 The original composition of *Type III* chromite from Los Guanacos, prior to
228 exsolution, was estimated in individual grains based on EPMA analyses (Table 2) and
the areal proportion of the exsolved phases in BSE images, using the Image-J software (Rasband 2007) (Table 1). The composition prior to exsolution of *Type II* chromite from Los Congos could not be integrated due to the fine networks of lamellae in the low-reflective domains (cf. Fig. 1d). The estimated original composition of *Type III* chromite has Cr# (0.55-0.60) and Mg# (0.42-0.49) similar to *Type I* chromite from disseminated chromitite (Cr# = 0.55-0.63 and Mg# = 0.42-0.52) (Fig. 2a and 2c; Table 2), but it shows the lowest Cr³⁺/R³⁺ ratio [Cr³⁺/(Cr³⁺+Fe³⁺+Al) in atomic ratio] (≈ 0.34) and the highest Fe³⁺/R³⁺ ratio (0.40-0.44) amongst all textural groups (Fig. 2b and 2d; Table 2).

The low-reflective phase in *Type II* and *Type III* chromite is rich in spinel ss. (Spl; MgAl₂O₄), with higher Mg# (0.56-0.63 in *Type II* chromite and 0.64-0.73 in *Type III* chromite) but lower Cr# (0.50-0.54 in *Type II* chromite and 0.41-0.43 in *Type III* chromite) than the high-reflective phase (Mg# = 0.21-0.23 and Cr# = 0.85-0.87 in *Type II* chromite; Mg# = 0.29-0.33 and Cr# = 0.76-0.79 in *Type III* chromite), which is rich in magnetite (Mag; Fe²⁺Fe³⁺₂O₄) (Fig. 2c; Table 2). This is depicted in the spinel prism (Fig. 2d), which shows that the exsolved phases plot along a compositional field between the spinel (Fe³⁺/R³⁺ = 0.12-0.21) and magnetite (Fe³⁺/R³⁺ = 0.55-0.65) corners (i.e. along the MgFe₁-Al₂Fe³⁺₂ exchange vector) (Table 2). In contrast to other exsolved phases in chromite from the literature (e.g., Loferski and Lipin 1983, Tamura and Arai 2004, 2005) the Cr³⁺/R³⁺ ratio in the exsolutions of spinel-rich and magnetite-rich phases from *Type II* and *Type III* chromite is rather constant (from 0.29 to 0.43 and from 0.30 to 0.37, respectively) (Fig. 2d; Table 2).

251 Crystal-crystal partition coefficients
Minor- and trace-element concentrations for *Type I* and *Type III* chromite from Los Congos and Los Guanacos are shown in Table 2. Analyses of *Type II* chromite from Los Congos were excluded from the following treatment because the beam size of LA-ICP-MS (~30-50 µm) is larger than the size of the exsolved phases (cf. Fig. 1d), and also because internal normalization with the EPMA data is not straightforward. Table 3 shows two statistical tests comparing the concentrations of minor and trace elements in the spinel-rich and magnetite-rich phases in *Type III* chromite. The differences in concentration between the two phases are statistically significant for all elements except for Cu and Sn. These tests also reveal that the difference in Mn is less robust than for the rest of the elements. Assuming that the exsolved phases represent an equilibrium assemblage at a given bulk composition, temperature, pressure and $fO_2$, a set of spinel-rich/magnetite-rich crystal-crystal partition coefficients can be evaluated from the relative change in concentration of the two exsolved phases. Crystal-crystal empirical partition coefficients are calculated based on the LA-ICP-MS data according to the following expression (1):

$$D_i^{Spl/Mag} = \frac{C_i^{Spl}}{C_i^{Mag}} \quad (1)$$

where the spinel-rich/magnetite-rich partition coefficient ($D_i^{Spl/Mag}$) of an element $i$ is defined as the ratio of the concentration of the element (in ppm) in the pair of exsolutions of spinel-rich ($C_i^{Spl}$) and the magnetite-rich ($C_i^{Mag}$) phases. Given the restricted range in major-element composition of the exsolved phases (see above) two approaches can be followed to estimate $D_i^{Spl/Mag}$: (1) averaging partition coefficient values for individual pairs (5 pairs of spatially related phases assumed to have been exsolved from the same grain) and (2) computing partition coefficient values based on
275 the average composition of six spinel-rich and seven magnetite-rich phases (where no
276 assumptions of pairs are made). The two approaches give nearly identical results,
277 however the averaged partition coefficient for individual pairs was used for comparison
278 (Table 3).

279 **Minor elements.** Ti, Ni, V, Mn, Co and Zn have been listed in increasing order
280 of compatibility with the spinel-rich phase (Fig. 3; Table 3). Empirical partition
281 coefficients span more than an order of magnitude, from D<sup>Spl/Mag</sup><sub>Ti</sub> = 0.30 ± 0.06 to
282 D<sup>Spl/Mag</sup><sub>Zn</sub> = 5.48 ± 0.63 (Table 3). Minor elements obtained in this work can be readily
283 compared to EPMA analyses of other exsolved phases in chromite from the literature
284 (Fig. 3; see Appendix for sources), where the exchange vector is systematically along
285 the spinel-magnetite corners and with rather constant Cr<sup>3+</sup>/R<sup>3+</sup> ratio for each pair
286 (although variable from pair to pair). All elements are in good agreement except for
287 D<sup>Spl/Mag</sup><sub>v</sub>, D<sup>Spl/Mag</sup><sub>Co</sub> and D<sup>Spl/Mag</sup><sub>Zn</sub> which are higher in this study compared to the
288 literature data (0.59 ± 0.11 vs 0.31 ± 0.14 for V; 1.85 ± 0.23 vs 0.92 ± 0.11 for Co and
289 5.48 ± 0.63 vs 3.32 ± 1.98 for Zn, respectively) (Fig. 3). The general agreement is
290 nevertheless remarkable considering that no attempt has been made to subdivide the
291 literature data based on geological setting, temperature or oxygen fugacity conditions
292 and that Cr<sup>3+</sup>/R<sup>3+</sup> ratio in the literature data spans the whole possible range along the
293 miscibility gap (Sack and Ghiorso 1992).

294 **Trace elements.** Sc, Ge, Cu, Sn and Ga are listed in increasing order of
295 compatibility with the spinel-rich phase (Table 3), spanning also almost an order of
296 magnitude (from D<sup>Spl/Mag</sup><sub>Sc</sub> = 0.33 ± 0.09 to D<sup>Spl/Mag</sup><sub>Ga</sub> = 2.23 ± 0.40; Table 3).

DISCUSSION

297
298 **Temperature of the exsolution process**

299 The exsolution of spinel-rich and magnetite-rich phases from a precursor
300 chromite from Los Congos and Los Guanacos seems to be independent of the texture of
301 the chromitite body but always it is restricted to clinopyroxene-free assemblages (Table
302 1). It is interesting to note that the observed exsolutions are in chromitite samples where
303 the main silicate is antigorite rather than clinochlore (Table 1). Clinochlore (but not
304 antigorite) is the expected phyllosilicate produced during hydrous metamorphism of
305 chromite-olivine bearing rocks under amphibolite- to greenschist-facies conditions (e.g.,
306 Gervilla et al. 2012). The extensive occurrence of antigorite might indicate an
307 orthopyroxene-bearing chromitite as the original protolith hosting *Type II* and *Type III*
308 chromite prior to hydration (e.g., Bach et al. 2006; Frost and Beard 2007). However, it
309 is clear that the main factor controlling the exsolution was the $Cr^{3+}/R^{3+}$ ratio of the
310 original chromite (Fig. 2 and 4). Only chromite with $Cr^{3+}/R^{3+}$ ratio < 0.35 (and
311 concomitant high $Fe^{3+}/R^{3+} > 0.40$) show exsolutions of spinel-rich and magnetite-rich
312 phases (Table 2). This feature has been observed in all previously reported studies of
313 exsolution in chromite (Purvis et al. 1972; Muir and Naldrett 1973; Loferski and Lipin
314 1983; Eales et al. 1988; Zarkrzewski 1989; Jan et al. 1992; van der Veen and Maaskant
315 1995; Appel et al. 2002; Garuti et al. 2003; Tamura and Arai 2004, 2005; Krause et al.
316 2007; Ahmed et al. 2008). The position of the miscibility gap defined by the exsolved
317 phases should be indicative of the temperature of exsolution if some constraints can be
318 put on the composition of the ferromagnesian silicate in equilibrium with the chromite
319 prior to its exsolution (Sack and Ghiorso 1991).

