Chromitite and anorthosite layers in Critical Zone, UG1 of the Bushveld Complex, in the classic Mononono (formerly Dwars) River outcrop, near Steelpoort, Mpumalanga province, South Africa.
Photo by: Kevin Walsh
The thing about trying to cling to the childish belief that the Universe and Earth were created out of nothing by magic just 10,000 years ago, is that that leaves the remaining 99.9975% of Earth's history to refute the daft idea.
This means it is trivially easy for science to refute creationism without even trying simply by revealing the facts of events from that vast period of Earth history that wouldn't be there if creationists superstition was correct.
And yet another of those fact has just been revealed by a team of geologists and geophysicists led by Duncan Keller and Cin-Ty Lee, working at Rice University, Houston, Texas, USA.
They have shown how the massive rock formations known as anorthosites were formed. These rocks can cover as much as 42,000 Km2. The team focused on the Marcy and Morin anorthosites from the 1.1-billion-year-old Greenville orogen.
Can you translate the following into plain English and explain the significance of the isotope analyses, please:The research team have just published their findings, open access, in the journal Science Advances and explained it in a Rice University press release by Macy De Luna:
Using B, O, Nd, and Sr isotope analyses, bulk chemistry, and petrogenetic modeling, we demonstrate that the magmas parental to the Marcy and Morin anorthosites, classic examples from North America’s Grenville orogen, require large input from mafic melts derived from slab-top altered oceanic crust. Certainly! Here's a plain English translation and an explanation of the significance of the isotope analyses:
Plain English Translation:
Using different scientific techniques, including the analysis of specific isotopes (forms of elements with different atomic weights), chemical composition, and models of how rocks form, we show that the molten rock that eventually formed the Marcy and Morin anorthosites (types of rocks found in North America's Grenville mountain range) needed a significant amount of material from mafic (rich in magnesium and iron) melts. These mafic melts originated from oceanic crust that had been altered by the conditions at the top of a subducting slab (a piece of oceanic crust sinking under another tectonic plate).
Explanation of the Significance:
- Isotope Analyses (B, O, Nd, Sr): The isotopes of elements like boron (B), oxygen (O), neodymium (Nd), and strontium (Sr) are used to trace the origins and processes involved in the formation of rocks. Different environments (like the deep ocean, the Earth's crust, or the mantle) leave specific isotopic signatures in the rocks that form there. By analyzing these isotopes, scientists can figure out where the material that made up the Marcy and Morin anorthosites came from and what processes it underwent.
- Mafic Melts from Altered Oceanic Crust: The study suggests that a significant portion of the magma that formed these rocks came from mafic melts. Mafic melts are typically produced in the mantle (the layer beneath the Earth's crust) but in this case, the source was specifically the oceanic crust. This oceanic crust had been chemically altered by water and other elements as it moved along the top of a subducting slab (a process where one tectonic plate slides under another into the Earth's mantle).
- Petrogenetic Modeling: This involves creating models to understand how rocks formed, including the conditions under which the magma originated, what materials were involved, and how the process evolved over time.
Significance:
- Understanding Rock Formation: This analysis helps geologists understand the complex processes that formed the Marcy and Morin anorthosites. These rocks are significant because they are classic examples from the Grenville orogen, a mountain-building event that shaped much of North America's geology.
- Plate Tectonics Insight: The study provides evidence that the magmas forming these rocks were influenced by subduction processes (where oceanic plates are pushed under continental plates). This is important for understanding how different types of Earth's crust interact and the role of subduction in creating new types of rocks.
- Isotopic Signatures: The specific isotopic signatures help pinpoint the sources of the materials in these rocks, giving a clearer picture of the geological history of the area. This information can be used to compare with other regions and contribute to the broader understanding of Earth's geological processes.
Is it possible to infer the age of the rocks from this?
