Extensive Thermal Metamorphism Research Articles

The iron oxidation state of Ryugu samples

AbstractThe Hayabusa2 mission sampled Ryugu, an asteroid that did not suffer extensive thermal metamorphism, and returned rocks to the Earth with no significant air exposure. It therefore offers a unique opportunity to study the redox state of carbonaceous Cb‐type asteroids and evaluate the overall redox state of the most primitive rocks of the solar system. An analytical framework was developed to investigate the iron mineralogy and valence state in extraterrestrial material at the micron scale by combining x‐ray diffraction, conventional Mössbauer (MS), and nuclear forward scattering (NFS) spectroscopies. An array of standard minerals was analyzed and cross‐calibrated between MS and NFS. Then, MS and NFS spectra on three Ryugu grains were collected at the bulk and the micron scales. In Ryugu samples, iron is essentially accommodated in magnetite, clay minerals (serpentine–smectite), and sulfides. Only a single set of Mössbauer parameters was necessary to account for the entire variability observed in MS and NFS spectra, at all spatial scales investigated. These parameters therefore make up a fully consistent iron mineralogical model for the Ryugu samples. As far as MS and NFS spectroscopies are concerned, Ryugu grains are overall similar to each other and share most of their mineralogical features with CI‐type chondrites. In detail however, no ferrihydrite is found in Ryugu particles even at the very sensitive scale of Mössbauer spectroscopy. The typical Fe3+/Fetot of clay minerals is much lower than typical redox ratios measured in CI chondrites (Fe3+/Fetot = 85%–90%). Furthermore, magnetite from Ryugu is stoichiometric with no significant maghemite component, whereas up to 12% of maghemite was previously identified in the Orgueil's so‐called magnetite. These differences suggest that most CI meteorites suffered terrestrial alteration and that the preterrestrial composition of these carbon‐rich samples was less oxidized than previously measured. However, it is not clear yet whether or not the parent bodies of CI chondrites were as reduced as Ryugu. Finally, the high spatial resolution of NFS allows to disentangle the redox state and the crystal chemistry of iron accommodated in serpentine and smectite. The most likely polytype of serpentine is lizardite, containing <35% of Fe3+, a fraction of which being tetrahedrally coordinated. Smectite is more oxidized (Fe3+/Fetot > 65%) and mainly contains octahedral ferric iron. This finding implies that these clays formed from highly alkaline fluids and the spatial variability highlighted here may suggest a temporal evolution or a spatial variability of the nature of this fluid.

Pristinity and petrogenesis of eucrites

AbstractNew petrography, mineral chemistry, and whole rock major, minor, and trace element abundance data are reported for 29 dominantly unbrecciated basaltic (noncumulate) eucrites and one cumulate eucrite. Among unbrecciated samples, several exhibit shock darkening and impact melt veins, with incomplete preservation of primary textures. There is extensive thermal metamorphism of some eucrites, consistent with prior work. A “pristinity filter” of textural information, siderophile element abundances, and Ni/Co ratios of bulk rocks is used to address whether eucrite samples preserve endogenous refractory geochemical signatures of their asteroid parent body (i.e., Vesta), or could have experienced exogenous impact contamination. Based on these criteria, Cumulus Hills 04049, Elephant Moraine 90020, Grosvenor Range 95533, Pecora Escarpment 91245, and possibly Queen Alexander Range 97053 and Northwest Africa 1923 are pristine eucrites. Eucrite major element compositions and refractory incompatible trace element abundances are minimally affected by metamorphism or impact contamination. Eucrite petrogenesis examined through the lens of these elements is consistent with partial melting of a silicate mantle that experienced prior metal–silicate equilibrium, rather than as melts associated with cumulate diogenites. In the absence of the requirement of a large‐scale magma ocean to explain eucrite petrogenesis, the interior structure of Vesta could be more heterogeneous than for larger planetary bodies.

