Trachyandesitic volcanism in the early Solar System

Significance

Volcanism is a fundamental geological process on planets and was substantial during crustal growth on planetary bodies in the early Solar System, as witnessed by ubiquitous rocks of basaltic composition, e.g., on Earth, Moon, Mars, and asteroids. Besides basaltic volcanism, trachyandesite lavas are generated on Earth. The first occurrence of a trachyandesite lava in the meteorite collections demonstrates that trachyandesitic, alkali-, and silica-rich volcanism takes place not only on Earth today but already occurred on a small planetesimal ∼4.56 billion years ago. It sets new constraints on mechanisms and styles of early Solar System volcanism.

Abstract

Volcanism is a substantial process during crustal growth on planetary bodies and well documented to have occurred in the early Solar System from the recognition of numerous basaltic meteorites. Considering the ureilite parent body (UPB), the compositions of magmas that formed a potential UPB crust and were complementary to the ultramafic ureilite mantle rocks are poorly constrained. Among the Almahata Sitta meteorites, a unique trachyandesite lava (with an oxygen isotope composition identical to that of common ureilites) documents the presence of volatile- and SiO₂-rich magmas on the UPB. The magma was extracted at low degrees of disequilibrium partial melting of the UPB mantle. This trachyandesite extends the range of known ancient volcanic, crust-forming rocks and documents that volcanic rocks, similar in composition to trachyandesites on Earth, also formed on small planetary bodies ∼4.56 billion years ago. It also extends the volcanic activity on the UPB by ∼1 million years (Ma) and thus constrains the time of disruption of the body to later than 6.5 Ma after the formation of Ca–Al-rich inclusions.

A large number of planetary embryos, tens to hundreds of kilometers in size, accreted within the early Solar System. In some of these embryos, internal heating triggered melting and differentiation, giving rise to a varied suite of lithologies as documented by the achondritic meteorites. Planetary crustal growth occurs via both volcanic eruptions and plutonic intrusions. Constraining these processes and the diversity of crustal materials that formed the outermost solid shell of planetary bodies is crucial for understanding Solar System planetary processes and evolution.

Ureilites are among the most common achondrites and represent remnants of the mantle from a planetary body from which magmas have been extensively extracted (1–4). Several details of ureilite petrogenesis (e.g., the mode of melt extraction) remain controversial (e.g., refs. 1 and 2) because crustal rocks from the ureilite parent body (UPB) have not yet been discovered. Although tiny remnants of feldspathic and felsic melts from ureilitic breccias have been interpreted as UPB basalts or products of partial melting of plagioclase-bearing cumulates (e.g., refs. 5 and 6), it is generally assumed that the complementary melts were lost to space during explosive eruptions (e.g., refs. 7–9).

A unique opportunity to gain new insights into ureilite petrogenesis was provided by the polymict asteroid 2008 TC₃ that impacted our planet October 7, 2008, in the Nubian Desert, Sudan, containing various ureilitic and ureilite-related fragments (10, 11). Among its remnant fragments collected in the strewn field, collectively named the “Almahata Sitta” meteorites, the sample ALM-A (Almahata Sitta trachyandesitic meteorite) was recovered (12). ALM-A weights 24.2 g and is covered with a greenish and shiny fusion crust (Fig. 1).

Fig. 1.

(A) Greenish ALM-A hand specimen, (B) overview of the ALM-A thin section (polarized light, crossed nicols) showing high abundances of feldspar (mainly gray) and displaying a subdoleritic texture. Anhedral anorthoclase and subhedral, zoned plagioclase laths occur. Low-Ca and Ca pyroxene (both colored) are frequently encountered. (C) Close-up backscattered electron image illustrating the textural relation of feldspar (Fsp) and pyroxenes (Cpx, Ca rich; Px, low Ca). Ca pyroxene bears abundant inclusions of, e.g., feldspar and SiO₂-rich glass. Cl apatite (Ap) is developed as laths. (D) Quartz-normative, alkali-rich glass inclusions occur within large Ca pyroxene crystals (arrows; polarized light).

