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Anais da Academia Brasileira de Ciências

Print version ISSN 0001-3765On-line version ISSN 1678-2690

An. Acad. Bras. Ciênc. vol.90 no.1 Rio de Janeiro Mar. 2018  Epub Feb 01, 2018

http://dx.doi.org/10.1590/0001-3765201820170854 

Earth Sciences

Serra Pelada: the first Amazonian Meteorite fall is a Eucrite (basalt) from Asteroid 4-Vesta

MARIA ELIZABETH ZUCOLOTTO1 

AMANDA A. TOSI2 

CAIO V.N. VILLAÇA2 

ANDRÉ L.R. MOUTINHO3 

DIANA P.P. ANDRADE4 

FABIANO FAULSTICH1 

ANGELO M.S. GOMES5 

DEBORA C. RIOS6 

MARCILIO C. ROCHA7 

1LABET/MN/UFRJ, Laboratório Extraterrestre, Departamento de Geologia e Paleontologia, Museu Nacional, Universidade Federal do Rio de Janeiro, Quinta da Boa Vista, São Cristóvão, 20940-040 Rio de Janeiro, RJ, Brazil

2LABSONDA/IGEO/UFRJ, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 274, Cidade Universitária, 21941-972 Rio de Janeiro, RJ, Brazil

3Colecionador da International Meteorite Colector Association (IMCA #2731), R. Roberto dos Santos, 163, 12300-000 Jacareí, SP, Brazil

4OV/UFRJ, Observatório do Valongo, Universidade Federal do Rio de Janeiro, Ladeira Pedro Antônio, 43, Saúde, 20080-090 Rio de Janeiro, RJ, Brazil

5IF/UFRJ, Instituto de Física, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos,149, CT, Bloco A, Cidade Universitária, 21941-972 Rio de Janeiro, RJ, Brazil

6GPA, Universidade Federal da Bahia/UFBA, Instituto de Geociências, R. Barão de Geremoabo s/n, Ondina, 40170-290 Salvador, BA, Brazil

7Universidade Federal do Pará/UFPA, Departamento de Geociências e Engenharias, Rua Augusto Correa, nº 01, Campus Universitário do Guamá, 66075-110, Belém, PA, Brazil


ABSTRACT

Serra Pelada is the newest Brazilian eucrite and the first recovered fall from Amazonia (State of Pará, Brazil, June 29th 2017). In this paper, we report on its petrography, chemistry, mineralogy and its magnetic properties. Study of four thin sections reveals that the meteorite is brecciated, containing basaltic and gabbroic clasts, as well of recrystallized impact melt, embedded into a fine-medium grained matrix. Chemical analyses suggest that Serra Pelada is a monomict basaltic eucritic breccia, and that the meteorite is a normal member of the HED suite. Our results provide additional geological and compositional information on the lithological diversity of its parent body. The mineralogy of Serra Pelada consists basically of low-Ca pyroxene and high-Ca plagioclase with accessory minerals such as quartz, sulphide (troilite), chromite - ulvöspinel and ilmenite. These data are consistent with the meteorite being an eucrite, a basaltic achondrite and a member of the howardite-eucrite-diogenite (HED) clan of meteorites which most likely are from the crust asteroid 4 Vesta.

Key words: Serra Pelada; meteorite; eucrite; Vesta; Brazilian Meteorite

INTRODUCTION

Observed meteorite falls are very important for scientific studies, because the recovered material was not yet subjected to terrestrial weathering.

It is believed that the majority of meteorites are originated on parent bodies in the asteroid belts. Although few specific asteroids have been identified as meteorite sources, there is an association between Vesta (and other asteroids in the Vestoid Family) and the Howardite, Eucrite, Diogenite (HEDs) clan of achondrites (Binzel and Xu 1993). The achondrites are differentiated rocks and were derived from planetary bodies that partially or totally melted. In case of asteroid 4 Vesta, the most likely parent asteroid of the HEDs, this process formed a metallic core, a mafic mantle, and a basaltic crust. This differentiation took place in the very early history of the solar system, and the heat source was short lived 26Al (e.g., Srinivasan et al. 1998; Mittlefehldt et al. 1998; Keil 2002; Mittlefehldt 2005, 2014, 2015).

