Unlike their silicic counterparts, mafic eruptions are known for being on the low-end of the explosivity spectrum with eruption styles commonly ranging from effusive to Hawaiian fire fountaining. However, there are increasing discoveries of large mafic Plinian eruptions, sometimes generating ignimbrites, suggesting that this phenomenon might not be so uncommon. So, what processes lead a mafic magma to fragment violently enough to generate extensive ignimbrites? We sampled pumices from ignimbrites and PDCs with a compositional range from basaltic-andesite (Curacautín ignimbrite, Volcàn Llaima, Chile), andesite (Marapi, Indonesia) to trachyte (Gunungkawi ignimbrite, Batur, Indonesia). We use SEM imagery and X-ray Microtomography on pyroclasts from these deposits to characterize phenocryst, microlite and vesicle textures. From vesicle number densities we estimate fragmentation decompression rates in the range of 0.4–1.6 MPa/s for the three deposits. With a combination of EPMA and SIMS analyses we characterise pre-eruptive storage conditions. Based on the bulk and groundmass compositions, the storage temperature (1,050–1,100°C), pressure (50–100 MPa) and phenocryst content (1.0–2.5 vol%), we conclude that the basaltic-andesitic Curacautín magma was at sub-liquidus conditions, which allowed fast and widespread disequilibrium matrix crystallization (0–80 vol%) during ascent to the surface. Combined with the important decompression rate, this intense crystallization led to a magma bulk viscosity jump from 10 3 up to >10 7 Pa s and allowed it to fragment brittlely. Conversely, for the Marapi PDC and Gunungkawi ignimbrite, similar decompression rates coupled with larger initial bulk viscosities of 10 5 –10 6 Pa s were sufficient to fragment the magma brittlely. The fragmentation processes for these latter two deposits were slightly different however, with the Marapi PDC fragmentation being mostly driven by vesicle overpressure, while a combination of bubble overpressure and intense strain-rate were the cause of fragmentation for the Gunungkawi ignimbrite. We conclude that mafic ignimbrites can form due to a combination of peculiar storage conditions that lead to strongly non-linear feedback processes in the conduit, particularly intense microlite crystallization on very short timescales coupled with intense decompression rates. Conversely, the high viscosity determined by pre-eruptive storage conditions, including temperature and volatile-content, are key in controlling the formation of more evolved magmas PDCs'.
Abstract Lunar meteorite MacAlpine Hills ( MAC ) 88105 is a well‐studied feldspathic regolith breccia dominated by rock and mineral fragments from the lunar highlands. Thin section MAC 88105,159 contains a small rock fragment, 400 × 350 μm in size, which is compositionally anomalous compared with other MAC 88105 lithic components. The clast is composed of olivine and plagioclase with minor pyroxene and interstitial devitrified glass component. It is magnesian, akin to samples in the lunar High Mg‐Suite, and also alkali‐rich, akin to samples in the lunar High Alkali Suite. It could represent a small fragment of late‐stage interstitial melt from an Mg‐Suite parent lithology. However, olivine and pyroxene in the clast have Fe/Mn ratios and minor element concentrations that are different from known types of lunar lithologies. As Fe/Mn ratios are notably indicative of planetary origin, the clast could either (1) have a unique lunar magmatic source, or (2) have a nonlunar origin (i.e., consist of achondritic meteorite debris that survived delivery to the lunar surface). Both hypotheses are considered and discussed.
