Mineral chemistry from the Alfeu-I lamproite (Southern Brazil) and its contribution to understand the mantle heterogeneity under South American Plate during the Gondwana breakup

Abstract The Alfeu-I lamproite is one of the few alkaline rock occurrences in the South of Brazil that represents the alkaline event related to the South Atlantic opening and the enormous magmatic activity that formed the Paraná basalts. Alfeu-I lamproite is a diatreme facies and exhibits an inequigranular texture with macrocrysts of mica, spinel, garnet, and ilmenite and microcrysts of mica, pyroxene, and rare olivine, all immersed in a groundmass of pyroxene, spinel, perovskite, rutile, ilmenite, and, more rarely, olivine. Major element compositions of Alfeu-I pyroxene, garnet, ilmenite, mica, and olivine were determined by electron microprobe analyses, and trace element concentrations of clinopyroxene, garnet, ilmenite, and mica were measured using laser-ablation inductively coupled plasma mass spectrometry techniques. Temperature, pressure, and oxygen fugacity (fO2) conditions during the crystallization of Alfeu-I lamproite were calculated with the geothermobarometers and olivine, spinel, garnet, and orthopyroxene. The resulting mean equilibrium temperature ranges from 1375°C at 4 GPa to 1395°C at 5 GPa, whereas the fO2 points to ΔFMQ = +2.4 (at 4 GPa) and ΔFMQ = +2.2 (at 5 GPa). Rb-Sr and Sm-Nd isotopic data together with the trace element concentrations of minerals suggest that melting of a mantle source enriched in incompatible elements and volatiles due to previous subduction events occurred during the Gondwana breakup around 125 Ma ago. Fluids that may have originated from subducting slabs in the old subduction zone are probably the cause of the high fO2 conditions in Alfeu-I lamproite.


INTRODUCTION
Lamproites are formed by partial melting of metasomatized lithospheric mantle (Scott Smith et al. 2018) and are usually classified according to the mineralogical and geochemical criteria given by Mitchell and Bergman (1991).However, the name lamproite was recently redefined by Scott Smith et al. (2018) to emphasize the common petrogenesis and eliminate petrological confusion with petrogenetically distinct kimberlite.Although they are volumetrically minor components of continental magmatism, lamproites are rare products of the melting of geochemically exceptional and variable lithospheric mantle sources.Mitchell (1995) underlined that kimberlites and related rocks cannot be identified only by petrography and that geochemical data are scant due to metasomatism, crustal contamination, and, perhaps most important, weathering.The best studied material for rock classification is obtained from the hypabyssal facies of these rocks, since they contain less crustal xenoliths and the minerals are well crystallized to allow a better understanding of the primary mineral assemblage.Classification based on whole rock chemistry of diatreme facies, as in the case of Alfeu-I lamproite, is more difficult because of the predominance of fragmented lapilli and the tendency of these rocks to weathering.In this case, mineral chemistry and in situ isotope characterization provide a better contribution to the understanding of the origin and magmatic history of the lamproite rocks.
The Alfeu-I lamproite, located in the southeastern portion of the Sul-Riograndense Shield, southern Brazil, is one of the Mineral chemistry from the Alfeu-I lamproite (Southern Brazil) and its contribution to understand the mantle heterogeneity under South American Plate during the Gondwana breakup rare occurrences of alkaline rocks that may further our understanding of the magma diversity production during the opening of the South Atlantic.There is scarce information on the petrogenesis of alkaline rocks from this part of Brazil, and no detailed studies of the trace element geochemistry and Sr-Nd isotope compositions of these rocks have been published.
Pressure, temperature, and redox conditions (fO 2 ) of lamproites and related rocks provide valuable insights on the mantle source and the melting regime of these rocks.However, the application of geothermobarometers and oxygen barometers in lamproites and related rocks is difficult due to the mostly low preservation grade of these rocks, which is usually the main obstacle, but also due to the diversified mineral assemblage, which sometimes does not contain all the minerals required for geothermobarometric calculations.In this study, we use pyroxene, garnet, ilmenite, mica, and olivine compositions to determine age, temperature, pressure, and fO 2 conditions for the Alfeu-I lamproite to constrain its mantle source and the geological context related to the Atlantic opening.Moreover, we also use these new data to infer the potential of these melts to carry and preserve diamonds.