The eucrites represent volcanic rocks and are subdivided into cumulate and basaltic, based on their pyroxene and plagioclase major-element compositions (McSween et al. 2011). Cumulate eucrites are plutonic rocks (coarse grained equigranular) and primarily composed of anorthite plagioclase (An90-96) and MgO-rich pyroxene (En46-65). Basaltic eucrites are defined by extrusive, i.e., fine- to very fine-grained textures. It is believed that they formed at or near Vesta’s surface quickly cooled. They contain pyroxene, FeO-rich (En<46), and more sodic plagioclase (An<90) than the cumulate (McSween et al. 2011).

Aiming to improve understanding of Vesta magmatic processes, this paper investigates this fresh eucrite fall and describes different textures in the meteorite, as well as the chemical analysis of its minerals.

HISTORY

The Serra Pelada meteorite fell on June 29, 2017 (10:35 local time - UTC-3) at Serra Pelada Village (a famous gold mining location) (5º 57.135’S, 49º 39.238’W), located in the State of Pará, northern Brazil. People reported seeing a fireball in the southeast sky of the state, apparently traveling from NE to SW. A large explosion followed by a series of minors ones was heard in nearby towns such as Marabá, Eldorado dos Carajás and Paraopebas. The inhabitants of these localities thought an airplane had fallen. Using helicopters, the Fire and Police departments searched in vain for the accident.

The students and staff of the Rita Lima Sousa Municipal School who were in the playground witnessed the fall of the rock on the sidewalk, after a sequence of four to six explosions. Also observed were a trail of smoke and a little dust caused by the impact. People went to the impact site and collected fragments of the broken stone leaving only a small impact pit.

A second mass of 5.4 kg was observed to fall and was recovered by an electrician while working at Serra Leste, a Vale do Rio Doce enterprise. This piece was sold to an anonymous buyer. The residents did not realize the importance of the event. They exchanged the fragments among themselves and shared them with local people.

The geologist Marcilio Cardoso Rocha, born at Serra Pelada, was contacted and, suspecting that it was a meteorite, he contacted Dra. Maria Elizabeth Zucolotto (MEZ), the senior author of this paper, who advised him to do some preliminary analyses. Even before Marcílio sent a sample to the Museu Nacional, information about the fall of the meteorite leaked into social medias and shortly dealers went to the place and got many pieces. This event happened while some authors were in the field trying to stablish the strewnfield of Três Irmãos meteorite, which fell in the State of Bahia a month before, which is an unusual fact.

Some of us went to Serra Pelada in order to determine the strewnfield and possibly retrieve other pieces. The task was complicated due to intrinsic difficulties associated with the region, such as the hot weather, a large area covered by forest or high grass and dangerous conditions, such as: find a clandestine gold prospecting, the need to make large amount of withdrawals, and be stopped at a road closed by the Movement of Landless Workers (MST, Movimento dos Trabalhadores Rurais Sem Terra, in Portuguese). The best place for searching was in the Serra Leste enterprise area, but we have not given permission to explore.

The meteorite classification and the name “Serra Pelada” were approved by the Meteorite Nomenclature Committee of the Meteoritical Society N° 106 as a brecciated eucrite.

MATERIALS AND METHODS

Many slices, such as in figure 1, were examined using a stereomicroscope Zeiss Discovery V8. Three polished thin sections were examined microscopically in transmitted and reflected light using a petrographic microscope (Zeiss Axioplan) and minerals in two polished thin sections were analyzed using a JEOL EPMA JXA-8230 Superprobe at LABSONDA/IGEO/UFRJ.

Figure 1 A cut section of Serra Pelada meteorite. It shows a typical eucrite with a partial, shiny, black fusion crust and a light colored interior with a brecciated texture. Also visible are some cracks and melt veins. Some clasts show clumps of white blades of plagioclase (size of cube 1 cm). 

Quantitative analyses of constituent phases we carried out using Wavelength Dispersive Spectrometry (WDS). Beam conditions included an accelerating voltage of 15 KeV, beam current of 20 nA and a spot size of 1 μm for silicates and 20 KeV, for opaque minerals. Well-characterized natural and synthetic phases were used as standards, and corrections for differential matrix effects were made with a ZAF factory supplied procedure. During the study, Energy Dispersive Spectrometer (EDS), back-scattered electrons (BSE) imaging and composition mapping image by WDS were also performed.

