Description: Plastic deformation of diamond has long been associated with the generation of color, specifically brown and pink. Extensive previous optical and spectroscopic characterization of natural pink Type I (nitrogen containing) diamonds has revealed two clear groupings, with distinct geographical origins. Group 1 pinks, which have low concentrations of nitrogen and are relatively highly aggregated (IaA ≤ B), have only been found in the Argyle lamproite pipe (Australia) and Santa Elena alluvial deposits (Venezuela). Group 2 pinks, which have much higher nitrogen concentrations and exhibit low levels of aggregation, have been found in deposits from southern Africa, Canada, and Russia. Pink color is intimately associated with deformation lamellae on the {111} crystal planes, and understanding their formation and structure has been a priority with respect to defining the source of this gemologically valuable color center. In group 2 pinks, these {111} lamellae have been characterized as deformation microtwins by both transmission electron microscopy and X-ray diffraction. Subsequently the {111} lamellae in group 1 pinks have been assumed to also be deformation microtwins. In this paper we report electron backscatter diffraction (EBSD) studies of three brown and six pink naturally deformed diamonds with varying nitrogen concentrations and aggregation states. The results show that there are no deformation microtwins in the group 1 pink or brown diamonds. The study also highlights the usefulness of orientation contrast imaging as a simple and rapid method for determining the presence of microtwins. Our results suggest that the color in the group 1 pink diamonds is not directly related to the presence of deformation twins. However, we propose that twins may have been present but subsequently removed by de-twinning, a process that utilizes the same Shockley partial dislocations involved in the original twinning event. Therefore, it may be the process of twinning (and de-twinning) that creates the defect responsible for pink color, as opposed to the actual structure of microtwins themselves. In addition, a large laboratory data set of pink diamond analyses reveals the occurrence of group 1 pink diamonds in the Namibian marine (secondary) deposits. This would appear to suggest an additional source of group 1 pink diamonds in southern Africa, but the antiquity of these diamonds means that a common source on the former Pangaea supercontinent cannot be ruled out.

Description: The Paleoproterozoic Mojave and Yavapai crustal provinces in southwestern Laurentia contain evolved and juvenile crust, respectively, but the nature of the province boundary remains uncertain. 1.78–1.35 Ga crystalline basement rocks of the Mojave Province preserve an evolved isotopic signature reflecting an Archean crustal component in several isotopic systems (Nd, Pb, Hf). However, no Archean rocks have been found, and hence the origin and tectonic significance of this Archean component are also unclear. This paper analyzes the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons (1.84–1.71 Ga) and the oldest metasedimentary rocks (1.75 Ga Vishnu Schist) across a 180-km-long cross-strike transect in Grand Canyon. This transect crosses the Crystal shear zone, which has been proposed as the location of a suture separating the provinces. Our results show that the characteristically bimodal population of detrital zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed Hf(t) values, primarily between +5 to –5, that are uniform across the transect. Another new finding is that the 1.84 Ga Elves Chasm pluton, on which the Vishnu Schist was deposited, yields juvenile Hf(t) values of +5 to +12 and was not the dominant source for the ca. 1.85 Ga peak in the 1.75 Ga Vishnu Schist. Instead, the Vishnu Schist was derived from an Archean craton mixed with intermediate to evolved 1.85 Ga crust. Metasediments show no evidence in support of the proposed suture. Paradoxically, plutons east and west of the Crystal shear zone do support models for a crustal suture. Plutons east of the Crystal shear zone dated at 1.74–1.71 Ga yield juvenile Hf(t) values of +5 to +12 that are characteristic of the Yavapai Province. Plutons west of the Crystal shear zone show juvenile to evolved Paleoproterozoic grains ( Hf(t) of –5 to +10) as well as xenocrystic Archean and 1.85 Ga grains ( Hf(t) of –12 to +10). These data support the proposition that the Crystal shear zone marks a sharp boundary between the Mojave and Yavapai crustal provinces. However, the overlapping Vishnu Schist suggests a more complicated crustal architecture. The depositional setting of the Vishnu Schist remains unclear; however, we interpret the ultimate geometry of the transect to reflect an ~200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across both Mojave and Yavapai crust. This system was subsequently imbricated in an accretionary complex. The ultimate architecture is of a distributed boundary with slivers of plutons that carry the isotopic signature of their respective provinces imbricated within metasediments.

Description: We suggest a new explanation for the presence of crustally derived zircons in the upper-mantle rocks of ophiolitic complexes, as an alternative to subduction-related models. Integrated isotopic (U-Pb, Hf, and O isotopes) and trace-element data for zircons from the Tumut ophiolitic complex (southeast Australia) indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust. These observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper-mantle rocks and the dynamics of crust-mantle interactions.

Description: Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (southeastern Tibet) contain diamond and a highly reduced trace-mineral association. Exsolution of diopside and coesite from chromite suggests inversion from the Ca-ferrite structure in the upper part of the mantle transition zone (〉400 km). However, the trace-element signatures of the chromites are typical of ophiolitic chromitites, implying primary crystallization at shallow depths. Os-Ir nuggets in the chromitites have Re-Os model ages ( T RD ) of 234 ± 3 Ma, while T RD ages of in situ Ru-Os-Ir sulfides range from 290 to 630 Ma, peaking at ca. 325 Ma. Euhedral zircons in the chromitites give U-Pb ages of 376 ± 7 Ma, Hf = 9.7 ± 4.6, and 18 O = 4.8–8.2. The sulfide and zircon ages may date formation of the chromitites from boninite-like melts in a supra-subduction-zone environment, while the model ages of Os-Ir nuggets may date local reduction in the transition zone following Devonian subduction. Thermo-mechanical modeling suggests a rapid (10 m.y.) rise of the buoyant harzburgites from 〉400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.

Description: The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from...