Mineral Data Research Articles

Metamorphic Indicators for Collision, Extension, and Shear Zone Geodynamic Settings of the Earth’s Crust

An effective method for identifying the tectonic-magmatic causes of metamorphism is the reconstruction and analysis of P–T–t paths, which can be used to develop a model of rock evolution with time in the “pressure–temperature–time” coordinates. This approach is most widely used in interpreting the development of orogens with a polycyclic history, where different types of metamorphism are combined in contrasting geodynamic settings. The progress in this direction in recent years is due to the improvement of the instrumental base, precision methods for the study of matter – in situ local microanalysis and dating of zonal metamorphic minerals, and the computing apparatus of geothermobarometric procedures using multi-equilibria. The article discusses these issues on the example of geological complexes of various geodynamic nature, which are typical for different types of metamorphism. Reaction microtextures, chemical zoning in minerals, geochronological dating as well as the trajectory of the Р-Т paths provide evidence for the polymetamorphic history. A generalized P-T diagram was built using data on well-studied metamorphic complexes, which are characterized by the simultaneous presence of prograde and retrograde segments of a P-T path. Diagnostic P-T-t paths characteristic of rocks formed in zones of active tectogenesis—under collision, extension and in shear zones of the earth’s crust—have been established. This paper discusses some of the ambiguous interpretations of P-T-t paths in areas with simultaneous manifestation of different metamorphic processes, which requires further research.

The world-class Nanling metallogenic belt (Jiangxi, China): W and Sn deposition at 160 Ma followed by 30 m.y. of hydrothermal metal redistribution

The W-Sn Maoping deposit and related W-Sn deposits from the world-class Nanling Range in Southeast China formed at ca. 160 Ma and experienced several phases of metal addition and metal redistribution between 160 and 130 Ma. Isotopic dating of ore mineral (wolframite) of Maoping demonstrates that W was deposited first, during a hydrothermal event at ca. 160 Ma. Successive fluid episodes, at ca. 156 Ma (Mo and Sn-rich fluid) and 152 Ma (REE-rich fluid), resulted in the formation of REE- and Fe-Cu-Zn-sulfide minerals. A last fluid event occurring at ca. 130 Ma was responsible for the deposition of Zr-REE-Nb-Ta minerals that are attributed to magmatic fluids derived from unexposed magmatic bodies. The three episodes of fluid circulation post-dating W deposition resulted in partial to complete resetting of the isotopic systems (mica, wolframite, xenotime), which are conventionally used for dating such deposits. We show that W and Sn mineralization in the Nanling Range formed during a unique fluid event at ca. 160 Ma. Later fluid episodes redistributed the previously deposited metals and sequentially introduced additional metals, including Mo-Sn, Fe-Cu-Zn, and Zr-REE-Nb-Ta. Direct dating of paragenetically well-constrained minerals is therefore critical for determining the age and the duration of mineralizing processes and for characterizing the fluid evolution of magmatic-hydrothermal systems, as exemplified in the Nanling Range.

Dating hypogene iron mineralization events in Archean BIF at Weld Range, Western Australia: insights into the tectonomagmatic history of the northern margin of the Yilgarn Craton

