Response of silicate chemical composition variation on thermal metamorphism of ordinary chondrites and classification of petrologic types: the case of L chondrites from Grove Mountains, Antarctica

Chuantong Zhang • Bingkui Miao • Zhipeng Xia • Qinglin Xie

Abstract Analysis of the thermal metamorphism of the ordinary chondrites is a key premise for gaining insights into the accretion and heating of rocky bodies in the early solar system. Such an analysis also represents an essential condition for constraining the early thermal and evolutionary histories of asteroids and terrestrial planets. Classifying ordinary chondrites into petrologic type (type 3–6)is the criterion for studying the thermal metamorphism of their parent bodies. However, the boundary between the unequilibrated (type 3) and equilibrated (type 4–6) chondrites is ambiguous at present, thus, limiting the understanding of their thermal metamorphism. In this study, the petrology, mineralogy and chemical composition of a set of seven ordinary chondrites with different degrees of thermal metamorphism collected from Grove Mountains (Antarctica) have been studied. The results demonstrated that these chondrite samples were L3.7, L3.8, L3.9, L3.9/4, L4, L5 and L6 type meteorites, with optimal correlations of Si,Mg, Fe, Mn and Ca with equilibrium degree of the olivine and low-calcium pyroxene and petrologic type. In this respect, the multi-parameter classification standard PMD(SiO2)-PMD (MgO)-PMD (MnO)-PMD (CaO) based on the percent mean deviation (PMD) of the chemical compositions of the olivine and low-calcium pyroxene was proposed to distinguish between the unequilibrated and equilibrated meteorites. The proposed standard exhibited high ‘‘resolution’’ in terms of classification, thus, also deepening the understanding of the effect of the silicate mineral composition in the thermal metamorphism of chondrites.

Keywords Grove mountains ∙Antarctica ∙Ordinary chondrite ∙Thermal metamorphism ∙Silicate ∙Petrologic type ∙Classification standard

1 Introduction

Ordinary chondrites are classified into three groups as H(high-Fe), L (low-Fe) and LL (low-Fe, low-metal), based on the accumulation of early condensed substances in the solar nebula (Weisberg et al. 2006). Thus, these planetary materials provide vital opportunities to reveal the early evolutionary history of the solar nebula (Dobrica et al.2016). During the accretion and early formation of the chondrite parent asteroids, the radioisotopes including26Al present in the internal constituent materials have decayed to produce the thermal energy (Gail et al. 2019; Telus et al.2014). This thermal energy has subsequently changed the original texture and mineral composition of chondrites,thus, triggered the thermal metamorphism of the chondrite parent body (Huss et al. 2006). Studying the thermal metamorphism of chondrites can not only retrieve the early history of the solar system but also represents a crucial requirement for restricting the accretion and later thermal evolution of the asteroids and terrestrial planets (Hutchison 2004). Moreover, the early components including presolargrains, water and organic matter of the solar nebula were depleted during thermal metamorphism (Sears 2016; Sears et al. 2013). Thus, the information about the formation of the solar system and the early evolution of nebulas can be effectively attained by analyzing the thermal metamorphism experienced by meteorites. Consequently, the thermal metamorphism process has gained widespread research attention and is being actively studied with chondritic meteorites.

Similar to using isotope dating as a tool to study the chronology of meteorites, determining ‘‘petrologic type’’ is also a criterion to gain insights into the thermal metamorphism of meteorites. Van Schmus et al. (1967) reported the following observations based on the petrologic texture and mineral composition with the increase in the intensity of thermal metamorphism in ordinary chondrites: 1) gradually blurred chondrule texture and outline, with gradually crystallized and coarsened matrix, 2) devitrification of the chondrule matrix, with gradually enlarged secondary feldspar and diopside, 3) enhanced proportion in the conversion of monoclinic to orthorhombic in low-calcium pyroxene, 4) gradually equilibrated and homogenized mineral composition and 5) progressive reduction in the content of the volatile components. Besides, Van Schmus et al. (1967) used the petrologic types (type 3, type 4, type 5 and type 6) to qualitatively reflect the intensity of thermal metamorphism experienced by chondrites. Among these,type 3 refers to the meteorites with minimal influence of thermal metamorphism, denoted as unequilibrated meteorites, whereas types 4–6 represent the equilibrated meteorites, which are significantly modified by thermal metamorphism (Sears et al. 1988). Recently some reports came on the presence of type 7 (Tait et al. 2014; Weisberg et al. 2006) and type 8 (Irving et al. 2019) chondrites, but they are not explored in this study because of their controversial formation mechanisms (Krot et al. 2014).

Although the research on the thermal metamorphism of chondrites has gradually developed and established, significant limitations have been identified in the accuracy of the classification of petrologic types (type 3–6). For instance, although the chondrites Bishunpur and Dhajala are type 3 meteorites, the former reveals only a slight extent of thermal metamorphism (Chan et al. 2012), while the latter exhibits the thermal metamorphism close to that of type 4 chondrites (Grossman et al. 2009; Noonan et al.1976). To this end, Sears et al. (1980) proposed to employ induced thermoluminescence (TL) to further subdivide the type 3 ordinary chondrites into types 3.0–3.9. The lower petrologic type represents a more primitive meteorite and the early matter of the solar nebula and the solar system.However, the TL measurement system is less prevalent owing to its complexity. Therefore, a method based on the variation in the chemical composition of the chondrite minerals has been gradually established according to the TL classification parameters to subdivide type 3 in practical applications. The main criteria have been established as follows: 1) the PMD of olivine Fa ((Fe + Mg)/Fe mol.%)[PMD (Fa)] was used to classify the subtype 3.0–3.9 chondrites (Sears et al. 1980, 1991; Sears et al. 1988); 2)Scott et al. (1994) delineated the subtypes of the meteorite petrologic types based on the olivine composition in the IA and IIA chondrules; 3) Grossman et al. (2005) used the distribution characteristics of the Cr element contained in olivine in the type II chondrules to subdivide the type 3.0–3.2 meteorites; 4) Kimura et al. (2008) delineated the petrologic type of the primitive chondrites based on the texture and composition of the Fe–Ni metal; 5) Bonal et al.(2016) subdivided the type 3.0–3.7 meteorites following the laser Raman spectra of the carbonaceous organic matter in chondrites.

