Parauapebas meteorite from Pará , Brazil , a “ hammer ” breccia chondrite

The Parauapebas meteorite, third official meteorite discovered in the Brazilian Amazon region, is a “hammer meteorite” which fell on December 9th, 2013, in the city of Parauapebas, Pará State, Brazil. Mineralogy is dominated by forsterite, enstatite, iron, troilite, and tetrataenite. Albite, chromite, diopside, augite, pigeonite, taenite, and merrillite are minor components. Two main clasts are separated by black shock-induced melt veins. One clast exhibits an abundance of chondrules with well-defined margins set on a recrystallized matrix composed mostly of forsterite and enstatite, consistent with petrologic type 4 chondrites. The other clast displays chondrules with outlines blurring into the groundmass as evidence of increasing recrystallization, consistent with petrologic type 5 chondrites. The clasts of petrologic type 4 have a fine-grained texture compared to those of type 5. It is a genomict breccia (indicated by shock melt veins) with the clasts and matrix of the same compositional group, but different petrologic types, H4 and H5. The melted outer crust of the Parauapebas meteorite is comprised of forsterite with interstitial dendritic iron oxide, and is rich in irregular vesicles, which are evidence of the rapid formation of the crust. The type specimen is deposited in the Museum of Geosciences of the University of São Paulo, Brazil.


INTRODUCTION
Parauapebas is the third official meteorite found in the state of Pará, along with the Ipitinga chondrite, H5, found in the Ipitinga mountain range, Almeirim city, in 1989 (Dreher et al. 1995), and Serra Pelada, a rare anomalous brecciated eucrite that fell in the famous village of Serra Pelada, city of Curionópolis, in 2017 (Zucolotto et al. 2018, Yin et al. 2019. Parauapebas is also the first recognized meteorite fall of Pará State, despite the fact that Serra Pelada meteorite was classified and made official first. This happened because the meteorite was kept with the finders for about 5 years before it was made available to researchers for classification. Parauapebas is a "hammer" meteorite, a term associated with meteorites that strike objects or people during their fall. There is one very well-documented case of a meteorite that struck and injured a person, the Sylacauga meteorite (Swindel Jr. and Jones 1954), which fell in 1954 in Alabama, USA, hitting and wounding Mrs. Ann Hodges. In the case of Parauapebas, the event took place about 5 years ago and it was not possible to obtain evidence such as photos of Mrs. Maria das Neves's injury (see below). However, both Maria das Neves's and her son's testimonials were obtained and recorded, reporting the occurrence of the injury.
For the classification and officialization of this meteorite by the Nomenclature Committee of the Meteoritical Society, a consortium was established between researchers of the Institute of Geoscience of the Universidade de São Paulo and the Brazilian National Museum, Universidade Federal do Rio de Janeiro.
During the submission of the Parauapebas meteorite, the Museum of Geoscience of the Universidade de São Paulo was registered in the Nomenclature Committee of the Meteoritical Society as an official repository of meteorite type specimens. The Parauapebas meteorite is the first meteorite whose type sample is deposited in this museum. The type specimen deposited in the Museum of Geosciences (34.8 g) belongs to the fragment that caused injuries to Mrs. Maria das Neves. Another 4g of the same fragment was deposited in the National Museum of Rio de Janeiro. Andre L. R. Moutinho holds the 210.3 g main mass and a 13.08 g slice of the Mrs. Maria das Neves fragment.
A witness reported seeing the bolide traveling from NE to SW direction. A stone hit the roof of a house with a loud noise that was heard by resident Mrs. Maria das Neves Silva Lima. She claims that a meteorite fragment hit and injured her right shoulder, but her son says that she was hit only by roof debris. Her son, Ilson Silva Lima, collected the stone on the roof. The stone broke into two pieces during the impact. One piece is lost, but the remaining fragment of 62 grams (Fig. 2) was kept by Mrs. Maria das Neves until 2015, when she handed it over to Dorília Cunha, who was working at the USP's Institute of Geosciences at the time. Mrs. Maria das Neves is the mother of Dorilia's sister-in-law. She took the meteorite to be studied by some people who did not show much interest. Finally, she managed to attract the attention of Professor Daniel Atencio and, together, they began the first analyses. The Parauapebas meteorite was finally approved on December 2 nd , 2018.
Another stone from this same fall, showing flight orientation features and weighing 210.3 g (Fig. 3), was also witnessed to fall by other residents of the city while they were in front of their houses. Meire C. Rosa, her husband Paulo T. Nunes, her mother Rosa C. Santos, and their neighbors heard a loud noise of thunder and the impact when the stone struck the wooden beam of the roof of their house and then fell, embedding itself on the ground. Meire immediately picked up the stone and noticed that it was still warm. In early 2018, she contacted André L.R. Moutinho, who was already part of the classification team of the 62 g fragment. The 210.3 g stone was purchased by André L. R. Moutinho.