320 The minimum temperature of the exsolution can be constrained using the
321 composition of homogenous *Type I* chromite from disseminated chromitite bodies from
322 Los Guanacos, which has the lowest $Cr^{3+}/R^{3+}$ ratio (0.44-0.48; Table 2; Fig. 4).
Temperature conditions equal to or higher than 600 °C are required to preserve this chromite as a homogenous phase, which is independent of the forsterite content of the olivine (Sack and Ghiorso 1991). Isothermal (isobaric) Cr-Al-Fe³⁺ sections of the spinel prism at 600 °C (Sack and Ghiorso 1991) show that the slope of the empirical tielines joining the pair of exsolutions of spinel-rich and magnetite-rich phases match with those calculated by Sack and Ghiorso (1991) when the equilibrium olivine composition is Fo₈₀ or Fo₉₅ (Fig. 4a and c). The first situation can apply to exsolved phases in chromite with very similar compositions reported from layered intrusions (Purvis et al. 1972; Muir and Naldrett 1973; Loferski and Lipin 1983; Eales et al. 1988; Zarkrzewski 1989; Jan et al. 1992; van der Veen and Maaskant, 1995; Appel et al. 2002; Garuti et al. 2003; Krause et al. 2007; Ahmed et al. 2008) but is at odds with the presumed setting of the ultramafic rocks from the Argentinian Central Andes (Proenza et al. 2008 and references therein). Therefore, we suggest that an olivine of ~Fo₉₅ was in equilibrium with the precursor chromite at around 600 °C in Los Guanacos (Fig. 4c). This interpretation is consistent with the fact that the relic clinopyroxene associated with *Type I* chromite is highly-magnesian (X<sub>Mg</sub> = 0.954 ± 0.004) and presumably, cogenetic. Tamura and Arai (2004, 2005) reached a similar conclusion from the Iwanai-dake peridotite complex from Hokkaido (Japan), which in this case was supported by the preservation of magmatic olivine with Fo₉₂-9₅.

The estimated minimum temperature for the exsolution (ca 600 °C) from Los Guanacos (and most likely also from Los Congos) matches with the thermal conditions of amphibolite-granulite facies metamorphism (590-730 °C) that is inferred to have affected the ultramafic bodies from the southern part of the Eastern Pampean Ranges of Córdoba (Rapela et al. 1998; Escayola et al. 2004; Martino et al. 2010). Under these conditions in the crust, the inferred minimum temperature for Los Guanacos and Los
348 Congos precludes the coexistence of hydrous phases with chromite during the
349 exsolution process (Gervilla et al. 2012).

350 **Crystal-chemistry constraints on the partition coefficients**

351 The observed crystal-crystal partition coefficients points to the effect that the
352 crystal-chemistry of the spinel-rich (MgAl₂O₄) and magnetite-rich phases (Fe²⁺Fe³⁺₂O₄)
353 has on all investigated elements except Cu and Sn (Table 3). The elements most
354 sensitive to crystal-chemistry could be Zn, Ga, Co, Ni, Ti and Sc (Table 3). On the other
355 hand, Mn is only slightly more compatible in the magnetite-rich phase (Table 3).

356 Oxide spinel minerals comprise a large group of end-members (e.g., Biagioni
357 and Pasero 2014) with the general formula AB₂O₄ (some of them are included in Table
358 4). The letters A and B represent a range of elements with different valences, and the
359 formulae are conventionally represented as (A²⁺)[B³⁺]₂O₄ (2-3 spinels) and
360 (A⁴⁺)[B²⁺]₂O₄ (4-2 spinels), where tetrahedrally and octahedrally coordinated sites are
361 represented with parenthesis () and brackets [] respectively. In normal spinels the
362 tetrahedrally coordinated sites are occupied by A whereas the octahedrally coordinated
363 sites are occupied exclusively by B. In inverse spinels the tetrahedrally coordinated sites
364 are occupied by B cations whereas the octahedrally coordinated sites are occupied by A
365 and B cations: (B³⁺)[A²⁺B³⁺]O₄ for 2-3 spinels and (B²⁺)[A⁴⁺B²⁺]O₄ for 4-2 spinels.
366 Cation distribution and degree of ordering in spinel-type structures have been
367 investigated intensely in the past half-century (e.g., see Price et al. 1982 and references
368 therein). Early models were based on crystal field stabilization energies (e.g., McClure
369 1957; Dunitz and Orgel 1957) but these were later superseded by calculations of
370 electrostatic lattice energy that showed the important role of ionic radii (O'Neill and
371 Navrotsky 1983), pseudopotential orbital radii (Price et al. 1982), or ionic potential
(Bosi et al. 2012; Fregola et al. 2012) in influencing cation site preferences in spinel structures. Different cations and their arrangements in octahedrally and tetrahedrally coordinated sites result in changes of the unit-cell parameter, *a*, as well as, in the oxygen fractional coordinate, *u*-parameter (e.g., Lavina et al. 2002; Stevanović et al. 2010). The latter parameter describes the deviation of the ideal cubic close packed (CCP) array and the angular distortion of the octahedrally coordinated oxygen polyhedron from the ideal octahedron; and it has been successfully used to predict the normal or inverse nature of 2-3 and 4-2 spinel end-members (e.g., O'Neill and Navrotsky 1983; Stevanović et al. 2010).

It can be envisaged that the observed differences in element partitioning between the spinel-rich and the magnetite-rich phases (and therefore their relative solubility) is ultimately related to their contrasting crystal-chemistry (i.e., larger *a* and lower *u* for magnetite relative to spinel, Table 4). Some possible end-members that might be involved in the observed partitioning are shown in Table 4. Contrary to the expectation there is not to clear correlation between *u* and $D_i^{Spl/Mag}$. For the end-member gahnite (ZnAl₂O₄) *a* and *u* are effectively similar to spinel (8.0860 vs 8.0832 Å and 0.2636 vs. 0.2624, respectively) (Table 4); this similarity probably explains the enrichment of Zn in the spinel-rich phase relative to the magnetite-rich phase ($D_{Zn}^{Spl/Mag} = 5.48 \pm 0.63$; Table 3). Compared to spinel, the Ga end-member ZnGa₂O₄ has also similar *u* (0.2617) but contrasting *a* (8.3300 Å) (Table 4) but it is still more compatible with the spinel-rich phase ($D_{Ga}^{Spl/Mag} = 2.23 \pm 0.40$; Table 3), suggesting that *u* rather than *a* controls the partitioning. However, this inference does not apply to Jacobsite (MnFe₂O₄) which is slightly more compatible with the magnetite-rich phase ($D_{Mn}^{Spl/Mag} = 0.81 \pm 0.15$; Table 3) despite having *u* (0.2615) similar to the Ga end-member (Table 4). The lack of
correlation between *u* (and/or *a*) and $D_i^{Spl/Mag}$ is corroborated by the partition of Cu,
which probably is controlled by cuprospinel ($CuFe_2O_4$). In this case, although *u* is
similar to magnetite (0.2550 vs. 0.2548, Table 4) Cu is equally partitioned in both
phases ($D_{Cu}^{Spl/Mag} = 1.11 \pm 0.57$; Table 3).

Compared to crystal-melt equilibrium where the ionic radii and the elasticity of
the crystal lattice are key parameters controlling partitioning (e.g., Blundy and Wood
1994), the factors controlling crystal-crystal partitioning are less well constrained. An
attempt to correlate ionic radii with $D_i^{Spl/Mag}$ shows only a very weak dependency (Fig.
5). Divalent cations with radius greater than 0.65 Å are partitioned in the magnetite-rich
phase (Fig. 5a). The correlation for trivalent and tetravalent cations is even less clear
(Fig. 5b and c). This is in part due to the degree of ordering (normal vs inverse) of the
spinel. In inverse and partially-inverse spinels the same trivalent cation in 2-3 spinels
(or divalent cations in 4-2 spinels) occupies two different sites, resulting in two apparent
ionic radii. This even makes the definition of element partitioning based on bulk
analyses ambiguous as by definition they do not distinguish between sites. Moreover,
the variable valence states of certain elements (such as V, Ti, Mn or Co) further
complicates this picture. The complex interplay between crystal-chemistry and the
partitioning of minor- and trace-elements suggests a relative dependence on cell
parameters and ionic radii of these elements in spinel-type structures. Nevertheless, the
ultimate influence of crystal-chemistry on the partitioning is not yet deciphered, due to
the lack of detailed studies about the site preference and valence states of minor- and
trace-elements in the spinel structures.

**Comparison with chromite/melt element partitioning from high temperature**
**experiments**
Since early experimental studies (e.g., Horn et al. 1994) it was noticed that the chromite/melt partitioning of certain minor- and trace-elements is a function of the composition of major-elements in chromite. A direct comparison with our calculated D<sup>Spl/Mag</sup> is not always straightforward as the main exchange vectors experimentally investigated regarding trivalent cations are mostly limited to Fe<sup>3+</sup>Cr<sub>.1</sub> (e.g., Horn et al. 1994) and AlCr<sub>.1</sub>, (Wijbrans et al. 2015; these authors also presented data for two compositions along the vector Fe<sup>3+</sup>Al<sub>.1</sub>). The remarkable agreement of the partitioning data from the exsolved phases in chromite from this study and from the literature (Fig. 3), spanning all possible ranges of Cr<sup>3+</sup>/R<sup>3+</sup> ratio of the solvus (from nearly zero to 0.6; Sack and Ghiorso 1991), suggest that the main factor controlling the partitioning in exsolved phases is not due to variations in Cr<sup>3+</sup> but due to the exchange vector Fe<sup>3+</sup>Al<sub>.1</sub>.