The current focus of research on the thermal metamorphism of chondrites involves 1) identifying the most primitive unequilibrated chondrites (type 3.0) to study the initial composition of the solar system (Huss et al. 1995)and 2) investigating the early thermal metamorphism and evolution histories of the asteroids and terrestrial planets based on the equilibrated chondrites (types 4–6) (Archer et al. 2019). However, the boundary between the unequilibrated and equilibrated chondrites in the existing studies is not well defined. The typical petrographic parameters such as chondrule definition, matrix recrystallization and devitrification degree are noted to be semi-empirical and it is troublesome to popularize TL and volatile content in practical applications (Grossman et al. 2009). Therefore,the chemical composition of the chondrite minerals is the main criterion for the classification as types 3 and types 4–6 meteorites. Among these, PMD (Fa) is the most important and widely used parameter for petrologic type classification. Thus, the meteorites with PMD (Fa) ＞5%are classified as types 3, while those with PMD (Fa) ≤5%are classified as types 4–6 (Huss et al. 2006). However, the classification of types 3 and types 4–6 meteorites based on PMD (Fa) demonstrates an increasing number of challenges and even controversial classifications have been obtained. For instance, the PMD (Fa) values of chondrite Dhajala (H group) measured by different researchers are 3.4% and 12.6% respectively, which led them to obtain different classifications of petrologic type (Grossman et al.2009). Hence, it is vital to further improve the petrologic type classification system based on the chemical composition of the minerals in chondrites. In fact, in recent years,the silicate composition-based criteria for type 3 and type 4 ordinary chondrites have also been explored and reported occasionally in the form of conference papers (Ragland et al. 2009). Though nill substantial results have been achieved so far, the proposed criteria still represent an important potential breakthrough.

To overcome the existing challenges and limitations, in this study, a set of seven L-group ordinary chondrites with different degrees of thermal metamorphism collected from the Grove Mountains (Antarctica) were analyzed. The variations in the silicate composition in the ordinary chondrites during thermal metamorphism were studied based on petrological and mineralogical observations and the resulting insights were applied for the classification of the unequilibrated and equilibrated chondrites. Finally,PMD (SiO2)-PMD (MgO)-PMD (MnO)-PMD (CaO) of the olivine and low-calcium pyroxene were proposed as a new criterion for the classification of types 3 and types 4–6 meteorites.

2 Analytical methods

The preparation of the polished thin sections, petrographic and mineralogical observation and chemical composition analysis were carried out in the Guilin University of Technology, China. An optical microscope (NIKON ECLIPSE 100POL) was utilized to investigate the texture and mineral assemblages of the meteorite specimens. The chemical composition of silicate in the meteorite samples was determined using an electro-probe microanalyzer(EPMA, JEOL JXA-8230). The large olivine and lowcalcium pyroxene crystals were selected from the polished thin sections and the chemical composition analysis was subsequently conducted using the electron probe (working voltage of 15 keV, 20 nA current and beam spot diameter of 3 μm) for 4 min per spot. For some minerals with compositional zoning (mainly found in most unequilibrated chondrite), the core and rim of these crystals have been analyzed. The national standard samples in the Institute of Mineral Resources (Chinese Academy of Geological Sciences) were adopted for the quantitative analysis. The silicate minerals and their oxides are natural standard mineral samples. The standard sample of Si, Mg and Fe elements is olivine and their detection limits are 130 ppm, 119 and 154 ppm, respectively. The standard sample of Na and Al are albite and their detection limits are 63 and 47 ppm,respectively. The standard sample of Ca is wollastonite with a detection limit of 91 ppm. The standard sample of Cr is chromic oxide and its detection limit is 259 ppm. The standard sample of Ti is rutile and the detection limit is 294 ppm. Manganese oxide is the standard sample of Mn,whose detection limit is 104 ppm. The obtained results were corrected by utilizing the ZAF method (atomic number, absorption effect and fluorescence effect). Adobe Photoshop software was employed to count the mineral modal abundance in the polished thin sections of the meteorite samples through a backscattering panoramic image.

3 Results

In this study, a set of seven ordinary chondrites (GRV 13008/13033/13062/13107/090572/090566/090438) with increasing degree of thermal metamorphism were collected from Grove Mountains (GRV) for analysis. We have applied for the registration of all the seven meteorites to the Meteoritical Bulletin Database, which was further approved by the Nomenclature Committee of the Meteoritical Society. The main mass and thin sections of meteorites were stored in the Polar Institute of China. The preliminary classification revealed that GRV 090572/090566/090438 were equilibrated chondrites,whereas the other 4 samples might be type 3 meteorites(Table 1).

3.1 Petrological characteristics

The hand specimen of GRV 13008 had yellowish-brown weathering products on the surface and black fusion crust residues were noticed on about 10% of the surface(Fig. 1a). There were a large number of chondrules or broken chondrules distributed in the opaque matrix with mild recrystallization (Fig. 2a). Besides, the chondrules had an extremely clear edge, with the presence of the primary glass, a fraction of which was devitrified (Figs. 3a,4a). The hand specimen of GRV 13033 was noted to be flat, with black fusion crust (about 30%) on the surface and brown weathering products on the remaining areas. There was the presence of a large number of chondrules, broken chondrules and opaque minerals (Fig. 1b). The matrix was opaque and exhibited mild recrystallization (Fig. 2b). The chondrules had an extremely clear edge (Fig. 4b), with the presence of the primary glass, a fraction of which was devitrified (Fig. 3b). There was some feldspathic recrystallization or a small number of ultrafine feldspar crystals in the local matrix. The hand specimen of GRV 13062 had black fusion crust on about half of the surface and brown weathering products on the remaining areas (Fig. 1c).There were a large number of chondrules or broken chondrules distributed in the thin section, which had a clear edge and obvious recrystallization (Fig. 4c). The matrix was observed to be translucent with moderate recrystallization (Fig. 2c). Further, feldspathic recrystallization or a small number of fine feldspar crystals were observed locally (Fig. 3c). There was a black fusion crust on 30% of the surface of the hand specimen GRV 13107, with brown weathering products on the remaining areas (Fig. 1d). The chondrules or broken chondrules, which exhibited a welldefined outline (Fig. 4d), were distributed in the translucent matrix with obvious recrystallization (Fig. 2d).Besides, the chondrules were mostly broken and compounded (Fig. 3d) and the production of the latter ones was attributed to the unclear edges or disintegration of the partial edges. There were various types of chondrules in the above four chondrites (Fig. 4), such as porphyritic olivine-pyroxene chondrule (POP), porphyritic olivine chondrule (PO), porphyritic pyroxene chondrule (PP),barred olivine chondrule (BO), cryptocrystalline chondrule(C), glass-rich chondrules (GR), radial pyroxene chondrule(RP), etc. Both type I and type II chondrules could be observed in GRV 13008 sample. The chondrules in GRV 13033 meteorite were mainly type II and there were also a few type I chondrules. In contrast, the chondrules in chondrite GRV 13062 and GRV 13107 were type II.