Mineralogy, geochemistry, and evolution of the Mivehrood skarn and the associated pluton, northwest Iran

An Upper Miocene (9.12 ± 0.19 Ma; biotite 40Ar/39Ar) shallow pluton and numerous dykes and sills of felsic–intermediate compositions intruded the Upper Cretaceous, flysch-type sediments in the Mivehrood area, northwest Iran. The intrusions caused extensive thermal metamorphism and metasomatism, leading to the formation of hornfels and skarn. A massive skarn, 1–10 m thick, immediate to the intrusive contact, is bordered by a banded skarn, 100–400 m thick, that grades outward into hornfels and original sediments. The Mivehrood pluton is characterized by steep REE pattern, high Al2O3 (14.64–16.4 wt%) and Sr (380–786 ppm), and low MgO (1.3–3.4 wt%), Y (4.8–10.7 ppm), and Yb (0.35–0.95 ppm), characteristics typical of high-silica adakites. Skarn formation started with thermal metamorphism, followed by anhydrous prograde and hydrous retrograde stages. Prograde and retrograde mineral assemblages are developed in both skarns, represented by garnet–clinopyroxene–wollastonite and epidote–actinolite–scapolite–chlorite, respectively. Granditic F-bearing garnet dominates clinopyroxene in both skarns. The banded skarn contains minor scapolite of marialite composition. The calc-silicate mineral assemblages and the mineral chemistry allow the Mivehrood skarn to be classified as a calcic, oxidized skarn. Mass balance assessments suggest that Fe, Si, and S were significantly enriched, and Na, LILEs, and LREEs were strongly depleted, in the massive skarn. In the banded skarn, Na, K, Si, and S were enriched. Significant dehydration and carbon degassing occurred in both skarns. Stockworks, veins, and replacement bodies of pyrite ± chalcopyrite locally occur in the pluton and the dykes and in the skarns. The δ34SCDT values for the sulfides fall in a narrow range around 0.0 ‰, suggesting a magmatic source for sulfur and possibly the hydrothermal water and solutes involved in the skarn formation.

Petrography, stable isotope compositions, microRaman spectroscopy, and presolar components of Roberts Massif 04133: A reduced CV3 carbonaceous chondrite.

Here, we report the mineralogy, petrography, C-N-O-stable isotope compositions, degree of disorder of organic matter, and abundances of presolar components of the chondrite Roberts Massif (RBT) 04133 using a coordinated, multitechnique approach. The results of this study are inconsistent with its initial classification as a Renazzo-like carbonaceous chondrite, and strongly support RBT 04133 being a brecciated, reduced petrologic type >3.3 Vigarano-like carbonaceous (CV) chondrite. RBT 04133 shows no evidence for aqueous alteration. However, it is mildly thermally altered (up to approximately 440 °C); which is apparent in its whole-rock C and N isotopic compositions, the degree of disorder of C in insoluble organic matter, low presolar grain abundances, minor element compositions of Fe,Ni metal, chromite compositions and morphologies, and the presence of unequilibrated silicates. Sulfides within type I chondrules from RBT 04133 appear to be pre-accretionary (i.e., did not form via aqueous alteration), providing further evidence that some sulfide minerals formed prior to accretion of the CV chondrite parent body. The thin section studied contains two reduced CV3 lithologies, one of which appears to be more thermally metamorphosed, indicating that RBT 04133, like several other CV chondrites, is a breccia and thus experienced impact processing. Linear foliation of chondrules was not observed implying that RBT 04133 did not experience high velocity impacts that could lead to extensive thermal metamorphism. Presolar silicates are still present in RBT 04133, although presolar SiC grain abundances are very low, indicating that the progressive destruction or modification of presolar SiC grains begins before presolar silicate grains are completely unidentifiable.