This study is the first to constrain the absolute timing of hypogene iron mineralization in Archean BIF located in the Yilgarn Craton. In situ SHRIMP U–Th–Pb geochronology on xenotime and monazite grains has been used to constrain the age of hypogene magnetite replacement ores at the Beebyn deposit and hypogene magnetite vein ores at the Madoonga deposit in the Weld Range study area. The Beebyn magnetite replacement ores (c. 2627 Ma) are younger than the published maximum depositional age of the BIF hosts of the Wilgie Mia Formation (2792 ± 9 Ma) and partially overlaps crystallization ages of granitic rocks (2757–2606 Ma) that intrude supracrustal rocks throughout the study area. These plutons, and their probable subvolcanic expressions, are considered to be likely sources of energy and fluids responsible for magnetite replacement ores. In contrast, Madoonga magnetite veins record multiple dates: the first at 2857 ± 41 Ma, followed by events at c. 2775 Ma through 1812 Ma. The two oldest monazite dates of 2857 ± 41 and 2832 ± 51 Ma are interpreted to be mineralization ages for magnetite veins, possibly related to subseafloor volcanism and base metal VMS systems; whereas the younger dates (i.e. 2775–1812 Ma) probably represent multiple episodes of phosphate mineral precipitation related to reactivation of structures and overprinting by discrete pulses of hydrothermal fluids. The Madoonga BIFs are genetically distinct from the Beebyn BIFs and are the oldest dated BIFs at Weld Range, being older than c. 2857 Ma, but younger than the c. 2970 Ma felsic volcanic rocks in the stratigraphic footwall to the Madoonga BIFs. Hydrothermal events at the Madoonga deposit (2215–2120 Ma) coincide with published dates for rifting, mafic–ultramafic magmatism, and basin development along the northern margin of the Yilgarn Craton (2215–2145 Ma), whereas the younger phosphate mineral dates correspond with the Glenburgh (2002–1947 Ma) and Capricorn Orogenies (1817–1772 Ma). Tectonic activity along the northern margin of the Yilgarn Craton coincides with reported ages for hematite mineralization in BIF-hosted iron-ore deposits in the Hamersley Basin and Pilbara Craton, suggesting the far-reaching effects of tectonism along paleocratonic boundaries as drivers for iron mineralization in BIF.

Vein fluorite U-Pb dating demonstrates post–6.2 Ma rare-earth element mobilization associated with Rio Grande rifting

AbstractNumerous studies have documented rare-earth element (REE) mobility in hydrothermal and metamorphic fluids, but the processes and timing of REE mobility are rarely well constrained. The Round Top laccolith in the Trans-Pecos magmatic province of west Texas, a REE ore prospect, has crosscutting fractures filled with fluorite and calcite along with a variety of unusual minerals. Most notably among these is an yttrium and heavy rare-earth element (YHREE) carbonate mineral, which is hypothesized to be lokkaite based on elemental analyses. While the Round Top laccolith is dated to 36.2 ± 0.6 Ma based on K/Ar in biotite, U-Pb fluorite and nacrite ages presented here clearly show the mineralization in these veins is younger than 6.2 ± 0.4 Ma (the age of the oldest fluorite). This discrepancy in dates suggests that fluids interacted with the laccolith to mobilize REE more than 30 m.y. after igneous emplacement. The timing of observed REE mobilization overlaps with Rio Grande rift extension, and we suggest that F-bearing fluids associated with extension may be responsible for initial mobilization. A later generation of fluids was able to dissolve fluorite, and we hypothesize this later history involved sulfuric acid. Synchrotron spectroscopy and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of minerals that record these fluids offer tremendous potential for a more fundamental understanding of processes that are important not only for REE but other ore deposits as well.

In situ Rb-Sr dating of K-bearing minerals from the orogenic Akçaabat gold deposit in the Menderes Massif, Western Anatolia, Turkey

The Akçaabat gold deposit is mainly composed of massive arsenopyrite veins in strongly foliated augen gneisses of the Çine Sub-massif in western Turkey. K-bearing minerals from orogenic gold veins were dated by the in situ Rb-Sr method in order to determine the formation age of the Akçaabat gold deposit. Textural relationships between the vein minerals suggest that arsenopyrite, K-feldspar, quartz, and muscovite formed in one paragenetic stage. Native gold, native bismuth, scorodite, and galena were introduced in a later paragenetic stage. The isochron derived from K-feldspar and muscovite occurring in the veins yielded 31.3 ± 4.7 Ma. In situ Rb-Sr dating of the same assemblage (K-feldspar and muscovite) in the host rock gave an isochron age of 40.8 ± 3.8 Ma, 10 Ma older than the vein ages, and corresponding to peak metamorphism of the regional main Menderes metamorphism during the Alpine-Himalayan orogenic event. Biotite from the host rock gave an isochron age of 28.1 ± 2.2 Ma and probably reflects the cooling age. Homogenization temperatures from fluid inclusions in quartz show that quartz formed at 280–390 ℃ but has a dominant mode in the 350–360 ℃ temperature interval. The age of vein formation was successfully distinguished from the age of the Alpine-Himalayan orogenic event associated with main Menderes metamorphism. This study demonstrates that in situ Rb-Sr isotopic study can be applied to resolve absolute dating of orogenic deposits in metamorphic complexes.