Inferring the age of rocks directly from the information provided would be difficult without additional context or data. However, isotopic analyses, especially involving elements like neodymium (Nd) and strontium (Sr), can indeed be used to estimate the age of rocks under certain conditions.
Here's how age determination might work:
- Radiometric Dating: Isotopes like Nd and Sr can be used in radiometric dating, a method that determines the age of rocks by measuring the ratio of parent to daughter isotopes. For example, neodymium isotopes are often used in a method called Sm-Nd dating, where samarium (Sm) decays into neodymium over time. By measuring the proportions of these isotopes, geologists can estimate when the rock formed.
- Isotopic Signatures and Geological History: The isotopic signatures mentioned (B, O, Nd, Sr) can also provide clues about the rock's history, including the age of the source material from which the magma was derived. For example, if the magma included material from ancient oceanic crust, the isotopic composition could reflect the age of that crust, giving indirect clues about the age of the rock itself.
Conclusion:
While the statement doesn't explicitly mention using these isotopes for age dating, the isotopic data combined with other geological information could potentially be used to infer the age of the Marcy and Morin anorthosites. If the study included isotopic ratios specifically for radiometric dating, then yes, the age of the rocks could be inferred. Otherwise, more data would be needed to directly determine the age.
Researchers unveil mysteries of ancient Earth
Rice-led geochemistry study advances understanding of ancient rock formations
A team of researchers has made strides in understanding the formation of massif-type anorthosites, enigmatic rocks that only formed during the middle part of Earth’s history. These plagioclase-rich igneous rock formations, which can cover areas as large as 42,000 square kilometers and host titanium ore deposits, have puzzled scientists for decades due to conflicting theories about their origins.
A new study published in Science Advances on Aug. 14 highlights the intricate connections between Earth’s evolving mantle and crust and the tectonic forces that have shaped the planet throughout its history. It also provides new ways to explore when plate tectonics began, how subduction dynamics operated billions of years ago and the evolution of Earth’s crust.
The research team, led by Rice’s Duncan Keller and Cin-Ty Lee, studied massif-type anorthosites to test ideas about the magmas that formed them. The research focused on the Marcy and Morin anorthosites, classic examples from North America’s Grenville orogen that are about 1.1 billion years old.
By analyzing the isotopes of boron, oxygen, neodymium and strontium in the rocks as well as conducting petrogenetic modeling, the researchers discovered that the magmas that formed these anorthosites were rich in melts derived from oceanic crust altered by seawater at low temperatures. They also found isotopic signatures corresponding to other subduction zone rocks such as abyssal serpentinite.
Our research indicates that these giant anorthosites likely originated from the extensive melting of subducted oceanic crust beneath convergent continental margins. Because the mantle was hotter in the past, this process directly connects the formation of massif-type anorthosites to Earth’s thermal and tectonic evolution.
Dr. Duncan Keller, led author
Clever Planets Postdoctoral Research Associate
Earth, Environmental and Planetary Sciences
Rice University, Houston, TX, USA.
The study, which combines classical methods with the novel application of boron isotopic analysis to massif-type anorthosites, suggests that these rocks formed during very hot subduction that may have been prevalent billions of years ago.
Because massif-type anorthosites don’t form on Earth today, the new evidence linking these rocks to very hot subduction on the early Earth opens new interdisciplinary approaches for understanding how these rocks chronicle the physical evolution of our planet.
This research advances our understanding of ancient rock formations and sheds light on the broader implications for Earth’s tectonic and thermal history.
Professor Cin-Ty A. Lee, co-author
The Harry Carothers Wiess Professor of Geology
Professor of Earth, environmental and planetary sciences
Rice University, Houston, TX, USA.
The study’s other co-authors include William Peck of the Department of Earth and Environmental Geosciences at Colgate University; Brian Monteleone of the Department of Geology and Geophysics at Woods Hole Oceanographic Institution; Céline Martin of the Department of Earth and Planetary Sciences at the American Museum of Natural History; Jeffrey Vervoort of the School of the Environment at Washington State University; and Louise Bolge of the Lamont-Doherty Earth Observatory at Columbia University.
