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.

KEYWORDS: Gondwana sub-lithospheric mantle; mineral chemistry; lamproite; mantle redox conditions

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 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 (fO₂) 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 fO₂ 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 and Comin-Chiaramonti 2017).

Figure 1.
Simplified geological map of the Alfeu-I lamproite based on Svisero and Chieregati (1991), Conceição et al. (2019), Morbidelli et al. (2000), and Carniel et al. (2020). Río de la Plata Craton limit from Santos et al. (2019).

The Alfeu-I rocks overlap the Canguçu Dorsal Transcurrent Shear Zone, intruded into granodioritic to monzogranitic orthogneisses from the Pinheiro Machado Granite-Gneiss Complex (780–610 Ma) (Fragoso-César 1991, Philipp 1991) (Fig. 2A). The samples are quite weathered (Fig. 2B); however, it is possible to identify preserved minerals and the original texture in thin sections. These rocks are composed of macrocrysts and microcrysts (pyroxene, garnet, spinel, ilmenite, biotite, and olivine) in an interstitial matrix with some imbricated pelletal lapilli, wall rock autoliths, and crustal xenoliths.

Figure 2.
(A) Alfeu-I lamproite emplaced in the Pinheiro Machado Suite of the Pelotas Batholith domain; (B) Alfeu-I weathered samples (Provenzano 2016).

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/cm² 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 reference glass measuring ¹³⁹La, ²³²Th, and ²³²Th¹⁶O to get stable signals and high sensitivity, as well as low oxide production rates (²³²Th¹⁶O/²³²Th < 0.1%) during ablation. A total of 32 elements were quantitatively analyzed. Masses monitored were ⁷Li, ²⁹Si, ⁴³Ca, ⁵¹V, ⁵³Cr, ⁵⁵Mn, ⁵⁹Co, ⁶⁰Ni, ⁶¹Ni, ⁶³Cu, ⁶⁶Zn, ⁶⁹Ga, ⁷²Ge, ⁷³Ge, ⁸⁵Rb, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ¹¹⁸Sn, ¹²¹Sb, ¹³³Cs, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, ¹⁷⁵Lu, ¹⁷⁸Hf, ¹⁸¹Ta, ¹⁸²W, ²⁰⁸Pb, ²³²Th, and ²³⁸U. The NIST 612 glass was used as an external reference material and ²⁹Si, ⁴³Ca (silicates), ⁴⁷Ti (Fe-Ti oxides), and ²⁶Mg (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 0.5–0.8 g 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 ⁸⁷Rb/⁸⁴Sr and ¹⁴⁹Sm/¹⁵⁰Nd tracers and then digested with HF, HNO₃, 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 H₃PO₄ 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 ⁸⁶Sr/⁸⁸Sr=0.1194 and to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219. Measurements of specific standards were performed for accurate analysis. The NIST standard NBS-987 resulted in ⁸⁷Sr/⁸⁶Sr = 0.710260 ± 0.000014, and the JNd-1 standard resulted in a ratio of ¹⁴³Nd/¹⁴⁴Nd = 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 fO₂ 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 Fe²⁺ and Fe³⁺ 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.

RESULTS

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.

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.

Pyroxene

Pyroxene microcrysts are classified as augite ((Al_0.2 Ti_0.03 Fe_0.32 Mn_0.004 Mg_0.58 Ca_0.62 Na_0.26)_2.01 (Si_1.94 Al_0.06)₂ O₆ on the basis of 6 oxygens) and enstatite ((Al_0.03 Ti_0.003 Fe_0.17 Mn_0.003 Mg_1.74 Ca_0.03 Na_0.01)_1.98 (Si_1.9Al_0.1)₂ O₆ on the basis of 6 oxygens) (Fig. 4; Suppl. Data). Enstatites have SiO₂ contents of 54–56 wt.%, Al₂O₃ contents of 3–4.4 wt.%, MgO of 32–35 wt.%, MnO up to 0.15 wt.%, Cr₂O₃ contents of 0.39–0.86 wt.%, Na₂O contents up to 0.17 wt.%, and Mg# ((100*Mg)/(Mg+Fe)) of 88–93, whereas augites have CaO contents ranging from 15 to 16 wt.%, Al₂O₃ contents of 4.8–6.6 wt.%, MgO from 9 to 11 wt.%, Na₂O contents of 3.2–4 wt.%, Mg# of 58–68, and Ca# of 49–54. Augites also have lower Cr₂O₃ contents than enstatites, up to 0.06 wt.%, and similar MnO contents, from 0.1 to 0.18 wt.%.