Eocene exhumation and extensional basin formation in the Copper Mountains, Nevada, USA

Abstract Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.

Clay mineral dating of displacement on the Sronlairig Fault: implications for Mesozoic and Cenozoic tectonic evolution in northern Scotland

AbstractTemporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

First in situ Re-Os dating of molybdenite by LA-ICP-MS/MS

Conventional dating of molybdenite (187Re-187Os) provides one of few options for direct dating of sulfide mineralization. Unfortunately, in situ dating of molybdenite is considered unreliable due to intra-granular decoupling of 187Re-187Os. In this study, we developed a new analytical protocol for studies of micron- to grain scale 187Re-187Os systematics in molybdenite. Online chemical separation using ICP-MS/MS technology enables in situ dating by β-decay systems (e.g., Rb-Sr and K-Ca in micas) using laser ablation. Here, the methodology is extended to the 187Re-187Os system, another β-decay system that cannot be resolved by mass spectrometry. Several reaction gases were evaluated, and production of OsCH2 by reaction with CH4 was found to produce strong separation of Os from Re. However, in contrast to the e.g., 87Rb-87Sr system, 1–2% of the parent isotope Re also reacted to ReCH2, leaving a significant interference. A mathematical correction of this remaining interference is possible, and 187Re-187Os (mass-shifted) can be measured accurately even for fairly extreme ratios. For laser ablation, standards were developed by pressing particulate pellets of conventionally dated molybdenite (Moly Hill and Merlin), because there are no appropriate reference materials available. Six natural molybdenite samples from a range of geological settings, containing > 10 ppm Re, were analyzed by 70 μm laser ablation spots, and ages were calibrated by analysis of molybdenite pellets. Contrary to our expectation, weighted average ages obtained were in agreement (within 1%) with conventional age determinations, with fairly good precision (from ~ 1 to 5% 2σ depending on Re concentration), suggesting limited or essentially nonexistent decoupling within crystals. Two important implications of this result are that decoupling Re-Os is not universal, and that our new analytical protocol is useful both for dating and for studies of decoupling. The benefit of in situ dating compared to conventional dating is, apart from lower cost and time consumption, the possibility of targeting smaller molybdenite crystals (≥ 100 μm) in thin sections and epoxy mounts. The youngest sample in the study is 920 Ma, but we see potential of dating significantly younger Re-rich molybdenite.

U–Pb geochronology of tin deposits associated with the Cornubian Batholith of southwest England: Direct dating of cassiterite by in situ LA-ICPMS