Micro-Raman spectroscopy was conducted with a Horiba Jobin Yvon LabRam HR at CETEM (Centro de Tecnologia Mineral). A 632.8 nm laser standardized with a Si chip and a confocal hole of 300. The analyses were performed using a 100 X objective lens on the Raman microscope. The recorded spectra were compared with the RRUFF database for phase identification.

The magnetic data were obtained in a Physical Properties Measurement System, from Quantum Design Inc with Option P500, which uses an extraction technique where the magnetized sample is moved through detection coils. All measurements were performed at room temperature, T=300K, and ramping the magnetic field up to 10 kOe. From the M vs H curve (M = Magnetization; H = Magnetic Field), we obtained the DC (direct current) susceptibility by angular coefficient of the linear fit up to 500 Oe. The resulting mass susceptibility is calculated according to the mass of each sample.

RESULTS

MORPHOLOGY

As first mentioned in the history of the fall, the first mass of Serra Pelada weighed about 6 kg. It has been broken and distributed among local inhabitants. The second mass, which weighs 5.4 kg was shaped like a rounded pyramid, having dimensions of 25 x 12.5 x 12 cm and was nearly 80% covered with fusion crust. Many fragments of the first mass showed that most of the samples have a glassy and shiny black fusion crust of about 1.0 mm in thickness. Some pieces also show a crust with flow lines, which are typical for eucrites.

Stereomicroscope examinations of the cut surfaces of various slices, hand specimens and thin sections reveal a heterogeneous, brecciated appearance (Figure 1). The clasts range in size from millimeters to centimeters, in shape from oblate to rectangular, and in texture from very fine to coarse grained. The matrix between the clasts is gray and comprises an overall magmatic intergrowth assemblage of plagioclase and pyroxene.

PETROGRAPHY AND MINERAL CHEMISTRY

Macroscopic analysis of Serra Pelada showed that this meteorite is brecciated in nature (Figs. 1-2) with different kinds of clasts and distinct textural features (Figs. 1-3). Petrographic study revealed different textures of rock fragments embedded within a well-consolidated matrix. Here the textural terminology and abbreviations follow that used by Yamaguchi et al. (1994): MM - clastic fine to medium grained matrix (0.05 to 0.1mm) rich in lithic clasts and mineral fragments ranging from 0.05 to 0.8mm. The matrix presents a fragmented aspect and it is difficult to identify the borders of most of the clasts (Fig 3a); FX - very fine grained clast of equigranular subophitic and ophitic textures composed of pyroxenes (<0.07mm) and plagioclase of the same size range (shock melt veins are present within this clast) (Fig 3b); CX - coarse grained clasts with gabbroic subophitic texture. Fractured anhedral to subhedral pyroxenes (0.6 to 1 mm) show mosaicism and opaque inclusions while plagioclase (0.25 to 0.8 mm) shows undulatory extinction (Fig 3c); MX - Medium grained clasts with subophitic texture. Fractured pyroxene (0.2 to 0.5mm) containing opaque inclusions and mosaicism. Needles of plagioclase ranging from 0.2 to 0.8 mm show undulatory extinction (Fig 3d); RX - recrystallized impact melts (Fig 3e-f).

Figure 2 Picture of a thin section in transmitted light. A variety of light and dark angular clasts range in size from millimeters to centimeters. The lithologies are outlined with dashed lines for better visualization. The matrix and clasts are identified by letters as indicated in the text of the paper. White areas are plagioclase, medium brown areas are clinopyroxene and small black spots are melts and opaque minerals. The abreviations are those presented in figure 3.  

Figure 3 Transmitted plane polarized light photomicrographs of Serra Pelada showing variability of clast type: (a) MM - clastic fine to medium grained matrix , b) FX - very fine grained clast , c) CX - coarse grained clasts, d) MX - Medium grained clasts, e-f) RX - recrystallized impact melts. 

Shock features in Serra Pelada are somewhat heterogeneous among the various lithologies. Mostly plagioclase crystals in the entire meteorite preserve their original lath shape and show weak undulatory extinction, although it is difficult to verify in the FX-clasts, due to their small grain size. Many pyroxenes of the CX-clasts are fractured and show mosaic texture. Over some areas, both in the matrix and in the clasts, there are small melt pockets and network-like glassy veins. These veins are composed of mafic glass containing very fine and partly molten minerals. Since no maskelynite is present nor are planar deformation features observed in pyroxenes, the shock stage is estimated to be S3, according to shock metamorphism features proposed for shock classification by Stöffler et al. (1991) and Rubin et al. (1997).

