Oxygen-isotope and trace element constraints on the origins of silica-rich melts in the subarc mantle

Peridotitic xenoliths in basaltic andesites from Batan island in the Luzon arc contain silica-rich (broadly dacitic) hydrous melt inclusions that were likely trapped when these rocks were within the upper mantle wedge underlying the arc. These melt inclusions have been previously interpreted to be slab-derived melts. We tested this hypothesis by analyzing the oxygen isotope compositions of these inclusions with an ion microprobe. The melt inclusions from Batan xenoliths have δ 18OVSMOW values of 6.45 ± 0.51‰. These values are consistent with the melts having been in oxygen isotope exchange equilibrium with average mantle peridotite at temperatures of ≥875°C. We suggest the δ 18O values of Batan inclusions, as well as their major and trace element compositions, can be explained if they are low-degree melts (or differentiation products of such melts) of peridotites in the mantle wedge that had previously undergone extensive melt extraction followed by metasomatism by small amounts (several percent or less) of slab-derived components. A model based on the trace element contents of Batan inclusions suggests that this metasomatic agent was an aqueous fluid extracted from subducted basalts and had many characteristics similar to slab-derived components of the sources of arc-related basalts at Batan and elsewhere. Batan inclusions bear similarities to “adakites,” a class of arc-related lava widely considered to be slab-derived melts. Our results suggest the alternative interpretation that at least some adakite-like liquids might be generated from low-degree melting of metasomatized peridotites

Fluid Flow, Brecciation, and Shear Heating on Faults: Insights from Carbonate Clumped-Isotope Thermometry

Slip on gently dipping detachments in the brittle crust has been enigmatic for decades, because fracture mechanics laws predict frictional resistance is too great for sliding to occur, except under rather unusual circumstances. The Miocene Mormon Peak detachment in Nevada and the Eocene Heart Mountain detachment in Wyoming are two well‐studied examples of upper crustal, carbonate‐hosted low‐angle detachments, with highly debated slip processes. Both low‐angle faults were active during regional magmatism, and a number of proposed slip mechanisms involve magmatic fluids, frictional heating, or both. To address the role that magmatic fluids and frictional heating may have played in reducing friction, we measured clumped‐isotope ratios on 137 carbonate samples from these faults. The majority of fault breccias and gouges on the detachment slip surface record temperatures that are colder than the host rock. Surprisingly, samples from within 5 m of the Heart Mountain detachment average just 65 °C, and not a single sample (out of 37 measurements, excluding metamorphosed host rock at White Mountain) records a temperature greater than 90 °C. Along both faults, most samples are depleted in δ^(18)O relative to the host rock, indicating that meteoric, not magmatic, fluids were present and interacting with the fault rock. However, a few samples preserve temperatures of over 160 °C, which, based on textural and geochemical criteria, are difficult to explain other than by frictional heating during slip. These temperatures are recorded in one sample directly on the Mormon Peak detachment slip surface and in two hanging wall localities above the Heart Mountain detachment

The Effects of Metamorphism on Iron Mineralogy and the Iron Speciation Redox Proxy

As the most abundant transition metal in the Earth’s crust, iron is a key player in the planetary redox budget. Observations of iron minerals in the sedimentary record have been used to describe atmospheric and aqueous redox environments over the evolution of our planet; the most common method applied is iron speciation, a geochemical sequential extraction method in which proportions of different iron minerals are compared to calibrations from modern sediments to determine water-column redox state. Less is known about how this proxy records information through post-depositional processes, including diagenesis and metamorphism. To get insight into this, we examined how the iron mineral groups/pools (silicates, oxides, sulfides, etc.) and paleoredox proxy interpretations can be affected by known metamorphic processes. Well-known metamorphic reactions occurring in sub-chlorite to kyanite rocks are able to move iron between different iron pools along a range of proxy vectors, potentially affecting paleoredox results. To quantify the effect strength of these reactions, we examined mineralogical and geochemical data from two classic localities where Silurian-Devonian shales, sandstones, and carbonates deposited in a marine sedimentary basin with oxygenated seawater (based on global and local biological constraints) have been regionally metamorphosed from lower-greenschist facies to granulite facies: Waits River and Gile Mountain Formations, Vermont, USA and the Waterville and Sangerville-Vassalboro Formations, Maine, USA. Plotting iron speciation ratios determined for samples from these localities revealed apparent paleoredox conditions of the depositional water column spanning the entire range from oxic to ferruginous (anoxic) to euxinic (anoxic and sulfidic). Pyrrhotite formation in samples highlighted problems within the proxy as iron pool assignment required assumptions about metamorphic reactions and pyrrhotite’s identification depended on the extraction techniques utilized. The presence of diagenetic iron carbonates in many samples severely affected the proxy even at low grade, engendering an interpretation of ferruginous conditions in all lithologies, but particularly in carbonate-bearing rocks. Increasing metamorphic grades transformed iron in carbonates into iron in silicate minerals, which when combined with a slight increase in the amount of pyrrhotite, drove the proxy toward more oxic and more euxinic conditions. Broad-classes of metamorphic reactions (e.g. decarbonation, silicate formation) occurred at distinct temperatures-pressures in carbonates versus siliciclastics, and could be either abrupt between metamorphic facies or more gradual in nature. Notably, these analyses highlighted the importance of trace iron in phases like calcite, which otherwise might not be included in iron-focused research i.e. ore-system petrogenesis, metamorphic evolution, or normative calculations of mineral abundance. The observations show that iron is mobile and reactive during diagenesis and metamorphism, and these post-depositional processes can readily overprint primary redox information held by iron speciation. However, in principle, additional mineralogical and petrographic approaches can be combined with iron speciation data to help untangle many of these post-depositional processes and arrive at more accurate estimates of paleoenvironmental redox conditions and processes, even for metamorphosed samples

