Noble Metals in Arc Basaltic Magmas Worldwide: A Case Study of Modern and Pre-Historic Lavas of the Tolbachik Volcano, Kamchatka

Platinum-group elements (PGE) and gold are a promising tool to assess the processes of mantle melting beneath the subduction zones. However, fractionation processes in magmas inevitably overwrite the initial metal budgets of magmas, making constraints on the melting processes inconclusive. Moreover, little is still known about the geochemical behavior of a particular metal in a single arc magmatic system, from mantle melting towards magma solidification. Here we compare noble metals in lavas from several eruptions of the Tolbachik volcano (Kamchatka arc) to better understand the effects of magma differentiation, estimate primary melt compositions and make constraints on the mantle melting. We show that Ir, Ru, Rh and, to a lesser extent, Pt are compatible during magmatic differentiation. The pronounced incompatible behavior of Cu and Pd, observed in Tolbachik magmas, rules out the significant influence of sulfide melts on the early magmatic evolution in this particular case. Gold is also incompatible during magmatic differentiation; however, its systematics can be affected by the inferred gold recycling in the plumbing system of Tolbachik. Although the Tolbachik lavas show only slightly higher PGE fractionation than in MORB, a notable negative Ru anomaly (higher Pt/Ru and Ir/Ru) is observed. We attribute this to be a result of greater oxidation in the subarc mantle (by 1–4 log units), which promotes crystallization of Ru-bearing phases such as Fe3+-rich Cr-spinel and laurite. The estimated Pd contents for the parental melt of the Tolbachik lavas approaches 6.5 ppb. This is several times higher than reported MORB values (1.5 ± 0.5 ppb), suggesting the enrichment of Pd in the mantle wedge. Our results highlight the influence of the subduction-related processes and mantle wedge refertilization on the noble metal budgets of arc magmas.

High-Precision 26A1-26Mg Systematics of Basaltic Achondrites, Chondrites and Ultramafic Achondrites