The Cornwall and Devon vein- and greisen-type copper and tin deposits of southwest England are spatially and genetically related to shallow-seated granitic intrusions. These late Variscan intrusions, collectively known as the Cornubian Batholith, extend over 200 km and form a continuous granitic spine from the Isles of Scilly Granite in the west to the Dartmoor Granite in the east. The granitic plutons of the Cornubian Batholith were intruded from ~ 295 to 270 Ma without a major hiatus. Twelve samples of cassiterite (SnO2) were obtained from tin deposits associated with seven different plutons within the Cornubian Batholith for in situ LA-ICPMS U–Pb dating. This study of cassiterite was undertaken to obtain the first results of direct dating of ore mineral to refine the geochronology of tin mineralization in this region. Of the cassiterite samples analyzed, the oldest ages were determined within the Kit Hill and Hingston–Gunnislake Granites in the central part of the Cornubian Batholith. The Hingston–Gunnislake cassiterite, from Drakewalls Mine, was the oldest sample dated at 291.8 ± 3.4 Ma. The next oldest dates, 290.5 ± 2.8 and 288.5 ± 2.9 Ma, were from two cassiterite samples extracted from the adjacent Kit Hill Consolidated Mines within the Kit Hill Granite. At the eastern end of the study area, two cassiterite samples within the Dartmoor Granite produced ages of 286.0 ± 1.8 and 284.1 ± 1.3 Ma. The youngest sample from this study, 275.4 ± 1.6 Ma, is from the Balleswidden Mine within the westernmost Land’s End Granite. The cassiterite dates do not reveal any readily observable relationship between ore ages and geographic relationship from west to east throughout the Cornubian Batholith. Incorporating the associated errors, the geochronology does indicate continuous mineralization within the granites for ~ 21 million years, from ca. 295 to 274 Ma. This span falls within the established period of granitic magmatism of ca. 295 to 270 Ma for the Cornubian Batholith and further confirms the reliability of in situ LA-ICPMS U–Pb dating of cassiterite.

Direct dating of hydrothermal tungsten mineralization using in situ wolframite U–Pb chronology by laser ablation ICP-MS

The wolframite series may contain relatively high U and low common Pb contents, and thus has been used for direct U-Pb dating of hydrothermal W mineralization. In this paper, we present in-situ laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of U–Pb isotopes and trace elements of wolframite crystals from the giant Yaogangxian and Piaotang W deposits in the extensive Nanling metallogenic belt, South China. The objective of this work is to directly constrain the timing of hydrothermal W mineralization in these two deposits. Wolframite crystals from both deposits show textural evidence for two generations. Early wolframite (Wol-1) has higher Fe and lower Mn contents relative to those in the late wolframite (Wol-2). Uranium is correlated positively with Nb5+, tetravalent (Ti, Sn, Zr, Hf), and trivalent (Sc, V, Y, REEs) cations in all wolframite samples, suggesting that the incorporation of U into wolframite is controlled by the coupled substitution mechanisms. The early and late wolframite domains from the Yaogangxian deposit yielded U–Pb ages of 159.1 ± 2.0 Ma (2σ; MSWD = 0.7) and 153.7 ± 0.7 Ma (2σ; MSWD = 0.5), respectively. Similarly, two wolframite generations from the Piaotang deposits yielded U–Pb ages of 159.5 ± 1.3 Ma (2σ; MSWD = 0.3) and 152.1 ± 0.9 Ma (2σ; MSWD = 0.5). The obtained U–Pb ages confirm two successive episodes of hydrothermal W mineralization at ~159 Ma and ~153 Ma in the Yaogangxian and Piaotang deposits. This study demonstrates the potential of U–Pb dating of wolframite by LA-ICPMS and highlights its importance for directly dating hydrothermal ore-forming processes.

Two episodes of REE mineralization in the Qinling Orogenic Belt, Central China: in-situ U-Th-Pb dating of bastnäsite and monazite

