Zircon (U-Th)/He Closure Temperature Lower than Apatite Ther-Mochronometric Systems: Reconciliation of a Paradox

We present here seven new zircon (U-Th)/He (ZHe) ages and three new zircon fission track ages (ZFT) analyzed from an age-elevation profile (Machu Picchu, Peru). ZFT data present older ages in comparison with the other thermochronological data, whereas the ZHe data interestingly present similar ages than the ones obtained with apatite (U-Th)/He (AHe). It has been proposed that He retention in zircon is linked to the damage dose, with an evolution of the closure-temperature from low values associated to low α-dose (<1016 α/g), subsequently increasing before decreasing again at very high α-dose (>1018 α /g). Studies have been focused on the He diffusion behavior at high α-dose, but little is known at low dose. We propose that the ZHe closure temperature at α-dose ranging from 0.6×1015 to 4×1016 α/g is in the range of ~60-80°C. This value is lower than the one proposed in the current damage model ZRDAAM and demonstrates that the ZHe and AHe methods could have similar closure temperatures at low α-dose (i.e. similar ages). These new data strengthen our previous geological conclusions and even highlight an about twice more important cooling rate than the one deduced from AHe and apatite fission-track data alone registered at Machu Picchu.

Deformation recorded in polyhalite from evaporite detachments revealed by ⁴⁰Ar ∕ ³⁹Ar dating

The Salt Range Formation is an extensive evaporite sequence in northern Pakistan that has acted as the primary detachment accommodating Himalayan orogenic deformation from the north. This rheologically weak formation forms a mylonite in the Khewra Mine, where it accommodates approximately 40 km displacement and is comprised of intercalated halite and potash salts and gypsiferous marls. Polyhalite [K2Ca2Mg(SO4)4⚫2H2O] grains taken from potash marl and crystalline halite samples are used as geochronometers to date the formation and identify the closure temperature of the mineral polyhalite using the 40Ar/39Ar step-heating laser and furnace methods. The diffusion characteristics measured for two samples of polyhalite are diffusivity (D0), activation energy (Ea), and %39Ar. These values correspond to a closure temperature of ca. 254 and 277 ∘C for a cooling rate of 10 ∘C Myr−1. 40Ar/39Ar age results for both samples did not return any reliable crystallisation age. This is not unexpected as polyhalite is prone to 40Ar* diffusion loss and the evaporites have experienced numerous phases of deformation resetting the closed K/Ar system. An oldest minimum heating step age of ∼514 Ma from sample 06-3.1 corresponds relatively well to the established early Cambrian age of the formation. Samples 05-P2 and 05-W2 have measured step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 and 415 Ma. We interpret the youngest measured step ages, between ca. 286 and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest measured step ages for samples 05-P2 and 05-W2 occur within the time of a major unconformity in the area. These dates may reflect partial resetting of the K/Ar system from meteoric water infiltration and recrystallisation during this non-depositional time. Otherwise, they may result from mixing of Ar derived by radiogenic decay after Cambrian precipitation with partially reset Ar from pervasive Cenozoic deformation and physical recrystallisation.

Two pulse intrusive events of the Pliocene Tanigawa-dake granites revealed from zircon U–Pb dating

We performed zircon U–Pb dating on the Pliocene Tanigawa-dake granites (Makihata and Tanigawa bodies) and the Cretaceous Minakami quartzdiorite, Northeast Japan Arc. Concordia ages were estimated to be 3.95 ± 0.11 Ma (± 2 sigma) for the Makihata body, 3.18 ± 0.13 Ma and 3.32 ± 0.15 Ma for the Tanigawa body, and 109.4 ± 2.2 Ma for the Minakami quartzdiorite. The Minakami quartzdiorite is possibly correlated to the bedrock in the Ashio belt because the age of the Minakami quartzdiorite is consistent with the zircon U–Pb ages of the earliest Tadamigawa granites (107–62 Ma) which are distributed to the northeast of the Tanigawa-dake region and belong to the Ashio belt. All the zircon U–Pb ages of the Tanigawa-dake granites are older than the previously reported cooling ages, i.e., K–Ar ages and zircon fission-track ages, being consistent with their difference in closure temperature. On the basis of these results, we concluded that the intrusive ages of the Tanigawa-dake granites are ~ 4–3 Ma, which are among the youngest exposed plutons on Earth. The U–Pb ages of the Makihata body and the Tanigawa body are different significantly in the 2 sigma error range. Thus, the Tanigawa body intruded later than the Makihata body by ~ 0.7 Myr. Graphical Abstract