GEOLOGICAL SETTINGS AND SAMPLES
Alfeu-I lamproite is a volcanic pipe emplaced in the Pinheiro Machado Suite of the Pelotas Batholith domain and occurs in the eastern portion of the Sul-Riograndense Shield, southern Brazil.Its location is around 300 km from the Rosário-6 alnöite and the Paraná basalts (Fig. 1).The alkaline rocks in this region occurred in four main stages: Permian to Triassic, probably caused by the stress propagation related to the Cabo La Ventana orogeny (Gomes et al. 1996, Milani 1997); lower Cretaceous, associated with the rifting of the Atlantic marginal basin; upper Cretaceous, contemporaneous to the Atlantic Ocean; and Cenozoic (Paleogene), linked to the evolution of continental rift systems in southeastern Brazil (Ribeiro 1980, Almeida 1983).The Mesozoic alkaline magmatism is conditioned by shear zones and discontinuities between cratonic limits that were reactivated by the Gondwana breakup tectonics (Barbieri et al. 1987, Gomes andComin-Chiaramonti 2017).

METHODS
The Alfeu-I minerals were selected with a binocular magnifying glass from the pan-concentrate that was collected from the altered rock.Microprobe analyses were performed on mineral separates of pyroxene, garnet, ilmenite, mica, and olivine macrocrysts (> 0.5-10 mm) and microcrysts (< 0.5 mm) using a CAMECA SX-five electron microprobe of the Laboratório de Microssonda Eletrônica (CPGq-IG/ UFRGS), Brazil.The analyses were performed using an acceleration voltage of 15 kV, a beam current of 10 nA, a beam size of 5 μm, and a counting time of 20 s on the peak and 5 s on each background.The standards used included sanidine (Si, Al), diopside (Mg, Ca), almandine (Fe), rutile (Ti), chromium oxide (Cr), and rhodonite (Mn).Details of the method are given in the Supplementary data.The fresh minerals were analyzed in the core and border, whereas the ones more altered were analyzed just in the core, as we pointed out in the Suppl.Data.
Trace element concentrations of the Alfeu-I minerals (clinopyroxene, garnet, biotite, and ilmenite) were determined with laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Institut für Mineralogie, Münster, Germany (Beyer et al. 2013, Wijbrans et al. 2015).Sample ablation was performed with a pulsed 193 nm ArF excimer laser (Analyte G2, Photon Machines).A repetition rate of 5 or 10 Hz and an energy of ∼3-4 J/cm 2 were used.The beam spot diameter varied between 15 and 30 μm.Elemental analysis has been carried out with an Element XR mass spectrometer (ThermoFisher Scientific).Forward power was 1300 W and reflected power was < 1 W; gas flow rates were about 1 L/min for He (carrier gas of ablated material), 0.8 L/min for the Ar-auxiliary gas, and 1 L/min for the sample gas, respectively.The cooling gas flow rate was set to 16 L/min.Before starting analysis, the system has been tuned on a NIST 612   232 Th, and 238 U.The NIST 612 glass was used as an external reference material and 29 Si, 43 Ca (silicates), 47 Ti (Fe-Ti oxides), and 26 Mg (spinels) as internal standards, which have been previously determined by electron microprobe.The overall time of a single analysis was 75 s (20 s for background, 40 s for peak after switching the laser on, 15 s washout time).Concentrations of measured elements were calculated using the Glitter software (Van Achterbergh et al. 2001, Griffin et al. 2008).Standard reference glasses BCR2-G and BIR1-G were analyzed as monitors for precision and accuracy for silicate phases of this study.Standard analyses were repeated every 20 analyses of unknown minerals (Suppl.Data).The obtained results match the published range of concentrations given in the GeoReM database (version 18) ( Jochum et al. 2005).
Rb-Sr and Sm-Nd isotopic analyses were performed using two different thermal ionization mass spectrometers (Sector 54,VG Scienta Holdings AB;and Triton,ThermoFisher Scientific) for isotopic characterization at the Laboratório de Geologia Isotópica (LGI-IG, UFRGS), Brazil.Around 0.5-0.8g of each mineral was crushed in an agate mortar and leached with HCl 0.1 N in order to eliminate the crustal alteration.Posteriorly, 0.1 g of the leached residue was spiked with mixed 87 Rb/ 84 Sr and 149 Sm/ 150 Nd tracers and then digested with HF, HNO 3 , and HCl until complete dissolution, followed by drying and homogenization of the residue in 3 mL of HCl 2.5N.Columns filled with cationic AG-50W-X8 (200-400 mesh) and anionic LN-B50-A (100-150 mesh) resins were used to separate Rb, Sr, and REE and Sm, Nd, respectively.Each sample was dried to a solid residue and then loaded with 0.25N H 3 PO 4 in appropriate filaments (single Ta filaments for Rb, Sr, Sm, and triple Ta-Re-Ta for Nd).Sr and Nd isotopic ratios were normalized to 86 Sr/ 88 Sr=0.1194 and to 146 Nd/ 144 Nd = 0.7219.Measurements of specific standards were performed for accurate analysis.The NIST standard NBS-987 resulted in 87 Sr/ 86 Sr = 0.710260 ± 0.000014, and the JNd-1 standard resulted in a ratio of 143 Nd/ 144 Nd = 0.512108 ± 0.000010.Blanks were < 60 pg for Sr, < 500 pg for Rb, < 200 pg for Sm, and < 500 pg for Nd.The errors do not exceed more than 1% of the reported value.
To determine the crystallization temperature and fO 2 of Alfeu-I, we used the olivine-spinel oxygen geothermobarometers of O' Neill and Wall (1987) and Ballhaus et al. (1991).The pressure of Alfeu-I was also calculated with the garnet-orthopyroxene geothermobarometer of Nickel and Green (1985).The Fe 2+ and Fe 3+ contents of minerals were calculated with the method of Droop (1987), based on stoichiometric criteria.We used analyses of Alfeu-I lamproite from mineral separates, as the Alfeu-I pipe is unfortunately very altered and it was not possible to collect cohesive samples.