Recent exploration revealed a number of rare earth element (REE) deposits that are distributed along the Qinling Orogenic Belt, Central China. These deposits have an estimated total reserve of about 2 Mt. REE2O3, thus making this belt a world-class REE metallogenic province. Understanding the metallogenesis of the belt requires direct dating of REE minerals. In this study, LA-ICP-MS U-Th-Pb dating on bastnasite and monazite from the Huangshuian, Taipingzhen, and Miaoya deposits in different units of the belt was used to precisely determine the timing of the REE mineralization. The Huangshuian deposit in the north is a carbonatite-related Mo-(REE) deposit in which the REE minerals are closely associated with molybdenite. After correction for common Pb and excessed 206Pb decayed from 230Th, a weighted average 206Pb/238U age of 207 ± 4 Ma (n = 17; MSWD = 1.9) is obtained for bastnasite grains from this deposit. Such an age is slightly younger than that of the final collision (peak at 230–220 Ma) of the Qinling Orogenic Belt, thus indicating that the Mo-REE mineralization is likely related to a post-collisional extension setting. The Taipingzhen REE deposit in the middle contains sheet-like ore bodies composed of veins where bastnasite, the dominant REE mineral, is closely associated with quartz, fluorite, and barite. In situ bastnasite U-Th-Pb dating shows that the REE mineralization in this deposit has formed at 421 ± 7 Ma (n = 17; MSWD = 1.5), synchronous with extension-related magmatism in the region. The Miaoya deposit in the south is the largest one in the belt, and it is essentially a REE-mineralized syenite-carbonatite complex. In this deposit, the monazite grains are closely associated with major minerals of the syenites or carbonatites (e.g., K-feldspar or calcite), but commonly exhibit complex internal textures. Different domains of monazite yield two groups of U-Pb ages at 414 ± 11 Ma (n = 5; MSWD = 0.91) and 231 ± 2 Ma (n = 21; MSWD = 3.1), whereas the bastnasite has an age of 206 ± 4 Ma (n = 14; MSWD = 1.5). The early age of 414 Ma is obtained from homogenous monazite grains and in good agreement with zircon U-Pb ages of the Miaoya syenite-carbonatite complex, and thus is considered to represent the timing of the major REE mineralization in the deposit. The younger ages of 231–206 Ma are obtained from monazite grains with complex internal textures and bastnasite in late veinlets, thus recording secondary, consecutive REE remobilization events likely related to the compression process during formation of the Qinling Orogenic Belt. Our new U-Th-Pb ages, in combination with previously geochronological data, demonstrate that there are two episodes of REE mineralization at 440–410 Ma and 220–200 Ma in the Qinling Orogenic Belt.

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero: Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks. The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades. The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny, making it unique among dome-and-keel provinces. However, a lack of appropriate lithologies, datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province. A clinopyroxene-bearing migmatite from the core of the Bação dome has peak P–T conditions of 5–7 kbar and 700–750 °C and a published age of ca. 2730 Ma based on U–Pb ages of zircon from leucosomes, suggesting that this age represents the migmatisation event. A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole, giving P–T conditions of 8–9 kbar and 550 °C with an associated titanite age of ca. 2050 Ma. A garnet-bearing amphibolite sample also from the core of the dome has peak P–T conditions of 7–8 kbar and 650–700 °C, and texturally late titanite from this sample produces an age of ca. 2060 Ma. Three additional samples were collected from the edges of the dome. A garnet-gedrite bearing felsic schist produces peak P–T conditions of 8–9 kbar and 650–700 °C on a clockwise P–T evolution. This sample has a U–Pb zircon age of ca. 2775 Ma, which could date metamorphism or be the age of its volcaniclastic protolith. Texturally unconstrained titanite from the sample gives an age of ca. 2040 Ma. A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca. 2050 Ma and has peak P–T conditions of 5–6 kbar and 650–700 °C on a near isobaric P–T path. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon U–Pb age of ca. 2760 Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775–2730 Ma) and again during the Transamazonian event. The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time. The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures, with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles. The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature, but they are susceptible to reworking, resulting in an enigmatic record of formation.

