P Olin

Crustal growth by magmatic overplating in the Galápagos

The isotopic compositions of xenoliths hosted in lavas from Floreana Island indicate that they formed from magmas unlike those at present-day Floreana. Instead, the xenoliths are geochemically more similar to magmas now erupting from Sierra Negra and Cerro Azul volcanoes, at the leading edge of the Galapagos hotspot. This is the first evidence for compositional evolution at a Galapagos volcano and indicates increasing contributions from an iso topically enriched source with time as the volcano is carried away from the focus of the hot-spot. Clinopyroxenes in many of the xenoliths exhibit positive anomalies of Sr and Eu, which are attributed to the breakdown of plagioclase. The growth of clinopyroxene at the expense of plagio clase results from compression as the crust cools. Compression is caused by growth mostly from above, as shallow intrusions and lavas load the middle and upper oceanic crust.

The use of biotite trace element compositions for fingerprinting magma batches at Las Cañadas volcano, Tenerife

Accurate identification of individual volcanic events in the field is crucial for constraining eruption volumes and calculating recurrence intervals between eruptive episodes. Due to complexities of pyroclastic transport and deposition and intra-unit textural variability, such identification can be challenging. We present a novel method for fingerprinting ignimbrites via trace element chemistry (V, Co, Nb) in biotite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Using samples from the alkaline magmatic series of Tenerife, we are able to demonstrate (1) clustering of previously characterized units into distinct, homogeneous groups based on V, Co, and Nb concentrations in biotite, despite the presence of extreme variation and zonation in other trace elements (Ba, Sr, Rb) that indicate complex petrogenetic processes, and (2) biotite compositions are similar throughout a deposit and relatively independent of stratigraphic height or juvenile clast texture (crystal-rich vs crystal-poor). Our results show that trace elements in biotite can be used to fingerprint eruptions and correlate geographically separated volcanic deposits, including those preserved in offshore turbidite records.

Matrix dependency for oxide production rates by LA-ICP-MS

The production rates of polyatomic oxygen interferents (MO+/M+) during LA-ICP-MS analysis were investigated in a range of silicate materials and metals. The total amount of oxygen in the ICP is significantly lower for laser ablation analysis compared to solution nebulisation analysis resulting in lower oxide production rates. However, these interferents can still be significant for some elements. The contribution of oxygen from the material being ablated was found to influence the oxide production rate (OPR). When using a well degassed system to minimise the entrainment of atmospheric oxygen, the OPR for Al, Si and W was up to 4 times lower when ablating the elements as a metal compared to when ablating oxygen-bearing minerals. There is a relationship between the MO+/M+ production rate and the cation–oxygen dissociation energy for elements measured by solution and by laser ablation ICP-MS. However, for Hf and Th the OPR varied significantly depending on the mineral being ablated under the same analytical conditions (0.007–0.02% for Hf and 0.09–0.2% for Th), whereas UO+/U+ was more consistent (0.058–0.063%). The effects of carrier gas flow rate and resulting differences in aerosol breakdown and ionisation in the ICP were investigated for U and Th oxides in NIST610, NIST612, zircon (ZrSiO4), monazite ([REE,Th]PO4) and uraninite (UO2). Increasing the Ar flow rate had a larger effect on the Th OPR (0.05 to 0.5%) compared to U (0.04 to 0.09%) when ablating the NIST610 glass. The relative differences in the OPR between minerals compared to NIST610 were small for U, with all minerals having the same OPR except for uraninite at high carrier gas flow rates (35% higher). In contrast the OPR for Th was highly variable between all minerals and showed differing responses to changes in the Ar flow. This study highlights the complexities in oxide production for LA-ICP-MS compared to solution analyses, and that the OPR for some elements is strongly dependent on the material being ablated. Also, that there can be a significant contribution to the MO+ production from ionisation of an incompletely atomised sample aerosol in the plasma.

Immiscible sulfide melts in primitive oceanic magmas: Evidence and implications from picrite lavas (Eastern Kamchatka, Russia)

Abstract Silicate-sulfide liquid immiscibility in mantle-derived magmas has important control on the budget of siderophile and chalcophile metals, and is considered to be instrumental in the origin orthomagmatic sulfide deposits. Data on primitive sulfide melts in natural samples, even those representing most voluminous magmatism in oceanic rifts, are very scarce due to the small size and poor preservation of incipient sulfide melt globules. Here we present the first detailed report of the crystallized sulfides melts in the oceanic picrites of the (presumably) Cretaceous age Kamchatsky Mys ophiolite complex in Eastern Kamchatka (Far East Russia). Sulfide melts are present in three forms; (1) as inclusions in olivine (87.1–89.6 mol% Fo), (2) interstitial to the groundmass minerals (clinopyroxene, plagioclase, and Ti-magnetite) of studied picrites, and (3) as daughter phases in silicate melt inclusions hosted by olivine and Cr-spinel phenocrysts. The sulfide melt inclusions in olivine and the groundmass of studied rocks are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss) and intermediate (Fe–Cu–Ni; Iss) solid solutions. Several <0.5 μm Pd–Sn, Pt–Ag, and Au–Ag phases are recorded within the matrix sulfides, commonly along phase boundaries and fractures. Major elements (S, Fe, Cu, Ni, Co), platinum group elements (PGE), and gold analyzed in the homogenized olivine-hosted sulfide melt inclusions, and phases identified in the matrix sulfides record the range of magmatic sulfide compositions. The most primitive sulfide liquids are notably enriched in Ni and Cu [(Ni+Cu)/Fe, at% > 0.5], continuously evolve with crystallization of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss. Although the compositional systematics found in this study are consistent with those previously recorded, the compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) “nuggets” that exsolve at subsolidus temperatures and form during serpentinization of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can be also used for modeling sulfide saturation at crustal pressures and understanding behavior of the noble metals in primitive oceanic magmas.