<p>A precise and accurate chronology of events that shaped the early Solar System is crucial in understanding its formation. One of the high-resolution chronometers that can be used to establish a relative chronology is the short-lived 26A1-to-26Mg clock (t1/2 = 0.73 Myr). This study developed new Mg chemical separation techniques for complex meteoritic matrices that produces Mg purities > 99% with > 99% yields. Mg was analysed by pseudo-high resolution multiple collector inductively coupled plasma mass spectrometry. These techniques make it possible to measure the mass-independent abundance of 26Mg (d26Mg*) that is related to 26A1 decay to very high-precision (+_ 0.0025 to 0.0050 per1000). These new techniques were then applied to three research objectives. The first part of this study presents Mg isotope data for thirteen bulk basaltic achondrites from at least 3 different parent bodies, as well as mineral isochrons for the angrites Sahara 99555 and D'Orbingy and the ungrouped NWA 2976. Model 26A1-26Mg ages based on bulk rock d26Mg* excesses for basaltic magmatism range from 2.6-4.1 Myr, respectively, after formation of calcium-aluminium-rich inclusions (CAIs) and the mineral isochrons for the angrites Sahara 99555 and D'Orbigny, and the ungrouped NWA 2976 yield apparent crystallisation ages of 5.06+0:06-0:05 Myr and 4.86+0:10-0:09 Myr after CAI formation. The elevated initial d26Mg* of the mineral isochron of NWA 2976 (+0.0175+ _0.0034h) likely reflects thermal resetting during an impact event and slow cooling on its parent body. However, in the case of the angrites, the marginally elevated initial d26Mg* (+0.0068 -0.0058h) could reflect d26Mg* in-growth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super-chondritic A1/Mg prior to impact melting and crystallisation of these angrites, or partial internal re-equilibration of Mg isotopes after crystallisation. 26A1-26Mg model ages and an olivine+pyroxene+whole rock isochron for the angrites Sahara 99555 and D'Orbigny are in good agreement with age constraints from 53Mn-53Cr and 182Hf-182W shortlived chronometers. This suggests that the 26A1-26Mg feldspar-controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar Mg isotope systematics. However, even the 26A1-26Mg angrite model ages cannot be reconciled with Pb-Pb ages for Sahara 99555/D'Orbigny and CAIs, which are ca. 1.0 Myr too old (angrites) or too young (CAIs) for reasons that are not clear. This discrepancy might indicate that 26A1 was markedly lower (ca. 40%) in the planetesimal- and planet-forming regions of the proto-planetary disk as compared to CAIs, or that CAI Pb-Pb ages may not accurately date CAI formation. The second part of this thesis focuses on investigating the homogeneity of (26A1/27A1)0 and Mg isotopes in the proto-planetary disk and to test the validity of the short-lived 26A1-to-26Mg chronometer applied to meteorites. Nineteen chondrites representing nearly all major chondrite classes were analysed, including a step-leaching experiment on the CM2 chondrite Murchison. d26Mg* variations in leachates of Murchison representing acid soluble material are <_30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in d26Mg* (-0.002 to +0.118h). Very small variations in d26Mg* anomalies in bulk chondrites (-0.006 to +0.019h) correlate with increasing 27A1/24Mg ratios and d50Ti, reflecting the variable presence of CAIs in some types of carbonaceous chondrites. Overall, the observed variations in d26Mg* are small and potential differences beyond those resulting from the presence of CAI-like material could not be detected. The results do not allow radical heterogeneity of 26A1 (>_+_ 30%) or measurable Mg nucleosynthetic heterogeneity (>_+_ 0.005h) to have existed on a planetesimal scale in the proto-planetary disk. The data imply that planets (i.e. chondrite parent bodies) accreted from material with initial (26Al/27A1)0 in the range of 2.1 to 6.7 x 10-5. The average stable Mg isotope composition of all analysed bulk chondrites is d25MgDSM-3 = -0.152 +_ 0.079 per1000(2 sd) and is indistinguishable from that of Earth's mantle. The third part of this study comprises a high-precision Mg isotope and mineral major and trace element study of 24 diogenites. Diogenites are ultramafic pyroxene and olivine cumulate rocks that are presumed to have resulted from magmatic differentiation on the howardite-eucritediogenite (HED) parent body. There are, however, no precise and independent age constraints on the formation of diogenites and, in particular, their age relationships to the basaltic eucrites. Mg isotope analysis of diogenites showed significant variability in d26Mg* anomalies that range from -0.0108 +_ 0.0018 to +0.0128 +_ 0.0018 per1000. These anomalies generally correlate with the mineral major and trace element chemistry and demonstrate active 26A1 decay during magmatic differentiation. Furthermore, it also suggests that diogenites are products of fractional crystallisation from a large scale magmatic system. Heating and melting of the HED parent body was driven by 26A1 decay and led to diogenite formation 0.7 to 1.3 Myr after CAIs depending on whether a heterogeneous or homogeneous (26Al/27A1)0 distribution is assumed between the proto-planetary disk and CAIs. These data show that diogenite formation pre-dates eucrite formation and indicate HED parent body accretion and core formation occurred within the first Myr of the Solar System. The lifetime of the magmatic evolution is less well constrained. The data suggest that the complete range of diogenites may have formed as quickly as ~ 0.2 Myr.</p>

High-Precision 26A1-26Mg Systematics of Basaltic Achondrites, Chondrites and Ultramafic Achondrites