Two Pulse Intrusive Events of the Pliocene Tanigawa-Dake Granites Revealed from Zircon U-Pb Dating.

We performed zircon U–Pb dating on the Pliocene Tanigawa-dake granites (Makihata and Tanigawa bodies) and the Cretaceous Minakami quartzdiorite, Northeast Japan Arc. Concordia ages were estimated to be 3.95 ± 0.11 Ma (± 2 sigma) for the Makihata body, 3.18 ± 0.13 Ma and 3.32 ± 0.15 Ma for the Tanigawa body, and 109.4 ± 2.2 Ma for the Minakami quartzdiorite. The Minakami quartzdiorite is possibly correlated to the bedrock in the Ashio belt because the age of the Minakami quartzdiorite is consistent with the zircon U–Pb ages of the earliest Tadamigawa granites (107–62 Ma) which are distributed to the northeast of the Tanigawa-dake region and belong to the Ashio belt. All the zircon U-Pb ages of the Tanigawa-dake granites are older than the previously reported cooling ages, i.e., K–Ar ages and zircon fission-track ages, being consistent with their difference in closure temperature. On the basis of these results, we concluded that the intrusive ages of the Tanigawa-dake granites are ~4–3 Ma, which are among the youngest exposed plutons on Earth. The U–Pb ages of the Makihata body and the Tanigawa body are different significantly in the 2 sigma error range. Thus, the Tanigawa body intruded later than the Makihata body by ~0.7 Myr.

Deformation recorded in polyhalite from evaporite detachments revealed by ⁴⁰Ar/³⁹Ar dating

The Salt Range Formation is an extensive evaporite formation in northern Pakistan that has acted as the primary detachment accommodating Himalayan orogenic deformation from the north. This rheologically weak formation forms a mylonite in the Khewra mines, where it accommodates approximate 40 km displacement and is comprised of intercalated halite and potash salts and gypsiferous marls. Polyhalite [K2Ca2Mg(SO4)4•2H2O] grains taken from potash marl and crystalline halite samples are used as geochronometers to date the formation and identify the closure temperature of the mineral polyhalite using the 40Ar/39Ar step heating method. The diffusion characteristics measured for two samples of polyhalite are diffusivity (D0), activation energy (Ea), and %39Ar. These values correspond to a closure temperature of ca. 281 and 296 °C for a cooling rate of 10 °C/Ma. 40Ar/39Ar age results for both samples did not return any reliable crystallization age. This is not unexpected as polyhalite is prone to 40Ar* diffusion loss and the evaporites have experienced numerous phases of deformation resetting the closed K/Ar system. An oldest minimum heating step age of 514 ± 3 Ma from sample 06-3.1 corresponds relatively well to the established early Cambrian age of the formation. Samples 05-P2 and 05-W2 have apparent step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 Ma and 415 Ma. We interpret the youngest apparent step ages, between ca. 286 Ma and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest apparent step ages for Samples 05-P2 and 05-W2 occur within the time of a major unconformity in the area. These dates may reflect partial resetting of the K/Ar system from meteoric water infiltration and recrystallisation during this non-depositional time. Else, they may result from mixing of Ar derived by radiogenic decay after Cambrian precipitation with partially reset Ar from pervasive Cenozoic deformation and physical recrystallisation.