MINERALOGY

The matrix and clasts of Serra Pelada are composed mainly of calcic plagioclase feldspar and clinopyroxenes, with variable composition. Accessory minerals are quartz, apatite and opaque minor phases that include Ni-poor iron, troilite, ilmenite and chromite.

Pyroxene: Pyroxene is identified to be dominantly pigeonite (MM: En26-29 Wo4-29 Fs34-61; FX: En25-33 Wo8-20 Fs40-60; MX: En33-39Wo7-14 Fs53-63; CX: En29-39 Wo7-37 Fs33-56) (Figure 4). Many of these grains in matrix and clasts have exsolved lamellae of augite (Figure 5a, b). The quantitative WDS analysis composition of the pyroxene for each lithology present in Serra Pelada are given in Table I. Some grains have a cloudy appearance due to many tiny opaque mineral inclusions (several microns to submicron dimension) which is a predominant feature in basaltic eucrites as reported in the literature (e.g. Hutchinson, 2004).

TABLE I Major and trace element composition of pyroxenes from the different lithologies present in Serra Pelada meteorite. Quantitative WDS analysis in wt. % . 

Al2O3 wt % MgO wt % SiO2 wt % FeO wt % CaO wt % TiO2 wt % Cr2O3 wt % MnO wt % CoO wt % Total wt %
MM fine to medium grained matrix 0.877 10.359 49.054 27.388 10.532 0.282 0.726 0.839 0.000 100.057
0.400 10.926 48.823 34.428 3.951 0.072 0.310 1.081 0.000 99.991
0.559 10.697 48.943 33.846 3.906 0.105 1.232 1.031 0.000 100.319
0.794 10.053 48.809 28.507 9.584 0.277 1.356 0.900 0.004 100.284
0.227 11.160 49.757 33.646 4.580 0.075 0.154 1.053 0.000 100.652
0.333 11.056 49.515 33.587 4.684 0.061 0.204 1.089 0.000 100.529
1.455 10.179 49.614 24.129 14.054 0.177 0.486 0.717 0.000 100.811
0.597 10.205 48.814 27.766 10.567 0.342 0.524 0.871 0.000 99.686
0.456 11.198 49.879 33.631 4.520 0.017 0.159 1.072 0.004 100.936
1.428 9.926 48.154 28.151 10.919 0.231 1.072 0.864 0.000 100.745
FX very fine grained clast 0.879 10.230 49.456 28.723 9.596 0.170 0.339 0.871 0.000 100.264
0.515 9.898 49.296 29.901 9.312 0.180 0.300 0.929 0.000 100.331
0.315 9.897 49.371 33.971 5.342 0.131 0.242 1.080 0.000 100.349
0.494 9.633 49.493 29.953 9.618 0.169 0.430 0.956 0.013 100.759
3.711 9.428 47.196 30.486 6.636 0.121 0.285 0.949 0.000 98.812
0.295 10.991 49.771 34.849 3.708 0.104 0.150 1.061 0.000 100.929
0.554 10.541 49.063 30.275 7.555 0.145 0.557 0.956 0.000 99.646
9.917 7.209 47.637 20.386 13.097 0.075 0.152 0.658 0.000 99.131
0.576 10.509 48.544 34.885 3.944 0.140 0.269 1.110 0.000 99.977
0.446 10.756 48.842 34.609 4.300 0.130 0.791 1.022 0.000 100.896
MX medium grained clasts 0.454 12.913 47.280 32.056 3.684 0.116 0.935 1.012 0.000 98.450
0.281 12.190 43.190 40.915 1.396 0.017 0.410 1.157 0.000 99.556
0.464 12.713 49.982 32.267 3.736 0.105 0.477 0.957 0.011 100.712
0.758 11.129 49.187 31.032 6.645 0.102 0.906 0.970 0.000 100.729
0.585 12.050 48.675 33.872 3.839 0.105 1.060 1.032 0.000 101.218
0.807 11.818 47.399 33.622 3.599 0.121 1.239 1.022 0.006 99.633
0.455 11.213 47.973 33.764 3.980 0.196 0.929 1.066 0.000 99.576
0.412 13.233 48.727 32.347 2.599 0.101 0.559 1.035 0.000 99.013
0.412 13.034 49.149 32.408 3.708 0.062 0.415 1.001 0.000 100.189
CX coarse grained clasts 0.708 13.094 49.300 31.585 4.036 0.068 0.787 0.983 0.000 100.561
0.455 12.411 49.569 33.181 3.191 0.022 0.346 1.058 0.000 100.233
0.580 12.626 49.658 30.397 5.454 0.084 0.308 1.022 0.000 100.129
0.378 12.377 49.085 33.360 3.497 0.081 0.625 1.093 0.000 100.496
1.026 11.571 49.804 31.210 5.152 0.097 0.279 1.021 0.000 100.160
0.466 11.002 49.501 33.226 5.297 0.087 0.366 1.036 0.000 100.981
1.205 11.820 49.884 25.598 10.824 0.167 0.450 0.850 0.000 100.798
0.471 12.405 49.602 33.415 3.033 0.080 0.437 1.081 0.011 100.535
1.002 11.333 49.846 26.339 10.592 0.118 0.584 0.839 0.000 100.653
1.186 10.610 49.823 24.595 12.780 0.157 0.441 0.773 0.012 100.377
RX recrystallized impact melts 0.854 10.043 48.688 26.711 12.053 0.244 1.042 0.821 0.000 100.456
6.852 8.558 47.822 25.582 9.647 0.126 0.433 0.816 0.000 99.836
7.809 7.801 46.940 22.861 11.490 0.242 0.938 0.685 0.003 98.769
0.809 10.031 48.364 25.225 13.714 0.304 0.843 0.756 0.000 100.046
4.893 9.485 49.144 28.005 7.867 0.017 0.068 0.887 0.000 100.366
3.190 9.315 44.911 30.414 7.595 3.980 0.074 0.852 0.000 100.331
0.317 10.707 48.947 35.324 3.778 0.233 0.300 1.118 0.000 100.724
5.252 9.946 48.903 30.448 5.706 0.185 0.061 0.937 0.000 101.438
6.535 9.039 48.544 30.239 5.460 0.095 0.070 0.925 0.000 100.907
6.473 9.807 48.144 31.850 3.700 0.044 0.059 1.036 0.000 101.113