QUE 94204: A primitive enstatite achondrite produced by the partial melting of an E chondrite‐like protolith

Abstract– Queen Alexandra Range (QUE) 94204, an enstatite achondrite, is a coarse‐grained, highly recrystallized, chondrule‐free and unbrecciated rock dominated (about 70 vol%) by anhedral, equigranular crystals of orthoenstatite of nearly endmember composition (Fs0.1–0.4, Wo0.3–0.4) with interstitial plagioclase, kamacite, and troilite. Abundance of approximately 120° triple junctions and the close association of metal–sulfide and plagioclase‐rich melts indicate that QUE 94204 has undergone limited partial melting with inefficient melt extraction. Mineral chemistry indicates a high degree of thermal metamorphism. Kamacite in QUE 94204 contains between 2.09 and 2.55 wt% Si, similar to highly metamorphosed EL chondrites. Plagioclase has between 4.31 and 6.66 wt% CaO, higher than other E chondrites but closer in composition to plagioclase from metamorphosed EL chondrites. QUE 94204 troilite contains up to 2.55 wt% Ti, consistent with extensive thermal metamorphism of an E chondrite‐like precursor. Results presented in this study indicate that QUE 94204 is the result of low degree, (about 5–20 vol%, probably toward the lower end of this range) partial melting of an E chondrite protolith. Textural and chemical evidence suggests that during the metamorphism of QUE 94204, melts formed first at the Fe,Ni‐FeS cotectic near approximately 900 °C, followed by plagioclase‐pyroxene silicate partial melts near approximately 1100 °C. Neither the Fe,Ni‐FeS nor the plagioclase‐pyroxene melts were efficiently segregated or extracted. QUE 94204 belongs to a grouplet of similar “primitive enstatite achondrites” that are analogous to the acapulcoites‐lodranites, but that have resulted from the partial melting of an E chondrite‐like protolith.

Compositional diversity in insoluble organic matter in type 1, 2 and 3 chondrites as detected by infrared spectroscopy

Insoluble organic matter (IOM) isolated from 22 carbonaceous and ordinary chondrites spanning a wide range of groups and petrologic types were analyzed using Fourier transform infrared spectroscopy (FTIR). Based on common IR spectral features, it is observed that IOM falls into 4 molecularly distinct groups (designated here as A through D). Spectral group A includes type 1 and 2 chondrites and exhibits intense aliphatic C–H and carboxyl vibrational peaks. Spectral group B includes the least metamorphosed type 3 chondrites and Tagish Lake, and exhibits weaker aliphatic and carboxyl vibrational intensity. Spectral groups C and D include metamorphosed type ⩾3.1 chondrites and a heated CM chondrite. The carbonyl stretching features in spectral groups C and D differ from that in spectral groups A and B and from each other. In spectral group C, the carbonyl stretching is assigned to cyclic unsaturated lactones; in spectral group D carbonyl exists predominantly in the form of unsaturated ketone moieties. Both spectral groups C and D have a relatively narrow band structure around 1210 cm −1 (assigned to aromatic skeletal modes) as compared with spectral groups A and B, which is consistent with the formation of more condensed aromatics by extensive thermal metamorphism. The differences in carbonyl structures in spectral groups C and D are not the result of different effective metamorphic temperatures, rather these differences likely result from variation in the activity of water and oxygen at different stages of parent body metamorphism. Such environmental variations must be local phenomena in the parent bodies as there is no correlation between spectral grouping and chondrite class or group.

Hammadah al Hamra 193: The first amphibole-bearing winonaite

The Hammadah al Hamra 193 winonaite was found in the Libyan desert in 1996. Unlike most winonaites with fine- to medium-grained equigranular textures, it consists predominantly of very large (up to 5 mm), optically continuous orthopyroxene grains enclosing smaller grains of olivine and plagioclase. It also contains large (up to 2 mm) poikilitic grains of amphibole enclosing clinopyroxene, plagioclase, olivine, and occasionally orthopyroxene, which occur interstitial to the large orthopyroxene grains. The amphibole is identified as fluoro-edenite, and textures indicate that it replaces clinopyroxene via a reaction in which diopside, olivine, and plagioclase form fluoro-edenite. Trace-element data are consistent with the formation of fluoro-edenite from clinopyroxene and plagioclase. Fluoro-edenite has a REE pattern similar to that of clinopyroxene, but has elevated abundances of Na, K, and Ba, elements typically enriched in plagioclase. The source of the F is uncertain, but may be apatite, which is fluor-apatite in this meteorite. The presence of fluoro-edenite in HaH 193, a meteorite that experienced extensive thermal metamorphism, indicates a significant stability field for this rare mineral.

