Comparative Study on the Origin and Characteristics of Chinese (Manas) and Russian (East Sayan) Green Nephrites

Green nephrites are widely pursued for their mild texture and vivid color. In recent years, many Russian green nephrites appeared in China (the world’s largest nephrite market) and competed with the Chinese Manas green nephrites, which are traditionally highly valued. In this study, we compared the appearance, mineralogy and geochemical features (with EPMA and LA-ICP-MS) of Chinese (Manas) and Russian (East Sayan) green nephrites to objectively characterize and distinguish between these two nephrites. Chinese (Manas) and Russian (East Sayan) green nephrites are mined from serpentinized ultramafic units in the northern Tian Shan and East Sayan orogen, respectively. In terms of appearance, the Manas green nephrites are slightly bluish or grayish, whilst their East Sayan counterparts are brighter (duck-egg cyan). The Manas nephrites commonly have a caramel color, crumple structure, characteristic white globules and sinuous veins, green stains and yellow–green veins, together with a local fibrous structure. The East Sayan green nephrites are more transparent, with a gentler fine texture, uniform color, many black spots and a few green spots. Some green nephrites from the Arahushun mine of East Sayan have an ice-like appearance. Microscopic petrography and EPMA analysis indicate that both the Manas and East Sayan green nephrites comprise mainly tremolite with minor actinolite. Minor minerals in the Manas samples include chromite, chlorite-group minerals, and uvarovite; whilst those in the East Sayan samples include actinolite, chromite, chlorite-group minerals, and bornite. Bornite is not found in any other sources of green nephrite, and thus is characteristic of Russian (East Sayan) green nephrites. LA-ICP-MS trace element data in their amphiboles and Single-Factor Analysis of Variance (ANOVA) results suggest that the differences in Cr, Zn, Y, Ba, and Sr contents and values of δEu, Eu/Sm, (La/Yb)N, (La/Sm)N, (Gd/Yb)N, ∑HREE, ∑LREE/∑HREE between the two nephrites are present, and can be used as their origin trace.

Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Assembling Dacite in a Continental Subduction Zone: A Case Study of Tauhara Volcano

<p>Mount Tauhara is the largest dacitic volcanic complex of onshore New Zealand and comprises seven subaerial domes and associated lava and pyroclastic flows, with a total exposed volume of ca. 1 km3. The dacites have a complex petrography including quartz, plagioclase, amphibole, orthopyroxene, clinopyroxene, olivine and Fe‐Ti oxides and offer an excellent opportunity to investigate the processes and timescales involved in assembling dacitic magma bodies in a continental subduction zone with in situ and mineral specific analytical techniques. Whole rock major and trace element data and Pb isotopes ratios define linear relationships indicating that the dacites are generated by mixing of silicic and mafic magmas. Two groups of samples define separate mixing trends between four endmembers on the basis of La/Yb ratios, 87Sr/86Sr ratios and Sr contents. The older Western and Central Domes have low 87Sr/86Sr (0.7042‐0.7046) and high LREE/HREE (LaN/YbN = 8.0‐11.5) and Sr (380‐650 ppm) compared to the younger Hipaua, Trig M, Breached and Main Domes, which have higher 87Sr/86Sr (0.7047‐0.7052) and lower LREE/HREE (LaN/YbN = 6.5‐7.5) and Sr (180‐400 ppm). In situ mineral major and trace element chemistry of mineral phases, as well as Sr and Pb isotope ratios of mineral separates have been used to: (i) fingerprint the origin of each crystal phase; (ii) constrain the chemistry of the four endmembers involved in the mixing events and; (iii) estimate the timing of mixing relative to eruption and the ascent rate of the dacitic magmas. The presence of quartz and analyses of quartz‐hosted melt inclusions are used to fingerprint the chemistry of the silicic endmembers, which is a rhyolitic melt with a major element chemistry similar to that of either the Whakamaru Group Ignimbrite melts (Western, Central and Trig M Domes) or intermediate between that of the Whakamaru and the Oruanui Ignimbrite melts (Hipaua, Breached and Main Domes). Similarly, Ba‐Sr concentrations and Sr isotopic signatures of plagioclase show that this phenocryst phase also predominantly crystallized from the rhyolitic melt. Variations in the Mg# and trace element chemistry of clinopyroxenes suggest they were formed both in the mixed dacitic melts and in a mafic endmember. The chemistry of the mafic endmembers have been traced using a combination of back‐calculated Sr melt concentrations from clinopyroxene with the highest Mg# in each sample group, and the linear trends between whole rock SiO2 content and most elements. These results indicate that dacites erupted from the Western and Central Dome were generated by the mixing of a high alumina basalt and a rhyolitic melt and Trig M Dome dacites were generated by the mixing of an andesite with a rhyolitic melt. Magmas erupted from Hipaua, Breached and Main Domes were also produced by the mixing of an andesitic melt and a rhyolitic body with a composition intermediate between that of the Whakamaru and the Oruanui melt bodies. Trace element data and 87Sr/86Sr ratios of amphibole demonstrate that it crystallized from the mixed dacitic melt. Thermobarometric conditions obtained from amphibole indicate that the magma mixing event that produced the dacites occurred within a magma chamber located at ca. 9 km depth and ca. 900°C with the exception of Trig M Dome which occurred deeper at 13 km and 950°C. Diffusion profiles of Ti in quartz and Fe‐Mg in clinopyroxene indicate the magma mixing events occurred < 6 months prior to eruption. Amphibole reaction rims show the magma to have ascended over 2‐3 weeks for each dome, with the exception of Main Dome where reaction rims were not present in the amphibole, suggesting the ascent rate was faster than 0.2 m/s (< 6 hours).</p>

Assembling Dacite in a Continental Subduction Zone: A Case Study of Tauhara Volcano