Figure 4.
Alfeu-I pyroxenes classified based on the Wo-En-Fs ternary, compiled after Morimoto et al. (1988).

Figure 5A (Cr₂O₃ × Al₂O₃ and Cr × Fe × Na molar) shows that Alfeu-I clinopyroxenes plot in the eclogites and Cr-poor megacrysts field and Alfeu-I orthopyroxenes in APIP field, which is the field of peridotite nodules in Alto Paranaíba Province. In Fig. 5B (Mg# × Ca#), Alfeu-I clinopyroxenes plot with lower Mg# and higher Ca# than pyroxene from lherzolites, wehrlites, PIC, and MARID rocks. Trace element contents of clinopyroxenes have homogeneous composition, 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).

Figure 5.
(A) Cr₂O₃ versus Al₂O₃ (wt.%) (modified after Nimis 1998, Barabino et al. 2007) and (B) Mg# ((100*Mg)/(Mg+Fe)) versus Ca# ((100*Ca)/(Ca+Mg)) (modified after Fitzpayne et al. 2019 and 2020) with Alfeu-I clinopyroxenes plotted as green triangles and orthopyroxenes as purple triangles. For comparison, in (A): black circles are high- and low-Cr clinopyroxenes from Lages diatremes (Barabino et al. 2007); KG-1: clinopyroxenes from kimberlites of Gibeon Province (Namibia) (Franz et al. 1996, Davies et al. 2001); Premier kimberlites field from Kaapvaal Craton, South Africa (Grégoire et al. 2005); AB clinopyroxenes from spinel; spinel+garnet and garnet-peridotites from alkali basalt-like magmas after Princivalle et al. (1989, 2000), Ionov et al. (1993), and Kempton et al. (1999); APIP: peridotite nodules from pipes of the Alto Paranaíba Province (Meyer and Svisero 1991, Meyer et al. 1991, Araújo et al. 2001) and Lamproites field from Mitchell and Bergman (1991). For comparison, in diagram (B): PIC (Phlogopite-Ilmenite-Clinopyroxene rocks) and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside rocks) (Fitzpayne et al. 2019 and 2020).

Figure 6.
(A) (K+Na) versus Ca (molar) for Alfeu-I orthopyroxenes (inverted pink triangles); (B) Ca/(Ca+Mg) versus Mg/(Mg+Fe) (molar) for Alfeu-I clinopyroxenes (green triangles).

Figure 7.
Chondrite normalized (McDonough and Sun 1995) trace element patterns of (A) Alfeu-I clinopyroxene, (B) garnet, (C) ilmenite, and (D) biotite. REE patterns are marked in the dotted squares.

Garnet

Alfeu-I garnets macrocrysts and microcrysts ((Mg_4.44 Fe²⁺_0.85 Mn_0.04 Ca_0.78)_6.1 (Al_3.67 Ti_0.02 Fe³⁺_0.06 Cr_0.24)_3.99 (Al_0.01 Si₆)_6.0 O₁₂ on basis of 24 oxygens) are classified as Cr-pyrope (Supplementary data), which are typical in peridotites, kimberlites, and lamproites (Mitchell 1995). They are homogeneous, Mg-rich, close to the end-member Pyrope (Mg₃Al₂Si₃O₁₂) (Fig. 8). The diagrams Ca/(Ca+Mg) versus Mg/(Mg+Fe) molar and Cr₂O₃ versus CaO (wt.%) (after Schulze 2003) show that the garnet grains have compositions similar to mantle-derived garnets (Figs. 9A and 9B). The garnets have MgO contents of 20–22 wt.%, Al₂O₃ of 21–23 wt.%, Cr₂O₃ of 1–4 wt.%, and MnO of 0.2–0.4 wt.%. Trace element contents in garnets are very homogeneous and the content of V ranges from 104 to 223 ppm and Ni from 54 to 90 ppm (Suppl. Data), whereas the trace element patterns (normalized to chondrite) show positive anomalies of V, Cr Mn, Y, and Zr, and negative anomalies of Ni, Sr, Nb, La, Ce, W, and Th (Fig. 7B).

Figure 8.
Garnet classification ternary: Mg₃Al₂Si₃O₁₂ versus Fe₃Al₂Si₃O₁₂ versus Ca₃Al₂Si₃O₁₂ with Alfeu-I garnets plotted as orange circles. For comparison, crosses are garnets from Janjão, Pandolfo, and Lambedor diatremes (Brazil) (Barabino et al. 2007), and blue squares are from Arkhangelsk Region (Russia) (Shchukina et al. 2019).