High temperature (HT) partitioning experiments for Ti and Sc are consistent among several studies and indicate an increase in D<sup>Spl/Mag</sup><sub>Ti</sub> and D<sup>Spl/Mag</sup><sub>Sc</sub> with increasing Fe<sup>3+</sup> under otherwise constant pressure and temperature conditions but variable oxygen fugacity (Irving 1978; Horn et al. 1994; Nielsen et al. 1994; Nielsen and Beard 2000). This is consistent with the preference of Ti and Sc for the magnetite-rich phase observed in this study (Fig. 3); these elements are amongst the most sensitive to spinel crystal-chemistry (D<sup>Spl/Mag</sup><sub>Ti</sub> = 0.30 ± 0.06 and D<sup>Spl/Mag</sup><sub>Sc</sub> = 0.33 ± 0.09; Table 3). Recent experiments by Wijbrans et al. (2015) further support the preference of Ti and Sc for Fe<sup>3+</sup>- and Cr-rich (Al-poor) spinels. These authors also observed only a small compositional effect on D<sup>Spl/melt</sup><sub>Mn</sub> in Fe<sup>3+</sup>-rich spinels, in line with our observations (D<sup>Spl/Mag</sup><sub>Mn</sub> = 0.81 ± 0.15; Table 3) and with the literature data (D<sup>Spl/Mag</sup><sub>Mn</sub> = 0.83 ± 0.42; Fig. 3). Ni is expected to be more compatible in magnetite (through the trevorite component, NiFe<sub>2</sub>O<sub>4</sub>, with inverse nature; Table 4) than in Al-rich spinels, as found in
the present study ($D_{Ni}^{\text{Spl/Mag}} = 0.42 \pm 0.09$; Table 3) and in other exsolved phases in chromite from the literature ($D_{Ni}^{\text{Spl/Mag}} = 0.46 \pm 0.28$; Fig. 3). However, chromite/melt partitioning experiments performed by Righter et al. (2006) and Wijbrans et al. (2015), did not reveal any clear relationship between the partitioning of Ni in chromite and variations in major-element components, except for the amount of Ni itself (Righter et al. 2006). Furthermore, the compositional effect on the partitioning of V cannot be inferred directly from experimental studies because it is strongly linked to the effects of temperature and oxygen fugacity (Horn et al. 1994; Nielsen et al. 1994; Canil 1999; Toplis and Corgne 2002; Lee et al. 2005; Righter et al. 2006; Mallmann and O'Neill 2009).

Contrary to our observations (Fig. 3; Table 3), the compositional effect on Ga, Co and Zn partitioning in exsolved phases in chromite with contrasting major-element compositions (Horn et al. 1994; Righter et al. 2006; Wijbrans et al. 2015) has not been reported so far. Wijbrans et al. (2015) reported some experimental challenges regarding these elements at high temperatures (> 1300 °C) and low oxygen fugacity, explaining it due to the volatility of Zn and Ga and alloying of Co with the Pt wire. Another possible explanation is that these elements are only sensitive to $Fe^{3+}$Al$_{1}$ variations, which have been poorly experimentally constrained. If so, partitioning data from the exsolved phases in chromite will represent a valuable source of information about the dependency of Ga, Co and Zn partitioning on compositions along the MgFe$_{1}$-Al$_{2}$Fe$^{3+}_{2}$ exchange vector; this information could be extrapolated with caution to chromite/melt partitioning.

In summary it can be concluded that the results of HT experiments and low temperature (LT) exsolutions in chromite are generally in agreement and suggest a significant dependence of Ti and Sc partitioning on $Fe^{3+}$ substitution in chromite. We
469 speculate that, rather than temperature, the compositional effect on Ga, Co and Zn
470 observed exclusively in the exsolved phases from chromite is due to crystal-chemistry
471 differences along the MgFe₁-Al₂Fe³⁺₂ exchange vector (spinel-magnetite vector). The
472 implications of this inference and some observations that might support it will be further
473 explained below.

474

IMPLICATIONS

475 The compositional effect that major-elements has on minor- and trace-element
476 partitioning in the exsolved phases in chromite, as deduced from this study, might give
477 some insights into the origin of some compositional trends (or lack of trends) of
478 chromite reported from different magmatic settings and those associated with
479 metamorphism (Fig. 6; see Appendix for data sources).

480 The linear positive covariation shown by Co and Zn in chromite from magmatic
481 suites is remarkable and suggests a constant relative solubility of Co and Zn (probably
482 as CoAl₂O₄ and ZnAl₂O₄, gahnite, end-members in a 1:3 molar proportion; Fig. 6a).
483 Moreover these elements are both correlated (particularly for layered intrusions) with
484 the MgAl₂O₄ component in the chromite (Fig. 6a). It is also noteworthy the strong
485 positive correlation between Co and Ga in chromite from layered intrusions (probably
486 suggesting a 6:1 molar mix of CoAl₂O₄ and ZnGa₂O₄) which is not observed in
487 ophiolitic peridotites nor in lavas (Fig. 6b). Compared to other magmatic settings,
488 major-element compositional variations in chromite from layered intrusions correspond
489 mostly to the exchange vectors MgFe₁-Al₂Fe³⁺₂, with minor variations of Cr/(Cr+Al)
490 atomic ratio (e.g., Barnes and Roeder 2001). This further supports our previous
491 suggestion that Zn, Co and Ga partitioning is mostly sensitive to variations in MgFe₁-
492 Al₂Fe³⁺₂. As a whole these trends probably reflect gradual compositional changes in
493 chromite in equilibrium with melt (and/or olivine) in response to variations in temperature and/or composition.
494

495 Our observations have also some implications for our understanding of the
496 disturbance of magmatic patterns in minor- and trace-elements during hydration and
497 metamorphism of chromitite bodies. Chromite in apparent textural equilibrium with
498 chlorite shows a distinctive departure from the magmatic trends discussed above (Fig.
499 6c and d). Chromite associated with chlorite, but particularly those enriched in Fe³⁺ (i.e.,
500 Fe³⁺-rich chromite), have considerably higher Co and Zn contents than magmatic
501 chromite. Moreover the Co:Zn ratio is higher and more scattered in chlorite-bearing
502 assemblages (Fig. 6c). This increase in Co and Zn is associated with a decrease in Ga
503 particularly in Fe³⁺-rich chromite (Fig. 6d). Whereas the decrease in Ga could be
504 explained by an important change in the chromite composition, the coeval enrichment
505 of Zn and Co is not consistent with the increase in the Fe₃O₄ component in the chromite,
506 which is less variable related to hydrous metamorphism (Barnes 2000; Barnes and
507 Roeder 2001; Mukherjee et al. 2010, 2015; Gervilla et al. 2012; Prabhakar and
508 Bhattacharya 2013; Singh and Singh 2013; Gargiulo et al. 2013). The most likely
509 explanation is that the contents of minor- and trace-elements in chromite affected by
510 hydrous metamorphism are no longer controlled by the compositional changes of major
511 elements in chromite but potentially to the chemistry of metamorphic fluids (e.g., fluids
512 enriched in Zn and Co; Barnes 2000), which produce an incomplete reaction with
513 magmatic chromite (i.e., presence of chromite cores) and the formation of hydrous
514 phases in equilibrium with metamorphosed chromite (see Figure 7 of Gervilla et al.
515 2012). Element partitioning between metamorphosed chromite and hydrous phases has
516 not yet been thoroughly investigated. Interestingly, Colás (2015) reported chlorite from
517 the metamorphosed chromitites of the Eastern Rhodope with relatively high contents of
Ga (up to 30 ppm) and low contents of Zn and Co (below 15 ppm and 18 ppm, respectively). Therefore, the formation of chlorite by interaction with fluids during hydrous metamorphism might explain the enrichment in Co and Zn coupled with strong depletion in Ga in chromite from metamorphosed chromitites of the Eastern Rhodope, as discussed by Colás et al. (2014).

## ACKNOWLEDGMENTS

This research was supported by the project CGL2010-15171 and F.P.I. grant BES-2011-045423 of the Ministerio de Econonía y Competitividad (Spain). Support for this study has also been provided by the FONDECYT #11140005 and 'Millenium Nucleus for Metal Tracing Along Subduction NC130065 to José María González-Jiménez. We wish to thank Steve Barnes and an anonymous reviewer for their careful and constructive comments and Ferdinando Bosi for his proficient editorial handling of this manuscript. The analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS, industry partners and Macquarie University. This is a contribution XXX from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and XXX in the GEMOC Key Centre (www.gemoc.mq.edu.au).

