Ultra-violet Laser Ablation Microprobe Research Articles

Exploring the variability of argon loss in Apollo 17 impact melt rock 77135 using high‐spatial resolution40Ar/39Ar geochronology

Abstract40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP)40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally,UVLAMPanalysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.

Phlogopite40Ar/39Ar geochronology of mantle xenoliths from the North China Craton: Constraints on the eruption ages of Cenozoic basalts

Mantle xenoliths provide direct information about lithospheric evolution and asthenosphere–lithosphere interaction, and therefore precise dating of the host basalts which carried the xenoliths is important. Here we report 40Ar/39Ar geochronology of phlogopite separates from five spinel lherzolite xenoliths collected from the North China Craton (Hannuoba of Hebei Province, Sanyitang of Inner Mongolia Autonomous Region and Hebi of Henan Province), as well as the groundmass of the host basalts. Argon extraction was performed by conventional step heating technique and ultra-violet laser ablation microprobe (UVLAMP) technique. 40Ar/39Ar incremental heating results on groundmass yielded geologically meaningless ages. However, conventional step heating on phlogopites produced Miocene cooling ages, identical to the eruption ages obtained from the K–Ar dating methods of the Hannuoba and Sanyitang basalts. Adopting procedures to exclude potential influence of excess radiogenic Ar from a deep fluid source on a phlogopite separate from lherzolite yielded results with a good agreement of ages suggesting that the argon isotopes are distributed homogenously in this mineral, with no influence of excess argon. Phlogopites from Hebi yield ages between 6.43 and 6.44Ma which are slightly older than those obtained from K–Ar method on whole-rocks. The discrepancy in the K–Ar ages obtained from the altered whole-rock samples suggests partial loss of 40Ar. As a consequence, phlogopite Ar–Ar ages are considered more accurate than that of the whole-rocks. These results suggest that 40Ar/39Ar chronology of phlogopite provides reliable and precise 40Ar/39Ar ages of host basalts.

Ultra-violet laser probe measurement of40Ar/39Ar age profile in phlogopite

Ultra-Violet Laser Ablation Microprobe (UVLAMP) extraction technique enables the direct investigation of Ar-Ar age profile in crystals, and yields more information on rates and durations of geological process than conventional single time snapshots. Phlogopite flakes from lamprophyre at Pishan dyke in western Kunlun were dated by using an UV laser (λ = 213 nm) microprobe with spot analyses. The results show good agreement with those from the conventional40Ar/39Ar step heating experiments. This indicates that the Ar isotopes are distributed homogenously in the phlogopite and the UVLAMP can be a powerful tool in the study of thermal history.

Excess argon evolution in HP–LT rocks: a UVLAMP study of phengite and K-free minerals, NW Turkey

Excess argon is a common problem in high-pressure metamorphic rocks. Here, we present 40Ar/ 39Ar data from metabasite, metachert and metapelite lithologies and a range of minerals. The results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high-pressure metamorphism. Samples were derived from blueschist and low-temperature eclogite facies rocks from the Tavsanli Zone of NW Turkey. Samples were analysed using the 40Ar/ 39Ar Ultra-Violet Laser Ablation Microprobe (UVLAMP) and yield ages ranging from 72±3 to 154±7 Ma. Detailed intra-grain profiling of phengites in situ, and analyses of sodic-amphibole, lawsonite, quartz and garnet, present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples, revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite. Whole rock XRF data indicates a scattered relationship between bulk K 2O content of the rock and the concentration of excess argon in phengite. We conclude that excess argon was derived in situ in a relatively closed, fluid-absent system, rather than introduced by fluids.