This study was supported by NASA and the U.S. National Science Foundation.
AbstractFor creationists wishing to refute the dating methods, the following is how the research team described their methodology in their paper. It is very technical but any creationists who understands the science should have no difficulty spotting the error(s), they believe are there:
Massif-type anorthosites, enormous and enigmatic plagioclase-rich cumulate intrusions emplaced into Earth’s crust, formed in large numbers only between 1 and 2 billion years ago. Conflicting hypotheses for massif-type anorthosite formation, including melting of upwelling mantle, lower crustal melting, and arc magmatism above subduction zones, have stymied consensus on what parental magmas crystallized the anorthosites and why the rocks are temporally restricted. Using B, O, Nd, and Sr isotope analyses, bulk chemistry, and petrogenetic modeling, we demonstrate that the magmas parental to the Marcy and Morin anorthosites, classic examples from North America’s Grenville orogen, require large input from mafic melts derived from slab-top altered oceanic crust. The anorthosites also record B isotopic signatures corresponding to other slab lithologies such as subducted abyssal serpentinite. We propose that anorthosite massifs formed underneath convergent continental margins wherein a subducted or subducting slab melted extensively and link massif-type anorthosite formation to Earth’s thermal and tectonic evolution.
INTRODUCTION
Anorthosites are intrusive igneous rocks composed of ≥90% plagioclase feldspar that represent accumulated crystals concentrated from a crystallizing magma (1). On the modern Earth, where mafic magmatism occurs in oceanic spreading centers, above subducting slabs, and at intraplate hotspots, anorthosites form only as minor lenses, layers, or intrusions closely associated with their parental magmas (1, 2). In contrast, an enigmatic style of anorthosite magmatism operated between 2.6 and 0.5 billion years ago (Ga). Only during this period did massif-type anorthosites, composite intrusions of plagioclase cumulates separated from their parental magma chambers and intruded into the continental crust, form worldwide. Between 1.8 and 0.9 Ga, during the assembly and persistence of the supercontinents Nuna and Rodinia, numerous massifs, including the largest known, were generated (Fig. 1). The largest massifs reach up to at least 42,000 km2 in preserved extent and several kilometers in thickness (3), making them comparable to the texturally similar composite granitoid batholiths generated at modern convergent margins. The spatiotemporal patterns of anorthosite occurrence on Earth hint at fundamental changes in Earth’s geodynamic and magmatic styles through time, but why massif-type anorthosites are restricted in time remains debated (2, 4–6). Factors such as interplays between mantle temperatures and lithospheric strengthening (4), slower plate velocities (5), and effects from long-lived supercontinents and high mantle heat flow during the Proterozoic (2, 6) have each been proposed as mechanisms favorable to the generation of the long-lived, voluminous mafic magmas parental to massif-type anorthosites.
Some key aspects of massif-type anorthosite genesis are well-constrained. Their primary mineralogy of intermediate-composition plagioclase (~An30–70) with or without pyroxenes, olivine, and oxides, and little to no primary amphibole points to crystallization from mafic parental magmas with relatively low water activities (1, 4, 7, 8). Restricted ranges of plagioclase composition at the massif scale (e.g., ~An47±8 for the Marcy and Morin) suggest compositional buffering of the magmatic system, perhaps through recharge of long-lived parental magma chambers (9). The anorthosite bodies were emplaced into the continental crust, apparently in multiple pulses, as evidenced by the cross-cutting generations of anorthosite cumulates that can be observed in outcrop in many massifs (e.g., Fig. 2A). Emplacement of some massifs is well-constrained to mid- to upper-crustal depths (1, 7, 10). However, barometric constraints such as from high-Al pyroxene megacrysts formed at ~9 to 13 kbar that were entrained by ascending anorthositic mushes (Fig. 2, B and C) (7, 11) suggest that plagioclase crystallization was a multistage process that happened, at least in part, at the base of the continental crust. These observations, as well as the observation that there is apparently insufficient preserved fossil mafic magma in or around the massifs to be complementary to the anorthosite cumulates, have led to the hypothesis that the anorthosites formed from ascending crystal-rich batches derived from a deeper system (1). Numerical modeling of these systems shows that buoyant rise of thick plagioclase cumulate mush piles could transport the anorthosites upward through the crust (12).