Abstract Rubinite (IMA 2016-110) is a recently discovered Ti3+-dominant refractory mineral in the garnet group from the solar nebula. It has the Ia3d garnet-type structure with a = 12.19(1) Å, and Z = 8, and end-member formula of Ca3Ti3+2Si3O12. Rubinite was identified as micrometer-sized crystals in five refractory Ca,Al-rich inclusions (CAIs) from the CV3 carbonaceous chondrites Allende, Efremovka, and Vigarano. In the Vigarano CAI V3, it occurs in the central portion of an ultrarefractory fragment with Zr,Y,Sc-oxide, spinel and davisite-diopside, all enclosed within an amoeboid olivine aggregate. In the Allende Compact Type A (CTA) CAI AE01-01, it occurs with gehlenitic melilite, perovskite, spinel, hibonite, davisite, grossmanite, and diopside. In Efremovka, rubinite occurs within gehlenitic melilite with perovskite, spinel, and grossmanite in three CTA CAIs E101, E105, and 40E-1 (in a compound CAI). Rubinite is present in spinel-poor regions in all four of the Efremovka and Allende CAIs but it is in contact with spinel in the Vigarano inclusion. The mean chemical composition of type rubinite in Allende is (in wt%) CaO 32.68, Ti2O3 14.79, TiO2 13.06, SiO2 28.37 Al2O3 3.82, Sc2O3 1.80, Na2O 1.01, ZrO2, 0.80, MgO 0.79, V2O3 0.61, FeO 0.53, Y2O3 0.07, Cr2O3 0.05, total 98.38, giving rise to an empirical formula of (Ca2.94Na0.08)(Ti3+1.04Ti4+0.59Sc0.13Mg0.10V0.04Fe0.04Zr0.03)(Si2.38Al0.38Ti4+0.24)O12, where Ti3+ and Ti4+ are partitioned based on stoichiometry. Efremovka rubinite has a similar composition with a mean empirical formula of (Ca2.97Na0.06)(Ti3+1.05Ti4+0.66Mg0.12Sc0.09Zr0.03V0.03Y0.01Fe0.01)(Si2.36Al0.48Ti4+0.16)O12. Vigarano rubinite is much more Y-, Sc-, and Zr-rich, having an empirical formula of (Ca1.89Y0.83Mg0.28)(Ti3+0.59Sc0.50Zr0.72Mg0.2V0.02Cr0.01)(Si1.64Al1.18Ti4+0.07Fe0.06)O12. All rubinites are Ti3+-rich but a significant amount (11–46%) of the Ti is 4+. In the Efremovka CTAs, spinel is 16O-rich (Δ17O ~ –24‰); rubinite and perovskite show limited ranges of Δ17O (from –24 to –16‰; most analyses range from –24 to –20‰); melilite and grossmanite are the most 16O-depleted minerals (Δ17O range from ~ –10 to –4‰ and from –8 to –5‰, respectively). In the Allende CTA AE01-01, spinel and hibonite are 16O-rich (Δ17O ~ –24‰); melilite, rubinite and perovskite show large ranges in Δ17O (from –23 to –3‰, from –21 to –6‰, and from –14 to – 2‰, respectively); grossmanite is uniformly 16O-depleted (Δ17O ~ –3‰). Rubinite formed under highly reducing conditions in the solar nebula by gas-solid condensation and by crystallization from a Ca, Al, and Ti-rich melt. Subsequently, most rubinite grains in the Allende CAI and some in the Efremovka CAIs may have experienced O-isotope exchange to a various degree with an 16O-depleted (Δ17O ~ – 2‰) aqueous fluid on the CV chondrite parent asteroid. However, crystallization from a Ca,Al,Ti-rich melt that recorded O-isotope exchange: with nebular gas with variable Δ17O or post-crystallization O-isotope with such gas cannot be excluded. The mineral name is in honor of Alan E. Rubin (b. 1953), a cosmochemist at University of California, Los Angeles (UCLA), USA, for his many contributions to research in cosmochemistry and mineralogy of meteorites.
Using chemical and petrologic evidence and modeling, we deduce that two chondrule-like particles named Iris and Callie, from Stardust cometary track C2052,12,74, formed in an environment very similar to that seen for type II chondrules in meteorites. Iris was heated near liquidus, equilibrated, and cooled at ≤ 100 °C/hr and within ≈ 2 log units of the IW buffer with a high partial pressure of Na such as would be present with dust enrichments of ≈ 103. There was no detectable metamorphic, nebular or aqueous alteration. In previous work Ogliore et al. (2012) reported that Iris formed late, > 3 Myr after CAIs, assuming 26Al was homogenously distributed, and was rich in heavy oxygen. Iris may be similar to assemblages found only in interplanetary dust particles and Stardust cometary samples called Kool particles. Callie is chemically and isotopically very similar but not identical to Iris.