Petrology of the Indian eucrite Piplia Kalan

Abstract— Piplia Kalan is an equilibrated eucrite consisting of 60–80 vol% lithic clasts in a subordinate brecciated matrix. Ophitic/subophitic clasts fall into two groups: finer‐grained lithology A and coarser‐grained lithology B. Very fine‐grained clasts with equigranular textures (lithology C) also occur and originally were hypocrystalline in texture. The variety of materials represented in Piplia Kalan suggests cooling histories ranging from quenching to slower crystallization. Despite textural differences, clasts and matrix have similar mineral and bulk compositions. Thus, Piplia Kalan is probably best classified as a genomict breccia that could represent fragments of a single lava flow or shallow intrusive body, including fine‐grained or glassy outer margin and more slowly cooled coarser‐grained interior. Bulk composition suggests that the meteorite is most closely related to the main group eucrites, but it probably was affected by minor amounts of fractional crystallization. Piplia Kalan displays evidence of an early shock event, including brecciated matrix and areas of lithic clasts that contain very fine‐grained, granular pyroxene between deformed feldspar laths. The meteorite also displays evidence of at least one episode of extensive thermal metamorphism: hypocrystalline materials are recrystallized to hornfelsic textures and minerals throughout the meteorite contain abundant inclusions that are relatively large in size. Veins of brown glass transect both clasts and matrix and indicate a second, postmetamorphism shock event.

Syncontractional Crustal Anatexis and Deformation during Emplacement of ~1435 Ma Plutons, Western Needle Mountains, Colorado

Mesoproterozoic deformation and biomodal magmatism in southwest Colorado offer important insight into the nature and tectonic regime of 1.4 Ga magmatism in the western United States. We document syncontractional emplacement of coeval gabbroic and granitic plutons in the middle crust at ~1435 Ma in the western Needle Mountains. This is incompatible with extension-dominated "anorogenic" models proposed for the generation of 1.4 Ga plutonic rocks in the Southwest but lends further support for synkinematic intracontinental magmatism in response to regional tectonic strains created by transpressive or compressive plate margin interactions between 1500-1300 Ma. Intrustion of the ~1435 Ma Electra Lake Gabbro and Eolus Granite into Paleoproterozoic rocks caused local crustal anatexis and extensive thermal metamorphism in the Needle Mountains. Deformation of Paleoproterozoic country rocks at ~1435 Ma is widespread but is concentrated in pluton aureoles where thermally softened crust accommodated the strain. We interpret the kinematic framework of structural elements in the deformed pluton aureoles to indicate a regional synmagmatic strain field involving subhorizontal N-S contraction and E-W extension. This is reflected by brittle fracture arrays and conjugate ductile shear bands filled with remelted trondhjemitic gneiss, transposed amphibolite layers and ~1435 Ma granite dikes, local solid-state ductile deformation in ~1435 Ma intrusive rocks, broad-scale folding of migmatites, and trends of ~1435 Ma granitic and gabbroic dikes. Aureole metamorphism and kinematics of deformation recorded in the Needle Mountains at ~1435 Ma are similar to those that are documented in other 1.4 Ga plutonic complexes throughout Colorado, Arizona, and New Mexico. This regional record argues for a widespread episode of synorogenic magmatism in the southwestern United States at 1.4 Ga.

