Chalcophile elements in Shatsky Rise basalts as potential indicators of the mantle plume origin
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Igneous oceanic plateaus, such as Shatsky Rise, represent one of the least understood types of basaltic magmatism on Earth. In the case of Shatsky Rise, two main models were proposed to explain its origin: (1) melting of deep-sourced mantle plume head, (2) mantle melting at a mid-oceanic ridge with unusual characteristics (e.g., anomalously high dynamics or spreading-induced upwelling of eclogite). In this project we proposed to carry out new geochemical investigations of the Shatsky Rise basaltic glasses with the focus on: (1) in-situ analysis of strongly chalcophile elements in pristine glasses (Cu, Ag, Au, Se), and (2) determination of the oxidation state of magmas. These data were planned to test the plume vs. non-plume origin of the Shatsky Rise oceanic plateau. In the course of this study we analysed major and trace elements, including strongly chalcophile and siderophile elements (CSE: Cu, Ag, Se, Re, Ir, Pt, Au) in 48 glass samples, determined Fe3+/ΣFe ratio in 19 glass samples, investigated V partitioning between olivine and glass for 3 samples, and analysed major elements and sulfur content in a series of melt inclusions trapped in plagioclase phenocrysts in Tamu Massif glasses. On the basis of this data we can show that Shatsky Rise magmas are enriched in CSE and in this respect are strikingly different from typical N-MORB magmas but show some similarities with magmas formed at mid ocean ridges in the vicinity of mantle plumes (P-MORB). Our new results support plume related origin of the Shatsky Rise. Furthermore, Shatsky magmas are more oxidized in comparison to MORB magmas and crystallized at fO2=FMQ+0.4 to FMQ+1. Primitive Shatsky Rise glasses are sulfide undersaturated and enriched in CSE, which is likely resulted from a large difference between the pressures of mantle melting and magma crystallization. In this respect these magmas are similar to primitive basalts at the Ontong Java Plateau. Evolved Shatsky glasses are either S-rich sulfide saturated (Ori Massif) or S-poor sulfide under-saturated (Tamu Massif) that correlates with depletion or enrichment in CSE respectively. Correlation between [S] and [Pt] in Tamu glasses suggests that these melts could be saturated with Pt metal phase, but not with the sulfide phase. Previously proposed magma degassing during eruption and dissolution of sulfide in Tamu magmas cannot explain the CSE systematics and also low sulfur content in Tamu melt inclusions, trapped in plagioclase before eruption. Alternatively, we propose that Tamu magmas could lost sulfur before eruption during fractionation in shallow magma chamber due to fluxing by high-CO2/S fluids from depth. Our results emphasize an important role of shallow magma fractionation which strongly affect the behavior of CSE in oceanic magmas.
Publications
- (2017) Chalcophile elements in Shatsky Rise basalts as potential indicators of the mantle plume origin? IODP/ICDP Kolloquium 2017, IGeo, Technische Universität Braunschweig, 14-16 March 2017, p.32
Almeev R, Portnyagin M, Garbe-Schönberg D
- (2018) Evidence for mantle plume origin of the Shatsky Rise from systematics of chalcophile and siderophile elements in volcanic glasses Gemeinsames IODP/ICDP Kolloquium 2018, Ruhr-Universität Bochum, 14-16 March 2018, p.67
Portnyagin M, Almeev R, Garbe-Schönberg D
- (2019) Plume versus ridge origin of Shatsky Rise (Pacific): Insight from noble metal systematics. Russian Ridge, Moscow, June 3-4, 2019
Portnyagin M, Almeev R, Garbe-Schoenberg D