Figure 9.
Garnet composition Ca/(Ca+Mg) versus Mg/(Mg+Fe) molar and Cr₂O₃ versus CaO (wt.%) for Alfeu-I (modified from Schulze 2003). The fields in (A) distinguish mantle-derived garnet from those from crustal rocks, and in (B) compositions of garnets from harzburgites, lherzolites, and wehrlites.

Ilmenite

Alfeu-I macrocryst and microcryst ilmenites are ubiquitous, with MgO contents of 0.36–4.9 wt.%, FeO of 42–47 wt.%, TiO₂ of 47–50 wt.%, and MnO contents of 0.2–0.6 wt.% (Suppl. Data). However, seven analyses of four microcryst grains showed higher MnO contents (from 0.62 to 4.2 wt.%), accompanied by lower FeO contents (29–32 wt.%) and elevated TiO₂ contents (54–57 wt.%) (Fig. 10). These analyses showed a lower total than the low-Mn ilmenites (Suppl. Data), which may be explained by a higher amount of trace elements (Kaminsky and Belousova 2009) such as Co, Nb, V, and Zr. The content of V in ilmenite ranges from 54 to 911 ppm, Zr from 17 to 646 ppm, and Nb from 15 to 463 ppm (details are given in Suppl. Data). According to the chondrite normalized trace element patterns (Fig. 7C), the low-Mn ilmenites show an 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 10.
Compositions (mol%) of Alfeu-I ilmenites plotted in the ternary system geikielite (MgTiO₃) – ilmenite (FeTiO₃) – pyrophanite (MnTiO₃) (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).

Mica

Alfeu-I mica macrocrysts and microcrysts have an average composition (Ca_0.007 Na_0.25 K_1.8)_2.06 (Ti_0.9 Al_0.01 Fe_1.92 Mn_0.008 Mg_2.38)_5.23 (Al_2.37 Si_5.64)_8.01 O₁₀ (F,OH)_4.7 on the basis of 22 oxygens, were classified as tri-octahedral biotite, following the ternary system Al-Mg-Fe_T (Mitchell 1995) (Fig. 11), and have SiO₂ contents of 35–40 wt.%, MgO of 8.8–12 wt.%, K₂O of 8.7–9.6 wt.%, Al₂O₃ of 12–14 wt.%, and FeO of 12–17 wt.% (details are given in Suppl. Data). In the diagram of the end-member phlogopite KMg₃AlSi₃O₁₀(OH)₂ and annite KFe₃AlSi₃O₁₀(OH)₂, the composition of Alfeu-I mica is rather different from the groundmass and microphenocryst phlogopites from the Rosário-6 alnöite (Conceição et al. 2019). Moreover, we compare our data to phlogopites from Alto Paranaíba kimberlites and kamafugites, Arkhangelsk kimberlite province, calciocarbonatites of Mela Field, Jacupiranga complex carbonatites, and Canadian and Russian kimberlites and carbonatites (Fig. 11).

Figure 11.
Compositions (22 oxygens) of micas from Alfeu-I lamproite plotted in the ternary diagram Al-Mg-Fe_T, modified after Mitchell (1995). Phlogopite of Rosário-6 alnöite groundmass (white triangles) and microphenocrysts (black triangles) (Conceição et al. 2019); Alto Paranaíba kimberlites and kamafugites (yellow circles) (Melluso et al. 2008); Arkhangelsk kimberlite province (purple triangles); calciocarbonatite of Mela Field (gray circles) (Beard et al. 2000); Jacupiranga carbonatite complex (red circles) (Brod et al. 2001); and Canadian and Russian kimberlites and carbonatites (purple and green stars, respectively) (Reguir et al. 2009).

The cation exchange in the structure of the tri-octahedral biotite with increasing Fe and decreasing Mg and K contents is shown in Fig. 12. The analyzed grains have a content of 294–839 ppm for Mn, 170–459 ppm for Zn, 322–797 for Rb, and 565–1611 ppm for Ba (details are given in Suppl. Data) and have positive anomalies of Rb, Sr, Ba, Zr, Nb, and Ta, as well as an enrichment in LREE (La, Ce, Pr, Nd, Sm, and Eu) compared to HREE (Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) (Fig. 7D).

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.

