A number of experiments have been conducted in order to study the equilibria between olivine and basaltic liquids and to try and understand the conditions under which olivine will crystallize. These experiments were conducted with several basaltic compositions over a range of temperature (1150-1300 ° C) and oxygen fugacity (10-°.~s-10 -12 arm.) at one atmosphere total pressure. The phases in these experimental runs were analyzed with the electron microprobe and a number of empirical equations relating the composition of olivine and liquid were determined. The distribution coefficient o, (X~o/ (Xreo) K/~- i'~-Liq \ O1 t-XFeoJ (X~go) relating the partioning of iron and magnesium between olivine and liquid is equal to 0.30 and is independent of temperature. This means that the composition of olivine can be used to determine the magnesium to ferrous iron ratio of the liquid from which it crystallized and conversely to predict the olivine composition which would crystallize from a liquid having a particular magnesium to ferrous iron ratio. A model (saturation surface) is presented which can be used to estimate the effective solubility of olivine in basaltic melts as a fune¢ioa of temperature. This model is useful in predicting the temperature at which olivine and a liquid of a particular composition can coexist at equilibrium.

http://www.mantleplumes.org/WebDocuments/RoederEmslie.pdf

Olivine-Liquid Equilibrium

Geology and petrology of the Voisey's Bay intrusion: reaction of olivine with sulfide and silicate liquids

The activities of NiO, CoO and FeO in silicate melts

Olivine from the Noril’sk I, Main and Northwestern Talnakh ore-bearing intrusions in the Noril’sk region occurs as inclusions, discrete grains, and poikilitic crystals. The original compositions of olivine inclusions have been well preserved by their host oikocrysts from reaction with interstitial silicate and sulfide liquids. In contrast, the compositions of discrete and poikilitic olivine crystals have been variably modified by reequilibration with trapped silicate and sulfide liquids. There are abrupt changes in the contents of nickel in olivine inclusions (500–1,500 ppm) between different rock units in all three intrusions, which indicates that the different rock units in these intrusions are not related to each other via in situ differentiation of a common parental magma but rather are consistent with separate pulses of magma with variable degrees of differentiation, possibly involving olivine crystallization and assimilation of siliceous crustal material in a staging chamber.

The δ 34S values of the samples with high sulfur contents (>800 ppm sulfur) from all three ore-bearing intrusions vary between 8 and 14 per mil. Two sulfur-poor samples (<600 ppm sulfur) from the Northwestern Talnakh intrusion yield δ 34S values of 1.5 and 3.6 per mil. These values are similar to the values of the sulfur-poor rocks from the ore-barren intrusions in the region and illustrate the fact that high sulfur contents couple with increasing δ 34S values. Such a relationship is consistent with the addition of crustally derived 34S-enriched sulfur to the magma of the ore-bearing intrusions.

In contrast to other models of sulfide segregation in closed systems, we propose that sulfide segregation occurred in an open system and the magma involved was undersaturated in sulfur and undepleted in chalcophile elements. We envision that the 34S-enriched sulfur was introduced via circulating fluids that transported sulfur from evaporite in the underlying sedimentary sequence to the magma in a dynamic magma conduit. The immiscible sulfide liquids that segregated during sulfur contamination settled down in the wider parts of the conduit as the magma continued to travel. Successive pulses of new magma entered the conduit, reacted with the early sulfide, and displaced much of the early magma in the conduit. The ore-bearing intrusions are thought to have crystallized within the wider parts of the conduits and their voluminous, sulfide-poor peripheral sills represent some (if not all) of the magma that flowed out of the conduits.

Compositional Variations of Olivine and Sulfur Isotopes in the Noril’sk and Talnakh Intrusions, Siberia: Implications for Ore-Forming Processes in Dynamic Magma Conduits

Subduction of oceanic crust generates chemical and lithological heterogeneities in the mantle. An outstanding question is the extent to which these heterogeneities contribute to subsequent magmas generated by mantle melting, but the answer differs depending on the geochemical behaviour of the elements under investigation: analyses of incompatible elements (those that preferentially concentrate into silicate melts) suggest that recycled oceanic crust is an important contributor, whereas analyses of compatible elements (those that concentrate in crystalline residues) generally suggest it is not. Recently, however, the concentrations of Mn and Ni—two elements of varying compatibility—in early-crystallizing olivines, have been used to infer that erupted magmas are mixtures of partial melts of olivine-rich mantle rocks (that is, peridotite) and of metasomatic pyroxene-rich mantle rocks (that is, pyroxenite) formed by interaction between partial melts of recycled oceanic crust and peridotite. Here, we test whether melting of peridotite alone can explain the observed trend in olivine compositions by combining new experimental data on the partitioning of Mn between olivine and silicate melt under conditions relevant to basalt petrogenesis with earlier results on Ni partitioning. We show that the observed olivine compositions are consistent with melts of fertile peridotite at various pressures—importantly, melts from metasomatic pyroxenites are not required. Thus, although recycled materials may well be present in the mantle source regions of some basalts, the Mn and Ni data can be explained without such a contribution. Furthermore, the success of modelling the Mn–Ni contents of olivine phenocrysts as low-pressure crystallization products of partial melts of peridotite over a range of pressures implies a simple new approach for constraining depths of mantle melting.

/pdf/the-roles-of-pyroxenite-and-peridotite-in-the-mantle-sources-xmmebf5ciw.pdf

F. E. Campbell

Papers

The stability of olivine and pyroxene in the Ni-Mg-Si-O system