The closure temperature(s) of zircon Raman dating

Zircon Raman dating based on irradiation damage is a debated concept but not an established geo-/thermochronological method. One issue is the temperature range of radiation-damage annealing over geological timescales. We conducted isochronal and isothermal annealing experiments on radiation-damaged zircons between 500 and 1000 ∘C for durations between 10 min and 5 d to describe the annealing kinetics. We measured the widths (Γ) and positions (ω) of the ν1(SiO4), ν2(SiO4), and ν3(SiO4) internal Raman bands, and the external rotation Raman band at ∼974, 438, 1008, and 356 cm−1 after each annealing step. We fitted a Johnson–Mehl–Avrami–Kolmogorov and a distributed activation energy model to the fractional annealing data, calculated from the widths of the ν2(SiO4), ν3(SiO4), and external rotation bands. From the kinetic models, we determined closure temperatures Tc for damage accumulation for each Raman band. Tc ranges from 330 to 370 ∘C for the internal ν2(SiO4) and ν3(SiO4) bands; the external rotation band is more sensitive to thermal annealing (Tc∼260 to 310 ∘C). Our estimates are in general agreement with previous ones, but more geological evidence is needed to validate the results. The Tc difference for the different Raman bands offers the prospect of a multi-closure-temperature zircon Raman thermochronometer.

Academic Reminiscences and Thermodynamics-Kinetics of Thermo-Barometry-Chronology

This article has three major components that include, in addition to the technical aspects, reminiscences of my academic upbringing, my move to the USA from India, and my professional career. I have recounted many stories that I hope convey some sense of time, especially in these two countries with vastly different cultures, my personal journey with its ups and downs and how I made the transition to an academic career path in USA even though that was not in my future plan as a young man. The development of the field of thermobarometry and its integration with diffusion and crystal kinetic modelling of compositional zoning (or lack thereof) and cation ordering in minerals have led to important quantitative constraints on the pressure-temperature-time evolution of terrestrial rocks and meteorites. I review the historical developments in these areas and a segment of my own research spanning the period of 1964-2021. The foundational works of the thermometry of metamorphic rocks and palaeothermometry were laid at the University of Chicago around 1950. Subsequently, the synergetic growth of thermodynamics and experimental studies in petrology in the 1960s and 1970s, along with the introduction of electron microprobe as a nondestructive analytical tool with micron scale resolution, gave a major boost to the field of thermobarometry. There were also significant new developments in the field of thermodynamics of solid solutions in the petrology community and demonstration from observational data, countering strong scepticism, that the principles of classical thermodynamics were applicable to “complex natural systems”. The section on thermodynamic basis of thermobarometry concludes with a discussion of the thermodynamics of trace element and single mineral thermometry. I further deal with the experimental protocols, along with selected examples, for phase equilibrium studies that provide the bedrock foundation for the field of thermobarometry based on elemental compositions of coexisting minerals in a rock. It is followed by an account of the controversies and international meetings relating to the aluminum silicate and peridotite phase diagrams that play crucial roles in the thermobarometry of metamorphic rocks and mantle xenoliths, respectively. The construction of quantitative petrogenetic grids to display stability relations of minerals in multicomponent–multiphase systems came into play in the field of metamorphic petrology in the mid-1960s and early 1970s. Augmented by experimental data, these petrogenetic grids led to important discoveries about the P-T-<em>f</em>(O₂) and bulk compositional controls on the stability of certain “index” minerals that are used to define metamorphic isograds and different types of regional metamorphism; one such grid also opened up a new field that came to be known as ultra-high temperature metamorphism. The construction of petrogenetic grids has now evolved to computer based calculations of complex equilibrium P-T phase diagrams, commonly referred to as “pseudosections”, by minimisation of Gibbs free energy of a system with fixed bulk composition. I discuss these historical developments and modern advancements. Subsequently I highlight some aspects of thermobarometry and diffusion kinetic modelling of selected natural samples along with their broader implications and present a critical discussion of different protocols for thermobarometry of natural assemblages. Following up on the introductory historical perspective of development of palaeothermometry, I discuss the modern advancements using density functional theory (DFT). Examples of DFT based calculations have been shown for hydrogen isotope fractionation in mineral-water/hydrogen systems and “clumped isotope” thermometry. The hydrogen isotope fractionation data led the development of new low temperature palaeothermometers using serpentine-talc/brucite mineral pairs. These results enable simultaneous solutions of both temperature and source of fluid in the serpentinisation process of rocks. The final section is devoted to high temperature thermochronology dealing with the problems of closure temperature of decay systems in minerals and the use of bulk and spatial resetting of mineral age according to a specific decay system to determine cooling rates of the host rocks. Complications arise in the interpretation of mineral ages determined by such decay systems as ¹⁷⁶Lu-¹⁷⁶Hf or the short- lived system ⁵³Mn-⁵³Cr in which the parent nuclide has a much lower closure temperature than the corresponding daughter product. Numerical simulations help explain the discrepancy between the ¹⁷⁶Lu-¹⁷⁶Hf and ¹⁴⁷Sm-¹⁴³Nd ages of garnets in metamorphic rocks and enable construction of the entire T-t cycle from the discrepant ages and some additional constraints.