Fig. 1 Hand specimens of meteorites (the quadrille paper:the grid is of 1 cm × 1 cm size,the smallest grid is 1 mm):a GRV 13008; b GRV 13033;c GRV 13062; d GRV 13107

Fig. 2 Panoramic images of the meteorites under planepolarized light (the scale bars are all 1 cm): a GRV 13008;b GRV 13033; c GRV 13062;d GRV 13107

Fig. 3 Backscattering panoramic image (left), pseudocolor image (middle) and structure diagram (right) of the meteorites (scale bar reads 1 mm): (In pseudo-color image,green: olivine, blue: lowcalcium pyroxene, red: glass and feldspathic recrystallized substance, white: opaque minerals and yellow: holes. In structure diagram, red:chondrule, black: matrix, white:opaque minerals and yellow:holes.). a GRV 13008; b GRV 13033; c GRV 13062; d GRV 13107

Fig. 4 Backscattering images of typical chondrule and matrix in chondrites. POP-Porphyritic olivine-pyroxene chondrule,PO-Porphyritic olivine chondrule, BO-Barred olivine chondrule. a GRV 13008;b GRV 13033; c GRV 13062;d GRV 13107

The meteorite GRV 090572 had black fusion crust and brown weathering products distributed on the surface in a mixed morphology. In the polished thin section, the chondrules were distributed in the transparent matrix, with a defined edge and obvious recrystallization. Besides, there was some fine secondary feldspar (＜2 μm). The meteorite GRV 090566 had black fusion crust on about 10% of the surface and brown weathering products on the remaining surface. The polished thin section exhibited a transparent matrix and recognizable chondrules with a fuzzy edge.Fine secondary feldspar (＜50 μm) could also be observed.For the meteorite GRV 090438, there was a black fusion crust on 15% of the surface and yellowish-brown weathering products on the remaining surface. Poorly defined chondrules with an extremely fuzzy edge were observed in the sample and the matrix revealed complete recrystallization to form the medium-grained minerals. This meteorite had large-sized secondary feldspar (＞50 μm).

e s i t chondr n ve se of e s ur a t fe ous r i va of ry 1 Summa e Tabl 090438 GRV 090566 GRV 09072 GRV 13107 GRV 13062 GRV 13033 GRV 13008 GRV e mpl Sa 3 7.×d.8 10 5fine nt 9.de re××μm ly pa.6.8 1 20 40 ans 2.10 Poor–＞50 Tr ––––8.×e.9 bl 13 nt 6 za re×6.μm pa.5×cogni.8 22 45 ans 3.8 9.Re–＜50 Tr––––12×.9.2 28 17 nt re××d μm pa.3.51.1 fine 4 58 42 24 De a ns–＜2 Tr––––5.×.1 d 10 5fine 28 0.65)ent×5.-de ±=luc.3.3×l l(n .6.4 9 12 00 ans 1.1 7.W e 50 0.6–Tr 42 43 1.～4 7.×.6 12 0 42 0.41)ent×6.luc×.0 r ±=.6(n .8.2.1 19 44 e a ans 2.9 6.C l 55 0.8–Tr 45 36 4.～4 17 r ×ea.7.4 c l 25 10 ly 28 0.241)××me ±=.1.3.2.04.7 re(n que.2.2.3 21 15 15 Ext 49 0.–Opa 36 39＞1～3 12×r.7 e a 28 6 c l 8.37 ly 0.97)×±=.7.7.86×me.8(n que.9.5 38 24 14 re E t 62 0.0–Opa 40 39 5.～3)(mm ).%ed iz l l r ta te (vol e ys 2)eame c r nc re ul di z e 3)s i ne c S(mm(g)(mm e r oxe pa thi c hondr Fe abunda z e ul pa z e s s s i lds l pyr lds and s i ma of fe um fe e ee ion chondr loy i t i t c t ry moda c i se ion ge c onda l or ra a l a ndnc ta a l i te or te ra ix ne nfini t-c s Ave Me Me Thi tr ne De ivi a s Se Ma M i O l Low G l subs–N Fe

3.2 Chemical composition of minerals

As observed, the meteorite samples were mainly composed of olivine and low-calcium pyroxene, along with a small amount of glass, feldspathic recrystallized substance and metal phases. The previous studies have manifested a small amount of the primary feldspar in the ordinary chondrites below type 4 (Huss et al. 2006). Further, the metals in meteorites have also been reported to be susceptible to weathering, thus, leading to variations in the original chemical composition (Hutchison 2004; Wlotzka 1993).For this reason, olivine and low-calcium pyroxene are preferred for studying the thermal metamorphism of meteorites and the classification of petrologic types. The chemical composition of olivine and low-calcium pyroxene in the meteorites has been listed in Tables 2 and 3,respectively. A brief description of the composition characteristics is also presented as follows.

