Ai-Cheng Zhang

Chemical evolution of Nb-Ta oxides and zircon from the Koktokay No. 3 granitic pegmatite, Altai, northwestern China

Abstract The Koktokay No. 3 granitic pegmatite, Altai, northwestern China, is a strongly zoned rare-element granitic pegmatite, where the petrographic zones were distinguished into two groups: outer zones (I to IV) and inner zones (V to IX). Nb-Ta oxides and zircon are investigated in this paper by using quantitative electron-microprobe analyses (EMPA) and backscattered-electron (BSE) imaging. Columbite-tantalite and zircon occur in most textural zones, whereas tapiolite and uranmicrolite are mainly restricted to zone VII. Manganocolumbite and zircon from the outer zones (zones II and IV) are homogeneous except for a few exceptions, whereas manganotantalite and hafnian zircon from the inner zones (V-VII) are obviously heterogeneous and strongly zoned. Chemically, Ta/(Nb+Ta) in columbitetantalite and Hf/(Zr+Hf) in zircon increase from the outer to the inner zones on one hand, and from core to rim in single zoned crystals on the other hand. Observations of intra-zonal variations of the chemical composition of Nb-Ta oxides and zircon in the Koktokay No. 3 granitic pegmatite may suggest that the outer zones crystallize under magmatic conditions, whereas the inner zones crystallize under fluid-rich magmatic conditions, and locally under hydrothermal conditions. The extreme enrichments of columbite-tantalite in Ta, and of zircon in Hf, as well as the occurrence of uranmicrolite and tapiolite, indicate an elevated evolution of the Koktokay No. 3 granitic pegmatite.

Pyroxene polymorphs in melt veins of the heavily shocked Sixiangkou L6 chondrite

Sixiangkou is a heavily shocked L6 chondrite that contains numerous shock-induced melt veins with varying widths. This paper focuses on pyroxene polymorphs in shock-induced melt veins in the meteorite and discusses their implication to the high pressure and temperature conditions during impact metamorphism. The major high-pressure polymorphs are majorite and majorite-pyrope solid solution. Diopside, akimotoite and jadeite were also observed in melt veins of the Sixiangkou chondrite. Diopside coexists with ringwoodite (+ akimotoite) in coarse-grained fragments of the melt veins. Two mechanisms were proposed for the lack of phase transformation of diopside: a) low local temperature and b) a very sluggish transformation rate. Majorite and majorite-pyrope occur as coarse-grained polycrystalline and fine-grained solid solution. They were formed through solid state transformation and crystallized from a melt under pressures, respectively. Akimotoite coexists with low-Ca pyroxene in coarse-grained fragments of melt veins. They have nearly identical chemical compositions to the host low-Ca pyroxene, implying a solid state transformation mechanism. Jadeite appeared as fine-grained and was originated through retrograde transformation of lingunite under moderate post-shock pressure and high temperature conditions. The coexistence of different high-pressure pyroxene polymorphs reflects an inhomogeneous distribution of temperature in the shock-induced melt veins and a kinetic effect of phase transformations of minerals during shock metamorphism.

Extreme fractionation from zircon to hafnon in the Koktokay No. 1 granitic pegmatite, Altai, northwestern China

Abstract The Koktokay No. 1 pegmatite is a Li-Cs-Ta-rich granitic pegmatite located in Altai, northwestern China. Zircon is present in most textural zones of this pegmatite and in its contact zone with surrounding metagabbro. Here we describe the detailed associations of zircon with other minerals, and the internal textures and chemistry of the zircons. Most zircon grains from the contact zone have relatively low HfO2 (<9.4 wt%), whereas the bright rim of one such grain has high HfO2 (18.0-18.7 wt%). Zircon grains from the aplite zone contain <9.6 wt% HfO2, although their thin and bright rims have higher HfO2 (10.8-13.0 wt%). Most zircon grains from the quartz-muscovite zone have complex internal textures and have HfO2 contents of <13.0 wt%. However, zircon grains from localized, nest-like, muscovite aggregates are highly enriched in HfO2 (up to 36.1 wt%). Zircon (sl) from the cleavelandite-quartz-spodumene zone can be divided into two types based on petrography and chemistry. One group of zircons appears to be typical magmatic zircon and are greater than 100 μm in size, closely associated with albite, and have HfO2 contents of 13.0-19.5 wt%. The second group of zircons is typically associated with muscovite and/or spodumene, is small in size (down to a few micrometers), and may exhibit zoning or alteration textures. The HfO2 contents of this second zircon group are 19.8-58.9 wt%, indicating the presence of hafnian zircon to zirconian hafnon. Large HfO2 content variations of up to 34.8 wt% were also observed within single zoned crystals. We suggest that the increase of HfO2 in the magmatic zircon from 9.4 wt% in the contact zone to 19.5 wt% in the cleavelandite-quartz-spodumene zone mainly reflects fractional crystallization of pegmatite magma. However, the occurrence of hafnian zircon and hafnon in the cleavelandite-quartz-spodumene zone is likely related to coupled Li-F fluxing effects in the pegmatite magma.

