Protogenetic clinopyroxene inclusions in diamond and Nd diffusion modeling—Implications for diamond dating

Abstract

Diamonds are witnesses of processes that have operated in Earth’s mantle over more than 3 b.y. Essential to our understanding of these processes is the determination of diamond crystallization ages. These cannot be directly determined on diamond, but they can be calculated using radiogenic isotopic systematics of suitable minerals included in a diamond. This method relies on the assumption that the mineral inclusions were in isotopic equilibrium with the diamond-forming medium. We evaluated the validity of Sm-Nd ages yielded by clinopyroxene inclusions by combining crystallographic orientation analyses and Nd diffusion modeling at the relevant conditions for Earth’s cratonic mantle. We investigated the crystallographic orientation relationships (CORs) for 54 clinopyroxene inclusions within 18 diamonds from South Africa and Siberia. Clinopyroxene inclusions in some diamonds showed specific CORs with their hosts, indicating possible syngenesis. Other samples had clusters of clinopyroxene inclusions sharing the same orientation but no specific orientation relative to their hosts, indicating that the inclusions are older than the diamond (i.e., they are protogenetic). Diffusion modeling in the temperature range typical for lithospheric diamonds (900–1400 °C) showed that resetting of the Sm-Nd isotopic system in clinopyroxene grains larger than 0.05 mm requires geologically long interaction with the diamond-forming fluid/melt (>3.5 m.y. at average temperature of ~1150 °C). Depending on inclusion size and temperature regime, protogenetic clinopyroxene inclusions may not fully reequilibrate during diamond-formation events. We suggest that small clinopyroxene inclusions (<0.2 mm) that equilibrated at temperatures higher than 1050–1080 °C may be the most suitable for age determinations.

A critique of using epitaxial criterion to discriminate between protogenetic and syngenetic mineral inclusions in diamond

“…Olivines were interpreted as protogenetic, being relicts of an original monocrystal that underwent dissolution during diamond growth. Same observations were reported on garnet and pyroxene inclusions 12 , 14 , 15 . Thus, the discovery of protogenetic inclusions with diamond-imposed morphology contradicts the MC criterion.…”

“…We will show here that also the third criterion (EC) cannot be considered as a reliable argument to demonstrate the syngenetic relationship between diamonds and their mineral inclusions. We will make use of recent experimental observations and some new considerations to correctly interpret the relative crystallographic orientations of mineral inclusions in diamond 2 , 4 , 6 , 10 , 13 – 15 , 17 – 27 . Especially, we will focus our attention on periclase [(Mg,Fe)O] and magnesiochromite (MgCr 2 O 4 ) inclusions, recently considered syngenetic by some researchers on the basis of the EC.…”

Is the Imposition of Diamond Morphology on Mineral Inclusions a Syngenetic or Post-Genetic Process with Respect to Diamond Formation?

“…This interpretation was subsequently strengthened by the discovery in a xenolith of a clinopyroxene (external to diamond) with the same composition and crystal orientation of the clinopyroxene relicts included in the diamond (again, this orientation was different from that of the diamond host) . Subsequently, Nimis et al documented two iso-oriented inclusions of magnesiochromite, which clearly are the remains of an original single crystal partially resorbed during the diamond growth, highlighting the protogenetic character of the iso-oriented inclusions. In all the papers reporting crystal orientation relationships between inclusions and diamonds, at most three distinct sets of similarly oriented inclusions have been found within a single diamond. ,,,,,,,− Iso-oriented protogenetic inclusions show both a diamond-imposed morphology [e.g., olivine with (pseudo) cubo-octahedral morphology] and a lobed morphology. , The coexistence of these two different morphologies in the same diamond can inform us about the variation of the diamond growth rate during its formation history. However, as discussed later, this information can only be extracted when the diamond growth zonation is known. By performing micro-Raman spectroscopy on DIs, Nimis et al found that typical mineral inclusions in diamonds from the Siberian and Kaapvaal cratons are surrounded by a film of hydrous silicic fluid having a maximum thickness of 1.5 μm.…”

“…In all the papers reporting crystal orientation relationships between inclusions and diamonds, at most three distinct sets of similarly oriented inclusions have been found within a single diamond. ,,,,,,,− …”

“…This criterion was initially challenged by Taylor et al , and subsequently by Nestola et al In particular, by measuring the crystallographic orientations of numerous olivine inclusions in diamond with imposed morphology, Nestola et al . were able to establish their protogenetic origin; the same approach was also applied to garnet and pyroxene inclusions. , Hence, this criterion has been refuted: the discovery of protogenetic inclusions with diamond-imposed morphology contradicts the initial hypothesis. This does not mean that all inclusions with diamond-imposed morphology are protogenetic, but it simply means that this criterion is no longer applicable to discriminate unequivocally between syngenesis and protogenesis.…”