METHODS
Slices were examined using a stereomicroscope Zeiss Discovery V8. Polished thin sections were examined microscopically in transmitted and reflected light using a Zeiss Axioplan petrographic microscope.
Quantitative analyses of the constituent phases were carried out using the JEOL EPMA JXA-8230 Superprobe at LABSONDA/IGEO/UFRJ, WDS mode. Beam conditions included an accelerating voltage of 15 keV for silicates and 20 keV for opaque minerals, beam current of 20nA, and a spot size of 1 μm. Natural and synthetic standards of well-known compositions were used as standards for wavelength dispersive spectrometry (Tab. 1).
Corrections for differential matrix effects were made with a ZAF factory-supplied procedure. In total, 45 olivine and 32 pyroxene grains were analyzed. Points in the cores and rims of the grains were analyzed in order to access their compositional ranges. Energy Dispersive Spectrometer (EDS) data were also obtained at the Institute of Geoscience, Universidade de São Paulo.
The cathodoluminescence (CL) color image was obtained by the electron beam source CITL MK5-2, attached to the petrographic microscope Zeiss Axio Imager 2 under such analytical condition: the voltage applied was 15 keV and the current was 0.7 mA in a vacuum condition. In this way, a cooled charge-coupled device (CCD) camera recorded the real color emitted by the minerals as a luminescence response. From the CL images obtained from each piece of the meteorite, it was possible to create a photomosaic through the Adobe-Bridge CS6-64bit and the final

Physical characteristics
The 210.3 g main mass is almost fully crusted. It measures 5.7 × 5.3 × 4.2 cm, and shows orientation features such as nose-cone shape and roll-over lips. A small portion of the secondary crust is also present. The 62 g fragment measures 4.8 × 3.5 × 2.6 cm.
Fusion crust is dull black with 0.1 to 0.2mm in thickness in the entire observed sample. A variation in thickness may be present on the different masses as a result of in-flight fragmentation. On one side, it shows markedly regmaglypts. The fresh fractures surfaces of the meteorite interior have a variable gray color with a rare indication of brecciation.
Very thin black veinlets can be discerned in the interior of some broken and cut surfaces and resemble shock veins. On some cut surfaces, the slice fast becomes stained due to the high iron content.

Texture
In the examined thin section, the meteorite shows a well-developed chondritic texture, being possible to observe two main clasts separated by some black shock-induced melt vein crossing the entire thin section (Fig. 4). One clast (above the melt vein) exhibits an abundance of chondrules with well-defined margins, ranging in size from 0.3 to 0.9 mm, set on a microcristalline partial recrystallized matrix composed mostly by olivine and pyroxene, consistent with petrologic type 4 chondrites. The other clast displays less discernible chondrules of the same apparent dimensions, but with the outlines blurring into the groundmass, as evidence of increasing recrystallization, consistent with petrologic type 5 chondrites. Both clasts exhibit chondrules with variable internal texture such as porphyritic chondrules: PO (porphyritic olivine), PP (porphyritic pyroxene), POP (porphyritic olivine and pyroxene); non-porphyritic chondrules: GOP (granular olivine and pyroxene), C (cryptocrystalline), RP (radiating pyroxene), BO (barred olivine); and many chondrule fragments. The clasts of petrologic type 4 have a fine-grained texture compared to those of type 5. It is a genomict breccia (indicated by shock melt veins) with the clasts and matrix of the same compositional group but different petrologic types, H4 and H5.