The olivine of these meteorites had significant variation in the uniformity of the composition depending on the specific chondrite. For instance, the olivine in GRV 13008/13033/13062 meteorites showed significantly nonuniform composition and the corresponding PMD (Fa)values were 24.6%, 10.7% and 6.0%, respectively. In contrast, the olivine in GRV 090572/090566/090438 meteorites displayed a uniform composition, with PMD(Fa) of 2.5%, 1.2% and 1.1%, respectively. The uniformity of the chemical composition of olivine in GRV 13107 was in between the two groups, with the PMD (Fa) value of 4.6%.

The low-calcium pyroxene of these meteorites exhibited more significant variation in the chemical composition as compared to olivine. For example, the low-calcium pyroxene in GRV 13008/13033/13062/13107 meteorites demonstrated markedly non-uniform chemical composition and their PMDFs: [Fe/(Fe + Mg + Ca) mol.%] values were 42.8%, 32.5%, 7.3% and 7.5%, respectively.Whereas, the low-calcium pyroxene in GRV 090572/090566/090438 meteorites displayed relatively homogeneous composition, with the PMD (Fs) values of 1.9%, 4.9% and 1.7%, respectively.

4 Discussion

4.1 Types of meteorites

To evaluate the effect of parent body thermal metamorphism on the silicate composition of the ordinary chondrites, petrologic types are explored for the equilibrated and unequilibrated meteorites. The chemical groups and petrologic types of the studied meteorites are discussed as follows.

4.1.1 Chemical groups

Ordinary chondrites include three chemical groups (H, L and LL). Besides the Fe–Ni metal, the chemical composition of olivine and low-calcium pyroxene is a major parameter defining the chemical groups of ordinary chondrites (Weisberg et al. 2006). The H group(17.3–20.2 mol.%, 15.7–18.1 mol.%), L group(23.0–25.8 mol.%, 18.7–22.6 mol.%) and LL group(26.6–32.4 mol.%, 23.2–25.7 mol.%) are generally classified using the mean Fa of olivine and mean Fs of lowcalcium pyroxene (Brearley et al. 1998; Rubin 1990).Based on this, both Fa and Fs values of GRV 090572/090566/090438 were observed to be within the range of these values for the L group (Fig. 5a), thus,leading to their classification as the typical L-group meteorites. Although the mean Fa and Fs of GRV 13033/13062/13107 were nearly within the L group range(or close to the boundary), their respective errors exceeded the range (Fig. 5a). The mineral composition of GRV 13008 was highly heterogeneous, with its mean Fa and Fs values close to the intersection of the H and L groups,however, its error could even encompass the three chemical groups (Fig. 5a). Therefore, it is challenging to determine the type of the chemical groups in GRV 13008/13033/13062/13107 based only on Fa and Fs, which requires further verification.

The statistics of the petrographic parameters such as metal content and chondrule diameter of the meteorites are troublesome, thus, the chemical groups of the ordinary chondrites are classified using Fa and Fs, which is a popular strategy nowadays (Hutchison 2004). However, the chemical group classification via Fa and Fs is mainly aimed at the equilibrated (type 4–6) chondrites with uniform chemical composition (Brearley et al. 1998; Rubin 1990).It is, thus, necessary to combine the petrographic parameters (such as metal content) to prove the chemical groups of the unequilibrated chondrites with highly variable composition. The metal (Fe–Ni + FeS) content (vol.%) and mean chondrule diameter (mm) of GRV 13008/13033/13062/13107 were subsequently recorded. The metal content was observed to be ～3.7 vol.%, 3.3 vol.%, 4.0 vol.% and 4.3 vol.%, respectively, whereas the mean chondrule diameter was about 0.62, 0.49, 0.55 and 0.50 mm, respectively, thus,falling within the L group range (～3.0 vol.%–4.1 vol.%, ～0.5–0.7 mm) (Krot et al. 2014; Rubin 2000;Scott et al. 2014) (Fig. 5b). In summary, considering the findings from petrography and mineral composition classification, the GRV 13008/13033/13062/13107 samples should belong to L-group chondrites. It needs to be emphasized that these four chondrites have different petrological characteristics, mineral modal abundances,mineral chemical compositions and so on (Tables 1, 2 and 3), which exclude the possibility that they are paired meteorites.

EPMA a vi ed lyz ana ne ivi ol of ion it 2 Compos e Tabl/PMD%STD 22)GRV 090438=(n/PMD%STD 15)GRV 090566=(n/PMD%STD 14)GRV 090572=(n/PMD%STD GRV 1310741)=(n/PMD%STD GRV 1306224)=(n/PMD%STD GRV 1303388)=(n/STD PMD%e GRV 1300856)=(n mpl Sa 61 0.15 1..2 60 11 11 0.1.24 0.25 0.05 0.23 0.02 0.38 0.27 0..7.1 46.7 05.9.2 39 22 0.38 0.100 24 60 0.54 1..8 33 23 11 0.1.24 0.32 0.06 0.13 0.01 0.32 0.29 0..7.0 49.4 03.6.5 92 39 0.0.99 0.95 21 1..5 37 38 46 23 10 1.2.36 0.41 0.05 0.52 0.02 0.50 0.58 0..2.3 49.2 03.2.8 15 39 0.0.99 1.11 21 3..7 05 38 59 23 14 3.4.44 0.66 0.07 0.23 1.01 0.73 0.05 1..2.4 45.4 01.4.9 38 21 0.40 0.100 22 39 2.99 4..7 35 98 20 3.5.92 0.10 1.08 0.33 1.02 0.65 0.42 1..5.1 37.7 03.6.8 38 22 0.39 0.100 23 79 1.46 9..1 02.7 25 5.10 68 0.02 2.10 0.04 2.05 0.82 0.46 2..0.4 39.6 03.3.9 38 21 0.40 0.100 22 61 2..0.8 43.6 23 25 8.24 01 1.21 4.12 0.62 3.06 0.76 0.80 4..7.4 45.9 06.4.5 38 18 0.42 0.100 19 O2 O O a l Si Fe MnO MgO Ca Tot Fa)STD/ge ra a ve×(100 ion a t vi De an Me ent rc rk)—Pe ma to ine r l unde symbol ing(Us PMD,ion a t vi De rd a nda STD—St s.nt poi ed lyz ana of r numbe the s ent e s pr re t ke a c br in r numbe The e:Not