Figure 4 Pyroxene ternary diagram showing the composition of pyroxene in matrix and clasts in Serra Pelada meteorite.  

Figure 5 Pyroxene of Serra Pelada showing pigeonite and augite exsolution lamellae as dark lines. a) Transmitted plane polarized light photomicrograph b) back scattered electron (BSE) image. Pgt: pigeonite; Aug: augite; An: Anothite-bytownite. 

Feldspar: Calcic feldspar occurs in all clasts and variable in average composition from An81 to An93, which is similar in composition to typical basaltic eucrites of An75-94 (e.g., Mittlefehldt 2015). Many clasts show ophitic/sub-ophitic textures, defined by the presence of subhedral laths of plagioclase feldspars embedded in pyroxene grains. Feldspar also shows a cloudy appearance due many opaque inclusions, also found in the pyroxenes. Most of the feldspar grains have a composition similar to bytownite, whereas a couple of them show anorthite composition (Figure 6). Table II shows the quantitative WDS analysis composition of the plagioclase present in the matrix and clasts.

TABLE II Major and trace element composition of plagioclase from the different lithologies present in Serra Pelada meteorite. Quantitative WDS analysis in wt. %. 

Na2O wt % Al2O3 wt % SiO2 wt % MgO wt % CaO wt % FeO wt % K2O wt % TiO2 wt % MnO wt % Cr2O3 wt % Total wt %
MM fine to medium grained matrix 0.537 36.256 44.002 0.043 18.772 0.354 0.063 0.000 0.000 0.000 100.027
0.853 35.018 45.495 0.001 17.913 0.277 0.091 0.000 0.000 0.010 99.658
0.782 35.372 45.533 0.022 17.799 0.248 0.053 0.000 0.000 0.014 99.823
2.874 31.056 51.160 0.068 13.957 0.292 0.575 0.022 0.000 0.000 100.004
0.746 35.949 44.734 0.009 18.346 0.233 0.054 0.007 0.000 0.004 100.082
0.575 36.336 44.281 0.032 18.708 0.153 0.072 0.020 0.049 0.000 100.226
0.816 35.573 45.553 0.018 17.899 0.151 0.099 0.011 0.000 0.011 100.131
0.771 35.283 45.584 0.129 17.847 0.480 0.082 0.000 0.032 0.002 100.210
1.945 33.495 47.150 0.264 16.610 0.857 0.164 0.054 0.042 0.017 100.598
1.201 34.641 46.980 0.016 16.750 0.367 0.180 0.038 0.018 0.005 100.196
FX very fine grained clast 1.195 33.570 47.835 0.088 16.363 0.584 0.160 0.000 0.000 0.019 99.814
1.071 34.198 46.858 0.057 16.706 0.418 0.142 0.011 0.000 0.036 99.497
1.869 33.548 46.954 0.145 15.988 0.988 0.200 0.000 0.000 0.016 99.708
1.789 34.077 47.419 0.059 16.498 0.351 0.147 0.042 0.048 0.000 100.430
0.928 31.495 50.456 0.036 15.745 0.287 0.096 0.035 0.000 0.000 99.078
0.975 35.125 46.282 0.033 17.432 0.337 0.065 0.003 0.011 0.019 100.282