U-Pb isotopic dating of columbite-tantalite minerals: Development of reference materials and in situ applications by ion microprobe

Columbite-tantalite group minerals are the most common Nb-Ta minerals. Columbite-tantalite is particularly suitable for U-Pb dating due to its high U and low common Pb contents. In situ isotopic dating of columbite-tantalite by LA-ICP-MS or SIMS requires certified reference material to properly account for potential matrix effects linked to substitutions between Nb and Ta and between Mn and Fe. Our study has two objectives: i) establish a database of reference materials for in situ U-Pb isotopic dating of columbite-tantalite minerals and ii) test the capability of SIMS to in situ U-Pb date columbite-tantalite minerals of different chemical composition. Tests of in situ U-Pb dating demonstrate that SIMS can easily be used to date columbite-tantalite minerals with errors and precisions overlapping the reference ID-TIMS age. There are, however, significant matrix effects for non-matching Nb-Ta-Fe-Mn compositions of sample and reference material. Matrix effects are highly correlated with the Ta/(Ta + Nb) ratio of columbite-tantalite, due to the significant difference in the atomic mass of Nb and Ta. The Mn/(Mn + Fe) ratio does not significantly contribute to the observed matrix effect as the two elements have similar atomic masses. The linear correlation between Ta/(Nb + Ta) and ((206Pb/238U)SIMS/(206Pb/238U)ID-TIMS) obtained for columbite-tantalite minerals of known ID-TIMS age demonstrates that the SIMS matrix-effect can be properly accounted for by using the chemical composition as determined by EMPA. The ability to measure 204Pb by SIMS also allows the use of reference materials with a small common lead contribution and to calculate accurate and precise ages for columbite-tantalite minerals with contributions of common lead.

盆地流体年代学研究新技术:方解石激光原位U-Pb定年法

Basin fluid is the most active geological agent in sedimentary basins, having a close relationship with the generation, migration and accumulation of hydrocarbon resources. Accurate determination of fluid flow history has been a challenging and frontier research topic. In general, the previous studies of basin fluids mainly rely on the analysis of fluid inclusions, which is difficult to successfully reconstruct the events of basin fluids. More seriously, this method is unable to determine the timing of fluid flow events. Authigenic calcite is the direct product of basin fluids. Thus, accurate dating of authigenic calcite provides a new approach to determine the history of fluid flow events. In the field of calcite geochronology, the most widely used dating method was the isotope dilution U-Pb dating approach. However, this approach is time-consuming, and has a low success rate. In recent years, laser ablation technology has greatly facilitated U-Pb dating of accessory minerals (including calcite) because of its high spatial resolution and rapid data acquisition. It has been confirmed that the newly-developed in-situ U-Pb dating method is able to accurately determine the age of calcite with U content less than 10×10 . This method has successfully reconstructed the history of fluid flow events in the sedimentary basins, suggesting that it has a good application prospect in the field of basin fluid geochronology. In the future, it can be expected that the application of in-situ U-Pb dating of calcite together with C-O isotope and rare earth element analysis will be a significant development direction in the field of basin fluid studies. It is worth noting that the determination of stage of authigenic calcite through systematic microscopic identification and diagenetic observation is the premise of application success.

Hydrothermal apatite SIMS Th[sbnd]Pb dating: Constraints on the timing of low-temperature hydrothermal Au deposits in Nibao, SW China

Precise geochronology is a crucial tool for understanding the genesis of low-temperature hydrothermal mineral deposits, but datable minerals are not always available; e.g., mineralization within sedimentary rocks, such as Carlin-type or Carlin-like Au deposits. Here, we demonstrate that hydrothermal apatite associated with Au from the large Nibao deposit in the Yunnan–Guizhou–Guangxi region of South China can be dated to yield the age of Au mineralization. Nibao is a Carlin-like deposit hosted in a Permian carbonate-bearing pyroclastic breccia. The auriferous minerals are predominantly zoned pyrite containing As-, Cu-, and Au-rich rims. Euhedral and subhedral apatite forms a mosaic texture and is intergrown with hydrothermal quartz, sericite, and zoned auriferous pyrite, indicating that the apatite is of hydrothermal origin and was coeval with Au mineralization. Unlike sedimentary and igneous apatite, hydrothermal apatite is relatively depleted in LREEs, enriched in MREEs, and slightly depleted in Eu. Hydrothermal apatite also has high Th/U ratios, negligible U concentrations, and very low concentrations of common Pb. This makes the apatite a potential candidate for ThPb dating. Our SIMS ThPb analyses yield a weighted mean 232Th/208Pb age of 141 ± 3 Ma (N = 23, MSWD = 2.2) for apatite coeval with auriferous pyrite, and this is the best estimate of the timing of Au mineralization. Hydrothermal apatite is also present in similar deposits in the Golden Triangle of South China. Therefore, ThPb dating of hydrothermal apatite potentially allows for the accurate dating of low-temperature hydrothermal deposits in China and worldwide.