## APPENDIX

### Data sources for calculated crystal-crystal partition coefficients

The crystal-crystal partition coefficients of exsolved phases in chromite from previously published data sets were calculated using reported EMPA data for minor-elements. Chromite with exsolution textures come from the layered complexes of Carr-Boyd Rocks Complex (Western Australia, Purvis et al. 1972), Giant Nickel Mine
(British Columbia, Muir and Naldrett 1973), Red Lodge district (Montana, USA, 541
Loferski and Lipin 1983), Kuså (Sweden, Zakrzewski 1989), Chilas (Pakistan, Jan et al. 542
1992), Isua Greenstone Belt (Greenland, Appel et al. 2002) and the Eastern Desert 543
(Egypt, Ahmed et al. 2008); from the Uralian-Alaskan-type complexes of Staré Ransko 544
(Czech Republic, van der Veen and Maaskant 1995), Uktus (Rusia, Garuti et al. 2003) 545
and the Central Ural Mountains (Krause et al. 2007); and from the peridotite complex of 546
Iwanai-dake (Japan, Tamura and Arai 2005). 547

548 Data source for minor- and trace-elements in chromite

The data sources for minor- and trace-elements in chromite used in this study 549
consist of previously published and new data sets. Chromite samples are classified 550
based on magmatic setting (ophiolitic peridotites, lavas and layered intrusions) and by 551
metamorphic assemblage (chromite and Fe³⁺-rich chromite in equilibrium with 552
chlorite). 553

Reported minor- and trace-element compositions of chromite in ophiolitic 554
peridotites come from the Thetford Mine Ophiolite (Canada; Pagé and Barnes 2009), 555
Ouen Island and Dyne (New Caledonia; González-Jiménez et al. 2011; Colás et al. 556
2014), Mercedita, Tres Amigos and Rupertina (Cuba; Colás et al. 2014; González- 557
Jiménez et al. 2015), and Luobusa (Tibet, Zhou et al. 2014). Chromite samples of lavas 558
are taken from the East Pacific Rise, Bonin Island (Japan), Thetford Mine Ophiolite 559
(Canada) (Pagé and Barnes 2009), and Solomon Island (Yao 1999) and those of layered 560
intrusions from the Bushveld Complex (South Africa) and the Great Dike (Zimbabwe) 561
(Yao 1999). Data for the metamorphosed chromite come from ophiolitic chromitites of 562
Los Congos and Los Guanacos (Argentina), Central and Eastern Rhodope (Bulgaria, 563
González-Jiménez et al. 2015; Colás et al. 2014), Ouen Island (New Caledonia; 564
565 González-Jiménez et al. 2011) and Southeastern Turkey (Akmaz et al. 2014); and those
566 from the greenstone belt of Nuggihalli (India, Mukherjee et al. 2015).

567 Matlab script to plot spinel prism in 3D

568 The following is a simple Matlab script to plot the spinel prism that reads an input csv
569 file consisting in four columns (without headers) as follow Cr/R³⁺, Al/R³⁺, Cr/R³⁺, and
570 X<sub>Mg</sub> in mole proportions.

571 clear all
572 data = importdata('CongosExs.csv');
573 lambda = data(:,1:3);
574 points_x = transpose(1-data(:,4));
575
576 % Cartesian components of the triangle vertices r1, r2, r3;
577 % ri = (yi, zi), i=1:3
578 %         r1 r2 r3
579 vertex = [0 1 0.5; % y
580           0 0 1]; % z
581
582 % transformation to cartesian coordiantes
583 points_y = zeros(1,length(lambda));
584 points_z = zeros(1,length(lambda));
585
586 for i=1:length(lambda)
587     points_y(i) = lambda(i,1)*vertex(1,1)...
588     +lambda(i,2)*vertex(1,2)...
589     +lambda(i,3)*vertex(1,3);
590     points_z(i) = lambda(i,1)*vertex(2,1)...
591     +lambda(i,2)*vertex(2,2)...
592
592 +lambda(i,3)*vertex(2,3);

593 end

594

595 scatter3(points_x,points_y,points_z)

596 xlabel('x'),ylabel('y'), zlabel('z'),

597 xlim([0 1]), zlim([0 1])

598 text(-0.05,-0.15,0,[ 'Pc'], 'FontSize',14)

599 text(-0.05,1.15,-0.1,[ 'Sp'], 'FontSize',14)

600 text(1,-0.15,0,[ 'Chr'], 'FontSize',14)

601 text(0,0.5,1.1,[ 'Mf'], 'FontSize',14)

602 text(1,0.5,1.1,[ 'Mt'], 'FontSize',14)

603 text(1.15,1.25,-0.1,[ 'Her'], 'FontSize',14)

604 hold on

605

606 daspect([1.7 1 1.1753])

607

608 % Prism

609 plot3([0 0 0 0 1 1 1 1 1 0 0 1],...

610 [0 1 0.5 0 0 1 0.5 0 0.5 0.5 1 1],...

611 [0 0 1 0 0 0 1 0 1 1 0 0])

612

613

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873 FIGURE CAPTIONS
**Figure 1.** Back Scattered Electron (BSE) images of the different textures of chromite identified in the studied chromitites. *Type I* chromite from Los Congos (**a**) and Los Guanacos (**b**) with relics of clinopyroxene; *Type II* chromite from Los Congos (**c**, **d**) and *Type III* chromite from Los Guanacos (**e**, **f**). Mineral abbreviations after Whitney and Evans (2010), where antigorite is Atg, chromite is Chr, clinopyroxene is Cpx, magnetite is Mag and spinel is Spl.

**Figure 2.** Compositional variations of the different textures of chromite from the studied chromitites in terms of Mg# [Mg/(Mg+Fe²⁺) atomic ratio] versus Cr# [Cr/(Cr+Al) atomic ratio] and the spinel prism. Legend provided as inset in the figure and Matlab script to plot spinel prism in 3D is provided in the Appendix.

**Figure 3.** Box chart diagrams illustrating the statistical crystal-crystal partition coefficient of certain minor- and trace-elements in exsolved phases within chromite from this study (grey boxes) and from the literature (white boxes). Elements arranged in increasing order of compatibility with the spinel-rich phase. Data sources are listed in the Appendix.

**Figure 4.** Isothermal (isobaric) sections determined at 600 and 550 °C through the spinel prism calculated by Sack and Ghiorso (1991) for fixed Fe-Mg potentials defined by Fo₈₀ (**a**, **b**) and Fo₉₅ (**c**, **d**). Legend provided as inset in the figure.

**Figure 5.** Plot of crystal-crystal partition coefficients vs ionic radii (Shannon 1976; O'Neill and Navrotsky 1983) for divalent cations (**a**), trivalent and tetravalent cations in normal (**b**) and inverse spinels (**c**). Legend provided as inset in the figure.

**Figure 6.** Compositional variation in terms of Co (mol·10⁻⁴) versus Zn (mol·10⁻⁴) and Ga (mol·10⁻⁴) in chromite from different magmatic settings (**a**, **b**), and affected by
897 hydrous metamorphism (c, d). Legend provided as inset in the figure and data sources
898 are listed in the Appendix.

899 TABLE CAPTIONS

900 **Table 1.** Characterization of the chromitite samples from Los Congos and Los
901 Guanacos ultramafic massifs (Argentina) investigated in this study.

902 **Table 2.** Average composition of major-, minor- and trace-elements in chromite grains
903 from Los Congos and Los Guanacos ultramafic massifs (Argentina) analyzed by EMPA
904 and LA-ICP-MS.

905 **Table 3.** Statistics of minor- and trace-elements of Type III chromite from Los
906 Guanacos ultramafic massif (Argentina).

907 **Table 4.** Oxide spinels, ideal composition and selected structural data.

908 SUPPLEMENTARY MATERIAL

909 **Figure 1S.** Counts signal vs time in LA-ICP-MS analysis of low-reflective and high-reflective phases in Type III chromite from Los Guanacos and mixtures of low-reflective and high-reflective phases in Type II chromite from Los Congos. Legend provided as inset in the figure.

913 **Table 1S.** Results of repeated analysis of the BCR-2g and LCR-1 standard by laser
914 ablation ICP-MS (New Wave UP 266) and comparison with literature values.
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5</figcaption></figure>
<caption>Figure 6</caption>

Magmatic chromite

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Metamorphosed chromite

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
<table><caption>**Table 1.** Characterization of the chromitite samples from Los Congos and Los Guanacos ultramafic massifs (Argentina) investigated in this study.</caption><thead><tr><th rowspan="2">Chromite type</th><th rowspan="2">Locality</th><th rowspan="2">Sample</th><th colspan="2">Chromitite body</th><th rowspan="2">Exsolved phase composition</th><th rowspan="2">Exsolved phase proportions<sup>a</sup></th></tr><tr><th>Texture</th><th>Silicate matrix</th></tr></thead><tbody><tr><td rowspan="10">Type I</td><td rowspan="3">Los Congos</td><td>M-25</td><td rowspan="6">Massive</td><td>Atg (10 vol%)<br/>Cpx (10 vol%)</td><td rowspan="10">Chromite-rich</td><td rowspan="10">100%</td></tr><tr><td>M-17</td><td>Atg (15 vol%)<br/>Cpx (3 vol%)<br/>Clc (2 vol%)</td></tr><tr><td></td><td>Atg (35 vol%)<br/>Cpx, (4 vol%)<br/>Clc (4 vol%)</td></tr><tr><td rowspan="3">Los Guanacos</td><td>3317</td><td rowspan="3">Semi-massive</td><td>Atg (55 vol%)<br/>Cpx (7 vol%)<br/>Clc (5 vol%)</td></tr><tr><td>2260a</td><td>Atg (50-70 vol%)</td></tr><tr><td></td><td>Atg (50-70 vol%)</td></tr><tr><td rowspan="2">Type II</td><td rowspan="2">Los Congos</td><td rowspan="2">M-27</td><td>Spinel-rich</td><td>na<sup>b</sup></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>Magnetite-rich</td><td>na</td></tr><tr><td rowspan="3">Type III</td><td rowspan="3">Los Guanacos</td><td rowspan="3">2226</td><td>Spinel-rich</td><td>39.73%</td><td rowspan="3"></td><td rowspan="3"></td></tr><tr><td>Magnetite-rich</td><td>60.27%</td></tr><tr><td>Chromite-rich</td><td>100%</td></tr></tbody></table>

Note: Mineral abbreviations after Whitney and Evans (2010), where antigorite is Atg, clinochlore is Clc and clinopyroxene is Cpx.

