On the origin of andesite in the northern Mariana Island Arc: Implications from Agrigan

Magmatic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt. Basaltic products predominate, but the ratio of andesites to basalts increases with time up to an event of voluminous andesitic pyroclastic ejection accompanied by caldera-collapse; post-collapse lavas are entirely basaltic. Moderate iron-enrichment is demonstrated for the volcanic suite, with indications of a progressive, pre-caldera decrease in iron-enrichment; post-caldera lavas display a return to moderate Fe-enrichment. Overall, the lavas are enriched in the LIL elements (K, Rb, Ba, Sr) and depleted in Ti, Mg, Cr, and Ni. From the oldest to the youngest pre-caldera volcanic sequence, the LIL elements increase 3-6X while Ca and Mg decrease by 50% or more. Approximately constant K/ Rb (430±60) and ⁸⁷Sr/⁸⁶Sr (0.7032–0.7034) indicate consanguinity of the basalts and the andesites. Cumulate plutonic xenoliths, common in the lavas, are composed of mineral phases also encountered as phenocrysts. The following order of crystallization is indicated: olivine; anorthite-bytownite; clinopyroxene; orthopyroxene and titanomagnetite. Co-existing xenolithic olivines (Fo_74–83) and plagioclase (An_88–96) are typical of calc-alkaline island-arc assemblages and contrast with assemblages in the tholeiites from the Mariana Trough to the west. The relatively fayalitic composition and low abundances of Ni in olivines and Cr in clinopyroxenes indicate equilibrium with an already-fractionated liquid. These data, along with structural evidence, high Ca in the olivines, and comparison of the observed assemblages with experimental studies, suggests that these xenoliths formed as crystal cumulates at the floor of a shallow ( 7 km) crustal magma chamber.Major element modeling studies using the separation of observed xenocrystic and phenocrystic phases from assumed parental liquids reproduce the observed temporal and geochemical variations in the lavas. Trace element modeling parallels this evolution with the exception of Cr and Ni in the andesites. An extensive (16.3 km³) gabbroic body is required by this modeling to be present beneath Agrigan to produce the inferred volumes of the various lithologies preserved in the volcano's evolution. The sum of stratigraphic, geochemical, and isotopic evidence on Agrigan supports the derivation of calc-alkaline andesite by the removal of about 75% solids from a high-alumina basalt accompanied by a process of K and Rb enrichment, such as volatile-transfer. Considerations of ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and ³He/⁴He isotopic data indicate that the source region of these parental liquids lies in the mantle, not subducted crust. In the northern Marianas, the model of a shallow-crustal origin for andesite is preferred over one requiring andesite generation in the deeper mantle and/or subducted slab.     

Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating

An igneous zircon reference material (OG1) was characterised for U-Pb isotopes by ID-TIMS, and utilised to evaluate SIMS (SHRIMP) instrumental mass fractionation (IMF) of radiogenic Pb isotopes (²⁰⁷Pb*/²⁰⁶Pb*). The TIMS ²⁰⁷Pb*/²⁰⁶Pb* reference value for OG1 was 0.29907 ± 0.00011 (95% confidence limit), 3465.4 ± 0.6 Ma. The high ²⁰⁷Pb* (∼ 30 μg g⁻¹), negligible common Pb, and isotopic homogeneity permitted precise (± 1–2‰) ²⁰⁷Pb*/²⁰⁶Pb* measurements within the analytical sessions. External reproducibility of mean ²⁰⁷Pb*/²⁰⁶Pb* ratios between sessions was demonstrated for one instrument, yielding a mean IMF of +0.87 ± 0.49‰. The mean ²⁰⁷Pb*/²⁰⁶Pb* ratios between instruments were dispersed beyond uncertainties, with session IMF values from +3.6 ± 1.7‰ to −2.4 ± 1.3‰, and a grand mean IMF value (twenty-six sessions) of +0.70 ± 0.52‰, indicating a tendency towards elevated ²⁰⁷Pb*/²⁰⁶Pb*. The specific causes of variability in IMF are unclear, but generally reflect subtle differences in analytical conditions. The common practice in SIMS of assuming that IMF for Pb⁺ is insignificant could result in systematic age biases and underestimated uncertainties, of critical importance for precise correlation of Precambrian events. Nevertheless, a zircon RM such as OG1 can be readily incorporated into routine dating to improve ²⁰⁷Pb*/²⁰⁶Pb* accuracy and external reproducibility.     

Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δC-N study of eclogite-hosted diamonds from the Jericho kimberlite

Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ¹³C values. While diamonds from high-MgO eclogites have low nitrogen contents (5-82 ppm) and extremely low δ¹³C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ¹³C values from −3.5‰ to −5.3‰.Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ¹³C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ¹³C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ¹³C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ¹³C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction.SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ¹³C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ¹³C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ¹³C-N yielded a partition coefficient (K_N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ¹³C values of diamond cores (−4‰) dictate the growth medium had higher δ¹³C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.