<p>A precise and accurate chronology of events that shaped the early Solar System is crucial in understanding its formation. One of the high-resolution chronometers that can be used to establish a relative chronology is the short-lived 26A1-to-26Mg clock (t1/2 = 0.73 Myr). This study developed new Mg chemical separation techniques for complex meteoritic matrices that produces Mg purities > 99% with > 99% yields. Mg was analysed by pseudo-high resolution multiple collector inductively coupled plasma mass spectrometry. These techniques make it possible to measure the mass-independent abundance of 26Mg (d26Mg*) that is related to 26A1 decay to very high-precision (+_ 0.0025 to 0.0050 per1000). These new techniques were then applied to three research objectives. The first part of this study presents Mg isotope data for thirteen bulk basaltic achondrites from at least 3 different parent bodies, as well as mineral isochrons for the angrites Sahara 99555 and D'Orbingy and the ungrouped NWA 2976. Model 26A1-26Mg ages based on bulk rock d26Mg* excesses for basaltic magmatism range from 2.6-4.1 Myr, respectively, after formation of calcium-aluminium-rich inclusions (CAIs) and the mineral isochrons for the angrites Sahara 99555 and D'Orbigny, and the ungrouped NWA 2976 yield apparent crystallisation ages of 5.06+0:06-0:05 Myr and 4.86+0:10-0:09 Myr after CAI formation. The elevated initial d26Mg* of the mineral isochron of NWA 2976 (+0.0175+ _0.0034h) likely reflects thermal resetting during an impact event and slow cooling on its parent body. However, in the case of the angrites, the marginally elevated initial d26Mg* (+0.0068 -0.0058h) could reflect d26Mg* in-growth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super-chondritic A1/Mg prior to impact melting and crystallisation of these angrites, or partial internal re-equilibration of Mg isotopes after crystallisation. 26A1-26Mg model ages and an olivine+pyroxene+whole rock isochron for the angrites Sahara 99555 and D'Orbigny are in good agreement with age constraints from 53Mn-53Cr and 182Hf-182W shortlived chronometers. This suggests that the 26A1-26Mg feldspar-controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar Mg isotope systematics. However, even the 26A1-26Mg angrite model ages cannot be reconciled with Pb-Pb ages for Sahara 99555/D'Orbigny and CAIs, which are ca. 1.0 Myr too old (angrites) or too young (CAIs) for reasons that are not clear. This discrepancy might indicate that 26A1 was markedly lower (ca. 40%) in the planetesimal- and planet-forming regions of the proto-planetary disk as compared to CAIs, or that CAI Pb-Pb ages may not accurately date CAI formation. The second part of this thesis focuses on investigating the homogeneity of (26A1/27A1)0 and Mg isotopes in the proto-planetary disk and to test the validity of the short-lived 26A1-to-26Mg chronometer applied to meteorites. Nineteen chondrites representing nearly all major chondrite classes were analysed, including a step-leaching experiment on the CM2 chondrite Murchison. d26Mg* variations in leachates of Murchison representing acid soluble material are <_30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in d26Mg* (-0.002 to +0.118h). Very small variations in d26Mg* anomalies in bulk chondrites (-0.006 to +0.019h) correlate with increasing 27A1/24Mg ratios and d50Ti, reflecting the variable presence of CAIs in some types of carbonaceous chondrites. Overall, the observed variations in d26Mg* are small and potential differences beyond those resulting from the presence of CAI-like material could not be detected. The results do not allow radical heterogeneity of 26A1 (>_+_ 30%) or measurable Mg nucleosynthetic heterogeneity (>_+_ 0.005h) to have existed on a planetesimal scale in the proto-planetary disk. The data imply that planets (i.e. chondrite parent bodies) accreted from material with initial (26Al/27A1)0 in the range of 2.1 to 6.7 x 10-5. The average stable Mg isotope composition of all analysed bulk chondrites is d25MgDSM-3 = -0.152 +_ 0.079 per1000(2 sd) and is indistinguishable from that of Earth's mantle. The third part of this study comprises a high-precision Mg isotope and mineral major and trace element study of 24 diogenites. Diogenites are ultramafic pyroxene and olivine cumulate rocks that are presumed to have resulted from magmatic differentiation on the howardite-eucritediogenite (HED) parent body. There are, however, no precise and independent age constraints on the formation of diogenites and, in particular, their age relationships to the basaltic eucrites. Mg isotope analysis of diogenites showed significant variability in d26Mg* anomalies that range from -0.0108 +_ 0.0018 to +0.0128 +_ 0.0018 per1000. These anomalies generally correlate with the mineral major and trace element chemistry and demonstrate active 26A1 decay during magmatic differentiation. Furthermore, it also suggests that diogenites are products of fractional crystallisation from a large scale magmatic system. Heating and melting of the HED parent body was driven by 26A1 decay and led to diogenite formation 0.7 to 1.3 Myr after CAIs depending on whether a heterogeneous or homogeneous (26Al/27A1)0 distribution is assumed between the proto-planetary disk and CAIs. These data show that diogenite formation pre-dates eucrite formation and indicate HED parent body accretion and core formation occurred within the first Myr of the Solar System. The lifetime of the magmatic evolution is less well constrained. The data suggest that the complete range of diogenites may have formed as quickly as ~ 0.2 Myr.</p>

Atypical Mineralization Involving Pd-Pt, Au-Ag, REE, Y, Zr, Th, U, and Cl-F in the Oktyabrsky Deposit, Norilsk Complex, Russia