Thermal annealing of implanted ²⁵²Cf fission tracks in monazite

A series of isochronal heating experiments were performed to constrain monazite fission track thermal annealing properties. The 252Cf fission tracks were implanted into monazite crystals from the Devonian Harcourt granodiorite (Victoria, Australia) on polished surfaces oriented parallel to (100) pinacoidal faces and perpendicular to the crystallographic c axis. Tracks were annealed over 1, 10, 100 and 1000 h schedules at temperatures between 30 and 400 ∘C. Track lengths were measured on captured digital image stacks and then converted to calculated mean lengths of equivalent confined fission tracks that progressively decreased with increasing temperature and time. Annealing is anisotropic, with tracks on surfaces perpendicular to the crystallographic c axis consistently annealing faster than those parallel to the (100) face. To investigate how the mean track lengths decreased as a function of annealing time and temperature, one parallel and two fanning models were fitted to the empirical dataset. The temperature limits of the monazite partial annealing zone (MPAZ) were defined as length reductions to 0.95 (lowest) and 0.5 (highest) for this study. Extrapolation of the laboratory experiments to geological timescales indicates that for a heating duration of 107 years, estimated temperature ranges of the MPAZ are −44 to 101 ∘C for the parallel model and −71 to 143 ∘C (both ±6–21 ∘C, 2 standard errors) for the best-fitting linear fanning model (T0=∞). If a monazite fission track closure temperature is approximated as the midpoint of the MPAZ, these results, for tracks with similar mass and energy distributions to those involved in spontaneous fission of 238U, are consistent with previously estimated closure temperatures (calculated from substantially higher energy particles) of < 50 ∘C and perhaps not much higher than ambient surface temperatures. Based on our findings we estimate that this closure temperature (Tc) for fission tracks in monazite ranges between ∼ 45 and 25 ∘C over geological timescales of 106–107 years, making this system potentially useful as an ultra-low-temperature thermochronometer.

The closure temperature(s) of zircon Raman dating

We conducted isochronal and isothermal annealing experiments on radiation-damaged zircons between 500 and 1000 °C for durations between ten minutes and five days. We measured the widths (Γ) and positions (ω) of the internal ν1(SiO4), ν2(SiO4), ν3(SiO4), and external rotation Raman bands at ~ 974, 438, 1008, and 356 cm−1. We fitted a Johnson-Mehl-Avrami-Kolmogorov and a distributed activation energy model to the fractional annealing data, calculated from the widths of the ν2(SiO4), ν3(SiO4), and external rotation bands. From the kinetic models, we determined closure temperatures Tc for damage accumulation for each Raman band. Tc range from 330 to 370 °C for the internal ν2(SiO4) and ν3(SiO4) bands; the external rotation band is more sensitive to thermal annealing (Tc ~ 260 to 310 °C). Our estimates are in general agreement with previous ones, but more geological evidence is needed to validate the results. The Tc difference for the different Raman bands offers the prospect of a multi-closure-temperature zircon Raman thermochronometer.