Fig. 1. Temporal distribution and size of massif-type anorthosites worldwide.
Gray symbols represent the ages of anorthosite in the massifs; age uncertainties are not shown but are, in most cases, smaller than the symbol. Brown diamonds represent the ages of pyroxene megacrysts that have been dated and are connected to their host massif by a dotted line. Massif sizes and ages are from (3, 9, 13, 128). Periods of supercontinent assembly are from (129). See (6) for discussion of failed or partial breakup of Nuna.Despite these constraints on massif-type anorthosite formation, debate continues regarding the origins of the parental magmas and the tectonic setting(s) in which they were generated. Some studies argue for mafic parental magmas derived mostly from mantle sources [e.g., (2, 8, 9, 13)], while other studies argue for mostly crustal sources [e.g., (7, 14, 15)]. Both divergent and convergent tectonic settings have been inferred to produce the massifs’ parental magmas [see reviews in (1, 2)], but most hypotheses invoke mantle melting through upwelling asthenosphere as the heat and/or melt source for anorthosite generation [e.g., (16)].
Fig. 2. Anorthosites and leucogabbros in outcrop and thin section.
(A) Marcy outcrop (Wolfjaw Mountain) with a leucogabbro block (Leuco) in anorthosite (Anorth), showing that multiple generations of cumulate mushes coalesced to form the pluton. (B) Cluster of orthopyroxene megacrysts ~1 m in size in the Marcy anorthosite (Woolen Mill locality). (C) Clinopyroxene megacryst in thin section showing exsolved plagioclase (sample 98MA1A, crossed polars). (D) Coarse Marcy plagioclase in thin section (sample 14AD9A, crossed polars). (E) Pyroxene (Pyx) and plagioclase (Plag) in a Morin gabbroic anorthosite (sample 95MR115, crossed polars).
Recent geochronological studies have shown that some massif-type anorthosites were formed over time intervals of up to ~120 million years (9, 13, 17). These are unusually long time intervals for magmatism associated with plumes or divergent settings but are similar to geochronologic constraints for convergent settings. These findings have inspired the hypothesis that massif-type anorthosites are the products of long-lived magmatic systems underneath continental volcanic arcs (9, 13). Phanerozoic continental arcs do not generate massif-type anorthosites, so if a convergent margin hypothesis is correct, then the profound implication is that Proterozoic continental arcs must have operated differently from those of the Phanerozoic (Fig. 1).
Here, we combine geochemical and petrological approaches to study the magmas parental to the Marcy and Morin anorthosites of the North American Grenville Province and test hypotheses of the massifs’ formation. Because a convergent margin setting has been the focus of several recent studies of other massifs, we pair analyses of B and O stable isotopes, which are sensitive to subduction-related processes, with analyses of Nd and Sr radiogenic isotopes, which are sensitive to source reservoir age. Boron and O stable isotope analyses are particularly useful for detecting input from low-temperature altered oceanic crust (LTAOC; the pillow basalts and sheeted dikes in the upper kilometer of the oceanic crust altered by seawater at ≤400°C). Assimilation of LTAOC by the Marcy and Morin parental magmas has been hypothesized (18) but has not been tested using B isotopes. The combination of a major element stable isotope system (O) that accounts for a large fraction of the rock’s mass with a trace element stable isotope system (B) that is highly sensitive to source lithology offers complementary information on magma sources. We integrate the B, O, Nd, and Sr isotope data with numerical modeling of magma crystallization and bulk rock chemistry to constrain the origins and evolution of the magmatic systems that produced the anorthosites.
