The effects of experimental heating and alteration on melt inclusions in magmatic zircon
Bibliographic record
Abstract
Melt inclusions (MIs) in zircon can preserve information on the evolution of magmas. However, MIs in plutonic zircons are typically devitrified, consisting of multiple phases that must be remelted to obtain a homogeneous glass for reconstruction of melt composition and magma processes. We undertake a systematic investigation of melt inclusions in zircons from a ~ 3300 Ma xenolith of tonalite gneiss from the Barberton Greenstone belt, a well-studied section of cratonic lithosphere with components dating back to 3500 Ma. To better understand the influence of experimental heating on zircon and MI chemistry, multiple aliquots of zircons were heated in an internally heated pressure vessel at 0.4 GPa and temperatures ranging from 900 to 1200 °C (T step = 100 °C). Homogeneous MIs in domains with low degrees of radiation damage and isolated from cracks in the zircon were found by examination of >5000 zircons by SEM (CL, BSE). Oxygen isotopes (δ 18 O), OH/O ratios, U-Pb isotopes, trace and rare earth element (TREE) concentrations in zircon, along with δ 18 O, H 2 O contents, and major element compositions in glassy MIs were measured by SIMS and EPMA. Investigated MIs have granitic compositions with 67 to 81 wt% SiO 2 . Both heated and unheated host zircon possess statistically identical and uniform δ 18 O values of 6.02 ± 0.45 ‰ (2SD), while OH/O ratios systematically decrease with increasing temperature of laboratory heating. Inclusion textures (BSE contrast homogeneity) and composition (H 2 O, δ 18 O) suggest that experimental heating at 1100 °C was the most successful in recovering initial MI chemistries. Inclusions in lower temperature experiments either did not homogenize (900 °C) or are rarely homogenized (1000 °C), while those at higher temperature (1200 °C) are systematically dehydrated. Twenty-two hydrous MIs from the 1100 °C experiment have 3.1–11.5 wt% H 2 O and an average δ 18 O of 7.5 ± 0.9 ‰. Four zircons have a Δ 18 O(MI-Zrn) fractionation inconsistent with equilibration at magmatic temperatures. In general, δ 18 O values and TREE concentrations measured in the zircons heated at 1100 °C show consistent behavior with unannealed zircons, indicating these systems are not significantly disturbed on the μm-scale during heating experiments, and support the use of anomalous TREE concentrations/patterns as indicators of alteration. These measurements combined with Δ 18 O(MI-Zrn) identify the MI-zircon pairs that are unlikely to represent the melt composition at the time of entrapment. Ti-in-zircon temperatures (αSiO 2 = 1, αTiO 2 = 0.3) and rhyoliteMELTS thermometry of the unaltered MI-zircon pairs return similar temperature ranges of 762–833 °C and 750–865 °C respectively, and combined with the granitic major element compositions, suggest that zircons crystallized relatively late during melt fractionation and entrapped residual evolved melt. More generally, these experiments represent the first direct reconstruction of H 2 O contents and oxygen isotopes of Archean melts from zircon-hosted MIs, and the approach described here can be used as a model for evaluating potential alteration during experimental heating and the geologic history for MI-zircon pairs from plutonic rocks. As compared with the TREE concentrations of similar-aged detrital zircons in the Barberton terrane, the tonalite xenolith zircons are distinct and attest to the diversity of magma compositions around 3300 Ma that formed the Barberton basement. Given that no comparable rock with zircons of the same age and TREE chemistry is exposed in the Barberton terrane, this tonalite likely represents an unknown component of the Barberton basement.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".