Ar and K partitioning between clinopyroxene and silicate melt to 8 GPa

The relative incompatibility of Ar and K are fundamental parameters in understanding the degassing history of the mantle. Clinopyroxene is the main host for K in most of the upper mantle, playing an important role in controlling the K/Ar ratio of residual mantle and the subsequent time-integrated evolution of 40Ar/36Ar ratios. Clinopyroxene also contributes to the bulk Ar partition coefficient that controls the Ar degassing rate during mantle melting. The partitioning of Ar and K between clinopyroxene and quenched silicate melt has been experimentally determined from 1 to 8 GPa for the bulk compositions Ab80Di20 (80 mol% albite-20 mol% diopside) and Ab20Di80 with an ultraviolet laser ablation microprobe (UVLAMP) technique for Ar analysis and the ion microprobe for K. Data for Kr (UVLAMP) and Rb (ion probe) have also been determined to evaluate the role of crystal lattice sites in controlling partitioning. By excluding crystal analyses that show evidence of glass contamination, we find relatively constant Ar partition coefficients (DAr) of 2.6 × 10−4 to 3.9 × 10−4 for the Ab80Di20 system at pressures from 2 to 8 GPa. In the Ab20Di80 system, DAr shows similar low values of 7.0 × 10−5 and 3.0 × 10−4 at 1 to 3 GPa. All these values are several orders of magnitude lower than previous measurements on separated crystal-glass pairs.DK is 10 to 50 times greater than DRb for all experiments, and both elements follow parallel trends with increasing pressure, although these trends are significantly different in each system studied. The DK values for clinopyroxene are at least an order of magnitude greater than DAr under all conditions investigated here, but DAr appears to show more consistent behavior between the two systems than K or Rb. The partitioning behavior of K and Rb can be explained in terms of combined pressure, temperature, and crystal chemistry effects that result in changes for the size of the clinopyroxene M2 site. In the Ab20Di80 system, where clinopyroxene is diopside rich at all pressures, DK and DRb increase with pressure (and temperature) in an analogous fashion to the well-documented behavior of Na. For the Ab80Di20 system, the jadeite content of the clinopyroxene increases from 22 to 75 mol% with pressure resulting in a contraction of the M2 site. This has the effect of discriminating against the large K+ and Rb+ ions, thereby countering the effect of increasing pressure. As a consequence DK and DRb do not increase with pressure in this system.In contrast to the alkalis (Na, K, and Rb), DKr values are similar to DAr despite a large difference in atomic radius. This lack of discrimination (and the constant DAr over a range of crystal compositions) is also consistent with incorporation of these heavier noble gases at crystal lattice sites and a predicted consequence of their neutrality or “zero charge.” Combined with published DAr values for olivine, our results confirm that magma generation is an efficient mechanism for the removal of Ar from the uppermost 200 km of the mantle, and that K/Ar ratios in the residuum are controlled by the amount of clinopyroxene. Generally, Ar is more compatible than K during mantle melting because DAr for olivine is similar to DK for clinopyroxene. As a result, residual mantle that has experienced variable amounts of melt extraction may show considerable variability in time-integrated 36Ar/40Ar.

40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.

Direct measurement of Ar diffusion profiles in a gem-quality Madagascar K-feldspar using the ultra-violet laser ablation microprobe (UVLAMP)

