High Temp Geochem

• • The element K (potassium) does not form its own minerals in mid ocean ridge basalts (MORB) or mantle peridotites. K is therefore a trace element in the ocean crust and the mantle
  • K is not a trace but a major element in granites and the continental crust, where it forms K-feldspars
• What are lithophile elements known for?


  They are "rock-loving" and partition into silicate phases.
• Where are lithophile elements concentrated in the Earth?


  They are concentrated in the silicate portions of the Earth, specifically the crust and mantle.
• What does the term "siderophile" refer to?


  Siderophile elements are "metal-loving" and partition into metallic liquids.
• Where are siderophile elements primarily found?


  They are strongly enriched in the core but depleted in the silicate Earth.
• What are chalcophile elements known for?


  Chalcophile elements are "sulfur-loving" and partition into sulfide liquids.
• What type of liquids do chalcophile elements partition into?


  They partition into sulfide liquids.
• What does the term "atmophile" refer to?


  Atmophile elements are generally highly volatile and like to form gases or liquids.
• Where are atmophile elements concentrated?


  They are concentrated in the atmosphere and the hydrosphere.
• What are the four types of elements based on their affinity in the Earth?

  • Lithophile elements: "rock-loving", found in silicate phases (crust & mantle)
  • Siderophile elements: "metal-loving", found in metallic liquids (core)
  • Chalcophile elements: "sulfur-loving", found in sulfide liquids
  • Atmophile elements: volatile, found in gases or liquids (atmosphere & hydrosphere)
• Elemental behaviour depends on chemical/physical properties:

  • Low electronegativity elements form ionic bonds with high electronegativity elements; hence form oxide and silicate minerals, therefore lithophiles.
  • Siderophile and chalcophile elements have intermediate electronegativities; so they prefer covalent bonds and metallic bonding
  • Noble gases unreactive so concentrated in the atmosphere.
• Chalcophile does not automatically imply that an element is found in sulfide ore deposits. Many sulfide deposits are precipitated from aqueous solutions but chalcophile means that an element partitions into magmatic sulfide liquids/phases.

  Magmatic sulfides and sulfide liquids are present in the Earth’s mantle. Many highly siderophile elements (HSE) – Rhenium (Re) and the platinum group elements (PGE) – are concentrated in mantle sulfides relative to silicate minerals. The HSE behave as chalcophile elements in the mantle.
• Trace element concentrations and ratios are governed by substitution into phases (e.g., minerals) that are formed by the major elements. Hence, they can provide information on how magmas and chemical sediments formed by fingerprinting which phases were involved and at what conditions such phases are stable. The partitioning of trace elements into minerals is primarily a function of ionic charge and radius.
• The ionic radii of the elements vary with atomic mass & charge, and they show systematic variation across the periodic table.

  
• • The ions of one element can extensively replace those of another element in ionic crystals (such as silicates) if their radii differ by less than ~15% (e.g. Rb and K)
  • Ions whose charges differ by one unit substitute readily for one another, provided electrical neutrality can be maintained in the crystal. If the charges differ by more than one unit, substitution is typically slight (e.g. Ba2+ and Sr2+ can substitute K+, but requires charge neutrality to be maintained by coupled substitution of Al3+ for Si4+)
• When two different ions occupy a particular position in a crystal lattice, the ion with the higher ionic potential ( = higher charge/radius ratio) forms a stronger bond with the anions surrounding the site. e.g. Sr2+ has a higher ionic potential than Rb+, therefore it is preferred.
• Substitution may be limited, even when the size and charge criteria are satisfied, when the competing ions have different electronegativities and when they form bonds of different ionic character.
• The Camouflage Principle applies where two ions have virtually identical charge, ionic radii, and electronegativity. e.g. The Zr4+ and Hf4+ ions are so similar with ionic radii of 80 pm and 79 pm, respectively, that zircon accepts Hf as readily as it accepts Zr4+. The Zr/Hf ratio is therefore generally similar for zircon crystals and the magmatic liquid from which the zircon crystallizes. In fact, almost all rocks and minerals have nearly identical Zr/Hf ratios (of ~36).
• • The Admission Principle applies to the incorporation of ions that have a lower ionic potential than the major elements.
  • The charges, ionic radii and electronegativities of Rb+ and K+ are sufficiently similar, so that when when K-feldspar crystallizes from a magma, Rb is incorporated too. However, the difference in ionic potential means that the larger Rb+ ion is not incorporated in the same proportion as K+ from the silicate liquid. This means that the feldspar has a higher K/Rb ratio than the magma.
• $K_{D} =$$C_{solid}/C_{liquid}$

  • If KD > 1, the element is compatible in a certain solid phase
  • If KD < 1, they are incompatible
  • <<1 they are termed highly incompatible
• KD values are most readily determined by trace element analyses of silicate liquids that are in equilibrium with individual mineral phases (phenocrysts, e.g., ol, plag, cpx, etc).