The ALM-A sample described here is the only SiO₂-rich, rapidly cooled volcanic rock among the meteorites in our collections. This rock is texturally completely different from the felsic achondrite Graves Nunatak (GRA) 06128/9 (13) and granitic lithologies that occur as fragments in meteorite breccias (14). GRA 06128/9 shows a plutonic, granoblastic texture, 120° triple junctions between coexisting phases, and chemically equilibrated silicates (13). ALM-A clearly demonstrates that SiO₂-rich lavas were formed on the UPB and sets new important constraints on ureilite petrogenesis.

Results

The oxygen isotope compositions of two ALM-A aliquots (Fig. 2) were constrained to δ¹⁸O = 8.1 ‰, δ¹⁷O = 3.2 ‰, Δ¹⁷O = −1.06 ‰ and δ¹⁸O = 8.0 ‰, δ¹⁷O = 3.2 ‰, Δ¹⁷O = −1.04 ‰ (for details, see SI Appendix). They fall in the range of other ureilites, including those from Almahata Sitta (10, 11, 15, 16), but at the ¹⁶O-poor end of the range of oxygen isotopes in ureilites, which is the more common oxygen isotope composition found among ureilites.

Fig. 2.

Three-oxygen isotope diagram illustrating the bulk composition (two aliquots) of ALM-A (yellow stars) in comparison with ureilite data from Almahata Sitta ureilites (gray dots). The oxygen isotope compositions of two ALM-A aliquots are compared with data on other ureilitic samples from Almahata Sitta (10, 11, 15, 16). Green field highlights the compositional range for oxygen isotope compositions of other ureilites (20). Gray fields give the compositional ranges for Rumuruti (R) chondrites, ordinary chondrites (H, L, LL), and enstatite (E) chondrites. TFL, terrestrial fractionation line; CCAM, carbonaceous chondrite−anhydrous mineral line.

ALM-A (Fig. 1; see SI Appendix, Tables S1− S3 and Figs. S1− S3 for mineralogical details and the compositions of the constituent phases) is a fine-grained rock rich in feldspars (∼70 vol%; subhedral, zoned plagioclase with ∼An_10–55 and anhedral anorthoclase with ∼An_5–12) embedding abundant Cr-bearing (Cr₂O₃: ∼0.8–1 wt%) Ca pyroxene (∼20 vol %; ∼Fs_19–25Wo_34–41) having glass inclusions, and low-Ca pyroxene (∼5 vol%; ∼Fs_32–42Wo_8–14) without any exsolution lamellae. Anorthoclase was identified by Raman spectroscopy (SI Appendix, Fig. S4). As accessory phases, euhedral Cl apatite (with ∼0.3 wt% H₂O), merrillite, ilmenite, Ti,Cr,Fe-spinel, FeS, and Ni-poor Fe metal (Ni <0.10 wt%) are observed (Fig. 1). Fine-grained intergrowths of albitic feldspar (An_<10), skeletal pyroxene, and a K₂O-rich, quartz-normative glass (with up to ∼4.5 wt% K₂O and up to ∼40 µm in size) are found in interstices between the major minerals. These glasses are rhyolitic in composition, indicating the possibility for formation of rocks with even higher SiO₂ concentrations on the UPB (granitoids or granites; compare ref. 17). Considering the studied thin sections, the ALM-A rock is free of vesicles and olivine phenocrysts.

ALM-A has a trachyandesitic bulk chemical composition. Trachyandesite is a lava, rich in feldspars and containing—depending on the concentrations of Na₂O + K₂O—between 53 wt% SiO₂ and 63 wt% SiO₂ using the International Union of Geological Sciences criteria. ALM-A has high concentrations of alkalis (∼7 wt% of Na₂O + K₂O), about 60 wt% SiO₂, and is Mg rich (MgO = 4.8 wt%; Table 1). The composition results in a Mg number (mg#) [=100×Mg/(Mg+Fe)] close to 61, which is similar to, e.g., those of some terrestrial primitive basalts or Mg-rich andesites. ALM-A displays enrichments of Ba, Be, Th, U, Nb, Ta, Eu, Ti, Zr, Hf, and Sr relative to the light rare earth elements (REE) [e.g., (Th/La)_n = 1.64, Eu/Eu*= 1.63, (Sr/Ce)_n = 1.79, (Hf/Sm)_n = 1.70], and a significant heavy REE enrichment [(Gd/Lu)_n = 0.78] (Fig. 3A and SI Appendix, Fig. S5).