Controversy surrounding the mechanisms and controls on argon diffusion in K-feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures, over a range of temperatures. Measurements of argon diffusion profiles in a gem-quality iron-rich orthoclase heated in a cold seal apparatus, have been undertaken in situ using an ultra-violet laser ablation microprobe (UVLAMP) technique. The results agree very closely with the previously determined bulk values for Benson Mines orthoclase (activation energy ( E)=43.8±1 kcal mol −1) and vacuum furnace cycle-heating studies of K-feldspars ( E=46±6 kcal mol −1). However, instead of defining a single activation energy ( E) and diffusion coefficient ( D o), the data yield two sets of parameters: a low-temperature (550–720°C) array with an E of 47.2±2.5 kcal mol −1 (198.2±10.5 kJ mol −1) and a D o of 0.0374 +0.1123 −0.0281 cm 2 s −1, and a high-temperature (725–1019°C) array with an E of 63.8±3.4 kcal mol −1 (268.0±14.3 kJ mol −1) and a D o of 55.0 +225.5 −44.2 cm 2 s −1. The new results closely reproduce two sets of apparent activation energies previously measured in cycle-heating studies of Madagascar K-feldspar (40±3 and 57±3 kcal mol −1). Previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion. However, the UV depth profile analyses indicate simple diffusion to the grain surface and importantly, diffusion radii calculated by combining the UVLAMP and cycle-heating data, are the same as the physical grain sizes used in the experiments, around 1 mm. Vacuum furnace stepped heating experiments on slowly cooled K-feldspars have been interpreted as showing diffusion radii of around 6 μm and indicate complex populations of sub-grains. This study indicates that Madagascar K-feldspar and thus probably all gem-quality K-feldspars act as single diffusion domains and that short-circuit (or pipe) diffusion was not an important loss mechanism. An apparent diffusion compensation relationship in the stepped heating data for Madagascar K-feldspar implies that similar relationships seen in other K-feldspars are a result of a range of diffusion mechanisms.

Preliminary UVLAMP determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts

An ultra-violet laser ablation microprobe (UVLAMP) has been applied for the first time to investigate argon partition coefficients for olivines and clinopyroxenes grown from silicate melts at 1 bar argon pressure. These preliminary measurements yield crystal/melt partition coefficients ranging from 0.138 (±0.01) to 0.013 (±0.003) for olivine and 0.589 (±0.003) to 0.0016 (±0.0005) for clinopyroxene. The higher values may indicate sub-microscopic melt inclusions, or some other heterogeneous distributions of `non-equilibrium' argon in the crystals. The lower values are probably more representative of true partition coefficients and fall at least an order of magnitude below the previously reported experimental data. The possibility of anomalous, high argon contents for crystals in previous studies is discussed in terms of surface adsorption, `trapped' argon and early partial melting.

Argon Geochronology and Phengites: A Story of Excess Argon, Diagrams and Paradoxes

Among the most debated questions in the world of metamorphic dating is certainly that of high pressure metamorphism which ultimately underpins most geodynamical reconstructions and mechanisms of orogenesis. Applying argon dating to high pressure material brings both good news and possibly bad. The good news is that during the HP peak potassium rich phases are likely to crystallise in most pelitic but also in many magmatic systems. This is often represented by the crystallisation of the silica rich white mica: phengites. On the other hand the bad news is that the dating of such a ubiquitous and highly valuable phase has lead to a growing controversy. Many authors suspect that large quantities of excess argon can be incorporated during the growth of phengite leading to anomalously old ages. Moreover several flaws in the analyses protocol or the graphical tools usually used often prohibit the identification and correction of the incorporated excess argon. However only rarely can clear evidence of that artefact be isolated and the controversy rises from contradictory interpretations of age data. Below we expose a geological case in which field data and independent age constraints lead to the proposal of either the presence of excess argon, or very unusual diffusion characteristics for phengite. This in turn implies geological consequences which can be tested. Finally we discuss the possible directions of research which should lead to a definitive assessment of the role of phengite in dating HP terrains. metamorphic rocks which have yielded 40Ar-39Ar phengite and glaucophane ages of 88 to 108 Ma (Okay and Kelley, 1994; Harris et al., 1995) and 72 to 124 Ma A single sample, 96/134, has yielded a range of isotopic ages for the single high-pressure metamorphic event, with a Rb-Sr mineral age of 80.2 -41.6 Ma. for the same sample ultra-violet laser ablation microprobe (UVLAMP) white mica analyses are in the range 78 • 2 to 109 + 3 Ma and classical furnace step heating yielded a convincing plateau age of 107 _+ 2 Ma (Fig. 1), consistent with many of the oldest UVLAMP ages but 25 Ma older than the RbSr age for the same mineral.