U-pb dating of niobium minerals from pyrochlor group (ilmeno-vishnevogorsk carbonatitis-miaskite complex, of the southern urals)

U-pb dating of niobium minerals from pyrochlor group (ilmeno-vishnevogorsk carbonatitis-miaskite complex, of the southern urals)

40Ar-39Ar age constraints on timing of a late hydrothermal event in the Hutti greenstone belt, Karnataka, India

Obtaining constraints on the timing of orogenic gold mineralization hosted in shear zones in greenstone belts is often difficult because of the lack of easily datable minerals and problems of overprinting of earlier events of mineralization by later hydrothermal events. In the orogenic Hutti gold deposit the U-Pb SHRIMP date obtained from a monazite grain within arsenopyrite that gave an age of 2547 ± 10 Ma, is generally accepted as the age of early hydrothermal mineralization. Nevertheless, other work has indicated that hydrothermal mineralization may have occurred in later episodes as well. This study reports new 40Ar-39Ar ages for hydrothermal biotite, amphibolite and hornblende in granites that occur along the northern and southeastern boundary of the greenstone belt. The 40Ar-39Ar ages of biotite samples are inferred to be the result of re-setting of the ages of alteration biotite by the last phase of hydrothermal activity in this area at ∼2332 Ma. The study indicates that the effects of the thermal event are more significant in areas of greater hydrothermal access such as within the Hutti shear zone. This late hydrothermal alteration event may be related to the D3 deformational event of Roy (1991). The results of the present work are important because it demonstrates a significant hydrothermal event much after the major 2.58–2.52 Ga accretionary event and accompanying TTG and felsic magmatism in the Eastern Dharwar Craton.

Water Vapor-Assisted "Universal" Nonmatrix-Matched Analytical Method for the in Situ U-Pb Dating of Zircon, Monazite, Titanite, and Xenotime by Laser Ablation-Inductively Coupled Plasma Mass Spectrometry.

The U-Pb geochronologic analysis of accessory minerals has played an important role in Earth and solar system science in constraining the ages of a wide variety of rocks and minerals. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is one of the most popular techniques for U-Pb geochronologic analysis. Currently, the significant matrix effects observed between different accessory minerals and the lack of high-quality standards for many minerals of interest are the major limitations of its geochronological applications. In this study, we investigated the effects of the addition of oxygen, nitrogen, and water vapor before and after the ablation cell on the accuracy of the U-Pb dating of different minerals (e.g., zircon, monazite, titanite, and xenotime) by LA-ICP-MS. We found that the addition of water vapor, unlike that of oxygen and nitrogen, before the ablation cell can significantly suppress the matrix effects on U-Pb dating. The deviations of the measured 206Pb/238U ratios in these accessory minerals were significantly reduced from 10 to 24% to less than 1-2% when using NIST 610 glass as an external standard. This can be attributed to the suppression of elemental fractionation in both the laser ablation and ICP ionization processes by the presence of water vapor. The developed water vapor-assisted LA-ICPMS U-Pb dating method has been successfully applied to the analysis of zircon, monazite, xenotime, and titanite with NIST 610 glass as a reference material in both the 193 nm excimer laser and 213 nm Nd:YAG laser ablation systems.