Table 1.

Major and trace element abundances in the ALM-A meteorite

Oxides	ALM-A	Elements	ALM-A	Elements	ALM-A	Elements	ALM-A
SiO2	60.07	Li	3.95	Nb	2.57	Ho	0.467
TiO2	0.67	Be	0.23	Mo	0.017	Er	1.40
Al2O3	14.66	Sc	21.4	Cs	1.11	Tm	0.221
Cr2O3	0.28	V	26.1	Ba	22.06	Yb	1.50
FeO	5.57	Co	1.55	La	1.44	Lu	0.224
MnO	0.27	Ni	7.33	Ce	3.54	Hf	1.22
MgO	4.81	Cu	1.44	Pr	0.546	Ta	0.133
CaO	7.29	Zn	148	Nd	2.92	W	0.011
Na2O	6.59	Ga	30.2	Sm	1.01	Pb	0.15
K2O	0.29	Rb	4.57	Eu	0.628	Th	0.284
P2O5	0.52	Sr	85.8	Gd	1.46	U	0.078
Total	101.02	Y	12.63	Tb	0.280
		Zr	38.27	Dy	1.96

Oxides in wt%; other elements in micrograms per gram.

Fig. 3.

(A) Bulk trace element pattern of ALM-A (200-mg aliquot) and (B) selected trace elements of ALM-A (black line) compared with those of the experimental melts produced by disequilibrium melting of an ordinary chondrite [Leedey, L6, red lines (29)] normalized to CI chondrite values (33).

²⁶Al-²⁶Mg systematics of seven feldspar grains indicate that ALM-A crystallized 6.5 (+0.5, −0.3) million years (Ma) after Ca–Al-rich inclusions (CAIs) (Fig. 4 and SI Appendix, Table S4 and Fig. S6) and is thus ∼1 Ma younger than the monomict Northwest Africa (NWA) 766 ureilite (18) and the albitic clasts found in polymict ureilites (19).

Fig. 4.

Al-Mg age of ALM-A compared with Al-Mg and Mn-Cr ages obtained from albitic clasts found in polymict ureilites (19) considering (34) and the Mn-Cr age of the NWA 766 ureilite (18).

Discussion

Although ALM-A is unlike all previously described achondrites, there is clear evidence that the rock originated from the ureilite parent body: (i) The oxygen isotope composition of ALM-A (Fig. 2) falls in the field of the ureilites and feldspathic clasts from polymict ureilites (3, 20, 21); (ii) the feldspar mineralogy is very similar to the rare feldspathic clasts found in some polymict ureilites, which are considered as remnants of UPB melts (5); and (iii) it is most likely part of asteroid 2008 TC₃, which was shown to be a fragment of an asteroid dominated by ureilitic lithologies (10, 11).

The presence of glass (SI Appendix, Fig. S3) and anorthoclase (SI Appendix, Fig. S4), a phase occasionally occurring in terrestrial lava bombs, indicates very rapid cooling of ALM-A (22, 23), in accordance with a lava origin on the UPB.

Contrary to terrestrial trachyandesites, which are generally residual melts formed from more mafic magmas by fractional crystallization, ALM-A was certainly not affected substantially by this process. It has been observed that the Δ¹⁷O of ureilites correlates with the FeO in their olivines (SI Appendix, Fig. S7). Assuming that Δ¹⁷O does not change during partial melting, ALM-A’s source rock (i.e., the UPB mantle) had olivine compositions of Fo_78–86. Using an olivine/melt distribution coefficient K_D(Fe/Mg) [=(Fe/Mg)_olivine/(Fe/Mg)_melt] of 0.28 (24), the conjugate melts in equilibrium with such sources have mg# numbers in the range 50–63. ALM-A’s mg# value is 61 and is well in this range and supports an origin from the UPB.

The compositions of the first melts produced during equilibrium melting of typical chondrites at low pressure are controlled by the enstatite−plagioclase−forsterite peritectic in the olivine−plagioclase−silica system and would be basaltic in composition (25). Intermediate melts were suggested by gabbroic, plagioclase-, and diopside-rich inclusions in the Caddo County iron meteorite (26) and the felsic achondrite GRA 06128/9 (13), but the inferred compositions of their parental melts are still a matter of debate (e.g., ref. 27). If the melted lithologies are more silica rich, as exemplified by the FeO-poor enstatite chondrites or other pyroxene-rich source rocks, the compositions of the melts are controlled by the enstatite−plagioclase−silica peritectic. Although such melts are more silica- and plagioclase-rich and have andesitic or trachyandesitic compositions (28), they will be much poorer in FeO than ALM-A. Therefore, an origin of ALM-A from an enstatite chondrite-like parent body is excluded.

