Elements Covers

Thematic Articles

Sedimentary Sulfides

Sedimentary sulfides constitute over 95% of the sulfide on the surface of the planet, and their formation, preservation and destruction largely determines the surface environment. The sulfide in sediments is mainly derived from the products of sulfate-reducing bacteria, which are currently responsible for oxidizing over half the organic matter flux reaching sediments. Pyrite is the mineral overwhelmingly produced. The geochemistry of pyrite, both in terms of its isotopic composition and its trace-element loading, has varied dramatically over geologic time. As such, it is a major source of our current understanding about the nature of the early Earth and of the Earth’s subsequent geochemical and biological evolution.

Read More

Chalcophile Elements and Sulfides in the Upper Mantle

Sulfides are among the most important petrogenetic agents in magmatic systems. They are ubiquitous in most upper-mantle rock types, common as inclusions in diamonds and they host significant amounts of geochemically and economically important chalcophile (‘sulfur-loving’) elements, such as Cu, Ni, Pb, In, Au and the platinum-group elements. Despite their low abundance (<< 1% of the bulk rock), residual sulfides have a disproportionate control over the chalcophile element budget in upper mantle lithologies, as well as that of melts derived from the Earth’s mantle.

Read More

Volcanic Sulfides and Outgassing

Sulfides are a major potential repository for magmatic metals and sulfur. In relatively reduced magmas, there may be a dynamic interplay between sulfide liquids and magma degassing as magmas ascend/erupt. Sulfide-bubble aggregates may segregate to shallow levels. Exsolved fluids may oxidize sulfides to produce SO2 gas and metals, which can vent to the atmosphere with chalcophile metal ratios reflecting those in their parent sulfide liquids. Sulfide breakdown and/or sequestration timing and balance define the role of sulfides in both ore formation and the environmental impacts of volcanic eruptions, including during the evolution of large igneous provinces, which are key periods of heightened volcanism during Earth history.

Read More

Sulfide Minerals in Hydrothermal Deposits

Hydrothermal ore deposits are large geochemical anomalies of sulfur and metals in the Earth’s crust that have formed at <1 to ~8 km depth. Sulfide minerals in hydrothermal deposits are the primary economic source of metals used by society, which occur as major, minor and trace elements. Sulfides also play a key role during magmatic crystallization in concentrating metals that subsequently may (or may not) be supplied to hydrothermal fluids. Precipitation of sulfides that themselves may have little economic value, like pyrite, may trigger the deposition of more valuable metals (e.g. Au) by destabilizing the metal-bearing sulfur complexes. We review why, where and how sulfide minerals in hydrothermal systems precipitate.

Read More

Magmatic Sulfide Ore Deposits

Magmatic sulfide ore deposits are products of natural smelting: concentration of immiscible sulfide liquid (‘matte’), enriched in chalcophile elements, derived from silicate magmas (‘slags’). Sulfide ore deposits occupy a spectrum from accumulated pools of matte within small igneous intrusions or lava flows, mined primarily for Ni and Cu, to stratiform layers of weakly disseminated sulfides within large mafic–ultramafic intrusions, mined for platinum-group elements. One of the world’s most valuable deposits, the Platreef in the Bushveld Complex (South Africa) has aspects of both of these end members. Natural matte compositions vary widely between and within deposits, and these compositions are controlled largely by the relative volumes of matte and slag that interact with one another.

Read More

Mineralogy of Sulfides

Metal sulfides are the most important group of ore minerals. Here, we review what is known about their compositions, crystal structures, phase relations and parageneses. Much less is known about their surface chemistry, their biogeochemistry, or the formation and behaviour of ‘nanoparticle’ sulfides, whether formed abiotically or biogenically.

Read More

Volcanoes: Characteristics, Tipping Points, and those Pesky Unknown Unknowns

The geological record of volcanic eruptions suggests that scientists are some way from being able to forecast eruptions at many of the world’s volcanoes. There are three reasons for this. First, continuing geological discoveries show that our knowledge is incomplete. Second, knowledge is limited about why, how, and when volcanic unrest turns into eruptions, and over what timescales. Third, there are imbalances between the studies of past eruptions, and the geophysical techniques and observations on modern events, versus the information needed or demanded by society. Scientists do not yet know whether there are other, presently unknown, factors that are important in controlling eruptions, or if there is an inherent unknowability about some volcanic systems.

Read More

Dynamic Magma Systems: Implications for Forecasting Volcanic Activity

Magma systems that supply volcanoes can extend throughout the crust and consist of mush (melt within a crystalline framework) together with ephemeral magma accumulations. Within a crystal-rich mush, slow processes of melt segregation and heat loss alternate with fast processes of destablisation and magma transport. Magma chambers form by two mechanisms: incremental magma intrusion into sub-solidus rocks or the segregation and rapid merging of melt-rich layers within mush regions. Three volcanic states reflect alternations of slow and fast processes: dormancy, unrest and eruption. Monitoring needs to detect processes of melt and fluid movements in the lower and middle crust during destabilisation to improve forecasting.

Read More

Volatiles and Exsolved Vapor in Volcanic Systems

The role of volatiles in magma dynamics and eruption style is fundamental. Magmatic volatiles partition between melt, crystal, and vapor phases and, in so doing, change magma properties. This has consequences for magma buoyancy and phase equilibria. An exsolved vapor phase, which may be distributed unevenly through reservoirs, contains sulfur and metals that are either transported into the atmosphere or into ore deposits. This article reviews the controls on volatile solubility and the methods to reconstruct the volatile budget of magmas, focusing particularly on the exsolved vapor phase to explore the role of volatiles on magma dynamics and on eruption style.

Read More

What Does a Magma Reservoir Look Like? The “Crystal’s Eye” View

Crystals within volcanic rocks contain records of the changing chemical and thermal conditions within the magma reservoirs in which they resided before eruption. Observations from these crystal records place fundamental constraints on the processes operating within the reservoirs. Data from volcanic crystals are in accord with recent conceptual models of magma reservoirs being composed dominantly of crystal mushes, with small volumes and/or small fractions of melt present. The implication is that magma reservoirs have differing modes of behavior: magmas are stored over the long term in largely crystalline, quiescent, conditions, punctuated by brief episodes of intense activity during the decades to centuries immediately prior to an eruption.

Read More