4.1.2 Petrologic type

GRV 090572/090566/090438 samples exhibited readily or poorly defined chondrule outlines, obvious recrystallization in the matrix, presenting of the secondary feldspars and the highly uniform chemical composition of olivine with PMD(Fa) of 1.1%–2.5%. Based on the classification standard of the petrologic types (Krot et al. 2014; Van Schmus et al.1967), GRV 090572/090566/090438 were equilibrated(types 4–6) chondrites with homogeneous composition.Among these, GRV 090572 revealed clear chondrule boundaries and the crystal size of the secondary feldspars was small (＜2 μm), thus, indicating the classification as type 4 meteorite. GRV 090566 demonstrated a readily delineated chondrule texture with blurred boundaries and the size of the secondary feldspars crystals (2–50 μm) was larger than that of GRV 090572, thus, confirms the classification as the type 5 chondrite. GRV 090438 revealed poorly defined chondrule texture and severe recrystallization in the matrix, with large sizes of the secondary feldspars crystals (＞50 μm), thus, indicating its classification as type 6 chondrite.

GRV 13008/13033/13062 samples exhibited sharply defined chondrule boundaries, with the locally present primary glass. The chemical composition of olivine and low-calcium pyroxene was observed to vary significantly,with PMD (Fa) and PMD (Fs) values of 6.0%–24.6% and 7.3%–42.8%, respectively, thus, indicating the classification as the unequilibrated type 3 meteorites based on the classification standard for the petrologic types of chondrites (Krot et al. 2014; Van Schmus et al. 1967). Furthermore, these could be subdivided into types 3.0–3.9 using PMD (Fa) (Sears et al. 1980, 1991; Sears et al. 1988).Specifically, the PMD (Fa) of GRV 13008/13033/13062 are corresponding to type 3.7, 3.8 and 3.9, respectively(Fig. 6a).

The matrix of GRV 13107 was translucent and the overall outline of its chondrules was clear. The PMD (Fa)value of this chondrite was 4.6%, which was close to the boundary of type 3 and types 4–6 meteorites (Fig. 6).However, its low-calcium pyroxene composition did not reach equilibrium and its PMD (Fs) (7.5%) was noted to be significantly higher than that of type 4 ordinary meteorites,even exceeding the types 3.7–3.9 meteorites (Fig. 6b).Therefore, GRV 13107 was most likely the transitional sample between type 3.9 and type 4. It was observed that the olivine and low-calcium pyroxene compositions of GRV 13107 were in between the unequilibrated GRV 13008/13033/13062 and equilibrated GRV 090572/090566/090438 (Sect. 4.2.1, Fig. 10). These observations also strongly corroborated that the degree of thermal metamorphism in GRV 13107 was lower than that of type 4 ordinary meteorites, thus, GRV 13107 could be classified as type 3.9/4.

/PMD%83 0.87 1..5 75.6 10 0.13 66 1.)STD/STD 26)GRV 090438=(n/STD PMD%14)GRV 090566=(n/PMD%STD 13)GRV 090572=(n/PMD%STD GRV 1310717)=(n/PMD%STD 47 0.03 0.03 0.02 0.25 0.05 0.21 0.11 0.01 0.55 0.34 0..7 19 13 11.6 46.5 81 02.5.7 56 0.0.0.13 0.28 0.0.100 20 98 0.03 5.65 9.39 1..3 90 17 4.56 0.03 0.03 0.04 0.66 0.05 0.39 0.16 0.03 0.51 0.00 1..7 17 16 13.1 51.1 95 03.8.4 79 56 0.0.0.0.0.0.99 0.96 13 1..4 90 28.7 93 20 11 0.14 1.44 0.06 0.03 0.04 0.26 0.06 0.25 0.13 0.01 0.52 0.39 0..5 20 15 13.0 51.2 85 03.6.2 31 56 0.0.0.0.0.0.99 1.20 13 6..7 87 28.9 52 20 11 3.46 7.72 0.11 0.09 0.06 0.83 0.06 0.14 1.10 0.01 0.50 0.52 1..9 19 16 10.4 49.5 22 01.9.2 11 54 0.0.0.0.0.0.98 2.26 13 7..8 01 29.2 25 20 25 3.48 7.15 1.21 0.31 0.10 0.97 0.10 0.90 0.13 0.01 0.53 0.44 1.ge ra ave×(100 ion a t vi De a n Me ent rc—Pe rk)ma to ine r l unde symbol ing(Us PMD,ion a t vi De rd anda STD—St EPMA a vi ed lyz ana ne oxe pyr um c i a l-c low of ion it 3 Compos e Tabl GRV 1306218)=(n/PMD%STD GRV 1303331)=(n/PMD%STD e GRV 1300845)=(n mpl Sa.5 15 23 15.4 40.0 27 01.0.9 07 54 0.0.0.13 0.30 0.0.99 19 3..0.1.3.5 32 48 10 124 32 69 1.10 0.88 0.28 0.04 4.19 0.04 3.73 0.13 0.63 0.22 6..2 08 52 41.6 39.6 59 08.5.2 01 55 0.0.0.12 0.29 0.0.99 19 3..7.8.6.8 40 58 14 126 42 65 1.11 0.82 0.32 0.21 5.26 0.35 4.76 0.04 0.81 0.32 8..7 08 56 50.8 44.8 60 03.5.5 54 0.0.0.12 0.29 0.0.99 19 O2 2O3 O O 2O a l Si O2 T i A l2O3 Cr Fe MnO MgO Ca Na Tot Fs s.nt poi ed lyz ana of r numbe the s e nt e s pr re t ke a c br the in r numbe The e:Not

4.2 Effect of silicate mineral composition variation in thermal metamorphism and classification of petrologic types

Since Van Schmus et al. (1967) introduced the concept of petrologic types for classifying the intensity of the thermal metamorphism of chondrites, the uniformity of the silicate mineral composition has been an important standard for distinguishing the unequilibrated and equilibrated ordinary chondrites. Further, other alternate classification parameters are challenging to be popularized or quantified. Thus,PMD (Fa) of olivine has become the most important classification parameter for the petrologic types (Huss et al.2006; Krot et al. 2014; Sears et al. 1980, 1991; Sears et al.1988). However, literature studies have reported that PMD(Fa) of a few type 3 ordinary chondrites is distributed within the range of the types 4–6 meteorites (Fig. 6b) and the low-calcium pyroxene composition of these meteorites also exhibits significant heterogeneity (Fs-PMD can reach 40%). Therefore, it is of vital need to expand and improve the classification standard for the mineral composition of meteorites, to accurately distinguish the type 3 and types 4–6 meteorites. With an increase in the degree of thermal metamorphism in meteorites, the silicate (olivine and lowcalcium pyroxene) mineral composition in meteorites gradually becomes uniform (Van Schmus et al. 1967).Thus, it is theoretically feasible to achieve the classification of type 3 and types 4–6 meteorites through the variation in the chemical composition of olivine and low-calcium pyroxene.