<footnote><sup>a</sup> Calculated proportion of each phase in exsolved chromite grains using BSE images on the software Image_J (Rasband 2007). <sup>b</sup> Not available (na)</footnote>
<table><caption>**Table 2.** Average composition of major-, minor- and trace-elements in chromite grains from Los Congos and Los Guanacos ultramafic massifs (Argentina) analyzed by EMPA and LA-ICP-MS.</caption><thead><tr><th rowspan="3">Chromite<br>Location<br>Texture<br>Sample</th><th colspan="4">Type I</th><th colspan="3">Type II</th><th colspan="2">Type III</th></tr><tr><th colspan="2">Los Congos<br>Massive</th><th colspan="2">Los Guanacos<br>Semi-massive</th><th colspan="2">Los Congos<br>Disseminated<br>M-27</th><th colspan="2">Los Guanacos<br>Massive<br>2226</th></tr><tr><th>M-25<br>n = 17</th><th>M-17<br>n = 16</th><th>3317<br>n = 7</th><th>2260a<br>n = 8</th><th>Spinel-rich<br>n = 6</th><th>Magnetite-rich<br>n = 5</th><th>Spinel-rich<br>n = 10</th><th>Magnetite-rich<br>n = 9</th><th>Chromite-rich<sup>a</sup><br>n = 19</th></tr></thead><tbody><tr><td>TiO<sub>2</sub> (wt%)<sup>b</sup></td><td>0.19 ± 0.05</td><td>0.14 ± 0.02</td><td>0.19 ± 0.05</td><td>0.19 ± 0.03</td><td>0.8 ± 0.83</td><td>0.62 ± 0.07</td><td>0.05 ± 0.02</td><td>0.25 ± 0.01</td><td>0.17</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>21.4 ± 2.07</td><td>14.16 ± 1.55</td><td>17.13 ± 3.04</td><td>17.07 ± 2.08</td><td>21.02 ± 1.25</td><td>2.41 ± 0.11</td><td>28.00 ± 1.14</td><td>4.50 ± 0.44</td><td>13.83</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>36.79 ± 1.30</td><td>42.93 ± 2.00</td><td>47.58 ± 2.89</td><td>35.92 ± 1.01</td><td>33.23 ± 0.74</td><td>21.12 ± 0.76</td><td>30.63 ± 0.56</td><td>23.36 ± 0.87</td><td>26.24</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub><sup>c</sup></td><td>14.65 ± 1.85</td><td>15.75 ± 0.89</td><td>7.03 ± 0.42</td><td>18.68 ± 2.08</td><td>16.8 ± 0.39</td><td>46.28 ± 0.76</td><td>14.50 ± 1.69</td><td>43.90 ± 1.41</td><td>32.22</td></tr><tr><td>FeO</td><td>13.63 ± 0.75</td><td>16.37 ± 0.33</td><td>17.99 ± 0.83</td><td>19.66 ± 1.12</td><td>15.71 ± 1.24</td><td>25.27 ± 0.22</td><td>12.56 ± 1.06</td><td>23.00 ± 0.40</td><td>18.86</td></tr><tr><td>V<sub>2</sub>O<sub>3</sub></td><td>0.12 ± 0.02</td><td>0.13 ± 0.01</td><td>0.19 ± 0.03</td><td>0.07 ± 0.02</td><td>0.19 ± 0.02</td><td>0.36 ± 0.02</td><td>0.07 ± 0.03</td><td>0.19 ± 0.01</td><td>0.14</td></tr><tr><td>MnO</td><td>0.46 ± 0.03</td><td>1.04 ± 0.06</td><td>0.40 ± 0.03</td><td>0.36 ± 0.05</td><td>0.87 ± 0.08</td><td>0.9 ± 0.07</td><td>0.45 ± 0.03</td><td>0.68 ± 0.05</td><td>0.59</td></tr><tr><td>MgO</td><td>14.18 ± 0.59</td><td>11.15 ± 0.34</td><td>11.01 ± 0.85</td><td>9.93 ± 0.95</td><td>12.68 ± 0.63</td><td>3.97 ± 0.15</td><td>15.32 ± 0.81</td><td>5.67 ± 0.29</td><td>9.50</td></tr><tr><td>ZnO</td><td>0.09 ± 0.08</td><td>0.26 ± 0.03</td><td>0.20 ± 0.04</td><td>0.12 ± 0.07</td><td>0.37 ± 0.07</td><td>0 ± 0.00</td><td>0.44 ± 0.04</td><td>0.09 ± 0.09</td><td>0.23</td></tr><tr><td>NiO</td><td>0.2 ± 0.05</td><td>0.22 ± 0.06</td><td>0.08 ± 0.07</td><td>0.18 ± 0.06</td><td>0.19 ± 0.02</td><td>0.75 ± 0.02</td><td>0.20 ± 0.02</td><td>0.75 ± 0.05</td><td>0.53</td></tr><tr><td>Total</td><td>101.8 ± 0.7</td><td>102.1 ± 0.4</td><td>101.8 ± 0.6</td><td>102.3 ± 0.7</td><td>101.9 ± 0.79</td><td>101.7 ± 0.16</td><td>102.2 ± 0.4</td><td>102.4 ± 0.4</td><td>102.3</td></tr><tr><td colspan="10">Atoms per formula unit calculated on the basis of 4 oxygens</td></tr><tr><td>Al</td><td>0.77</td><td>0.53</td><td>0.64</td><td>0.64</td><td>0.76</td><td>0.10</td><td>0.97</td><td>0.18</td><td>0.53</td></tr><tr><td>Cr</td><td>0.88</td><td>1.08</td><td>1.18</td><td>0.90</td><td>0.81</td><td>0.60</td><td>0.71</td><td>0.64</td><td>0.67</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.34</td><td>0.38</td><td>0.17</td><td>0.45</td><td>0.39</td><td>1.25</td><td>0.32</td><td>1.15</td><td>0.79</td></tr><tr><td>V</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.00</td><td>0.01</td><td>0.00</td></tr><tr><td>Mg</td><td>0.64</td><td>0.53</td><td>0.52</td><td>0.47</td><td>0.58</td><td>0.21</td><td>0.67</td><td>0.29</td><td>0.46</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.35</td><td>0.44</td><td>0.47</td><td>0.52</td><td>0.40</td><td>0.76</td><td>0.31</td><td>0.67</td><td>0.51</td></tr><tr><td>Ti</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.02</td><td>0.02</td><td>0.00</td><td>0.01</td><td>0.00</td></tr><tr><td>Mn</td><td>0.01</td><td>0.03</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.02</td></tr><tr><td>Ni</td><td>0.00</td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.02</td><td>0.00</td><td>0.02</td><td>0.01</td></tr><tr><td>Mg#<sup>d</sup></td><td>0.61-0.68</td><td>0.52-0.57</td><td>0.48-0.55</td><td>0.42-0.52</td><td>0.56-0.63</td><td>0.21-0.23</td><td>0.64-0.73</td><td>0.29-0.33</td><td>0.42-0.49</td></tr><tr><td>Cr#<sup>e</sup></td><td>0.47-0.58</td><td>0.62-0.72</td><td>0.60-0.73</td><td>0.55-0.63</td><td>0.50-0.54</td><td>0.85-0.87</td><td>0.41-0.43</td><td>0.76-0.79</td><td>0.55-0.60</td></tr><tr><td>Cr<sup>3+</sup>/R<sup>3+</sup><sup>f</sup></td><td>0.41-0.47</td><td>0.51-0.60</td><td>0.55-0.67</td><td>0.44-0.48</td><td>0.40-0.43</td><td>0.29-0.32</td><td>0.35-0.37</td><td>0.30-0.34</td><td>0.34</td></tr><tr><td>Fe<sup>3+</sup>/R<sup>3+</sup><sup>g</sup></td><td>0.13-0.21</td><td>0.17-0.20</td><td>0.08-0.10</td><td>0.18-0.26</td><td>0.19-0.21</td><td>0.63-0.65</td><td>0.12-0.18</td><td>0.55-0.62</td><td>0.40-0.44</td></tr><tr><td colspan="10">n = 11 n = 17 n = 7</td></tr><tr><td>Sc (ppm)</td><td>1.92 ± 0.97</td><td>6.00 ± 1.44</td><td>0.72 ± 0.38</td><td></td><td></td><td></td><td>1.43 ± 0.40</td><td>4.10 ± 0.61</td><td>3.04</td></tr><tr><td>Ti</td><td>1159 ± 135</td><td>835 ± 96</td><td>1011 ± 252</td><td></td><td></td><td></td><td>377 ± 94</td><td>1198 ± 165</td><td>872</td></tr><tr><td>V</td><td>830 ± 70</td><td>775 ± 58</td><td>1197 ± 50</td><td></td><td></td><td></td><td>668 ± 95</td><td>1135 ± 159</td><td>950</td></tr><tr><td>Mn</td><td>3113 ± 157</td><td>7457 ± 376</td><td>2789 ± 259</td><td></td><td></td><td></td><td>3580 ± 372</td><td>4455 ± 666</td><td>4108</td></tr><tr><td>Co</td><td>306 ± 10</td><td>341 ± 14</td><td>381 ± 9</td><td></td><td></td><td></td><td>664 ± 33</td><td>359 ± 41</td><td>480</td></tr><tr><td>Ni</td><td>1710 ± 337</td><td>1609 ± 315</td><td>750 ± 80</td><td></td><td></td><td></td><td>2133 ± 411</td><td>5106 ± 747</td><td>3925</td></tr><tr><td>Cu</td><td>0.82 ± 0.69</td><td>2.61 ± 1.87</td><td>0.12 ± 0.02</td><td></td><td></td><td></td><td>12.11 ± 3.79</td><td>14.26 ± 4.09</td><td>13.41</td></tr><tr><td>Zn</td><td>1345 ± 74</td><td>2526 ± 164</td><td>2261 ± 201</td><td></td><td></td><td></td><td>5476 ± 348</td><td>991 ± 75</td><td>2773</td></tr><tr><td>Ga</td><td>26.49 ± 2.54</td><td>14.77 ± 2.32</td><td>24.16 ± 3.49</td><td></td><td></td><td></td><td>28.28 ± 1.09</td><td>13.01 ± 1.80</td><td>19.08</td></tr><tr><td>Ge</td><td>2.07 ± 0.31</td><td>2.48 ± 0.26</td><td>1.86 ± 0.27</td><td></td><td></td><td></td><td>2.64 ± 0.48</td><td>4.01 ± 0.59</td><td>3.47</td></tr><tr><td>Rb</td><td>0.63 ± 0.12</td><td>1.77 ± 0.45</td><td>0.91 ± 0.28</td><td></td><td></td><td></td><td>0.50 ± 0.19</td><td>0.44 ± 0.05</td><td>0.46</td></tr><tr><td>Sn</td><td>0.80 ± 0.24</td><td>7.14 ± 0.93</td><td>1.27 ± 0.21</td><td></td><td></td><td></td><td>0.95 ± 0.26</td><td>0.94 ± 0.21</td><td>0.95</td></tr></tbody></table>