High-pressure minerals in eucrite suggest a small source crater on Vesta

High-pressure minerals in meteorites are important records of shock events that have affected the surfaces of planets and asteroids. A widespread distribution of impact craters has been observed on the Vestan surface. However, very few high-pressure minerals have been discovered in Howardite-Eucrite-Diogenite (HED) meteorites. Here we present the first evidence of tissintite, vacancy-rich clinopyroxene, and super-silicic garnet in the eucrite Northwest Africa (NWA) 8003. Combined with coesite and stishovite, the presence of these high-pressure minerals and their chemical compositions reveal that solidification of melt veins in NWA 8003 began at a pressure of >~10 GPa and ceased when the pressure dropped to <~8.5 GPa. The shock temperature in the melt veins exceeded 1900 °C. Simulation results show that shock events that create impact craters of ~3 km in diameter (subject to a factor of 2 uncertainty) are associated with sufficiently high pressures to account for the occurrence of the high-pressure minerals observed in NWA 8003. This indicates that HED meteorites containing similar high-pressure minerals should be observed more frequently than previously thought.

Tourmalines from the Koktokay No.3 pegmatite, Altai, NW China: spectroscopic characterization and relationships with the pegmatite evolution

Mossbauer spectra (MS), infrared spectra (IR) in the OH stretching region, and reflectance spectra (RS) were obtained for natural tourmalines from the Koktokay No.3 pegmatite, the contact zone, and the altered country rock, Altai, NW China. Black Fe-rich dravite in the altered country rock and black Fe-rich dravite + schorl-foitite have the lowest degree of Fe 2+ ordering at Y-site and a high Fe 3+ /Fe 2+ ratio (0.35 and 0.30, respectively). Black elbaite-schorl in zones I to IV has a moderate degree of Fe 2+ ordering at Y-site and a low Fe 3+ /Fe 2+ ratio (0.11–0.18). Green elbaite in the inner pegmatite has the highest degree of Fe 2+ ordering at Y-site and contains no ferric iron. Tourmalines from the altered country rock, the contact zone, and zones I to IV of the pegmatite contain only OH − and no molecular water, whereas those from zones V to VIII of the pegmatite contain both OH − and molecular water. Variations of degrees of Fe 2+ ordering at Y-site of tourmaline crystal structure are generally consistent with the magmatic to hydrothermal evolution of the Koktokay No.3 pegmatite. However, the lowest degrees of Fe 2+ ordering at Y-site of tourmalines from the altered country rock and the contact zone are probably caused by rapid consolidation rates. The presence of molecular water in tourmalines from zones V to VIII reflects an increase of water fugacity during fractionation evolution of the pegmatite. The fact that molecular water is absent in tourmaline from the altered country rock and the contact zone suggests that there was a very low water fugacity during the interaction between fluids from the pegmatite and the country rock. The variation of Fe 3+ /Fe 2+ ratio suggests a decreasing oxidation state from the altered country rock across the contact zone through zones I–IV to zones V–VIII of the pegmatite, and generally consistent with previous investigations.

Young asteroidal fluid activity revealed by absolute age from apatite in carbonaceous chondrite.

Chondritic meteorites, consisting of the materials that have formed in the early solar system (ESS), have been affected by late thermal events and fluid activity to various degrees. Determining the timing of fluid activity in ESS is of fundamental importance for understanding the nature, formation, evolution and significance of fluid activity in ESS. Previous investigations have determined the relative ages of fluid activity with short-lived isotope systematics. Here we report an absolute 207Pb/206Pb isochron age (4,450±50 Ma) of apatite from Dar al Gani (DaG) 978, a type ∼3.5, ungrouped carbonaceous chondrite. The petrographic, mineralogical and geochemical features suggest that the apatite in DaG 978 should have formed during metamorphism in the presence of a fluid. Therefore, the apatite age represents an absolute age for fluid activity in an asteroidal setting. An impact event could have provided the heat to activate this young fluid activity in ESS.

Formation of phosphorus-rich olivine in Dar al Gani 978 carbonaceous chondrite through fluid-assisted metamorphism

Abstract Phosphorus-rich olivine (P2O5 > 1 wt%) is a mineral that has been reported only in a few terrestrial and extraterrestrial occurrences. Previous investigations suggest that P-rich olivine mainly forms through rapid crystallization from high-temperature P-rich melts. Here, we report a new occurrence of P-rich olivine in an ungrouped carbonaceous chondrite Dar al Gani (DaG) 978. The P-rich olivine in DaG 978 occurs as lath-shaped grains surrounding low-Ca pyroxene and olivine grains. The lath-shaped olivine shows a large variation in P2O5 (0–5.5 wt%). The P-rich olivine grains occur in a chondrule fragment and is closely associated with chlorapatite, merrillite, FeNi metal, and troilite.Tiny Cr-rich hercynite is present as inclusions within the P-rich olivine. The lath-shaped texture and the association with Cr-rich hercynite indicates that the P-rich olivine in DaG 978 formed by replacing low-Ca pyroxene precursor by a P-rich fluid during a thermal event, rather than by crystallization from a high-temperature melt. The large variation of P2O5 within olivine grains on micrometer-scale indicates a disequilibrium formation process of the P-rich olivine. The occurrence of P-rich olivine in DaG 978 reveals a new formation mechanism of P-rich olivine.