Highly atypical mineralization involving Pd-Pt, Au-Ag, REE, Y, Zr, U, Th, and Cl-F-enriched minerals is found in zones with base metal sulfides (BMS; ~5 vol.% to 20 vol.%) in the eastern portion of the Oktyabrsky deposit in the Norilsk complex (Russia). The overall variations in Mg# index, 100 Mg/(Mg + Fe2+ + Mn), in host-rock minerals are 79.8 → 74.1 in olivine, 77.7 → 65.3 in orthopyroxene, 79.9 → 9.2 in clinopyroxene, and An79.0 → An3.7. The span of clinopyroxene and plagioclase compositions reflects their protracted crystallization from early magmatic to late interstitial associations. The magnesian chromite (Mg# 43.9) trends towards Cr-bearing magnetite with progressive buildups in oxygen fugacity; ilmenite varies from early Mg-rich to late Mn-rich variants. The main BMS are chalcopyrite, pyrrhotite, troilite, and Co-bearing pentlandite, with less abundant cubanite (or isocubanite), rare bornite, Co-bearing pyrite, Cd-bearing sphalerite (or wurtzite), altaite, members of the galena-clausthalite series and nickeline. A full series of Au-Ag alloy compositions is found with minor hessite, acanthite and argentopentlandite. The uncommon assemblage includes monazite-(Ce), thorite-coffinite, thorianite, uraninite, zirconolite, baddeleyite, zircon, bastnäsite-(La), and an unnamed metamict Y-dominant zirconolite-related mineral. About 20 species of PGM (platinum group minerals) were analyzed, including Pd-Pt tellurides, bismuthotellurides, bismuthides and stannides, Pd antimonides and plumbides, a Pd-Ag telluride, a Pt arsenide, a Pd-Ni arsenide, and unnamed Pd stannide-arsenide, Pd germanide-arsenide and Pt-Cu arseno-oxysulfide. The atypical assemblages are associated with Cl-rich annite with up to 7.54 wt.% Cl, Cl-rich hastingsite with up 4.06 wt.% Cl, ferro-hornblende (2.53 wt.% Cl), chlorapatite (>6 wt.% Cl) and extensive solid solutions of chlorapatite, fluorapatite and hydroxylapatite, Cl-bearing members of the chlorite group (chamosite; up to 0.96 wt.% Cl), and a Cl-bearing serpentine (up to 0.79 wt.% Cl). A decoupling of Cl and F in the geochemically evolved system is evident. The complex assemblages formed late from Cl-enriched fluids under subsolidus conditions of crystallization following extensive magmatic differentiation in the ore-bearing sequences.

Geophysical Properties of Precambrian Igneous Rocks in the Gwanin Vanadiferous Titanomagnetite Deposit, Korea

Precambrian igneous rocks (851–873 Ma) occur in Pocheon City, Korea. These rocks —crystallized during magmatic differentiation—formed vanadiferous titanomagnetite (VTM) deposit. Vanadium is a crucial element in vanadium redox flow batteries that are most appropriate for large-scale energy storage systems. We investigated the VTM deposit to evaluate its size and the possible presence of a hidden orebody. We demonstrated laboratory experiments of density, susceptibility, resistivity, and chargeability of the Precambrian igneous rocks to enhance the interpretation accuracy of geophysical surveys. The rocks consisting of underground ore (UO), discovered ore (DO), gabbro (GA), monzodiorite (MD), and quartz monzodiorite (QMD) were sampled from drilling cores and outcrops. The average density values were UO: 4.57 g/cm3, DO: 3.63 g/cm3, GA: 3.26 g/cm3, MD: 3.18 g/cm3, and QMD: 2.85 g/cm3. The average susceptibility values were UO: 0.8175 SI, DO: 0.2317 SI, GA: 0.0780, MD: 0.0126 SI, and QMD: 0.0007. The average resistivity values were UO: 2 Ωm, DO: 36 Ωm, GA: 257 Ωm, MD: 4571Ωm, and QMD: 7801 Ωm. The chargeability values were UO: 143 mV/V, DO: 108 mV/V, GA: 79 mV/V, MD: 42 mV/V, and QMD: 9 mV/V. We found that the properties of the mineralized rocks are considerably different from those of the surrounding rocks. This result may facilitate the mineral exploration of VTM deposits.

REE Tetrad Effect and Sr-Nd Isotope Systematics of A-Type Pirrit Hills Granite from West Antarctica

The Pirrit Hills are located in the Ellsworth–Whitmore Mountains of West Antarctica. The Pirrit Hills granite exhibits significant negative Eu anomalies (Eu/Eu* = 0.01~0.25) and a REE tetrad effect indicating intensive magmatic differentiation. Whole-rock Rb-Sr and Sm-Nd geochronologic analysis of the Pirrit Hills granite gave respective ages of 172.8 ± 2.4 Ma with initial 87Sr/86Sr = 0.7065 ± 0.0087 Ma and 169 ± 12 Ma with initial 144Nd/143Nd = 0.512207 ± 0.000017. The isotopic ratio data indicate that the Pirrit Hills granite formed by the remelting of Mesoproterozoic mantle-derived crustal materials. Both chondrite-normalized REE patterns and Sr-Nd isotopic data indicate that the Pirrit Hills granite has geochemical features of chondrite-normalized REE patterns indicating that REE tetrad effects and negative Eu anomalies in the highly fractionated granites were produced from magmatic differentiation under the magmatic-hydrothermal transition system.