<footnote><sup>a</sup>Composition of chromite grains prior to exsolution estimated from the average content and proportions (see Table 1) of spinel-rich and magnetite-rich phases. <sup>b</sup> Uncertainties are given as 1σ standard deviation. <sup>c</sup> Fe<sub>2</sub>O<sub>3</sub> contents of chromite were computed assuming R<sub>3</sub>O<sub>4</sub> stoichiometry and charge balance. <sup>d</sup> Range of Mg# [Mg/(Mg+Fe<sup>2+</sup>) atomic ratio]. <sup>e</sup> Range of Cr# [Cr/(Cr+Al) atomic ratio]. <sup>f</sup> Range of Cr<sup>3+</sup>/R<sup>3+</sup> ratio [Cr/(Fe<sup>3+</sup>+Al+Cr) in atomic ratio]. <sup>g</sup> Range of Fe<sup>3+</sup>/R<sup>3+</sup> ratio [Fe<sup>3+</sup>/(Fe<sup>3+</sup>+Al+Cr) in atomic ratio].</footnote>
<table><caption>**Table 3.** Statistics of minor- and trace-elements of Type III chromite from Los Guanacos ultramafic massif (Argentina)<sup>a</sup></caption><thead><tr><th></th><th></th><th></th><th><sup>45</sup>Sc</th><th><sup>47</sup>Ti</th><th><sup>51</sup>V</th><th><sup>55</sup>Mn</th><th><sup>59</sup>Co</th><th><sup>60</sup>Ni</th><th><sup>65</sup>Cu</th><th><sup>66</sup>Zn</th><th><sup>71</sup>Ga</th><th><sup>72</sup>Ge</th><th><sup>118</sup>Sn</th></tr></thead><tbody><tr><td rowspan="3">C<sub>i</sub><sup>Spl b</sup><br>(n = 6)</td><td>Mean</td><td></td><td>1.43</td><td>377</td><td>668</td><td>3580</td><td>664</td><td>2133</td><td>12.11</td><td>5476</td><td>28.28</td><td>2.64</td><td>0.95</td></tr><tr><td>Median</td><td></td><td>1.26</td><td>331</td><td>620</td><td>3444</td><td>673</td><td>1977</td><td>12.49</td><td>5456</td><td>28.01</td><td>2.63</td><td>1.05</td></tr><tr><td>Std. Error</td><td></td><td>0.16</td><td>38</td><td>39</td><td>152</td><td>14</td><td>168</td><td>1.55</td><td>142</td><td>0.45</td><td>0.20</td><td>0.11</td></tr><tr><td rowspan="3">C<sub>i</sub><sup>Mag c</sup><br>(n = 7)</td><td>Mean</td><td></td><td>4.10</td><td>1198</td><td>1135</td><td>4455</td><td>359</td><td>5106</td><td>14.26</td><td>991</td><td>13.01</td><td>4.01</td><td>0.94</td></tr><tr><td>Median</td><td></td><td>4.17</td><td>1202</td><td>1131</td><td>4481</td><td>357</td><td>5172</td><td>15.38</td><td>1000</td><td>13.34</td><td>4.03</td><td>0.94</td></tr><tr><td>Std. Error</td><td></td><td>0.23</td><td>63</td><td>60</td><td>252</td><td>16</td><td>283</td><td>1.54</td><td>28</td><td>0.68</td><td>0.22</td><td>0.08</td></tr><tr><td colspan="13" style="text-align: center; font-weight: bold;">Comparison of Means</td></tr><tr><td rowspan="2">T test</td><td><i>p</i></td><td>2.11E-06</td><td>7.45E-07</td><td>7.11E-05</td><td>1.45E-02</td><td>1.65E-08</td><td>5.44E-06</td><td>3.46E-01</td><td>2.74E-07</td><td>3.72E-09</td><td>7.07E-04</td><td>9.22E-01</td></tr><tr><td>Result</td><td>R</td><td>R</td><td>R</td><td>R</td><td>R</td><td>R</td><td>FR</td><td>R</td><td>R</td><td>R</td><td>FR</td></tr><tr><td rowspan="2">Mann-<br>Whitney U</td><td><i>p</i></td><td>3.41E-03</td><td>3.41E-03</td><td>5.34E-03</td><td>3.83E-02</td><td>3.41E-03</td><td>3.41E-03</td><td>2.84E-01</td><td>3.41E-03</td><td>3.41E-03</td><td>8.22E-03</td><td>9.43E-01</td></tr><tr><td>Result</td><td>R</td><td>R</td><td>R</td><td>R</td><td>R</td><td>R</td><td>FR</td><td>R</td><td>R</td><td>R</td><td>FR</td></tr><tr><td colspan="13" style="text-align: center; font-weight: bold;">Partition coefficients</td></tr><tr><td rowspan="2">D<sub>i</sub><sup>Spl/Mag d</sup></td><td>Mean</td><td>0.35</td><td>0.31</td><td>0.59</td><td>0.80</td><td>1.85</td><td>0.42</td><td>0.85</td><td>5.52</td><td>2.17</td><td>0.66</td><td>1.01</td></tr><tr><td>Error<sup>e</sup></td><td>0.04</td><td>0.04</td><td>0.05</td><td>0.06</td><td>0.09</td><td>0.04</td><td>0.14</td><td>0.21</td><td>0.12</td><td>0.06</td><td>0.14</td></tr><tr><td rowspan="2">D<sub>i</sub><sup>Spl/Mag f</sup></td><td>Mean</td><td>0.33</td><td>0.30</td><td>0.59</td><td>0.81</td><td>1.85</td><td>0.42</td><td>1.11</td><td>5.48</td><td>2.23</td><td>0.64</td><td>1.12</td></tr><tr><td>Std. Dev.</td><td>0.09</td><td>0.06</td><td>0.11</td><td>0.15</td><td>0.23</td><td>0.09</td><td>0.57</td><td>0.63</td><td>0.40</td><td>0.12</td><td>0.70</td></tr></tbody></table>