0.900 34.386 45.765 0.315 17.037 1.627 0.041 0.073 0.062 0.026 100.232
0.864 34.586 46.358 0.096 17.370 0.597 0.122 0.000 0.004 0.006 100.003
0.044 0.691 97.472 0.020 0.250 0.222 0.027 0.091 0.004 0.003 98.824
0.907 34.638 46.260 0.119 17.446 0.366 0.109 0.050 0.029 0.000 99.924
MX medium grained clasts 1.390 33.224 47.740 0.278 16.024 0.801 0.208 0.023 0.020 0.000 99.708
2.045 34.001 47.409 0.090 16.299 0.629 0.193 0.000 0.002 0.014 100.682
1.832 34.265 47.041 0.157 16.662 0.657 0.142 0.000 0.000 0.000 100.756
1.182 34.312 47.137 0.049 16.818 0.409 0.160 0.002 0.077 0.000 100.146
0.962 34.971 46.015 0.035 17.169 0.248 0.088 0.000 0.066 0.000 99.554
0.608 35.928 44.611 0.101 17.999 0.555 0.101 0.000 0.032 0.000 99.935
1.194 33.533 47.054 0.219 16.422 1.018 0.204 0.004 0.003 0.025 99.676
1.082 35.015 46.434 0.022 17.152 0.312 0.134 0.019 0.000 0.009 100.179
2.354 33.835 47.801 0.104 15.766 0.664 0.250 0.014 0.031 0.000 100.819
1.177 34.362 46.802 0.085 16.467 0.466 0.196 0.001 0.019 0.019 99.594
CX coarse grained clasts 0.817 35.385 45.087 0.013 17.707 0.223 0.079 0.000 0.006 0.000 99.317
1.157 34.381 46.767 0.100 16.847 0.243 0.177 0.003 0.000 0.000 99.675
0.732 35.943 44.557 0.037 18.064 0.179 0.057 0.006 0.004 0.024 99.603
2.031 34.177 47.224 0.104 16.403 0.431 0.190 0.017 0.000 0.000 100.577
0.812 35.704 45.736 0.022 17.724 0.124 0.122 0.022 0.000 0.020 100.286
0.846 35.823 44.982 0.000 17.960 0.203 0.070 0.015 0.000 0.009 99.908
0.945 35.606 45.651 0.011 17.436 0.208 0.127 0.000 0.037 0.000 100.021
1.079 34.492 46.201 0.206 16.932 1.095 0.118 0.000 0.064 0.010 100.197
0.759 35.992 44.958 0.027 18.022 0.196 0.083 0.022 0.000 0.000 100.059
1.871 34.350 47.319 0.025 16.558 0.250 0.184 0.011 0.001 0.000 100.569
RX recrystallized impact melts 0.984 34.893 46.716 0.048 16.433 0.751 0.145 0.013 0.017 0.004 100.004
0.647 31.417 50.526 0.010 15.762 1.299 0.077 0.105 0.039 0.000 99.882
0.531 24.776 60.393 0.005 12.280 2.714 0.073 0.141 0.000 0.007 100.920
1.404 29.591 50.363 0.650 13.824 2.986 0.329 0.015 0.079 0.000 99.241
2.087 30.185 50.990 0.459 14.281 2.174 0.360 0.009 0.000 0.016 100.561
2.164 33.428 47.333 0.225 15.602 1.199 0.318 0.038 0.000 0.001 100.308
1.132 31.397 49.133 0.184 15.518 1.405 0.211 0.059 0.112 0.020 99.171
0.515 18.274 66.710 0.413 9.326 3.376 0.101 0.189 0.070 0.027 99.001
1.292 31.458 49.068 0.289 14.998 1.862 0.267 0.027 0.088 0.038 99.387
2.501 31.211 51.415 0.029 14.334 0.734 0.477 0.033 0.032 0.001 100.767