<p>Mount Tauhara is the largest dacitic volcanic complex of onshore New Zealand and comprises seven subaerial domes and associated lava and pyroclastic flows, with a total exposed volume of ca. 1 km3. The dacites have a complex petrography including quartz, plagioclase, amphibole, orthopyroxene, clinopyroxene, olivine and Fe‐Ti oxides and offer an excellent opportunity to investigate the processes and timescales involved in assembling dacitic magma bodies in a continental subduction zone with in situ and mineral specific analytical techniques. Whole rock major and trace element data and Pb isotopes ratios define linear relationships indicating that the dacites are generated by mixing of silicic and mafic magmas. Two groups of samples define separate mixing trends between four endmembers on the basis of La/Yb ratios, 87Sr/86Sr ratios and Sr contents. The older Western and Central Domes have low 87Sr/86Sr (0.7042‐0.7046) and high LREE/HREE (LaN/YbN = 8.0‐11.5) and Sr (380‐650 ppm) compared to the younger Hipaua, Trig M, Breached and Main Domes, which have higher 87Sr/86Sr (0.7047‐0.7052) and lower LREE/HREE (LaN/YbN = 6.5‐7.5) and Sr (180‐400 ppm). In situ mineral major and trace element chemistry of mineral phases, as well as Sr and Pb isotope ratios of mineral separates have been used to: (i) fingerprint the origin of each crystal phase; (ii) constrain the chemistry of the four endmembers involved in the mixing events and; (iii) estimate the timing of mixing relative to eruption and the ascent rate of the dacitic magmas. The presence of quartz and analyses of quartz‐hosted melt inclusions are used to fingerprint the chemistry of the silicic endmembers, which is a rhyolitic melt with a major element chemistry similar to that of either the Whakamaru Group Ignimbrite melts (Western, Central and Trig M Domes) or intermediate between that of the Whakamaru and the Oruanui Ignimbrite melts (Hipaua, Breached and Main Domes). Similarly, Ba‐Sr concentrations and Sr isotopic signatures of plagioclase show that this phenocryst phase also predominantly crystallized from the rhyolitic melt. Variations in the Mg# and trace element chemistry of clinopyroxenes suggest they were formed both in the mixed dacitic melts and in a mafic endmember. The chemistry of the mafic endmembers have been traced using a combination of back‐calculated Sr melt concentrations from clinopyroxene with the highest Mg# in each sample group, and the linear trends between whole rock SiO2 content and most elements. These results indicate that dacites erupted from the Western and Central Dome were generated by the mixing of a high alumina basalt and a rhyolitic melt and Trig M Dome dacites were generated by the mixing of an andesite with a rhyolitic melt. Magmas erupted from Hipaua, Breached and Main Domes were also produced by the mixing of an andesitic melt and a rhyolitic body with a composition intermediate between that of the Whakamaru and the Oruanui melt bodies. Trace element data and 87Sr/86Sr ratios of amphibole demonstrate that it crystallized from the mixed dacitic melt. Thermobarometric conditions obtained from amphibole indicate that the magma mixing event that produced the dacites occurred within a magma chamber located at ca. 9 km depth and ca. 900°C with the exception of Trig M Dome which occurred deeper at 13 km and 950°C. Diffusion profiles of Ti in quartz and Fe‐Mg in clinopyroxene indicate the magma mixing events occurred < 6 months prior to eruption. Amphibole reaction rims show the magma to have ascended over 2‐3 weeks for each dome, with the exception of Main Dome where reaction rims were not present in the amphibole, suggesting the ascent rate was faster than 0.2 m/s (< 6 hours).</p>

Ore and Geochemical Specialization and Substance Sources of the Ural and Timan Carbonatite Complexes (Russia): Insights from Trace Element, Rb–Sr, and Sm–Nd Isotope Data

The Ilmeno–Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are localized in the folded regions of the Urals and Timan. These complexes differ in geochemical signatures and ore specialization: Nb-deposits of pyrochlore carbonatites are associated with the IVC, while Nb–REE-deposits with the Buldym complex and REE-deposits of bastnäsite carbonatites with the Chetlassky complex. A comparative study of these carbonatite complexes has been conducted in order to establish the reasons for their ore specialization and their sources. The IVC is characterized by low 87Sr/86Sri (0.70336–0.70399) and εNd (+2 to +6), suggesting a single moderately depleted mantle source for rocks and pyrochlore mineralization. The Buldym complex has a higher 87Sr/86Sri (0.70440–0.70513) with negative εNd (−0.2 to −3), which corresponds to enriched mantle source EMI-type. The REE carbonatites of the Chetlassky сomplex show low 87Sr/86Sri (0.70336–0.70369) and a high εNd (+5–+6), which is close to the DM mantle source with ~5% marine sedimentary component. Based on Sr–Nd isotope signatures, major, and trace element data, we assume that the different ore specialization of Urals and Timan carbonatites may be caused not only by crustal evolution of alkaline-carbonatite magmas, but also by the heterogeneity of their mantle sources associated with different degrees of enrichment in recycled components.

Supplemental Material: Metamorphism and geochronology of high-pressure mafic granulites (retrograded eclogites?) in East Cathaysia terrane of South China: Implications for Mesozoic tectonic evolution

Supplemental tables contain the whole-rock and mineral compositions, PT estimates, zircon SIMS Th-U-Pb isotopic data and trace element data.

Supplemental Material: Metamorphism and geochronology of high-pressure mafic granulites (retrograded eclogites?) in East Cathaysia terrane of South China: Implications for Mesozoic tectonic evolution

Supplemental tables contain the whole-rock and mineral compositions, PT estimates, zircon SIMS Th-U-Pb isotopic data and trace element data.