Petrography and mineral compositions
The Alfeu-I lamproite rocks exhibit an inequigranular texture with macrocrysts (> 0.5-10 mm) and microcrysts (< 0.5 mm) of biotite (~25 vol%), spinel (~10 vol%), garnet (~5 vol%), and ilmenite (~5 vol%), microcrysts of pyroxene (~3 vol%), and rare olivine (~2 vol%), all immersed in a groundmass (~35 vol%) composed of pyroxene, chromite, perovskite, rutile, ilmenite, and, more rarely, olivine.Perovskite and rutile were identified by MEV-EDS in the Alfeu-I thin sections, but not found in rock concentrates.The macrocrysts and microcrysts are fractured and show corroded edges.Macrocrysts and microcrysts of biotite have curved cleavage planes that indicate the flow orientation and rounded edges (Fig. 3A).Garnet macrocrysts and microcrysts are rounded and fractured, with corroded edges, and they are surrounded by mica (Fig. 3B).Groundmass spinel and perovskite are rare (Figs.3C and 3D), whereas clinopyroxene is also found in the pelletal lapilli (Figs.3E and 3F).Olivine is rare and appears only as a serpentine pseudomorph and found also in pelletal lapilli (Fig. 3G).Xenoliths from the wall rock are rare, but two of them were observed in thin sections with an angular sub-rounded shape, from 1 to 3 mm in size, and with granitic composition.Xenocrysts consisting of polycrystalline quartz, microcline, and rare plagioclase are also present and occur with rounded shapes and well-defined edges, up to 1 mm in diameter.These xenocrysts are found aligned with the flow texture (Fig. 3H).Due to the lack of fresh rock in the Alfeu-I lamproite, we need to consider that all minerals underwent secondary alteration, and this open-system process produced changes in the mineral composition, as we will discuss below.The xenoliths, xenocrysts, and lapilli compose ~15 vol% of the rock.

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and the content for V, Sr, and Zr in augites ranges from 213 to 326 ppm, from 91 to 142 ppm, and from 103 to 269 ppm (Supplementary data), respectively.Figure 6A shows that Ca content in orthopyroxenes increases with the increase of alkalis, whereas Fig. 6B shows that Ca content in clinopyroxenes increases with the decrease of Mg. Figure 7A shows chondrite normalized (McDonough and Sun 1995) trace element patterns for clinopyroxenes, with positive anomalies of V, Sr, Y, and Zr, negative anomalies of Ni, Nb, and Pb, and an almost flat REE pattern, with only a slight enrichment of HREE relative to LREE (Fig. 7A).