Figure 6 An-Ab-Or ternary diagram depicting the plagioclase compositions in Serra Pelada compared with NWA 6105,2 (McSween et al. 2011). 

Accessory minerals: The presence of SiO2 has been identified as quartz by its Raman spectrum and is a common accessory mineral noticed mostly as inclusions within the plagioclase and also less common within pyroxene. In addition, it occurs in the interstices between plagioclase and pyroxene. Apatite also occurs in minor amounts.

The main opaque phases are troilite and ilmenite, minor amounts of Ni-poor Fe and chromite. There are no systematic differences in chemical compositions between mineral phases in different textural regions. Average composition of ilmenite (TiO2 = ~51 wt % FeO = ~43 wt %) is consistent with this mineral in other basaltic eucrites (Mayne et al. 2009; Mittlefehldt 2015). Chromite and ulvöspinel occur as accessory phases in sizes up to tens of microns. These chromite/ulvöspinel grains are common minor minerals in basaltic eucrites (Mittlefehldt 2015).

The measurements of the magnetic susceptibility showed that Log χ (10-9 m3/kg) is about 2.9 and the density is 2.77 g/cm3.

DISCUSSION

Petrographic properties and mineral chemistry data of Serra Pelada are typical of eucrites of the HED clan and confirm its origin from asteroid 4 Vesta (Figure 7 and Table III). It is a monomict breccia with lithic and mineral clasts set in a matrix of fine to medium-grained mineral fragments. As observed in most of the known eucrites, there are no systematic differences in chemical compositions of minerals among different textural regions.

TABLE III Comparing Serra Pelada pyroxene and plagioclase data with others planetary basalts. These literature data were taken from Papike et al. (2003). Pyx: pyxroxene; An: Anothite-bytownite. 

Earth Moon Mars 4-Vesta Serra Pelada
Pyx Fe/Mn An % Pyx Fe/Mn An % Pyx Fe/Mn An % Pyx Fe/Mn An % Pyx Fe/Mn An %
x = 40 x = 69 x = 62 x = 89 x = 32 x = 49 x = 30 x = 87 x = 32 x = 89
sd = 11 sd = 12 sd = 18 sd = 3 sd = 6 sd = 5 sd = 2 sd = 2 sd = 2 sd = 5
N = 513 N = 474 N = 37 N = 243 N = 33 N = 39 N = 38 N = 35 N = 65 N = 62

Figure 7 Fe vs. Mn per 6-oxygen formula unit for pyroxene from planetary basalts, modified from Papike et al. (2003). The angular coefficient of Serra Pelada fits well with an origin from asteroid 4 Vesta. 

The chemical data are broadly consistent with literature values for noncumulate eucrites, although TiO2 values are lower than those presented by Barrat et al. (2000) (Figure 8).

Figure 8 Plot of FeO/MgO vs. TiO2 for Serra Pelada eucrite, showing different ratios for each clast and matrix. This plot is compared with literature data taken from Barrat et al. (2000), the small figure inside the graph. 

As indicated in figure 4, pyroxene compositions of matrix and clasts plot along a single tie line in the pyroxene quadrilateral regardless of the texture. However, the Mg concentration of pyroxene in the crystalline MX clasts is slightly higher than in other regions.