Duncan S. Keller et al. ,
Mafic slab melt contributions to Proterozoic massif-type anorthosites. Sci. Adv. 10, eadn3976 (2024). DOI:10.1126/sciadv.adn3976
Copyright: © 2024 The authors.
Published by American Association for the Advancement of Science. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
MATERIALS AND METHODS SamplesSo, all creationists have to do now is explain, with evidence, why the dating method for the Marcy and Morin anorthosites made them look like they were 1.1 billion years old and not the 10,000 years old or less that they would be if creationism is correct. For those tempted to try the evidence-free claim that decay rates were much higher in those days than now, all they need do is explain how that was true within a range of nuclear forces that still allowed atoms to be formed when their putative creator created living organisms, or how it did it without atoms and just elementary particles and electromagnetism.
Rock samples of anorthosite and associated plagioclase-rich rocks (e.g., leucogabbro) were sourced from material collected for prior work (77, 78).
Boron isotope analysis and sample preparation
For B isotope analyses, petrographic thin sections were prepared using alumina grit rather than diamond paste due to the B contamination hazard posed by diamonds (79). Before SIMS analysis, 1″ diameter polished sections were cleaned in an ultrasonic bath first with deionized water and then with Milli-Q \(\small \ce{H2O}\) for 10 min at each stage and then dried in an oven at ~40°C before gold coating.
One set of boron isotope analyses was performed at Woods Hole Oceanographic Institution using the Cameca IMS 1280 SIMS in the Department of Geology and Geophysics. Analytical methods followed those in (79). Analyses used a 12-keV \(\small \ce{^16O−}\) primary ion beam, ~50- to 80-nA beam current, 10-kV secondary acceleration voltage, a 4000 μm by 4000 μm field aperture, and produced a ~50- to 100-μm-diameter beam spot. Sample surfaces were presputtered for 300 to 500 s before analysis, followed by counting \(\small \ce{^10B+}\)(15 s), \(\small \ce{^11B+}\) (5 s), and \(\small \ce{^28Si++}\) (1 s) over 40 cycles in monocollector mode using an electron multiplier. Total analysis time for each spot was 32 min. The relatively high primary beam current was necessary for transmitting sufficient signals of \(\small \ce{^10B+}\) and \(\small \ce{^11B+}\) to generate counting statistics for adequately precise ratios on grains with B (<1 μg/g). The strong primary beam, combined with a relatively long presputtering time, sputtered through potential surface B (contamination) before measurement. In addition, the 4000 μm by 4000 μm field aperture blocked the transmission of secondary B+ ions from outside of the innermost 40 μm by 40 μm from the center of the crater during analysis, thereby minimizing the transmission stray surface B+ ions from the crater periphery. Analysis of Boron-free Herasil glass on the standard mount produced <5 counts/s \(\small \ce{^11B+}\) ions and <1 count/s \(\small \ce{^10B+}\), demonstrating that the analytical setup effectively negated surface background contamination on cleaned, well-polished surfaces. The primary standard used between samples was GOR132 in session one and StHs6/80 in session two, with GOR132, GOR128, B6, StHs6/80, and NIST612 used as standards for calibrations. Comparison of instrumental mass fractionation factors between mafic GOR128 and GOR132 and andesitic StHs6/80 shows no matrix dependence on instrumental mass fractionation, as previously demonstrated (79). Fractionation factors of these three glasses were within error and reproducible to 1.2‰ relative standard deviation (RSD; 2σ). Fractionation factors were smaller than those previously determined by SIMS for a range of glasses and salts concluded to represent negligible matrix effects (80). The NIST612 standard, which was also used for LA-MC-ICP-MS analyses (see below), has been confirmed to produce similar \(\small \ce{δ^11B}\) values by SIMS and positive thermal ionization mass spectrometry (81). Analyses of megacrysts in all samples except 14AD9A were conducted from crystal fragments mounted in epoxy with standards; all other analyses were conducted in situ using 1″ round thin sections. Standards produced identical results within analytical uncertainty when analyzed in the epoxy mount compared to an indium mount. Using the \(\small \ce{SiO2}\) contents measured by EPMA, B concentrations of unknowns were calculated relative to the \(\small \ce{^11B+}\)/\(\small \ce{^28Si^2+}\) ion yields from the standard used to bracket samples (26, 79).
