Calculation of radiogenic 26Mg of CAI minerals under high precision isotope measurement by SIMS

NASA's Genesis Mission returned solar wind (SW) to the Earth for analysis to derive the composition of the solar photosphere from solar material. SW analyses control the precision of the derived solar compositions, but their ultimate accuracy is limited by the theoretical or empirical models of fractionation due to SW formation. Mg isotopes are “ground truth” for these models since, except for CAIs, planetary materials have a uniform Mg isotopic composition (within ≤1‰) so any significant isotopic fractionation of SW Mg is primarily that of SW formation and subsequent acceleration through the corona. This study analyzed Mg isotopes in a bulk SW diamond‐like carbon (DLC) film on silicon collector returned by the Genesis Mission. A novel data reduction technique was required to account for variable ion yield and instrumental mass fractionation (IMF) in the DLC. The resulting SW Mg fractionation relative to the DSM‐3 laboratory standard was (−14.4‰, −30.2‰) ± (4.1‰, 5.5‰), where the uncertainty is 2ơ SE of the data combined with a 2.5‰ (total) error in the IMF determination. Two of the SW fractionation models considered generally agreed with our data. Their possible ramifications are discussed for O isotopes based on the CAI nebular composition of McKeegan et al. (2011).

Section: Introductionmentioning

confidence: 99%

Section: Introduction Science Behind Sw Mg Isotope Measurementsmentioning

confidence: 99%

Magnesium isotopes of the bulk solar wind from Genesis diamond‐like carbon films

Jurewicz

Rieck

Hervig

et al. 2020

“…The excess of radiogenic 26 Mg, δ 26 Mg*, was calculated via an exponential fractionation law with coefficient α natural = 0.5128 because natural fractionation for Mg-isotopes is considered to be controlled by evaporation processes (Davis et al 2015). However, the natural mass fractionation deviates from the instrumental mass fractionation of SIMS, and the instrumental mass fractionation also differs among target minerals and among analytical sessions under the measurement conditions (Itoh et al 2008;Kawasaki et al 2017Kawasaki et al , 2018Kawasaki et al , 2019Kawasaki et al , 2020. We determined instrumental mass fractionation, α SIMS , through measurements of terrestrial standards to calculate the excess radiogenic 26 (Catanzaro et al 1966) were used for δ 26 Mg* calculations, although the final corrected δ 26 Mg* values are independent of the reference ratios used.…”

Section: Al-mg Isotope Analysis In Multicollection Modementioning

confidence: 99%

“…However, the natural mass fractionation deviates from the instrumental mass fractionation of SIMS, and the instrumental mass fractionation also differs among target minerals and among analytical sessions under the measurement conditions (Itoh et al. 2008; Kawasaki et al. 2017, 2018, 2019, 2020).…”

Section: Experimental Techniquesmentioning

confidence: 99%

Oxygen and Al‐Mg isotopic constraints on cooling rate and age of partial melting of an Allende Type B CAI, Golfball

Kawasaki

Itoh

Sakamoto

et al. 2021

Self Cite

Coarse-grained, igneous Ca-Al-rich inclusions (CAIs) in CV chondrites formed through multiple melting events. We conducted in situ O-isotope analysis and Al-Mg systematics by secondary ion mass spectrometry of relict and overgrown minerals from a partial melting event in an Allende Type B CAI, Golfball. Golfball has a Type B CAI bulk composition and a unique structure: a fassaite-rich mantle enclosing a melilite-rich core. Many of the blocky melilite crystals in the core have irregularly shaped, Al-rich (Åk 5-15 ) cores enclosed in strongly zoned (Åk 30-70 ) overgrowths. Since the Al-rich melilite grains could not have formed from a melt of Golfball, they are interpreted as relict grains that survived later melting events. The O-isotopic compositions of the blocky melilite crystals plot along the carbonaceous chondrite anhydrous mineral line, ranging between Δ 17 O~−14‰ and −5‰. The Al-rich relict melilite grains and their overgrowths exhibit the same O-isotopic compositions, while the O-isotopic compositions are varied spatially among melilites. We found that the O-isotopic compositions steeply change across several melilite crystals within few tens of micrometers, indicating the O-isotopic compositions of the melt could not have been homogenized during the partial melting in that scale. According to the time scale of O self-diffusivity in the melt, the cooling rate of the partial melting event is calculated to be >6 × 10 4 K h −1 . Al-Mg isotope data for core minerals plot on a straight line on an Al-Mg evolution diagram. A mineral isochron for Golfball gives initial 26 Al/ 27 Al of (4.42 AE 0.20) × 10 -5 and initial δ 26 Mg* of −0.035 AE 0.050‰. The chemical and Oisotopic compositions of melilite and those initial values imply that its precursor consisted of fluffy Type A and/or fine-grained CAIs. The partial melting event for Golfball may have occurred in very short order after the precursor formation.

“…Primary beam current is about 10nA. Detailed experimental procedures are described in Itoh et al (2008). Measurements were carried out at a mass resolving power of approximately 2000, sufficient to resolve all molecular ion interferences (e.g., 24 MgH + , 48 Ca 2+ ).…”

Section: Sims Measurements Of Oxygen and Al-mg Isotope Analysesmentioning

confidence: 99%

XANES and Mg isotopic analyses of spinels in Ca‐Al‐rich inclusions: Evidence for formation under oxidizing conditions

Paque

Sutton

Simon

et al. 2013

Abstract-Ti valence measurements in MgAl 2 O 4 spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti +4 . Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti +3 -rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti +4 -rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average D 25 Mg of 4.25 AE 0.028&, consistent with formation of the spinel from an evaporating liquid. D 25Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The D 25 Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti +4 concentrations in spinel.