Sr-Nd isotope data
We performed Rb-Sr and Sm-Nd isotopic analyses of biotite, ilmenite, garnet, and clinopyroxene macrocrysts from Alfeu-I lamproite (Tables 1 and 2), which are used to constrain the Alfeu-I mantle source composition.However, the data from biotite and ilmenite did not result in meaningful data, which indicates the possibility of an open system (Workman and Hart 2005).The 87 Rb/ 86 Sr ratio was calculated from the elemental concentrations of Rb and Sr. Isotopic data for pyroxene and garnet microcrysts are plotted in Fig. 13A, where we consider the age of 128 Ma obtained via U-Pb in groundmass perovskite from the Rosário 6 alnöite, also found in southern Brazil (Conceição et al. 2019), to calculate the 87 Sr/ 86 Sr and 143 Nd/ 144 Nd initial ratios for Alfeu-I minerals.The Rosário-6 alnöite age of 128 ± 8 Ma was chosen because it is one of the few alkaline rocks in the area, and its age agrees well with the age of the Paraná flood basalts, which leads us to infer that Alfeu-I lamproite is probably related to both the Rosário-6 and Paraná basalts.We compared these data with the MORB end-member, DMM, HIMU, and EM major mantle reservoirs, and worldwide rocks (Proto Tristan da Cunha plume, Rosário-6 alnöite, Namibia lamprophyres and carbonatites, Alto Paranaíba Igneous Province, Jacupiranga alkaline rocks, sodic and potassic rocks from Paraguay, Patagonian xenoliths, high-and low-Ti Paraná basalts, and Siberian meimechites).Alfeu-I clinopyroxenes plot on the mantle array, within the limits between DMM and EMI reservoirs, close to Rosário-6 alnöite, and within the Proto Tristan da Cunha plume field, whereas garnet has a lower 87 Sr/ 86 Sr radiogenic ratio but also a lower 143 Nd/ 144 Nd radiogenic ratio.Based on trace elements patterns (Fig. 7), we suggest that clinopyroxene and garnet were equilibrated with the melt, because both minerals have similar partition coefficients for a range of elements as most REE, Sr, and Y (Harte and Kirkley 1997).If garnet and clinopyroxene crystallized from the same magma at a given time, Figure 10.Compositions (mol%) of Alfeu-I ilmenites plotted in the ternary system geikielite (MgTiO 3 ) -ilmenite (FeTiO 3 ) -pyrophanite (MnTiO 3 ) (Mitchell 1995): red squares are from ilmenites with low Mn contents, whereas purple squares are ilmenites with high Mn contents.For comparison, ilmenites from Janjão, Pandolfo, and Lambedor diatremes (Brazil) (Barabino et al. 2007) and from diamondiferous Zolotitsa field, Ti-Fe-rich kimberlites, Mela carbonatites, and kimberlites (Arkhangelsk Region, NW Russia) (Beard et al. 2000).they should display similar initial 87 Sr/ 86 Sr ratios (Nowell et al. 2004, Blackburn et al. 2008).Radiogenic ingrowth from 87 Rb decay through time in each phase will modify the 87 Sr/ 86 Sr proportionally, leading to a linear correlation on a 87 Sr/ 86 Sr versus 87 Rb/ 86 Sr plot that has a slope proportional to the crystallization age.To perform isochron linear regressions and age calculation, we employed Isoplot/Ex Version 3.75 (Ludwig 2012), which in turn uses the algorithm of York (1969) and error propagation via the maximum-likelihood estimation algorithm of Titterington and Halliday (1979).Albeit threepoint isochrons are of doubtful reliability (Ludwig 2012), the crystallization age for the Alfeu-I lamproite magma is 125 ± 11 Ma (Fig. 13B), which closely matches the age of 128±8 Ma obtained with U-Pb dating of perovskite from the Rosário 6 alnöite (Conceição et al. 2019).