Fig. 5 Classification of the chemical groups of the ordinary chondrites. a Olivine Fa (mol.%)—low-calcium pyroxene Fs (mol.%) classification standards (Brearley et al. 1998; Rubin 1990); b Fe–Ni and FeS content (vol.%) – mean chondrule diameter (mm) classification standards (Krot et al. 2014; Rubin 2000; Scott et al. 2014). Fa and Fs errors in GRV 13008/13033/13062/13107 exceeded the range of a single chemical group and their types need to be verified by the petrographic parameters

4.2.1 Effect of the varying chemical composition of olivine and low-calcium pyroxene

With a gradual increase in the petrologic type, the composition (PMD) of olivine and low-calcium pyroxene in the seven meteorite samples displayed obvious variations(Fig. 7). The PMD (Fa) of olivine shows a decrease with an increase in the petrologic type of meteorites. However, it was difficult to distinguish between the type 3 and type 4 meteorites through 5% of PMD (Fa) (Fig. 7a). In fact, with the increase in the petrologic type, the PMD of olivine SiO2(Fig. 7b), MgO (Fig. 7c), FeO (Fig. 7d) and MnO (Fig. 7e)contents have decreased clearly in the meteorite samples,eventually fluctuating within a narrow range and reaching equilibrium. PMD (SiO2), PMD (MgO), PMD (FeO) and PMD (MnO) in the olivine of types 3.7–3.9 GRV 13008/13033/13062 were observed to fluctuate within 1.8%–2.6%, 3.4%–8.4%, 5.0%–23.0% and 20.7%–25.8%,respectively. On the other hand, PMD (SiO2), PMD (MgO),PMD (FeO) and PMD (MnO) in olivine of types 4–6 GRV 090572/090566/090438 varies within 0.6%–0.9%, 0.3%–1.4%, 1.2%–2.0% and 10.5%–11.8%, respectively. These indices for the type 3.9/4 GRV 13107 were observed between the indices of types 3.7–3.9 and types 4–6.

With an increment in the degree of thermal metamorphism in meteorites, the PMD (Fs) of low-calcium pyroxene was reduced, however, it was still difficult to effectively distinguish between type 3 and types 4–6 chondrites through 5% of PMD (Fig. 7a). It was noted that the PMD of low-calcium pyroxene SiO2(Fig. 7b), MgO(Fig. 7c), FeO (Fig. 7d), MnO (Fig. 7e) and CaO (Fig. 7f)decreased on increasing the petrologic type of meteorites,eventually, the values vary within a narrow range and reached equilibrium. In this respect, PMD (SiO2), PMD(MgO), PMD (FeO), PMD (MnO) and PMD (CaO) values in the low-calcium pyroxene of types 3.7–3.9 GRV 13008/13033/13062 distributed within 2.1%–3.1%, 3.0%–14.6%, 7.3%–40.7%, 25.8%–58.8% and 48.2%–126%,respectively. Likewise, PMD (SiO2), PMD (MgO), PMD(FeO), PMD (MnO) and PMD (CaO) in low-calcium pyroxene of types 4–6 GRV 090572/090566/090438 varied within 0.8%–1.0%, 0.8%–1.4%, 1.9%–5.0%, 9.7%–11.4%and 13.6%–17.3%, respectively. In addition, for type 3.9/4 GRV 13107, the PMD (MgO) and PMD (FeO) in lowcalcium pyroxene were noted to be within the range associated with types 3.7–3.9, whereas the other indices lied between the indices of types 3.7–3.9 and types 4–6.

The variation ranges of PMD (SiO2), PMD (MgO) and PMD (MnO) in olivine and low-calcium pyroxene of types 4–6 GRV 090572/090566/090438 were similar, however,low-calcium pyroxene PMD (SiO2), PMD (MgO) and PMD (MnO) in GRV 13008 (type 3.7), GRV 13033 (type 3.8) and GRV 13062 (type 3.9) were observed to be significantly higher than those in olivine (Fig. 7). In addition,low-calcium pyroxene PMD (FeO) was higher than PMD(FeO) in olivine for all the meteorite samples. The observed findings suggested that during thermal metamorphism, olivine Si, Mg, Fe and Mn in the ordinary chondrites reached equilibrium more easily than the corresponding elements in low-calcium pyroxene.

Fig. 6 Classification of the petrologic types of the ordinary chondrites. a Classification of petrologic subtypes and classification standard based on PMD (Fa) (Sears et al. 1988; Sears et al. 1991); b comparison of PMD (Fa) of olivine and PMD (Fs) of low-calcium pyroxene in the ordinary chondrites with different petrologic types (the data was taken from Connolly et al. 2007; Ninagawa et al. 2005, 2002, 2000; Russell et al.2003, 2005; Wang et al. 2020; Zhang et al. 2013)), the dashed line represents 5% PMD (Fa) boundary between the type 3 and types 4–6 meteorites (Krot et al. 2014)