Stable isotopic disequilibrium in high-T metamorphic systems

A principal use of stable isotopes in metamorphic rocks is as thermometers, or as tests for isotopic equilibrium where metamorphic temperatures are known. Applications are often complicated when apparent isotopic temperatures are discordant and disagree with petrologic temperatures, indicating a failure of isotopic systems to record and/or preserve equilibrium, peak-T fractionations. In low-T, fluid-hosted environments such features often clearly reflect open system exchange. However, in high-T metamorphic environments a slow cooling history can be sufficient to produce such features by retrograde, closed system exchange between coexisting minerals

An absolute reference frame for clumped isotope thermometry

Analysis of multiply substituted isotopologues of molecules (‘clumped isotope geochemistry’) presents special challenges to both precision and accuracy. Previous discussions have focused on mass spectrometric precision for these rare species and intralaboratory reference frames. This discipline has spread, demanding interlaboratory standardization. We present a four-laboratory study of the calibration of mass-47 anomalies (Δ_(47) values) in CO_2 (especially extracted from carbonate). We consider: instrument linearity, source fragmentation/recombination reactions (which vary between mass spectrometers and with time and instrument settings), and differences in methods, materials and conditions for sample preparation. We address these problems by developing a method for standardizing Δ_(47) measurements to an absolute reference frame based on theoretical predictions of the abundances of multiply-substituted isotopologues of gaseous CO_2 that has reached a thermodynamic equilibrium at a known temperature. By analyzing CO_2 gases that have been subjected to established laboratory procedures known to promote isotopic equilibration (i.e., heated gases and water-equilibrated CO_2), and by reference to the statistical thermodynamic predictions of equilibrium isotopic distributions, it is possible to construct an empirical transfer function that can then be applied to CO_2 samples with unknown Δ_(47) values. This reference frame may be unique in that it is based on thermodynamic equilibrium, rather than the isotopic composition of an arbitrary reference material. We describe the protocol necessary to construct such a reference frame, the method for converting gases with unknown clumped isotope compositions to this frame, and suggest a protocol for ensuring that reported Δ_(47) values can be compared among different laboratories, independent of laboratory-specific analytical or methodological artefacts. Application of this approach to measurements of CO_2 extracted from several carbonate reference materials results in interlaboratory agreement on their Δ_(47) values to within est. ±0.01 ‰, 1σ. Finally, we present a revised paleotemperature scale that applies when using the absolute reference frame described here, as opposed to the previous paleotemperature equation based on data from a single laboratory. More generally, this study presents a model for how interlaboratory standardization might be approached for other ‘clumped isotope’ measurements

Use of Clumped-Isotope Thermometry To Constrain the Crystallization Temperature of Diagenetic Calcite

We describe an approach to estimating the crystallization temperatures of diagenetic calcites using clumped-isotope thermometry, a paleothermometer based on the ^(13)C–^(18)O-bond enrichment in carbonates. Application of this thermometer to calcified gastropod shells and calcite cements in an early Eocene limestone from the Colorado Plateau reveals a record of calcite precipitation and replacement at temperatures varying from 14 to 123°C. The early Eocene host sediments were never deeply buried, but they experienced a significant thermal pulse associated with the emplacement of a late Miocene basalt flow. The combination of independent constraints on thermal history with clumped-isotope thermometry, petrographic (including cathodoluminescence) observations, and oxygen isotopic data provides an improved basis for estimation of the temperature and timing of diagenetic events and fluid sources. The petrography and calcite δ^(18)O values, taken alone, suggest that the aragonite-to-calcite transformation of gastropod shell material occurred simultaneously with early formation of cements and lithification of the matrix in the same sample. However, addition of clumped-isotope thermometry demonstrates that this phase transformation of shell material occurred at temperatures of 94–123°C in a highly rock-buffered microenvironment (i.e., with the isotopic composition of fluid buffered by coexisting carbonate), millions of years after lithification of the matrix and formation of initial low-temperature (14–19°C) calcite cements within shell body cavities. Clumped-isotope temperatures in excess of reasonable Earth-surface conditions recorded by later-formed cements demand that cement growth occurred in association with the lava emplacement. Our results illustrate the potential for clumped-isotope thermometry to constrain conditions of diagenesis and guide interpretations that would not be possible on the basis of conventional stable-isotopic and petrographic data alone, and demonstrate how petrographic characterization of clumped-isotope thermometry samples can benefit paleoclimate studies