The CI1-normalized element abundances in ALM-A show some anomalies. The concentrations of Sr, Eu, Zr, Hf, and Ti are slightly higher than expected for equilibrium partial melting of a source rock of chondritic composition, whereas concentrations of Rb and K are lower (SI Appendix, Fig. S8). Experimental data on disequilibrium partial melt compositions (51−56 wt% SiO₂, up to 4.4 wt% Na₂O) of an ordinary chondrite starting material [10–15% melting (29)], however, are close to the observed silica- and Na-rich composition of ALM-A, and well match the observed Sr, Eu, Zr, and Ti concentrations (Fig. 3). It cannot be completely ruled out that the Sr, Ba, and Eu concentrations may result from volatility-related processes during the experiments and are not an effect of the rapidity of melting. In any case, ALM-A’s trace element features (Fig. 3) must be genuine characteristics of the melt. Recent experimental works have demonstrated that low-degree melting (<5%) of an oxidized chondrite (R chondrite) even better matches the bulk compositions of ALM-A [and GRA 06128/9 (30)], in terms of major element chemistry (∼61 wt% SiO₂, ∼9 wt% Na₂O). Generation of ALM-A by disequilibrium melting of chondritic source rock suggests that the ALM-A melt was rapidly extracted. Rapid melt extraction was also considered in previous models of magma genesis on the UPB (1, 31).

The abundances of volatiles (F, Cl, and OH) displayed by ALM-A’s apatites (SI Appendix, Table S3) indicate that these elements were involved during the melting on the UPB, and have certainly favored the segregation of the melts from their sources and their transport to the surface.

The discovery of ALM-A shows that a crust with an intermediate composition on the UPB was at least partially preserved and that the lavas were not entirely lost to space by explosive volcanism as generally assumed (e.g., refs. 7–9). Beyond that, ALM-A is ∼1 Ma younger than the albitic clasts found in polymict ureilites [5.5 Ma after CAIs (18)], extending the igneous activity on the ureilite parent body for at least another 1 Ma. Although the duration of the entire magmatic activity of the UPB cannot be directly estimated, the thermal history of the ureilites indicates that their parent body was impact-disrupted while still hot [∼1200–1300 °C (32)]. These temperatures are in the range of temperatures required to produce melts on the UPB. This implies that magmatic activity was still ongoing on the UPB at the time, when the planetesimal was destroyed by a giant collision. The age of the catastrophic event is not known, but ALM-A constrains the time of disruption to ≥ 6.5 Ma after the formation of CAIs.

The unique ALM-A trachyandesite demonstrates that silica-rich magmas were generated on small planetary bodies early in Solar System history.

Methods

For mineralogical characterization, the sample was studied by optical and electron microscopy. A JEOL 6610-LV electron microscope (SEM) was used to resolve the fine-grained textures, and quantitative mineral analyses were obtained using a JEOL JXA 8900 Superprobe electron microprobe (EPMA) operated at 15 keV and a probe current of 15 nA at the Westfälische Wilhelms-Universität Münster. For the bulk chemical analysis, a 200-mg chip was powdered. An aliquot of 115 mg was used for the determination of trace elements by inductively coupled plasma (ICP)-MS, and with the remaining material the major elements were determined by ICP-atomic emission spectroscopy. The oxygen isotope compositions of two aliquots of ALM-A were measured by laser fluorination gas mass spectrometry (see SI Appendix for details). At the CRPG-CNRS (Nancy) the Mg isotopic compositions and Al/Mg concentration ratios were measured with the Cameca IMS 1280-HR2 ion microprobe. Raman spectroscopy was performed at the Institut für Mineralogie of the Westfälische Wilhelms-Universität Münster. The spectra of selected areas within grains of interest were acquired on a Horiba Scientific XploRA confocal Raman microscope. Details on the procedures and standards are given in SI Appendix.