Fig. 7 Effect of the chemical composition variation of olivine and low-calcium pyroxene in the ordinary chondrites during thermal metamorphism. The meteorites have been arranged on the abscissa in an ascending order based on the petrologic type shown in brackets:008-GRV 13008, 033-GRV 13033, 062-GRV 13062, 107-GRV 13107, 572-GRV 090572, 566-GRV 090566, 438-GRV 090438. Dark gray:olivine composition, light gray: low-calcium pyroxene composition, dashed line: 5% of the PMD (Fa) boundary between the type 3 and types 4–6 meteorites (Krot et al. 2014). a PMD (Fa) or PMD (Fs); b PMD (SiO2); c PMD (MgO); d PMD (FeO); e PMD (MnO); f PMD (CaO)

4.2.2 Classification standard for the petrologic types of the unequilibrated and equilibrated ordinary chondrites

The findings observed in this study demonstrate that PMD(Fa), PMD (SiO2), PMD (MgO), PMD (FeO) and PMD(MnO) in olivine decrease with increasing the degree of thermal metamorphism in meteorites, whereas PMD (Fs),PMD (SiO2), PMD (MgO), PMD (FeO), PMD (MnO) and PMD (CaO) in low-calcium pyroxene also reduce with upgradation in the petrologic types of meteorites. As PMD(Fa) in olivine can be used to distinguish the type 3 and types 4–6 ordinary chondrites (Krot et al. 2014; Scott et al.2014; Sears et al. 1980, 1991; Sears et al. 1988), the development and improvement of the classification standard for the petrologic types of the unequilibrated and equilibrated ordinary chondrites based on the variation of the chemical composition of olivine and low-calcium pyroxene are worthy of discussion.

Based on PMD (Fa) in olivine, the distribution areas of PMD (SiO2), PMD (MgO), PMD (FeO) and PMD (MnO)in olivine in the meteorite samples were compared (Fig. 8).In Fig. 8, the dashed line refers to 5% of the PMD (Fa)boundary between the types 3 and types 4–6 meteorites proposed previously, however, the meteorites with PMD(Fa) close to the 5% boundary cannot be clearly distinguished by solely relying on this boundary. However, on incorporating the additional parameters (PMD (SiO2),PMD (MgO), PMD (FeO) and PMD (MnO)), the distribution areas of type 3 and types 4–6 meteorites were significantly different, more obviously in the PMD (Fa)-PMD(SiO2) and PMD (Fa)-PMD (MnO) systems. Unlike the other meteorites, GRV 13107 was observed to lie out of the area range of type 3 or types 4–6 but tended to be between the two (Fig. 8), thus, confirming the earlier classification of its petrologic type as type 3.9/4.

Similarly, based on PMD (Fa) in olivine, the distribution areas of PMD (SiO2), PMD (MgO), PMD (FeO), PMD(MnO) and PMD (Fa) in low-calcium pyroxene of the meteorites were also compared (Fig. 9). In PMD (Fa)-PMD(Fs), PMD (Fa)-PMD (MgO) and PMD (Fa)-PMD (FeO)systems, the boundary of the distributed phases was very close to each other, in unequilibrated GRV 13008/13033/13062 and equilibrated GRV 090572/090566/090438.However, in PMD (Fa)-PMD (SiO2), PMD (Fa)-PMD(MnO) and PMD (Fa)-PMD (CaO) systems, the distribution areas of type 3 and types 4–6 meteorites exhibited more obvious differences. In addition, GRV 13107 (type 3.9/4) was not positioned within the area range of type 3 or types 4–6 meteorites, thus, proving the earlier classification of its petrologic type.

Fig. 8 Comparison of the compositional variations of olivine and petrologic types of the ordinary chondrites. Dashed line: 5% of the PMD (Fa)boundary between the type 3 and types 4–6 meteorites (Krot et al. 2014), light purple area: types 3.7–3.9 meteorites and light green area: types 4–6 meteorites. a PMD (Fa) vs. PMD (SiO2); b PMD (Fa) vs. PMD (MgO); c PMD (Fa) vs. PMD (FeO); d PMD (Fa) vs. PMD (MnO)

PMD (Fa) in olivine is a vital parameter used to distinguish the petrologic types of type 3 and types 4–6 ordinary chondrites (Krot et al. 2014; Scott et al. 2014;Sears et al. 1980, 1991; Sears et al. 1988), however, its‘‘resolution’’ for the classification boundary of the unequilibrated and equilibrated meteorites is not accurate enough. In particular, the classification results for the meteorites with PMD (Fa) close to 5% of the boundary tend to be controversial (Grossman et al. 2009). Based on the role of the chemical composition PMD in olivine and low-calcium pyroxene in thermal metamorphism obtained in this study, a multi-parameter classification standard for distinguishing the petrologic type of the unequilibrated and equilibrated ordinary chondrites was proposed (Fig. 10). In olivine PMD (MgO)-PMD (SiO2) and PMD (MnO)-PMD(SiO2) systems (Fig. 10a, b) and low-calcium pyroxene PMD (SiO2)-PMD (CaO) and PMD (MnO)-PMD (CaO)systems (Fig. 10c,d), type 3 GRV 13008/13033/13062 were observed to be distributed in a large area on the upper right, thus, suggesting the unequilibrated mineral composition. Types 4–6 GRV 090572/090566/090438 were noted to be distributed in a small area on the lower left, thus,indicating that the mineral chemical composition in meteorites was uniform and reached equilibrium under the effect of thermal metamorphism.

Fig. 9 Comparison of the compositional variations of low-calcium pyroxene and petrologic types of the ordinary chondrites. Dashed line: 5% of the PMD (Fa) boundary between the type 3 and types 4–6 meteorites (Krot et al. 2014), light purple area: types 3.7–3.9 meteorites and light green area: types 4–6 meteorites. a PMD (Fa) in olivine vs. PMD (Fs) in pyroxene; b PMD (Fa) in olivine vs. PMD (SiO2) in pyroxene; c PMD (Fa) in olivine vs. PMD (MgO) in pyroxene; d PMD (Fa) in olivine vs. PMD (FeO) in pyroxene; e PMD (Fa) in olivine vs. PMD (MnO) in pyroxene;f PMD (Fa) in olivine vs. PMD (CaO) in pyroxene

4.2.3 The prospect

The previous literature studies have reported that PMD(Fa) values of some type 3 ordinary chondrites (classified by TL, volatile content and so on) are distributed within the area of the types 4–6 meteorites (Fig. 6b). Thus, it is of vital need to expand the classification standard for the mineral composition of meteorites, to improve the ‘‘resolution’’ of the existing PMD (Fa) classification system of the petrologic types.