Geothermobarometry and oxygen barometry results
Crystallization temperatures of Alfeu-I lamproite were determined using the olivine-spinel Mg-Fe 2+ exchange geothermometer of O' Neill and Wall (1987) and Ballhaus et al. (1991), who applied their geothermometer to peridotite xenoliths.We used chromites and olivines microcrysts analyses of Alfeu-I lamproite (Chaves et al. 2014, Provenzano 2016; and this study -Table 3 and Suppl.data) from mineral separates, as the Alfeu-I pipe is unfortunately very altered and it was not possible to collect preserved samples.The Fe 2+ and Fe 3+ contents of chromites were calculated with the method of Droop (1987), based on stoichiometric criteria and assuming that all Fe is the only multivalent element and oxygen is the only anion.The calculated mean temperature of Alfeu-I lamproite crystallization is 1,375(± 65)°C at 4 GPa and 1,395(± 65)°C at 5 GPa (Table 3).To constrain the depth at which the Alfeu-I lamproite melt crystallized, we used the mineral assembly, which suggests a crystallization pressure higher than the crystallization pressure of Rosário-6 alnöite (Carniel et al. 2020).This depth range was confirmed by garnet-orthopyroxene geothermobarometry (Nickel and Green 1985).The monticellite in Rosário-6 assembly indicates that the pressure limit for its mantle source is around 3 GPa, whereas the presence of garnet in Alfeu-I assembly suggests a higher source depth.If we compare the results of Alfeu-I lamproite and Rosário-6 alnöite, we can conclude that Alfeu-I crystallization conditions are very similar to those of Rosário-6 alnöite, which gave us more confidence in the use of mineral separates for this geothermobarometry and also suggests that these minerals should be in equilibrium.We also applied the garnet-orthopyroxene geobarometer of Nickel and Green (1985) to Alfeu-I minerals.Calculated Alfeu-I pressure, using temperatures between 1,375 and 1,395°C (Ballhaus et al. 1991), is 4.0(± 0.2) GPa, which corresponds to a depth of around 120 km.
The oxygen barometer developed by Ballhaus et al. (1991) can be applied to a variety of mantle-derived rocks and spinel-bearing primitive melts.However, the equation is valid only if the silica activity (a SiO2 ) is buffered by the presence of both olivine and orthopyroxene.In this case, the calculated mean of Alfeu-I oxygen fugacity relative to the fayalite-magnetite-quartz buffer (ΔFMQ) for 4 GPa is 2.4, and for 5 GPa it is 2.2 (Table 3).

Alfeu-I mantle source constraints based on mineral composition and isotope data
Based on the major and trace elements of Alfeu-I minerals, we can interpret that its mantle source has been metasomatized, which is reflected by the almost flat REE pattern of clinopyroxene, with a slight enrichment of MREE relative to LREE and HREE (McDonough and Frey 1989) (Fig. 7A).The Alfeu-I pyroxenes suggest two distinct genetic episodes, as the clinopyroxenes have an eclogite signature and the orthopyroxenes contain a peridotite signature (Fig. 5A), which is associated with the high Cr content.Figure 5A (Cr 2 O 3 versus Al 2 O 3 and Cr versus Fe versus Na molar) shows that Alfeu-I clinopyroxenes plot in the eclogites and Cr-poor megacrysts fields, which indicates that their mantle source may have been metasomatized by fluids derived from a subducted slab (Shu et al. 2018, Skuzovatov et al. 2022).The chondrite-normalized garnet and clinopyroxene trace element patterns are similar (Figs.7A and 7B), which may suggest that both minerals were equilibrated in the melt (Harte and Kirkley 1997).The Alfeu-I pyropes have a depleted mantle signature, evidenced by the enrichment in HREE relative to LREE, and this reflects the incompatible element-depleted nature of the upper mantle from which these magmas are derived or passed by (Klein-BenDavid and Pearson 2009).The positive anomalies of Rb, Sr, Ba, Zr, Nb, and Ta in high-Mn ilmenites and the enrichment of LREE relative to HREE of low-Mn ilmenites occur due to structural reasons.However, it may also indicate a metasomatic process that may be caused by slab contamination of the mantle.The slabs may have been partially melted at high temperature and pressure, releasing HFS elements and causing the positive anomalies of, for example, Nb and Ta.
These metasomatic processes in the depleted mantle may have been caused by fluids from recycled oceanic crust, low degree melts in the upper mantle that act as enriching agents for the peridotite source region, or the presence of detached sub-continental lithospheric mantle that remained in the asthenosphere after the breakup (Hawkesworth et al. 1986, Peate et al. 1999).The Gondwana breakup probably initiated in the mid-Jurassic; however, the exact timing is uncertain, considering that the oldest magnetic anomalies on the South Atlantic oceanic crust have associated ages from 135 and 126 Ma (Nürnberg and Müller 1991, Turner et al. 1994, Hall et al. 2018).The Paraná flood basalts are one of the most important events that occurred during the continental separation, and they provide significant information about the sub-continental mantle in this region.These basalts are divided into low-and high-Ti groups, the latter with higher Fe, P, Ti, Zr, Ce, La, Ba, and Sr concentrations (Bellieni et al. 1984).This compositional variation is interpreted by the authors as evidence of a large-scale heterogeneous mantle source beneath this region during the breakup.In addition to previous studies (Marques et al. 1999, Peate et al. 1999) that proposed heterogeneous lithospheric mantle melting as the source of the Paraná flood basalts, Rocha-Júnior et al. (2012, 2013) demonstrated, based on Re-Os and Sr-Nd-Pb isotopic data, that the asthenospheric source of the basalts was enriched by fluids or magmas related to the Neoproterozoic subduction processes.The authors also suggested that the Tristan da Cunha mantle plume could have acted as a heat source that may have triggered the generation of the Paraná flood basalts ( Jennings et al. 2019).According to a number of authors (Comin-Chiaramonti et al. 1997, 2002, Gibson et al. 1995, 2006, Marques et al. 2016), extensional tectonic movements caused by the Gondwana breakup triggered the basaltic lava eruption that covered the Paraná basin sediments.These movements may also have been responsible for smaller alkaline events such as the Rosário-6 alnöite (Conceição et al. 2019, Carniel et al. 2020) and the Alfeu-I lamproite in greater depths.
As clinopyroxene prefers to incorporate Sr over Rb (Beattie 1993, Foley et al. 1996, Leitzke et al. 2017), the low Rb/Sr ratio of clinopyroxene will constraint the initial 87 Sr/ 86 Sr for the kimberlite and other-alkaline related magmas (Blackburn et al. 2008).Pyroxenes plot between DMM and EMI plots, close to Rosário-6 alnöite and inside the Proto Tristan da Cunha plume field, whereas garnet has a lower 87 Sr/ 86 Sr and 143 Nd/ 144 Nd radiogenic ratio.The values for Sr isotopes acquired in biotite are high, indicating upper continental crust origin.It is likely that clinopyroxene reflects the source of the kimberlite, while biotite was generated in a shallower, crustal environment due to metasomatic processes.The crystallization age suggested in this study (125 Ma) also indicates a possible correlation between Alfeu-I lamproite and Rosário-6 alnöite (128 Ma) events.