A B isotope dataset which included duplicate analysis on some grains analyzed by SIMS was collected using an LA-MC-ICP-MS. Analyses were made using a New Wave UP-193-FX ATL excimer laser attached to a Thermo Fisher Scientific Neptune Plus MC-ICP-MS located at Lamont-Doherty Earth Observatory. Analytical methods followed those in (82). Operating conditions for all analyses were 10-Hz repetition rate, 3-μm/s stage speed during the linear scans, and a He flow of 1.45 liters/min. Spot sizes were chosen to produce similar strength of \(\small \ce{^11B}\) signal across phases with differing B contents: 10 to 80 μm for pyroxene megacrysts, 100 μm for nonmegacrystic pyroxene, and 150 μm for plagioclase. Laser fluence was dependent on spot size and ranged from ~28,000 mJ/cm2 (150-μm spot) to ~1,000,000 mJ/cm2 (25-μm spot). Analysis locations were pre-ablated with a spot size ~20 μm larger in diameter before analysis. Calculated \(\small \ce{δ^11B}\) values were corrected on the basis of spot size following the approach in (82); in all cases, this resulted in a change of ≤2.5‰. B concentrations (c) were estimated semiquantitatively using the following relationship, where I represents the sample intensity and Ø represents the spot size (82)
\[ [c]_{\text{sample}} = [c]_{\text{standard}} \times \frac{I_{\text{sample}}}{I_{\text{standard}} \times \left(\frac{\phi_{\text{sample}}}{\phi_{\text{standard}}}\right)^2} \]
Boron stable isotope data are presented as \(\small \ce{δ^11B}\), the ratio of \(\small \ce{^11B}\) to \(\small \ce{^10B}\) relative to that of the SRM951 standard as a per mil (‰) value (83)
\[ \delta^{11}\text{B} = \left(\frac{\left(\frac{^{11}\text{B}}{^{10}\text{B}}\right)_{\text{sample}}}{\left(\frac{^{11}\text{B}}{^{10}\text{B}}\right)_{\text{SRM951}}} - 1 \right) \times 10^3 \]
The NIST612 glass standard was used as a bracketing standard between groups of five unknown analyses. The calibration curves used to calculate final \(\small \ce{δ^11B}\) values were constructed from analyses of the NIST612 standard using different spot sizes. NIST612 and the in-house MVE04-4-3 pyroxene, plagioclase, and amphibole standard were used to monitor and validate measured \(\small \ce{δ^11B}\) values of unknowns. NIST612 yielded \(\small \ce{δ^11B}\) = +0.98 ± 2.91‰ (2 SD, n = 34), consistent with previously published values of +0.68 ± 3.31‰ (2 SD) (82) and −0.51 ± 0.52‰ (84), both using the same analytical in situ method. This analysis also agrees with the wet chemistry values ranging from −1.07 ± 0.85‰ (minimum value) (81) to +0.10 ± 0.40‰ (maximum value) (85). MVE04-4-3 yielded \(\small \ce{δ^11B}\) = −13.79 ± 4.19‰ (2 SD, n = 7), in agreement with the previously published value of \(\small \ce{δ^11B}\) = −13.80 ± 1.71‰ (30).