The Mn-Fe correlation of pyroxene in figure 7 shows that Serra Pelada has a slope of 0.03115, which is close to 0.0336 value found by Papike et al. (2003) for 4 Vesta (but in fact this is an average trend line between diogenites and eucrites). Primitive bodies in the solar system can have different Fe/Mn ratios in their silicate minerals as a consequence of volatility and oxidation state (Figure 9). The manganese enrichments in bodies such as 4 Vesta and Mars compared with the Earth and Moon have also been noted by Drake et al. (1989). These observations suggest that the Mn/Fe ratio of materials can be used as a fingerprint of planetary provenance (Papike 1998).

Figure 9 Pyroxene Fe/Mg vs. Fe/Mn diagram. Basaltic eucrite, cumulate eucrite, diogenite and Ibitira data taken from Barret et al. (2015). Serra Pelada fits well within the low Ca basaltic eucrites. 

The different textures among the CX, MX and FX clasts may be due to the location within different lava units. Comparing the percentage of TiO2 and FeO/MgO ratio of Serra Pelada with the literature (Figure 8), the Serra Pelada FeO/MgO ratio is located in the region close to that of noncumulate eucrites (Barrat et al. 2000), although the TiO2wt% is lower than expected for these meteorites.

In our analysis, the amount of elemental titanium in chromite (ulvöspinel) is higher than in other eucrites, which may be linked to different degrees of oxidation. The reduced chrome and titanium in oxidizing conditions could be more abundant during the formation (Bunch and Keil 1971). High-Ti chromite grains are also a typical feature of highly metamorphosed eucrites due to the decomposition of spinel (Yamaguchi et al. 2001).

Using plagioclase as a classification method (Table III), the values for Serra Pelada, agree with those of 4 Vesta (Papike et al. 2003). Our results confirm that this can also be a useful method for planetary origin sorting.

Pyroxenes and plagioclases in the CX- and FX-clasts have a cloudy appearance. This feature has been reported by Harlow and Klimentidis (1980) to be a predominant feature of basaltic eucrites, although there are variations for each meteorite. The observed cloudy appearance can be explained by metamorphism without recrystallization. As a result, iron in plagioclase exsolved into blebs and rods of iron oxides (Harlow and Klimentidis 1980).

Just as chondritic meteorites can be classified on the basis of thermal metamosphism, the eucrites also are classified into six exsolved types by Takeda and Graham (1991). These types reflect the cooling history of the eucrites, which is controlled by its depth in the parent body. The Serra Pelada fits well with type 5 mainly because of: 1) homogeneous chemical composition; 2) occurrence of cloudy pyroxene; 3) exsolution lamellae of augite in pigeonite; 4) recrystallized matrix, and 5) absence of inverted orthopyroxene.

The magnetic susceptibility Log χ (10-9 m3/kg) about 2.9 corresponds well within the level of confidence to eucrite meteorites in the alignment chart given by Folco et al. (2006), revealing in this way, that it is a “magmatic” meteorite that comes from the asteroid belt.

Dawn’s mission, which covered a large fraction of Vesta’ surface, shows that the mineralogy is consistent with howardite-eucrite-diogenite (HED) meteorites (McSween et al. 2013). The howardite regolithic soil exhibits variable proportions of eucrite and diogenite material (De Sanctis et al. 2012; Ammannito et al. 2013). The study of HEDs allows a better understanding of the early differentiation of small bodies in the solar system subjected to low-gravity environments and post crystallization effects of impact metamorphism (Jasmeet et al., 2013).

CONCLUSIONS

Some remarkable attributes observed in Serra Pelada meteorite provide mineralogical and compositional information on diverse fragments. On the basis of petrography and mineral chemistry, we conclude that Serra Pelada is a basaltic monomict eucrite of Type 5 S3. It contains lithic and mineral clasts (mostly different varieties) indicating that the original igneous lithologies were subjected to post-crystallization thermal processing. These results help understand the surface of Vesta, covered by debris that resulted from impacts.

ACKNOWLEDGMENTS

Special thanks are given to Ricardo Josefides for providing samples for our studies and to the people of Serra Pelada for the attention they paid to this meteorite fall. Special thanks are also to Felipe Abrahão Monteiro for help with text, to Yara Ornellas for the assistance with EPMA analysis, and to Clarice Paixão for her wonderful coffee.

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Received: October 02, 2017; Accepted: December 11, 2017

Correspondence to: Maria Elizabeth Zucolotto E-mail: meteoritos@mn.ufrj.br

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