A hydrothermal origin for isotopically anomalous cap dolostone cements from south China

The release of methane into the atmosphere through destabilization of clathrates is a positive feedback mechanism capable of amplifying global warming trends that may have operated several times in the geological past. Such methane release is a hypothesized cause or amplifier for one of the most drastic global warming events in Earth history, the end of the Marinoan ‘snowball Earth’ ice age, ~635 Myr ago. A key piece of evidence supporting this hypothesis is the occurrence of exceptionally depleted carbon isotope signatures (δ^(13)C_(PDB) down to −48‰; in post-glacial cap dolostones (that is, dolostone overlying glacial deposits) from south China; these signatures have been interpreted as products of methane oxidation at the time of deposition. Here we show, on the basis of carbonate clumped isotope thermometry, ^(87)Sr/^(86)Sr isotope ratios, trace element content and clay mineral evidence, that carbonates bearing the ^(13)C-depleted signatures crystallized more than 1.6 Myr after deposition of the cap dolostone. Our results indicate that highly ^(13)C-depleted carbonate cements grew from hydrothermal fluids and suggest that their carbon isotope signatures are a consequence of thermogenic methane oxidation at depth. This finding not only negates carbon isotope evidence for methane release during Marinoan deglaciation in south China, but also eliminates the only known occurrence of a Precambrian sedimentary carbonate with highly ^(13)C-depleted signatures related to methane oxidation in a seep environment. We propose that the capacity to form highly ^(13)C-depleted seep carbonates, through biogenic anaeorobic oxidation of methane using sulphate, was limited in the Precambrian period by low sulphate concentrations in sea water. As a consequence, although clathrate destabilization may or may not have had a role in the exit from the ‘snowball’ state, it would not have left extreme carbon isotope signals in cap dolostones

Characterizing Early Solar System Fluids on the Allende (CV3) Parent Body: Nanosims Study of Phosphate Volatile Contents

Much of our knowledge about the early Solar System comes from the study of CV3 meteorites, in particular Allende. Hydrothermal alteration is recognized as an important, ubiquitous process within these rocks (e.g. [1]). Nevertheless, we know little regarding the nature of these fluids. Volatile element abundances in phosphates from carbonaceous chondrites could constrain relative fugacities of H_2O, HF and HCl in the fluid present during metasomatism in primitive planetesimals

A 600-Million-Year Carbonate Clumped-Isotope Record from the Sultanate of Oman

Carbonate clumped-isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks. This study utilizes a 600-million-year record of marine carbonate rocks from the subsurface and surface of the Sultanate of Oman to explore how burial and exhumation affected the carbonate clumped-isotope thermometer. Samples span 6 km of burial depth, and include calcite and dolomite mineralogies and a range of carbonate rock textures. We find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial. The first group of carbonates yield water δ^(18)O_(VSMOW) compositions slightly enriched or equal to an expected “ice-free” seawater composition of –1.2‰ and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment–water interface. Temperatures from the second group sit on the present-day geotherm, yield highly enriched water δ^(18)O_(VSMOW) compositions, and display fair to poor textural preservation. We find no evidence for solid-state reordering in paired analyses of calcites and dolomites. Our results contribute to a growing body of work that indicates that the seawater δ^(18)O_(VSMOW) composition has not changed significantly over 600 Myr and was not –6‰ in the Ediacaran

A 600-Million-Year Carbonate Clumped-Isotope Record from the Sultanate of Oman

Carbonate clumped-isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks. This study utilizes a 600-million-year record of marine carbonate rocks from the subsurface and surface of the Sultanate of Oman to explore how burial and exhumation affected the carbonate clumped-isotope thermometer. Samples span 6 km of burial depth, and include calcite and dolomite mineralogies and a range of carbonate rock textures. We find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial. The first group of carbonates yield water δ^(18)O_(VSMOW) compositions slightly enriched or equal to an expected “ice-free” seawater composition of –1.2‰ and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment–water interface. Temperatures from the second group sit on the present-day geotherm, yield highly enriched water δ^(18)O_(VSMOW) compositions, and display fair to poor textural preservation. We find no evidence for solid-state reordering in paired analyses of calcites and dolomites. Our results contribute to a growing body of work that indicates that the seawater δ^(18)O_(VSMOW) composition has not changed significantly over 600 Myr and was not –6‰ in the Ediacaran