Mineralogy and chemical composition
Mineralogy is dominated by forsterite, enstatite, iron, troilite, and tetrataenite. Albite, chromite, diopside, augite, pigeonite, taenite, and merrillite are minor components (Fig. 5). Microprobe analyses of each mineral both in the chondrules and in the matrix, in type 4, type 5 and the veins were treated separately, in an attempt to verify whether there were compositional differences among the lithologies. However, a rather homogeneous chemistry was found in the whole section, even presenting discernible textures.

Forsterite, Mg 2 (SiO 4 )
Most forsterite crystals show undulatory extinction and irregular fractures, and few have weak mosaicism. Chemical data for forsterite are represented in Figure 6 and in  Figure 6. Most Fs indices for enstatite are characteristic for H chondrite, except for a few typical points of L-chondrites.

Albite, Na(AlSi 3 O 8 )
Albite forms small secondary crystals. Chemical data and Ab-An-Or values are in Table 4 and Figure 6. Iron, Fe; Tetrataenite, FeNi; Taenite, (Fe,Ni); and Troilite, FeS Iron analyzed under reflected light shows Neumann bands, which are mechanical twins on the {211} planes. Twinning is induced by shock, either due to collisions in space, or during atmospheric passage. The ubiquity of Neumann bands indicates that they are easily formed at relatively low levels of shock (Buchwald 1975). In the past, the name "kamacite" was applied to iron from meteorite, but this unnecessary name was discredited by Burke (2006). Intimate intergrowth of iron and taenite ("plessite texture") with associated tetrataenite was observed. Zoned taenite could also be seen.
Chemical data and the calculated empirical formulas for iron, tetrataenite, and taenite are reported in Table 5. Troilite chemical data and the calculated empirical formulas are in Table 6.
Chromite, Fe 2+ Cr 2 O 4 ; and Merrillite, Ca 9 NaMg(PO 4 ) 7 Chromite and merrillite are rare in the Parauapebas meteorite and only were identified by scanning electron microscopy/ energy dispersive X-Ray spectroscopy ( SEM/ EDS) in broken fragments produced during the making of slides. There is no reference about where (H4, H5, vein, chondrule/matrix) chromite and merrillite were analyzed. Chromite occurs as platelet-shaped inclusions, which is relatively common in meteorites, or as irregular grains (Fig. 5

Fusion crust
The melted outer crust of the Parauapebas meteorite (Fig. 7) is comprised of forsterite with interstitial dendritic iron oxide. We cannot confirm if this iron oxide is magnetite, wüstite or other species because the dendrites are too small for accurate quantitative analyses. Qualitative EDS analyses indicate the occurrence of not only Fe but also of Mg, Si, and other elements due to matrix overlap with the forsterite surrounding phase. The molten crusts of meteorites are usually metal free, that is, the dendrites are probably not composed by metallic iron. The melted crust   of the Parauapebas meteorite is rich in irregular vesicles. The presence of vesicles and iron oxide dendrites are evidence of the rapid formation of the crust. Detailed studies of meteorite fusion crusts were made by Ramdohr (1967) and Hezel et al. (2015).