Olivine

Olivine microcrysts ((Fe_0.2 Mg_1.78 Mn_0.002)_1.98 SiO₆ on basis of 4 oxygens) from the Alfeu-I lamproite are rare, with a homogeneous composition of Fo_89-90 (Suppl. Data). The analyzed minerals have FeO contents of 9.4–9.6 wt.%, MgO of 48–49 wt.%, MnO of 0.13–0.16 wt.%, and NiO of 0.36–0.4 wt.%.

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 ⁸⁷Rb/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd 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 ⁸⁷Sr/⁸⁶Sr radiogenic ratio but also a lower ¹⁴³Nd/¹⁴⁴Nd 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, they should display similar initial ⁸⁷Sr/⁸⁶Sr ratios (Nowell et al. 2004, Blackburn et al. 2008). Radiogenic ingrowth from ⁸⁷Rb decay through time in each phase will modify the ⁸⁷Sr/⁸⁶Sr proportionally, leading to a linear correlation on a ⁸⁷Sr/⁸⁶Sr versus ⁸⁷Rb/⁸⁶Sr 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 three-point 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).

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.

Sample	Mineral	Rb (ppm)	Sr (ppm)	Rb/Sr	87Rb/86Sr	Error (abs)	87Sr/86Sr	Error (abs)
AF-02-F	Biotite	1167.3806	62.0040	18.8274	54.9073	7.0224	0.7188	0.000093
AF-03-I	Ilmenite	6.6838	4.0622	1.6453	4.8552	0.8914	0.7186	0.000065
AF-04-I	Ilmenite	6.6279	3.9925	1.6600	4.8975	0.9811	0.7186	0.000273
AF-07-P	Pyroxene	1.7830	94.1018	0.0189	0.0552	0.0023	0.7038	0.000219
AF-08-P	Pyroxene	1.4970	92.8910	0.0161	0.0469	0.0018	0.7038	0.000069
AF-05-G	Garnet	6.6426	3.1623	2.1005	6.1663	0.2407	0.7146	0.000271

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

Sample	Mineral	Age (Ma)	Sm (ppm)	Nd (ppm)	147Sm/144Nd	143Nd/144Nd	143Nd/144Nd (t)	Error (ppm)	Eps Nd	TDM 125 Ma
AF-07-P	Pyroxene	128	23.43	108.71	0.130300	0.512950	0.512841	8	7.18	210
AF-08-P	Pyroxene	128	23.36	90.81	0.155497	0.512830	0.512699	11	4.41	585
AF-05-G	Garnet	128	1044.94	12980.88	0.048669	0.511845	0.511804	16	-13.06	1105

Figure 13.
(A) Sr and Nd isotopic data for pyroxene (green diamonds) and garnet (blue diamond) from the Alfeu-I lamproite. Averages of mantle reservoirs from Hart et al. (1992): DMM (Depleted MORB Mantle: open green triangle), MORB (Mid-Ocean Ridge Basalt: blue triangle), HIMU (High-μ: purple circle), EM I (Enriched Mantle I: orange square), and EM II (Enriched Mantle II: red circle). For comparison, the field of Proto Tristan da Cunha plume (~130 Ma Paraná-Etendeka magmas) (Cohen and O’Nions 1982, Gibson et al. 1999), Rosário-6 alnöite bulk rock (purple diamonds) (Conceição et al. 2019), Namibia lamprophyres and carbonatites (green squares and stars, respectively) (Le Roex and Lanyon 1998), Alto Paranaíba Igneous Province (yellow circles) (Guarino et al. 2013), Jacupiranga alkaline rocks (pink diamonds) (Chmyz et al. 2017), sodic and potassic rocks from Paraguay (“x” symbols and red squares, respectively) (Comin-Chiaramonti et al. 1997), and Patagonian xenoliths (blue crosses) (Jalowitzki et al. 2017) are calculated to 128 Ma, while high- and low-Ti Paraná basalts (gray diamonds and traces, respectively) (Marques et al. 1999) are calculated to 133 Ma and Siberian meimechites are calculated to 245 Ma (gray circles) (Arndt et al. 1995). (B) Rb-Sr mineral isochron for mineral separates (pyroxene, garnet, ilmenite, and biotite) from the Alfeu-I lamproite. Uncertainties are 1σ of the mean value and given in Table 1. Isochron regressions considering both biotite and ilmenite (white diamonds) did not yield meaningful results (see text for details).

Geothermobarometry and oxygen barometry results

Crystallization temperatures of Alfeu-I lamproite were determined using the olivine-spinel Mg-Fe²⁺ 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 Fe²⁺ and Fe³⁺ 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.