Alfeu-I lamproite tectonic settings
The fO 2 conditions during Paraná flood basalt genesis are representative of the mantle redox conditions in this region during the beginning of the continental breakup.As the most expressive event that occurred during the Gondwana breakup in southern Brazil, its redox conditions may contribute to evaluating how oxidized the Alfeu-I lamproite is compared to this mantle source.The fO 2 conditions calculated for the Alfeu-I lamproite and Rosário-6 alnöite are significantly higher than the fO 2 of the Paraná flood basalts, as described by Bellieni et al. (1984) (Fig. 14).The latter authors used the method of  Buddington and Lindsley (1964) to determine the fO 2 of the Serra Geral basalts based on ilmenite and Ti-magnetite compositions.They found that high-Ti basalts, which have fO 2 values between the nickel-nickel oxide (NNO) and fayalite-magnetite-quartz (FMQ) buffers, are slightly more oxidized than the low-Ti basalts, with fO 2 values between the FMQ and wüstite-magnetite (WM) buffers, which can be correlated to the Ti-enrichment and the source depth of these rocks.High-Ti basalts have been derived from greater depths (90-120 km depth), with a lower degree of melting than the low-Ti basalts (30-60 km depth) (Garland et al. 1996).
Recent studies (Conceição et al. 2019, Carniel et al. 2020) on the mantle source of alkaline rocks in southern Brazil show a close link between these occurrences and the Gondwana continental breakup and the opening South Atlantic, which started at ca. 135 Ma ago (Hall et al. 2018).Following these authors, subducting slabs that contained carbonated sediments and metabasalts could be responsible for metasomatic processes in the mantle, which may have been oxidized and chemically enriched by carbonatite and/or silicate melt metasomatism (Fumagalli andKlemme 2015, Gervasoni et al. 2017).The origin of these hydrated and carbonated fluids or melts that caused such a melt oxidation process in Alfeu-I lamproite may be related to subducting materials from old subduction processes similar to the Rosário-6 alnöite (Conceição et al. 2019).
Based on our calculations, Alfeu-I lamproite may have crystallized at a pressure of 4-5 GPa, which corresponds to around 120-150 km depth, temperatures between 1,375 and 1,395°C, and at ΔFMQ = 2.4-2.2.Alfeu-I temperatures and fO 2 are plotted in the diagram (Fig. 15), where D/GCO is the diamond/graphite-carbon oxide buffer from Frost and Wood (1997), EMOD/G is the enstatite-magnesite-olivine-diamond/ graphite buffer from Zhao et al. (1999), and NNO is the nickel-nickel oxide buffer from Ballhaus et al. (1991).The GCO and DCO oxygen buffers describe the upper fO 2 stability of graphite or diamond with respect to a free C-O fluid (Frost and Wood 1997).The EMOD/G curve defines the stability field between diamond/graphite and magnesite (MgCO 3 ) in the mantle.Considering this, the area below this curve represents the oxygen fugacity for melts in equilibrium with diamond or graphite, and the Alfeu-I oxygen fugacity plots above the graphite stability curve (Fig. 15).At such conditions, carbon can be oxidized to produce carbonate melt through the reduction of Fe 3+ in silicate minerals during upwelling (Rohrbach andSchmidt 2011, Stagno et al. 2013).From 4 to 5 GPa (120-150 km depth), diamonds can be stable only at very low oxygen fugacities and at low temperatures (see D/G limit in Fig. 15).