In this study, it is found that 1) For olivine in type 3 and types 4–6 chondrites, PMD (SiO2) varies within 2.3–2.8%(type 3) and 0.7–1.2% (type 4–6), respectively. On the other hand, PMD (MgO) and PMD (MnO) vary within 3.6–8.2% (type 3) and 0.7–1.6% (type 4–6), 22.9–24.8%(type 3) and 10.1–11.8% (type 4–6), respectively; 2) For low-calcium pyroxene in the type 3 and types 4–6 chondrites, PMD (SiO2) varies within 2.0–3.7% (type 3) and 0.8–1.1% (type 4–6), respectively. Likewise, PMD (MnO)and PMD (CaO) vary within 25.8–59.3% (type 3) and 9.5–12.5% (type 4–6) and 50.7–118% (type 3) and 13.6–17.6% (type 4–6), respectively. Thus, a multi-parameter classification standard has been exploratively confirmed to distinguish the petrologic types of type 3 and type 4–6 ordinary chondrites. And in this system, the distribution areas of type 3 and type 4–6 chondrites can be completely separated (Fig. 10). That is, it has a higher‘‘resolution’’ of classification than the single parameter of 5% PMD (Fa) in olivine (Figs. 8 and 9).

However, the GRV 13107 chondrite was noted within the intermediate region between type 3 and type 4–6(Fig. 10). The thermal metamorphism of chondrites was a continuous variation process. Therefore, no matter how the petrologic type classification standards and their ‘‘resolution’’ were improved, there would always be some meteorites that were located exactly in the intermediate region of type 3 and type 4–6 chondrites. These meteorites(type 3.9/4) represent transitional samples between unequilibrated and equilibrated chondrites. Thus, the GRV 13017 chondrite represents the transitional behavior between type 3.9 and type 4. Only by finding enough transitional samples similar to GRV 13107, can we further improve the multi-parameter classification standard proposed in this article and increase its ‘‘resolution’’ based on the chemical composition characteristics of these chondrites.

Fig. 10 Multi-parameter classification standard of the petrologic type based on the chemical composition of olivine and low-calcium pyroxene of the unequilibrated and equilibrated ordinary chondrites. Light purple area: types 3.7–3.9 meteorites, light green area: types 4–6 meteorites.a PMD (MgO) in olivine vs. PMD (SiO2) in olivine; b PMD (MnO) in olivine vs. PMD (SiO2) in olivine; c PMD (SiO2) in pyroxene vs. PMD(CaO) in pyroxene; d PMD (MnO) in pyroxene vs. PMD (CaO) in pyroxene

In the future, more ordinary chondrites will be incorporated to verify and improve the developed multi-parameter classification standard. In any case, the multiparameter classification standard developed in this study complemented and improved the existing classification system for the mineral composition of the petrologic type of the ordinary chondrites. And it enhances our understanding of the effect of the variation in the chemical composition of the silicate minerals in thermal metamorphism in the ordinary chondrites.

5 Conclusions

1. Based on the comprehensive petrological, mineralogical and chemical composition analyses, GRV 13008,GRV 13033, GRV 13062, GRV 13107, GRV 090572,GRV 090566 and GRV 090438 are observed to belong to the L3.7, L3.8, L3.9, L3.9/4, L4, L5 and L6 meteorite types, respectively.

2. For olivine in the type 3 and types 4–6 chondrites,PMD (SiO2) varies within 2.3–2.8% (type 3) and 0.7–1.2% (type 4–6), respectively. On the other hand,PMD (MgO) and PMD (MnO) vary within 3.6–8.2%(type 3) and 0.7–1.6% (type 4–6), 22.9–24.8% (type 3)and 10.1–11.8% (type 4–6), respectively.

3. For low-calcium pyroxene in the type 3 and types 4–6 meteorites, PMD (SiO2) varies within 2.0–3.7% (type 3) and 0.8–1.1% (type 4–6), respectively. Likewise,PMD (MnO) and PMD (CaO) vary within 25.8–59.3%(type 3) and 9.5–12.5% (type 4–6) and 50.7–118%(type 3) and 13.6–17.6% (type 4–6), respectively.

4. The olivine PMD (SiO2)-PMD (MgO)-PMD (MnO)and low-calcium pyroxene PMD (CaO)-PMD (SiO2)-PMD (MnO) multi-parameter classification system has been exploratively confirmed (Fig. 10) to distinguish the petrologic types of the unequilibrated (type 3) and equilibrated (types 4–6) ordinary chondrites. The developed standard has a higher ‘‘resolution’’ of classification than the single parameter PMD (Fa) in olivine, thus, enhancing our understanding of the effect of the chemical composition variations of the silicate minerals in the thermal metamorphism process in the ordinary chondrites.

AcknowledgementsWe thank the Polar Research Institute of China and the Resource-sharing Platform of Polar Samples for the allocation of the meteorites samples. We also thank two anonymous reviewers and editors for their helpful comments and suggestions. In addition,we thank Dr. Ranjith P.M. for his help on the English writing of this study.

Authors’ contributionsWriting—original draft preparation,Chuantong Zhang; writing—review and editing, Bingkui Miao; data curation, Zhipeng Xia and Qinglin Xie. All authors have read and agreed to the published version of the manuscript.

FundingThis research was funded by Strategic Priority Research Program of Chinese Academy of Sciences (XDB 41000000), Project funded by China Postdoctoral Science Foundation(2020M673557XB), Guangxi Natural Science Foundation under Grant No. 2020JJB150056, Civil Aerospace Pre Research Project(D020302 and D020206), Guangxi Scientific Base and Talent Special Projects (No. AD1850007), Foundation of Guilin University of Technology (GUTQDJJ2019165) and the grant from Key Laboratory of Lunar and Deep Space Exploration, CAS (LDSE201907).

Declarations

Conflicts of interestThe authors declare no conflict of interest.