Table 3.
Alfeu-I equilibrium temperatures and oxygen fugacities calculated based on olivine-chromite geothermometer and the oxygen geobarometer of Ballhaus et al. (1991). Olivine compositions are from this study. Chromite compositions from Chaves et al. (2014). Olivines (Ol), chromites (Chr), ΔFMQ (fO₂ relative to FMQ buffer), and standard deviations (stdev) as last significant digits in brackets.

Sample		Ol-sp T (°C)		Ol-sp Δlog(fO2)FMQ
		4 GPa	5 GPa	4 GPa	5 GPa
Ol	01	1,489	1,509	2.2	2.0
Chr	Alf-Cr-01
Ol	02	1,404	1,420	2.8	2.6
Chr	Alf-Cr-02
Ol	03	1,281	1,301	2.4	2.1
Chr	Alf-Cr-04
Ol	04	1,322	1,341	2.4	2.1
Chr	Alf-Cr-05
Ol	05	1,314	1,333	2.3	2.1
Chr	Alf-Cr-06
Ol	06	1,343	1,362	2.3	2.1
Chr	Alf-Cr-07
Ol	07	1,623	1,433	2.5	2.4
Chr	Alf-Cr-08
Ol	08	1,415	1,439	2.3	2.1
Chr	Alf-Cr-11
Ol	09	1,393	1,412	2.3	2.1
Chr	Alf-Cr-12
Mean		1,375	1,395	2.4	2.2
(stdev)		(65)	(65)

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).

DISCUSSION

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 (Cr₂O₃ versus Al₂O₃ 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 ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd 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 fO₂ 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 fO₂ conditions calculated for the Alfeu-I lamproite and Rosário-6 alnöite are significantly higher than the fO₂ 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 fO₂ of the Serra Geral basalts based on ilmenite and Ti-magnetite compositions. They found that high-Ti basalts, which have fO₂ values between the nickel-nickel oxide (NNO) and fayalite-magnetite-quartz (FMQ) buffers, are slightly more oxidized than the low-Ti basalts, with fO₂ 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).

Figure 14.
LogfO₂ versus temperature with calculated T and fO₂ for Alfeu-I lamproite (inverted orange triangles); Rosário-6 (purple triangles); high-Ti Serra Geral basalts (green circles); and low-Ti Serra Geral basalts (yellow circles) (Bellieni et al. 1984) results. Buffers: NNO (nickel-nickel oxide); FMQ (fayalite-magnetite-quartz) (Ballhaus et al. 1991); and WM (wüstite-magnetite) (O’Neill 1988).

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 and Klemme 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 fO₂ 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 fO₂ 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 (MgCO₃) 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 Fe³⁺ in silicate minerals during upwelling (Rohrbach and Schmidt 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).

Figure 15.
Stability fields of graphite and CO₂ in logfO₂ versus temperature diagram at 4 and 5 GPa. D/GCO buffer from Frost and Wood (1997); EMOD/G buffer from Zhao et al. (1999); and NNO buffer from Ballhaus et al. (1991). The D/G (diamond/graphite) limit is from Kennedy and Kennedy (1976). Alfeu-I results are plotted as inverted orange triangles.

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.

ACKNOWLEDGMENTS

This research was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico – Brazil (CNPq – 141241/2013-6), Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul – Brazil (FAPERGS – Edital 05/2019), and the project “Diamante Brasil” from the Geological Survey of Brazil (CPRM). We are very grateful to Prof. Márcia E. B. Gomes, Susan Drago, MSc, and Daniel Grings Cedeño, MSc, for their help with Alfeu-I microprobe data acquisition, and to Beate Schmitt for her assistance with LA-ICP-MS data. We also thank João Wustrow, who separated the Alfeu-I minerals, and Mariana da Silva Assis, MSc, for her help with the isotope analyses.

Supplementary data

Supplementary data associated with this article can be found in the online version: http://sfbjg.siteoficial.ws/Sf/2023/415220220092.pdf

ARTICLE INFORMATION

• Manuscript ID: 20220092. Received on: 23 NOV 2022. Approved on: 06 JUN 2023.
  How to cite: Carniel L.C., Conceição R.V., Provenzano C.A.S., Sander A., Leitzke F.P., Silva A.B., Berndt J., Klemme S. (2023). 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. Brazilian Journal of Geology, 53(3), e20220092. https://doi.org/10.1590/2317-4889202320220092

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Publication Dates

• Publication in this collection
  23 Oct 2023
• Date of issue
  2023

History

• Received
  23 Nov 2022
• Accepted
  06 June 2023