CONCLUSION
We present new mineralogical and geochemical data on the Alfeu-I lamproite.It exhibits an inequigranular texture with macrocrysts of mica, chromite, garnet, and ilmenite and microcrysts of mica, pyroxene, and rare olivine, all immersed in a groundmass composed of pyroxene, chromite, perovskite, rutile, ilmenite, and, more rarely, olivine.Major and trace elements of Alfeu-I minerals indicate a depleted mantle source that was re-fertilized by metasomatic processes in the lithosphere.The Sr-Nd isotopic data and the Rb-Sr isochron, based on the pyroxene and garnet isotope compositions, indicate a metasomatized mantle source and crystallization age close to Rosário-6 alnöite.An enriched mantle source is considered to have been the product, for example, of metasomatism acting on the subcontinental lithospheric mantle after subduction ceases, which would lead to the formation of alkali-enriched magmas that can have a mantle signature.Therefore, we consider that the negative εNd values imply an enriched mantle source, i.e., a mantle that was metasomatized by fluids in a previous subduction setting (Zi et al. 2012).Garnet and clinopyroxene could come from two distinct sources, which would not be surprising given that this rock occurs in diatremes and the mantle beneath Gondwana during the breakup was not homogeneous.The Alfeu-I lamproite may have crystallized at pressures between 4 and 5 GPa, which corresponds to around 120-50 km depth, at high temperatures (from 1,375 to 1,395°C) and relatively oxidized conditions, at ΔFMQ = +2.4 to +2.2.The origin of the metasomatic agents that caused such a melt oxidation process in Alfeu-I lamproite may be related to subducted slab materials from old subduction processes in the mantle source.

Figure 5A (Figure 3 .
Figure 3. Microscope features of the Alfeu-I rocks: (A, B) uncrossed polarized image of fragments of clinopyroxene (Cpx) in pelletal lapilli (B is a detail of A); (C, D) uncrossed polarized image of large spinel and small perovskite (Prv) grains of the groundmass; (E) uncrossed polarized image of macrocryst of garnet (Grt) with corroded edges; (F) uncrossed polarized image of the flow orientation indicated by mica microcrysts; (G) uncrossed polarized image of pseudomorphs of olivine outlined in red; (H) crossed polarized image of granite xenolith with polycrystalline quartz.
enrichment in LREE relative to HREE, whereas the high-Mn ilmenites show the opposite pattern, which may indicate a depleted source.Both ilmenite groups show positive anomalies of V, Zr, Nb, and Ta.

Figure 12 .
Figure 12.Mg+Si versus Fe+Al and K+Si versus Fe+Al showing the cation exchange in the structure of the tri-octahedral biotite of Alfeu-I lamproite.

Table 1 .
Rb-Sr isotopic and concentration data obtained through ID-TIMS for pyroxene, garnet, ilmenite, and biotite mineral separates from the Alfeu-I lamproite.

Table 2 .
Sm-Nd isotopic and concentration data obtained through ID-TIMS for pyroxene and garnet mineral separates from the Alfeu-I lamproite.