Abstract Cosmochemists have relied on CI carbonaceous chondrites as proxies for chemical composition of the non‐volatile elements in the solar system because these meteorites are fine‐grained, chemically homogeneous, and have well‐determined bulk compositions that agree with that of the solar photosphere, within uncertainties. Here we report the discovery of a calcium‐aluminum‐rich inclusion (CAI) in the Ivuna CI chondrite. CAIs are chemically highly fractionated compared to CI composition, consisting of refractory elements and having textures that either reflect condensation from nebular gas or melting in a nebular environment. The CAI we found is a compact type A CAI with typical 16 O‐rich oxygen. However, it shows no evidence of 26 Al, which was present when most CAIs formed. Finding a CAI in a CI chondrite raises serious questions about whether CI chondrites are a reliable proxy for the bulk composition of the solar system. Too much CAI material would show up as mismatches between the CI composition and the composition of the solar photosphere. Although small amounts of refractory material have previously been identified in CI chondrites, this material is not abundant enough to significantly perturb the bulk compositions of CI chondrites. The agreement between the composition of the solar photosphere and CI chondrites allows no more than ~0.5 atom% of CAI‐like material to have been added to CI chondrites. As the compositions of CI chondrites, carbonaceous asteroids, and the solar photosphere are better determined, we will be able to reduce the uncertainties in our estimates of the composition of the solar system.
Abstract– We review recent results on O‐ and Mg‐isotope compositions of refractory grains (corundum, hibonite) and calcium, aluminum‐rich inclusions (CAIs) from unequilibrated ordinary and carbonaceous chondrites. We show that these refractory objects originated in the presence of nebular gas enriched in 16 O to varying degrees relative to the standard mean ocean water value: the Δ 17 O SMOW value ranges from approximately −16‰ to −35‰, and recorded heterogeneous distribution of 26 Al in their formation region: the inferred ( 26 Al/ 27 Al) 0 ranges from approximately 6.5 × 10 −5 to <2 × 10 −6 . There is no correlation between O‐ and Mg‐isotope compositions of the refractory objects: 26 Al‐rich and 26 Al‐poor refractory objects have similar O‐isotope compositions. We suggest that 26 Al was injected into the 26 Al‐poor collapsing protosolar molecular cloud core, possibly by a wind from a neighboring massive star, and was later homogenized in the protoplanetary disk by radial mixing, possibly at the canonical value of 26 Al/ 27 Al ratio (approximately 5 × 10 −5 ). The 26 Al‐rich and 26 Al‐poor refractory grains and inclusions represent different generations of refractory objects, which formed prior to and during the injection and homogenization of 26 Al. Thus, the duration of formation of refractory grains and CAIs cannot be inferred from their 26 Al‐ 26 Mg systematics, and the canonical ( 26 Al/ 27 Al) 0 does not represent the initial abundance of 26 Al in the solar system; instead, it may or may not represent the average abundance of 26 Al in the fully formed disk. The latter depends on the formation time of CAIs with the canonical 26 Al/ 27 Al ratio relative to the timing of complete delivery of stellar 26 Al to the solar system, and the degree of its subsequent homogenization in the disk. The injection of material containing 26 Al resulted in no observable changes in O‐isotope composition of the solar system. Instead, the variations in O‐isotope compositions between individual CAIs indicate that O‐isotope composition of the CAI‐forming region varied, because of coexisting of 16 O‐rich and 16 O‐poor nebular reservoirs (gaseous and/or solid) at the birth of the solar system, or because of rapid changes in the O‐isotope compositions of these reservoirs with time, e.g., due to CO self‐shielding in the disk.