Unprecedented 34 S-enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks.

In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show that, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in 34 S relative to 32 S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ34 Spyrite values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ34 Spyrite values (-54‰) suggest very large fractionation during MSR from an initial sulfate with δ34 S values (δ34 Ssulfate,0 ) of +14 to +28‰. Fractionation of this magnitude requires a slow MSR rate, a feature we attribute to nutrient and electron donor shortage as well as initial sulfate abundance. The superheavy δ34 Spyrite values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120±15‰) within single fracture intercepts in the boreholes, associated heavy average values up to +75‰ and heavy minimum δ34 Spyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ34 Spyrite variability and overall average δ34 Spyrite values (+11 to +16‰) lower than the anticipated δ34 Ssulfate,0 support this hypothesis. The superheavy pyrite found locally in the borehole intercepts thus represents a late stage in a much larger fracture system undergoing Rayleigh fractionation. Microscale Rb-Sr dating and U/Th-He dating of cogenetic minerals reveal that most pyrite formed in the early Paleozoic era, but crystal overgrowths may be significantly younger. The δ13 C values in cogenetic calcite suggest that the superheavy δ34 Spyrite values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrite has been proposed to be linked to anaerobic oxidation of methane. The findings provide new insights into MSR-related S-isotope systematics, particularly regarding formation of large fractions of 34 S-rich pyrite.

Structural, geochronological and P-T constraints on subduction-accretion processes in a Pan-African accretionary wedge – The Deep Level Southern Zone of the Damara Belt in Namibia

The Southern Zone (SZ) accretionary prism of the Damara Belt in Namibia records the burial, offscraping and accretion of marine and original trench sediments during the Pan-African (∼590–500 Ma) convergence of the Congo and Kalahari cratons. Although the structure and metamorphic conditions of the prism sediments have been documented, the processes that led to the imbrication of 100 km2 slivers of Mesoproterozoic basement gneisses with the overlying prism metasedimentary rocks are poorly understood. This study combines structural data with P-T estimates and in-situ LA-ICP-MS U-Pb xenotime dating and Ar-Ar age determinations of mineral separates, to give insight into the dynamics of subduction and exhumation in the deeply eroded, tectonically interleaved Deep Level Southern Zone (DLSZ). Peak metamorphic assemblages in garnet-bearing amphibole gneiss from the Ekuja basement dome record P-T conditions of 8.5–9.1 kbar and 635–655 °C, suggesting burial to ca. 35 km. Garnet zoning patterns and breakdown textures show that peak metamorphism was followed by a clockwise retrograde P-T-t evolution. Exhumation of the Ekuja Dome is supported by structural observations and the retrogression of amphibole-plagioclase assemblages to biotite and epidote in the Omitiomire Shear Zone (OSZ). 40Ar/39Ar dating of biotite from the OSZ constrain the timing of exhumation of the Ekuja Dome to between 526.4 ± 3.5 and 521.9 ± 3.6 Ma. Peak metamorphic assemblages in garnet-kyanite schists from the overlying prism metasedimentary rocks at Hochberg record P-T conditions of 7.0–9.3 kbar and 640–675 °C, suggesting burial to 27–35 km. U-Pb dating of xenotime constrains the timing of peak metamorphism in the overlying metasedimentary rocks to ca. 515 Ma, ca. 10 myr after the onset of exhumation of the Ekuja Dome. This indicates that the Ekuja Dome was already on its exhumation path from larger depths while the overlying Hochberg metasedimentary unit was still undergoing burial, and reflects the dynamics of burial and exhumation along intraprism thrusts at the interface between the downgoing plate and the overlying prism sediments in accretionary complexes. The structural, metamorphic and geochronological similarities between the Southern Zone of the Damara Belt and the Lufilian Arc point to a contiguous convergent margin along the leading edge of the Congo Craton that was active to at least ca. 515 Ma.