<footnote><sup>a</sup> Reported descriptive statistics are the mean, median, and standard errors of "n" samples of each phase from Type III chromite of Los Guanacos. Two statistical tests are also reported comparing the minor and trace elements of spinel-rich with those of magnetite-rich: T test: results of the T test for unequal and unknown variances; Mann-Whitney U test: results of the nonparametric (i.e., no assumptions about the distribution of the underlying population) Mann-Whitney U test. In both tests the null hypothesis is the two means are equal, and the level of significance is 5; *p* is the empirical significance test of a given test such as *p* < 0.05 indicates a confidence level greater than 0.99; *R* and *FR* stand for "reject" and "fail to reject" the null hypothesis, respectively. If rejected, it is likely that the two means are different (i.e., two populations) with a confidence level of *p*. All statistical computations were carried out using statistical tools from OriginPro (version 15) software. <sup>b, c</sup> Content of the element (in ppm) in the spinel-rich (C<sub>i</sub><sup>Spl</sup>) and the magnetite-rich (C<sub>i</sub><sup>Mag</sup>) phases. <sup>d</sup> Average of partition coefficient no assuming pairs. <sup>e</sup> Error of D<sub>i</sub><sup>Spl/Mag</sup> was calculated using the error propagation formulae for a division: δD<sub>i</sub><sup>Spl/Mag</sup> = D<sub>i</sub><sup>Spl/Mag</sup>√(δC<sub>i</sub><sup>Spl</sup>/C<sub>i</sub><sup>Spl</sup>)<sup>2</sup>+(δC<sub>i</sub><sup>Mag</sup>/C<sub>i</sub><sup>Mag</sup>)<sup>2</sup>, where δC<sub>i</sub><sup>Spl</sup> and δC<sub>i</sub><sup>Mag</sup> are the standard error of the content of the element (in ppm) in the spinel-rich and magnetite-rich phases, respectively. <sup>f</sup> Average of partition coefficient for individual pairs.</footnote>
<table><caption>**Table 4.** Oxide spinels, ideal composition and selected structural data.</caption><thead><tr><th rowspan="2">Mineral name</th><th colspan="2">Cation</th><th rowspan="2">Formulae</th><th rowspan="2">x<sup>a</sup></th><th rowspan="2">a (Å)<sup>b</sup></th><th rowspan="2">u<sup>c</sup></th></tr><tr><th>A</th><th>B</th></tr></thead><tbody><tr><td colspan="7" style="text-align:center;">2-3 spinels (A<sup>2+</sup> B<sup>3+</sup> O<sub>4</sub>)</td></tr><tr><td>Hercynite</td><td>Fe</td><td>Al</td><td>(Fe)[Al]<sub>2</sub>O<sub>4</sub><sup>d</sup></td><td>N(0.00)<sup>h</sup></td><td>8.1490</td><td>0.2650</td></tr><tr><td rowspan="3">Spinel</td><td>Mg</td><td>Al</td><td>(Mg)[Al]<sub>2</sub>O<sub>4</sub></td><td>N(0.01-0.07)<sup>f,h</sup></td><td>8.0832</td><td>0.2624</td></tr><tr><td>Co</td><td>Al</td><td>(Co<sup>2+</sup>)[Al]<sub>2</sub>O<sub>4</sub></td><td>N(0.15-0.20)<sup>f,h</sup></td><td>8.0950</td><td>0.2637</td></tr><tr><td>Cu</td><td>Al</td><td>(Cu<sup>2+</sup>)[Al]<sub>2</sub>O<sub>4</sub></td><td>N (0.40)<sup>f,h</sup></td><td>8.0860</td><td>na<sup>j</sup></td></tr><tr><td>Galaxite</td><td>Mn</td><td>Al</td><td>(Mn)[Al]<sub>2</sub>O<sub>4</sub></td><td>N (0.30)<sup>f,h</sup></td><td>8.2410</td><td>0.2650</td></tr><tr><td>Gahnite</td><td>Zn</td><td>Al</td><td>(Zn)[Al]<sub>2</sub>O<sub>4</sub></td><td>N(0.00-0.03)<sup>f,h</sup></td><td>8.0860</td><td>0.2636</td></tr><tr><td rowspan="3">Chromite</td><td>Fe</td><td>Co</td><td>(Co<sup>3+</sup>)[Fe<sup>2+</sup>Co<sup>3+</sup>]O<sub>4</sub></td><td>I(1.00)<sup>f,h</sup></td><td>8.2540</td><td>na</td></tr><tr><td>Fe</td><td>Cr</td><td>(Fe)[Cr]<sub>2</sub>O<sub>4</sub></td><td>N(0.00)<sup>h</sup></td><td>8.3920</td><td>na</td></tr><tr><td>Mg</td><td>Cr</td><td>(Mg)[Cr]<sub>2</sub>O<sub>4</sub></td><td>N(0.00)<sup>h</sup></td><td>8.3330</td><td>0.2612</td></tr><tr><td rowspan="2">Magnesiochromite</td><td>Fe</td><td>Ga</td><td>(Fe)[Ga]<sub>2</sub>O<sub>4</sub></td><td>I (1.00)<sup>f,h</sup></td><td>8.3600</td><td>na</td></tr><tr><td>Zn</td><td>Ga</td><td>(Zn)[Ga]<sub>2</sub>O<sub>4</sub></td><td>N(0.00)<sup>f,h</sup></td><td>8.3300</td><td>0.2617</td></tr><tr><td>Magnetite</td><td>Fe</td><td>Fe</td><td>(Fe<sup>3+</sup>)[Fe<sup>2+</sup>Fe<sup>3+</sup>]O<sub>4</sub></td><td>I(1.00)<sup>h</sup></td><td>8.3940</td><td>0.2548</td></tr><tr><td rowspan="2">Magnesioferrite</td><td>Mg</td><td>Fe</td><td>(Fe<sup>3+</sup>)[MgFe<sup>3+</sup>]O<sub>4</sub></td><td>I(0.90)<sup>h</sup></td><td>8.3600</td><td>0.2570</td></tr><tr><td>Co</td><td>Fe</td><td>(Co<sup>2+</sup>)[Fe]<sub>2</sub>O<sub>4</sub></td><td>I(1.00)<sup>f,h</sup></td><td>8.3500</td><td>0.2560</td></tr><tr><td>Cuprospinel</td><td>Cu</td><td>Fe</td><td>(Fe<sup>3+</sup>)[CuFe<sup>3+</sup>]O<sub>4</sub></td><td>I(0.66-1.00)<sup>f,h</sup></td><td>8.3690</td><td>0.2550</td></tr><tr><td>Jacobsite</td><td>Mn</td><td>Fe</td><td>(Mn)[Fe<sup>3+</sup>]<sub>2</sub>O<sub>4</sub></td><td>N(0.10-0.15)<sup>f,h</sup></td><td>8.5110</td><td>0.2615</td></tr><tr><td>Trevorite</td><td>Ni</td><td>Fe</td><td>(Fe<sup>3+</sup>)[NiFe<sup>3+</sup>]O<sub>4</sub></td><td>I(1.00)<sup>f,h</sup></td><td>8.3250</td><td>0.2573</td></tr><tr><td>Coulsonite</td><td>Fe</td><td>V</td><td>(Fe<sup>2+</sup>)[V<sup>3+</sup>]<sub>2</sub>O<sub>4</sub></td><td>N(0.00)<sup>f,h</sup></td><td>8.4160</td><td>0.2598</td></tr><tr><td>Magnesiocoulsonite</td><td>Mg</td><td>V</td><td>(Mg)[V<sup>3+</sup>]<sub>2</sub>O<sub>4</sub></td><td>N(0.00)<sup>f,h</sup></td><td>8.4530</td><td>0.2610</td></tr><tr><td colspan="7" style="text-align:center;">4-2 spinels (A<sup>4+</sup> B<sup>2+</sup> O<sub>4</sub>)</td></tr><tr><td>Ulvöspinel</td><td>Ti</td><td>Fe</td><td>(Fe<sup>2+</sup>)[TiFe<sup>2+</sup>]O<sub>4</sub></td><td>I(1.00)<sup>e,f,g,h</sup></td><td>8.5300</td><td>0.2650</td></tr><tr><td rowspan="3">Qandilite</td><td>Ti</td><td>Mg</td><td>(Mg)[TiMg]O<sub>4</sub></td><td>I(1.00)<sup>f,g,h</sup></td><td>8.4450</td><td>0.2650</td></tr><tr><td>V</td><td>Fe</td><td>(Fe<sup>2+</sup>)[V<sup>4+</sup>Fe<sup>2+</sup>]O<sub>4</sub></td><td>I(1.00)<sup>f,h</sup></td><td>8.4210</td><td>na</td></tr><tr><td>V</td><td>Mg</td><td>(Mg)[V<sup>4+</sup>Mg<sup>2+</sup>]O<sub>4</sub></td><td>I(1.00)<sup>h</sup></td><td>8.3840</td><td>0.2610</td></tr><tr><td rowspan="3">Brunogeierite</td><td>Ge</td><td>Fe</td><td>(Ge)[Fe<sup>2+</sup>]<sub>2</sub>O<sub>4</sub></td><td>N(0.00)<sup>f,h</sup></td><td>8.4110</td><td>0.2500</td></tr><tr><td>Ge</td><td>Mg</td><td>(Ge)[Mg]<sub>2</sub>O<sub>4</sub></td><td>N(0.00)<sup>f,h</sup></td><td>8.2496</td><td>0.2508</td></tr><tr><td>Sn</td><td>Mg</td><td>(Mg)[SnMg]O<sub>4</sub></td><td>I(1.00)<sup>f,h</sup></td><td>8.6000</td><td>0.2500</td></tr></tbody></table>