  • Need to make sure that equilibrium assemblages are used!
  • Suitable equilibrium systems can be either natural samples (basaltic rock w/ phenocysts) or they are produced in laboratory experiments (experimental petrology)
  • The analyses commonly utilize in-situ analytical techniques 24 (electron or ion microprobe, LA-ICP-MS)
• Rocks typically consist of more than one mineral phase. Hence, during partial melting of such a rock, several phases participate in the melting reactions.

  The degree to which an element is distributed between the liquid and the solid phases depends on the sum of the individual partition coefficients, weighed according to the relative proportions of the mineral phases present in the solid. The bulk partition coefficient D of a specific element is therefore defined as:

  $D_{s/l} =$$(K_{D}^i f^i) +$$(K_{D}^j f^j)+$$...$

  where f is the mass fraction of a specific mineral.
• The mineralogy of Earth's upper mantle is: olivine (~60%), orthopyroxene (~25%), clinopyroxene (~10%). Trace elements are compatible in the mantle if they can substitute for the major cations of these minerals (Mg, Fe and Ca). Ni,Co,Cr and V fir the criteria. Other elements are incompatible.
• The platinum group elements (PGE) are a group of noble metals that includes ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt). The group of highly siderophile elements (HSE) is made up of the PGE plus Re and Au.

  They are present in the Earth's core, but only in the mantle at about 1-10ng/g
• • Precise trace element data can be obtained on small samples or in situ with ion probes or by laser ablation-ICP-MS. Such data provide unprecedented insights into igneous processes.
• $C_{0} =$$C_{L}F +$$C_{S}(1-F)$ where:

  • C0 is the original concentration of the solid (and the whole system)
  • CL is the concentration of the liquid
  • CS is the concentration of the solid
  • F is the melt fraction (mass of melt/mass of system)
• with D = $C_{S}/C_{L}$, we can rearrange to:

  • $C_{L}/C{0} =$$1/F+$$D(1-F)$ , where D is the partition coefficient
  • this describes the enrichment/ depletion of a trace element in a melt as a function of degree in the melting
• As the D value decreases, the enrichment of the trace element increases.

  
• • If D<<F, then $C_{L}/C_{0} =$$1/F$, the degree of enrichment for a highly incompatible trace element is inversely proportional to the degree of melting
  • As F approaches 0 (little melting), then $C_{L}/C_{0} =$$1/D$. This means that the degree of enrichment depends on the compatibility of an element. The maximum enrichment that can be achieved is 1/D.
  • If D is large (>1; compatible element) the depletion in the melt is 1/D when F is small. Result insensitive to F

  
• Given that $C_{L}/C{0} =$$1/F+$$D(1-F)$ where $D =$$C_{S}/C_{L}$:

  • the trace element concentration of the residual solid rock that is in equilibrium with a partial batch equilibrium melt is straightforward to calculate:
  • $C_{S}/C_{0} =$$D/F+$$D(1-F) =$$C_{L}D/C_{0}$
• • If melting is very rapid and the liquids are sufficiently low in viscosity, the melts can be quickly driven away from the source rock by buoyancy forces. In this case, the solid will be unable to equilibrate with the total amount of melt that is produced during the melting process. Hence, we have incomplete equilibration between solid and melt.
  • In such case, give the solid an infinitesimally small amount of liquid:
  • $C_{L}/C_{0} =$$( 1/D )(1-F)^1/D-1$
  • This describes the composition of a single melt increment that is formed at a particular value of F

  
• • Note how quickly the concentrations of the incompatible elements (D < 1) decline in melt increments that are formed at higher degrees of partial melting (e.g., F > 1% for D = 0.001).
  • An aggregate liquid is produced by accumulating and mixing the various melt increments that are formed over the melting interval F = 0 to F.

  
• Equilibrium crystallization:

  • Occurs when the total liquid and solid remain in equilibrium throughout the crystallization of the magma.
  • Defining F as the fraction of the system that is still liquid, then we derive $C_{L}/C{0} =$$1/F+$$D(1-F)$, identical to the batch melting equation
  • Note; not applicable to solid formation for silicate/oxide mineral crystals in a magma chamber, because it requires crystal interiors to be in continuous equilibrium with the melt. This is only achieved by solid-state diffusion, and this is too slow over mineral-sized distances to permit equilibration.
• Why is the equilibrium crystallization model considered useful?


  It can be applied to the equilibration of two immiscible melts.
• In what context is the equilibrium between a silicate melt and a metallic melt relevant?


  It is relevant for core formation on Earth.
• What does an Fe-rich melt segregate from during core formation?


  It segregates from the molten silicate mantle.
• What values are used for the system of metallic melt-silicate melt?

  $K_{D}$ values
• What is the significance of the equilibrium between a silicate melt and an immiscible sulfide melt?


  It is relevant for the formation of sulfide-ore deposits.
• How does the equilibrium between a silicate melt and an immiscible sulfide melt relate to basaltic melts?


  It relates to the differentiation of many basaltic melts in near-surface magma chambers.
• What values are used for the system of sulfide melt-silicate melt?

  $K_{D}$ values