Results for the three grains analyzed by both SIMS and LA-MC-ICP-MS agree closely between the methods, as has been shown by previous work (30, 82). Analyses of grains 14AD9A_littlepyx and 14AD9A_HAOM1 gave \(\small \ce{δ^11B}\) values and B concentrations overlapping within 2σ SE uncertainty between the techniques (table S1 and Fig. 3). Grain averages determined for grain 98MA1A differ by 5.5‰, but this may reflect the inherent variability of both B concentrations and \(\small \ce{δ^11B}\) values measured within some single pyroxene megacryst grains (table S2).
Oxygen isotope analysis and sample preparation
For O isotope analyses, rock and plagioclase megacryst samples were crushed and then sieved. About 2 mg mineral separates of samples with a 14AD- prefix was handpicked by K. Varga; all other samples were handpicked by W.H.P. Plagioclase separates were analyzed for oxygen isotopes by laser fluorination in the Department of Geoscience at the University of Wisconsin-Madison following the methods in (41, 86). Precision of standards and duplicate samples on the days of analyses was ≤±0.18‰ (2σ). Oxygen isotope data are presented as \(\small \ce{δ^18O}\), the ratio of \(\small \ce{^18O}\) to \(\small \ce{^16O}\) relative to that of the VSMOW standard as a per mil (‰) value
\[ \delta^{18}\text{O}\ (\text{VSMOW}) = \left(\frac{\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right)_{\text{sample}}}{\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right)_{\text{VSMOW}}} - 1 \right) \times 10^3 \]
Neodymium and strontium isotope analysis and sample preparation
For Nd and Sr isotope analyses, plagioclase separates were prepared from rock samples at Rice University. Rock fragments with coarse, glassy plagioclase were selected and washed. The rocks were wrapped in clean paper and coarsely crushed by hand using a clean hammer. Fragments with the largest plagioclase crystals were then wrapped in new paper and crushed to a finer grain size using a pestle or hammer. This fine crushed fraction was sieved using a 1-mm mesh sieve and hand-picked using tweezers under an illuminated binocular microscope. Only plagioclase fragments without overgrown inclusions of other minerals (e.g., pyroxene) were selected. Crystal fragments were rinsed with ethanol after separation. All tools were washed between samples and then cleaned with deionized water and ethanol. Plagioclase separates for each sample were crushed using a CoorsTek 99.5% alumina mortar and pestle (#60374), which was washed, and then cleaned with deionized water and ethanol, between samples. Approximately 500 mg to 1 g of crushed plagioclase was prepared from each sample.
Neodymium and Sr isotope analyses were performed in the RIGL at Washington State University. Both Nd and Sr isotope compositions were determined on the same sample aliquots. For each aliquot, approximately 0.25 g of finely ground whole-rock powder was dissolved in high-pressure, steel-jacketed Teflon bombs at 150°C for 7 days using a ~10:1 mixture of concentrated HF and \(\small \ce{HNO3}\). After dissolution and conversion to chloride form using \(\small \ce{H3BO3}\), sample solutions were spiked with a mixed \(\small \ce{^149Sm}\)-\(\small \ce{^150Nd}\) tracer, heated in sealed Savillex capsules to facilitate sample-spike equilibration, and passed through initial columns filled with 8 ml of Dowex AG50W-X8 cation exchange resin. This column isolates Sr from the light rare earth elements, Sm and Nd. Sr was subsequently purified using microcolumns with 0.18-ml Eichrom Sr spec resin. Sm and Nd were isolated using columns with 1.7-ml Eichrom Ln Spec resin.