Determination of high spatial resolution argon isotope variations in metamorphic biotites

The circumstances of preservation of relict metamorphic minerals and fabrics which have subsequently been involved in later metamorphic cycles remain enigmatic because the kinetic parameters are not well understood. Textures and chemistry cannot always give direct information on the conditions attained in the successive events. We show in this study that 40 Ar 39 Ar laser studies can, in contrast, provide thermal history determinations and successfully discriminate between geological models. Two samples have been examined from the Seconda Zona Diorito Kinzigitica (IIDK) of the Sesia Zone, Italian Alps. A unit which preserves relict Hercynian metamorphic assemblages and is now located structurally above rocks which preserve Alpine eclogite and greenschist facies assemblages. The circumstances of this preservation remain controversial, with proposed models falling into two main categories. One hypothesis advocates that the IIDK was juxtaposed into its present position late in the Alpine orogeny and experienced no significant heating event. Alternatively the IIDK has been in its present position for the duration of the Alpine orogenic cycle and has been exposed to high temperatures but due to sluggish reaction kinetics, it failed to reequilibrate and develop Alpine metamorphic minerals. A high spatial resolution 40 Ar 39 Ar study using the ultraviolet laser ablation microprobe (UVLAMP) technique was undertaken with the aims of reassessing the metamorphic history of the IIDK and investigating the behavior of argon within the unit during its thermal evolution. 128 analyses have been obtained from ten profiles, between 60 and 275 μm long, across biotite grains which show apparent ages ranging from 514 to 161 Ma within individual crystals. These apparent age variations are systematic according to location in the grain, with the oldest ages at the grain edges and the youngest in the cores. These apparent age variations can not be explained by chemical alteration and require diffusion of excess argon into the grains. The age profiles show a remarkable fit to theoretically derived curves for volume diffusion of argon. Time integrated diffusion values obtained for the profiles fall between 2 × 10 −6 cm −6 and 5 × 10 -6 cm 2; this variation is interpreted as being due to spatial and temporal fluctuations in the availability of argon from the grain boundary network. Diffusion, and hence profile development, initiated at different times within the biotites, consequently producing the range of time integrated diffusion values. The temperatures at which the profiles developed have been estimated using published biotite diffusion parameters to model possible thermal histories. Using the assumptions of either a short thermal pulse event or continuous cooling from high temperatures, the models suggest that the profiles developed at temperatures less than 300°C. Possible geological models are proposed which document the thermal and excess argon history of the IIDK from the Hercynian through to the Alpine orogeny. The study shows the key role of detailed argon isotope studies in assessing the thermal evolution of rocks with relict metamorphic assemblages.

Evidence for excess argon during high pressure metamorphism in the dora maira massif (Western Alps, Italy), using an ultra-violet laser ablation microprobe 40Ar-39Ar technique

Ultra-high pressure eclogite/amphibolite grade metamorphism of the Dora Maira Massif in the western Alps is a well established and intensively studied event. However, the age of peak metamorphism and early cooling remains controversial. The 40Ar-39 Ar step-heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 Ma which have been interpreted as the time of early cooling after the high pressure event. Recent U/Pb and Sm/Nd results challenge this assertion, indicating a much younger age for the event, around 45 Ma, and hence a radically different timing for the tectonic evolution of the western Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39 Ar dating. The new technique allowed selective in situ analysis, at a spatial resolution of 50 urn, of quartz, phengite, biotite and K-feldspar. The results demonstrate the frequent occurrence of excess argon with high 40Ar-39 Ar ratios (1000–10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar/36Ar ratios. Step-heating analysis of minerals with this mixture would produce linear arrays on a40Ar/36Ar versus 40Ar/36 Ar correlation diagram, leading to geologically meaningless plateau ages, older than the true closure age. In the present case, some ages in the range 60–110 Ma could be explained by the presence of excess argon incorporated around 40–50 Ma ago. Similar results found in other high pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40Ar/ 36 Ar ages in many high pressure terrains.