DISCUSSION
According to the textural scheme of Gooding and Keil (1981), recognizable chondrule texture are: BO, PO chondrules, and RP crystals. Olivine crystals with undulatory extinction and irregular fractures, and a weak mosaicism,         Neumann lines in iron, in addition to the presence of shock melt veins and melt pockets indicate a shock stage S3/S4 of Stöffler et al. (1991). The mol percent fayalite vs. the mol percent ferrosilite plots within the H group, according to Figure 8.
The cobalt content in iron (average composition in weight percent) vs. fayalite in olivine (mol percent) in the chondrules also lies within the H group range (Fig. 9).
Using the pyroxene analyses, we calculated the percent mean deviation (PMD) of ferrosilite in enstatite for each clast and they present no visible difference seeming to represent H4 low and high borders. PMD of 5.44,9.16 and 18.13 (at vein), respectively, corresponding to petrologic type 4 chondrites.
Olivine, low-Ca, and high-Ca pyroxene chemical compositions were analyzed in each of the major clasts and in the melt vein. The mol% compositions of ferrosilite and fayalite plot within the H group. Its petrologic type matches those of other clasts with the matrix as petrologic type 4.  As a typical result for a thermal metamorphic chondrite, such as petrologic type 4 to 6, the striking feature of cathodoluminescence of Parauapebas meteorite is the blue color dominance in the mesostasis of chondrules opposing the lack of luminescence into the olivine and pyroxenes grains (Fig. 10). DeHart et al. (1992) and Sears et al. (1992) developed the compositional classification scheme for chondrules, taking into account the CL colors emitted by minerals. In terms of CL, there are two main trends, one that starts with luminescent chondrules and one that starts with non-luminescent ones. Thus, group A was designated to the brightest chondrules while group B gathered chondrules with low or no luminescence. DeHart (1989) correlated the petrologic types of chondrites 1 to 6 from those developed by Van Schmus and Wood (1967) and subtypes into the most primitive meteorites type 3 (3.0 to 3.9), as suggested by Sears et al. (1980) with CL color as well as with the mineral composition. These petrologic subtypes were created to split the type 3 meteorites, which exhibited slight mineral chemical and texture differences due to subtle thermal metamorphism events, where type 3.0 is designated to represent the most pristine materials, whereas types 3.1 to 6 indicate an increasing degree of petrologic equilibration and recrystallization (Weisberg et al. 2006). Thereby, the chondrules belonging to type A show highly blue plagioclase mesostasis (except the yellow Ca-rich mesostasis from the lowest petrologic subtypes, such as 3.0 to 3.2) and red olivine and pyroxene associated with Fe-poor minerals, whereas type B presents quartz mesostasis and Fe-rich olivine and pyroxenes.
According to Sears et al. (2013), as metamorphism proceeds, the chondrules undergo systematic changes until the two trends evolve into the uniform blue CL characteristic of equilibrated chondrules (Fig. 11). As shown in the figure, the three types present in chemically unequilibrated chondrules are A1, B1, and an unequilibrated type A5 (A5un) with reddish edges. Under the progress of metamorphism, these converge to the equilibrated type A5, passing through this process by intermediary groups A2, A3, A4, B2, and B3. For instance, the unequilibrated ordinary chondrites (UOC) as Sermakona (3.0) and Bishuspur (3.1) show colorful and brilliant CL results inside the chondrules with a dark glass matrix and the most metamorphosed ones have the blue recrystallized matrix and mesostasis with no luminescence olivine and pyroxenes, such as the Parauapebas meteorite. This behavior is explained by the fact that in the most primitive meteorites, mesostasis and olivine have a varied composition, whereas in meteorites of greater petrologic type, the composition is more homogeneous . The smallest yellow and red CL grains in the bulk are apatite that exhibit a great CL signal as their luminescence properties.
Therefore, based on this CL compositional classification, Parauapebas is clearly a high-petrologic type meteorite due to the absence of CL class A1, A2, A3, B1, B2, and B3 with the dominance of A5 (Fig. 12), the almost entirely CL signal of Parauapebas is related to type A5, even the melted vein, which splits both the petrological types of the meteorite, classified mainly by the subtle petrological differences in the matrix and in visible chondrites. This convergence to A5 equilibrated state occurs because the heat (> 300°C) turns Fe-poor in Fe-rich olivine and pyroxene with no CL and the glass around begins to recrystallize to plagioclase phases showing blue luminescence, as can be seen in the most part of this mass with dominance of the blue CL signal. Fayalite becomes chemically homogeneous from UOC 3.4 and ferrosilite from UOC 3.8 up to 4, whereas the matrix and mesostasis glasses recrystallize from the petrologic type 3.2 (Huss et al. 2006).
It confirms the highest petrologic classifications as type 5 for the biggest part of the Parauapebas meteorite, due to a sequence of soft chemical modifications along the thermal events submitted to the large mass of its parent body.     DeHart et al. 1992 in different petrologic types of chondrites from 3.0 to 5. Parauapebas is dominated by equilibrated chemical class A5 and classified as a high petrologic type 5. Clearly, Parauapebas is a high petrologic type meteorite due to the absence of CL class A1, A2, A3, B1, B2, and B3 with the dominance of A5. Scheme adapted from Keil et al. (2015).