The isotopic compositions of Sr and spiked Sm and Nd solutions were determined using RIGL’s Thermo Fisher Scientific Neptune Plus MC-ICP-MS. Samarium and Nd concentrations and Sm/Nd ratios were determined by isotope dilution. Rubidium and Sr concentrations and Rb/Sr ratios were determined on aliquots of the initial sample solution before column chemistry on RIGL’s Element2 high-resolution ICP-MS. Samarium and Nd isotope ratios were corrected for interference, spike subtraction, and mass bias using an off-line, in-house data reduction program. Strontium isotope compositions were interference- and mass bias–corrected online as part of the Neptune Plus analysis. During the period of the analyses, the Nd and Sr isotope standards JNdi-1 [\(\small \ce{^143Nd/^144Nd}\) = 0.512115 (87)] and NBS 987 Sr [\(\small \ce{^87Sr/^86Sr}\) = 0.710248 (88)] gave \(\small \ce{^143Nd/^144Nd}\) = 0.512090 ± 4 (2 SD) and \(\small \ce{^87Sr/^86Sr}\) = 0.710281 ± 13 (2 SD), respectively, in line with long-term RIGL averages. All Nd and Sr isotope values were normalized relative to these published standard values based on these small differences.
Initial Nd and Sr isotope compositions were calculated using the \(\small \ce{^147Sm}\) decay constant of 6.524 × 10−12 year−1 (89) and the \(\small \ce{^87Rb}\) decay constant of 1.3972 × 10−11 year−1 (90). Epsilon Nd values, the \(\small \ce{^143Nd/^144Nd}\) compositions relative to the chondritic uniform reservoir (CHUR) at the same age, were calculated using the model in (40) and the Sm-Nd CHUR parameters of \(\small \ce{^147Sm/^144Nd}\) = 0.1960 and \(\small \ce{^143Nd/^144Nd}\) = 0.512638 (91).
\[ \varepsilon_{\text{Nd}}(T) = \left(\frac{\left(\frac{^{143}\text{Nd}}{^{144}\text{Nd}}\right)_{\text{sample}}(T)}{\left(\frac{^{143}\text{Nd}}{^{144}\text{Nd}}\right)_{\text{CHUR}}(T)} - 1\right) \times 10^4 \]
Further details on relevant chemical and analytical procedures can be found in (92, 93).
Electron probe microanalysis
Analytical conditions
EPMA was conducted using a JEOL JXA 8530F Hyperprobe in the Rice University, Department of Earth, Environmental and Planetary Sciences. The instrument is equipped with a field emission assisted thermo-ionic (Schottky) emitter, a silicon drift electron dispersive spectrometry detector, and five wavelength dispersive spectrometers (WDS). The WDS EPMA spots used to measure mineral chemistry were placed within ~100 μm of SIMS pits.
Quantitative WDS analysis
The WDS major and minor element analyses for all minerals used conditions of 15-kV acceleration voltage and 20-nA beam current. An x-ray counting time of 20 s was used for each element, with 10 s used for the peak and 5 s each for upper and lower backgrounds. One analytical recipe was used to analyze inosilicates, and another recipe was used to analyze plagioclase. Pyroxene analyses used a focused beam spot (~250 nm), and plagioclase analyses used a 20-μm-diameter beam to avoid loss of Na and K during analysis. Primary and secondary standards of pyroxenes and feldspars were analyzed as unknown and yielded reproducibility errors below 1% and ~2 to 5% for major and minor elements, respectively. Primary natural mineral standards sourced from SPI Supplies were used in calibrations before analysis: \(\small \ce{SiO2}\): olivine (inosilicates) and plagioclase (plagioclase); \(\small \ce{TiO2}\): rutile; \(\small \ce{Al2O3}\): almandine (inosilicates) and plagioclase (plagioclase); \(\small \ce{Cr2O3}\): chromite; \(\small \ce{FeO}\): olivine; \(\small \ce{MnO}\): rhodonite; \(\small \ce{MgO}\): olivine; \(\small \ce{Na2O}\): plagioclase; \(\small \ce{CaO}\): plagioclase; and \(\small \ce{K2O}\): biotite (inosilicates) and orthoclase (plagioclase). A JEOL Phi-Rho-Z (PRZ) matrix correction was applied to inosilicate analyses, and a ZAF method was used for plagioclase. Mineral formulas presented in tables S3 and S4 were calculated by normalizing to 6 oxygen atoms for pyroxenes and 8 oxygen atoms for plagioclase.
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