<footnote><sup>a</sup>Inversion parameter, where N (x ≤ 0.5) and I(x ≥ 0.5) denotes predominantly normal and inverse spinels, respectively. <sup>b</sup>Unit-cell parameter; data from Hill et al. (1979). <sup>c</sup>Oxygen fractional coordinate; data from Hill et al. (1979). <sup>d</sup>The parentheses ( ) and [ ] are used to denote the tetrahedrally and octahedrally coordinated sites, respectively. <sup>e</sup>Forster and Hall (1965). <sup>f</sup>Muller and Roy (1974).</footnote>

<footnote><sup>g</sup>Lindsley (1976). <sup>h</sup>Hill et al. (1979). <sup>i</sup>Wechsler and Von Dreele (1989). <sup>j</sup>Not available (na).</footnote>
<figure><img src="image_12.png" /><figcaption>Figure 1S</figcaption></figure>
<table><caption>**Table 1S.** Results of repeated analysis of the BCR-2g and LCR-1 standard by laser ablation ICP-MS (New Wave UP 266) and comparison with literature values.</caption><thead><tr><th></th><th><sup>45</sup>Sc (ppm)</th><th><sup>47</sup>Ti</th><th><sup>51</sup>V</th><th><sup>55</sup>Mn</th><th><sup>59</sup>Co</th><th><sup>60</sup>Ni</th><th><sup>65</sup>Cu</th><th><sup>66</sup>Zn</th><th><sup>71</sup>Ga</th><th><sup>72</sup>Ge</th><th><sup>118</sup>Sn</th></tr></thead><tbody><tr><td>BCR-2g-1</td><td>33.82</td><td>13794</td><td>425</td><td>1551</td><td>38.34</td><td>11.79</td><td>18.45</td><td>143</td><td>22.49</td><td>3.06</td><td>2.54</td></tr><tr><td>BCR-2g-2</td><td>32.44</td><td>12774</td><td>430</td><td>1531</td><td>39.94</td><td>12.50</td><td>18.11</td><td>151</td><td>23.28</td><td>2.58</td><td>2.41</td></tr><tr><td>BCR-2g-3</td><td>33.61</td><td>12706</td><td>435</td><td>1567</td><td>40.40</td><td>13.76</td><td>19.14</td><td>172</td><td>23.78</td><td>2.55</td><td>2.66</td></tr><tr><td>BCR-2g-4</td><td>33.25</td><td>12823</td><td>431</td><td>1551</td><td>39.34</td><td>13.37</td><td>18.45</td><td>158</td><td>23.25</td><td>2.95</td><td>2.53</td></tr><tr><td>BCR-2g-5</td><td>33.14</td><td>13983</td><td>434</td><td>1560</td><td>38.39</td><td>12.08</td><td>18.66</td><td>149</td><td>22.28</td><td>2.8</td><td>2.51</td></tr><tr><td>BCR-2g-6</td><td>31.79</td><td>12716</td><td>431</td><td>1551</td><td>39.00</td><td>12.46</td><td>18.94</td><td>151</td><td>23.52</td><td>2.45</td><td>2.23</td></tr><tr><td>Minimum</td><td>31.79</td><td>12706</td><td>425</td><td>1531</td><td>38.34</td><td>11.79</td><td>18.11</td><td>143</td><td>22.28</td><td>2.45</td><td>2.23</td></tr><tr><td>Maximum</td><td>33.82</td><td>13983</td><td>435</td><td>1567</td><td>40.40</td><td>13.76</td><td>19.14</td><td>172</td><td>23.78</td><td>3.06</td><td>2.66</td></tr><tr><td>Mean</td><td>33.01</td><td>13133</td><td>431</td><td>1552</td><td>39.24</td><td>12.66</td><td>18.63</td><td>154</td><td>23.10</td><td>2.73</td><td>2.48</td></tr><tr><td>Median</td><td>33.20</td><td>12799</td><td>431</td><td>1551</td><td>39.17</td><td>12.48</td><td>18.56</td><td>151</td><td>23.27</td><td>2.69</td><td>2.52</td></tr><tr><td>Std. Dev.</td><td>0.8</td><td>590</td><td>3.3</td><td>12.1</td><td>0.8</td><td>0.8</td><td>0.4</td><td>10.1</td><td>0.6</td><td>0.2</td><td>0.1</td></tr><tr><td>Rel. Dev. (%)</td><td>2.3</td><td>4.5</td><td>0.8</td><td>0.8</td><td>2.1</td><td>6.0</td><td>2.0</td><td>6.6</td><td>2.6</td><td>8.9</td><td>5.9</td></tr><tr><td colspan="12" style="text-align: center;">Comparison of preferred published value and analyzed value</td></tr><tr><td>Norman et al. (1998)</td><td>33</td><td>13700</td><td>414</td><td>na<sup>a</sup></td><td>35.8</td><td>10.8</td><td>19.4</td><td>147</td><td>22.7</td><td>na</td><td>na</td></tr><tr><td>Diff. publ. value & this study<sup>b</sup></td><td>-0.01</td><td>567</td><td>17</td><td>na</td><td>3.44</td><td>1.86</td><td>-0.77</td><td>7</td><td>-0.4</td><td>na</td><td>na</td></tr><tr><td>Dev. publ. value & this study<sup>c</sup></td><td>0.0</td><td>4.1</td><td>4.1</td><td>na</td><td>9.6</td><td>17.2</td><td>-4.0</td><td>4.7</td><td>-1.8</td><td>na</td><td>na</td></tr><tr><td colspan="12" style="text-align: center;">Comparison of USGS reference value and analyzed value</td></tr><tr><td>USGS reference value<sup>d</sup></td><td>33</td><td>13500</td><td>416</td><td>1520</td><td>37</td><td>na</td><td>19</td><td>127</td><td>23</td><td>na</td><td>na</td></tr><tr><td>Diff. USGS value & this study<sup>e</sup></td><td>-0.01</td><td>367.18</td><td>15</td><td>32</td><td>-2.24</td><td>na</td><td>0</td><td>27</td><td>-0.10</td><td>na</td><td>na</td></tr><tr><td>Dev. USGS value & this study<sup>f</sup></td><td>0.0</td><td>2.7</td><td>3.6</td><td>2.1</td><td>-6.0</td><td>na</td><td>-2.0</td><td>21.2</td><td>-0.43</td><td>na</td><td>na</td></tr><tr><td colspan="12" style="text-align: center;">Comparison of preferred published value and analyzed value</td></tr><tr><td>Locmelis et al. (2011)</td><td>na</td><td>17117</td><td>2244</td><td>1803</td><td>234</td><td>1167</td><td>na</td><td>na</td><td>na</td><td>na</td><td>na</td></tr><tr><td>Diff. publ. value & this study<sup>h</sup></td><td>na</td><td>723</td><td>88</td><td>38</td><td>9.87</td><td>30.70</td><td>na</td><td>na</td><td>na</td><td>na</td><td>na</td></tr><tr><td>Dev. publ. value & this study<sup>i</sup></td><td>na</td><td>4.2</td><td>3.9</td><td>2.1</td><td>4.2</td><td>2.6</td><td>na</td><td>na</td><td>na</td><td>na</td><td>na</td></tr></tbody></table>

Note: Al of the NIST610 glass was used as internal standard.

<footnote><sup>a</sup> Not analyzed (na). <sup>b</sup> Difference between values published by Norman et al. (1998) and analyzed values. All values were determined by laser ablation ICP-MS. <sup>c</sup></footnote>

<footnote>Deviation between values by Norman et al. (1998) and analyzed values. <sup>d</sup> USGS website, September 2015. <sup>e</sup> Difference between USGS reference values and analyzed values. <sup>f</sup> Deviation between USGS reference values and analyzed values. <sup>g</sup> Ratio between USGS reference values and analyzed values. Ratios similar to 0 indicates good reproducibility and ratios near to ±1 indicates non-good reproducibility. <sup>h</sup> Difference between values published by Locmelis et al. (2011) and analyzed values. All values were determined by laser ablation ICP-MS. <sup>i</sup> Deviation between values by Locmelis et al. (2011) and analyzed values.</footnote>