A comparative petrologic study of iron meteorites with 107 Ag ∗ anomalies

We have made a petrographic study of the iron meteorites: Derrick Peak (IIB), Cape York (IIIA), Grant (IIIB), Gibeon (IVA) and Santa Clara (IVB). Each of these meteorites has been reported to contain excess 107Ag from the decay of now extinct 107Pd. Isotopic data on metal and sulfide phases for groups IIB and IIIAB iron meteorites have been explained using conventional interpretations which show a correlation between 107Ag 109Ag and Pd/Ag while data on sulfide nodules in meteorites from groups IVA and IVB are very complex. We have studied the extent to which shock and thermal events may have caused the redistribution of Ag and disturbed the original relationship between 107Ag ∗ and Pd, particularly in sulfide-rich regions. The petrography of the iron meteorites studied is consistent with the Pd-Ag isotopic data. The textures observed in the metal and sulfide in groups IIA and IIIAB were found to be indicative of shock but not extensive thermal metamorphism. The contacts between metal and sulfide are sharp and the sulfide and metal have remained discrete homogeneous phases. These meteorites yield isochrons between metal and sulfide and it is concluded that no significant Ag redistribution occurred after their formation. The petrography of group IVA and IVB meteorites reveals a history involving extensive shock and thermal metamorphism. Contacts between metal and sulfide nodules are irregular and the two phases are intimately mixed. In Gibeon, unequilibrated α 2-martensitic structures in the metal matrix suggest that some reheating affected entire individual fragments of this meteorite. While bulk samples of the metal phases of the IVA and IVB meteorites appear well behaved with regard to Pd-Ag systematics, the sulfide-rich regions show extreme isotopic heterogeneities and do not yield well-defined isochrons. The Pd-Ag data reflect complicated postformation Ag transport. This petrographic study has found that the complexity of Pd-Ag isotopic data is related to the shock and thermal histories of the iron meteorites. However, no self-consistent explanation has been found to explain the Pd-Ag results observed for the nodules. The mineral phase(s) in which Ag resides in both the metal and sulfide phases is as yet undetermined, though late-forming Cu-precipitates and chalcopyrite found in IIIAB irons are possible getters of volatiles (Ag, Au) and may be host to 107Ag ∗.

Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites

Precise determination of REE and Ba abundances in three carbonaceous (Orgueil Cl, Murchison C2 and Allende C3) and seven olivine-bronzite chondrites were carried out by mass spectrometric isotope dilution technique. Replicate analyses of standard rock and the three carbonaceous chondrites demonstrated the high quality of the analyses (accuracies for REE are ±1–2 per cent). Certain carbonaceous chondrite specimens showed small positive irregularities in Yb abundance. The Yb ‘anomaly’ (approximately + 5 per cent relative to the average of 10 ordinary chondrites) in Orgueil may relate to high temperature components. The REE pattern of Guareña (H6) exhibits comparatively extensive fractionation (about factor 2) with a negative anomaly for Eu (17 ± 1 per cent ) compared to the average H chondrite. This could be interpreted in terms of extensive thermal metamorphism leading to melting. Apart from absolute abundance differences, there appears to be small but recognizable fractionation among the average relative REE abundances of Cl, E, H and L chondrites. However, individual chondrites within these groups showed more or less fractionated REE patterns relative to each other. The distinction between H and L chondrites was well demonstrated in Eu-Sm correlation curves and absolute abundance differences of REE and major elements. Si-normalized atomic ratios of the REE abundances in different kinds of chondrites to those in Orgueil (Cl) chondrite were 0.58 (E), 0.75 (H), 0.81 (L), 1.07 (C2) and 1.32 (C3).

Bushveld Igneous Complex, South Africa: Absence of Shock-Metamorphic Effects in a Preliminary Search

A field and laboratory search for shock-metamorphic features associated with the Bushveld Igneous Complex, South Africa, was made to test the hypothesis that the structure formed by meteorite impact. Samples collected from inliers of basement rock within the complex (the Assen and Marble Hall fragments) and from the Rooiberg series exhibit no megascopic or microscopic shock effects. In particular, shatter cones, Sudbury-type breccias ( = Vredefort pseudotachylite), and planar shock lamellae in quartz are absent. Any shock-metamorphic features originally present have probably been obscured by the emplacement of the widespread Bushveld igneous rocks and by the resulting extensive thermal metamorphism of both the basement rocks and the Rooiberg series. More extensive sampling is needed to test the impact hypothesis adequately.