The Tethyan Himalaya igneous province: Early melting products of the Kerguelen mantle plume

The Kerguelen large igneous province (LIP) has been related to mantle plume activity since at least 120 Ma. There are some older (147–130 Ma) magmatic provinces on circum-eastern Gondwana, but the relationship between these provinces and the Kerguelen mantle plume remains controversial. Here we present petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb–Os isotopic data for high-Ti mafic rocks from two localities (Cuona and Jiangzi) in the eastern Tethyan Himalaya igneous province (147–130 Ma). Zircon grains from these two localities yielded concordant weighted mean 206Pb/238U ages of 137.25 ± 0.98 and 131.28 ± 0.78 Ma (2σ), respectively. The analyzed mafic rocks are enriched in high field strength elements and have positive Nb–Ta anomalies relative to Th and La, which have ocean island basalt-like characteristics. The Cuona basalts were generated by low degrees of melting (3–5%) of garnet lherzolites (3–5 vol.% garnet), and elsewhere the Jiangzi diabases were formed by relatively lower degrees of melting (1–3%) of garnet lherzolite (1–5 vol.% garnet). The highly radiogenic Os and Pb isotopic compositions of the Jiangzi diabases were produced by crustal contamination, but the Cuona basalts experienced the least crustal contamination given their relatively low γOs(t), 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi values. Major and trace element geochemical and Sr–Nd–Hf–Pb–Os isotope data for the Cuona basalts are similar to products of the Kerguelen mantle plume head. Together with high mantle potential temperatures (>1500°C), this suggests that the eastern Tethyan Himalaya igneous province (147–130 Ma) was an early magmatic product of the Kerguelen plume. A mantle plume initiation model can explain the temporal and spatial evolution of the Kerguelen LIP, and pre-continental break-up played a role in the breakup of eastern Gondwana, given the >10 Myr between initial mantle plume activity (147–130 Ma) and continental break-up (132–130 Ma). Like studies of Re-Os isotopes in other LIPs, the increasing amount of crustal assimilation with distance from the plume stem can explain the variations in radiogenic Os.

The Subantarctic lithospheric mantle

We present a summary of peridotite in the Subantarctic (46 to 60°S) surrounding the Antarctic Plate. Peridotite xenoliths occur on Kerguelen and Auckland islands. Kerguelen islands are underlain by a plume whereas the Auckland Islands are part of continental Zealandia, which is a Gondwana-rifted fragment. Small amounts of serpentinised peridotite has been dredged from fracture zones on the Southeast Indian, Southwest Indian, and Pacific Antarctic Ridge, and represents upwelled asthenosphere accreted to form lithosphere. Supra-subduction zone peridotite has been collected from two locations on the Sandwich Plate. Peridotites from most Subantarctic occurrences are moderately to highly depleted, and many show signs of subsequent metasomatic enrichment. Os isotopes indicate that the Subantarctic continental and oceanic lithospheric mantle contains ancient fragments that underwent depletion long before formation of the overlying crust.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5424956

Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere.

Supplemental Material: Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

Description of Rietveld refinements and composition of the Mss, reciprocal crystallographic orientations and Os diffusion model, unit-cell parameters of pyrrhotite single crystals, and orientation matrices for both inclusions and hosts.<br>

Supplemental Material: Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

Description of Rietveld refinements and composition of the Mss, reciprocal crystallographic orientations and Os diffusion model, unit-cell parameters of pyrrhotite single crystals, and orientation matrices for both inclusions and hosts.<br>

Fractionation of Highly Siderophile Elements during reactive melt infiltration in lower oceanic crust

&lt;p&gt;The olivine-rich troctolites are Mg-rich rocks forming by open-system magmatic crystallization in response to primitive melt injections into the growing lower oceanic crust (e.g., Renna et al., 2016).&lt;/p&gt;&lt;p&gt;In the present study, whole-rock highly siderophile (HSE: Os, Ir, Ru, Rh, Pt, Pd, Au and Re) and chalcogen (S, Se and Te) element compositions, and Re-Os isotopes of the olivine-rich troctolites from the Jurassic Alpine ophiolites were determined with the aim to investigate the control that the formation of lower oceanic crust may exert on the fractionation of HSE and other incompatible chalcophile elements in MORB.&lt;/p&gt;&lt;p&gt;The olivine-rich troctolites have initial &amp;#947;Os (160 Ma) ranging from +0.2 to +5.9, and Primitive Mantle (PM)-normalized HSE-Te-Se-S patterns showing a gradual increase from Os to Au, and nearly flat Au-Te-Se patterns. These patterns are similar to those of little-fractionated mantle melts and are parallel, at higher concentrations levels, to those typical of MORB. The olivine-rich troctolites have higher Te and Os/Ir, and lower Se/Te than MORB, which may be reconciled with a process of sulfide accumulation. Sulfide precipitation could be promoted by interaction between melts interstitial to olivine and melts relatively rich in silica, which could migrate from an underlying gabbroic framework (cf. Renna et al., 2016). Melts residual to the formation of olivine-rich troctolites are inferred to have a markedly HSE-fractionated signature comparable to that of MORB.&lt;/p&gt;&lt;p&gt;Renna M.R., Tribuzio R., Ottolini L. (2016). J Geol Soc Lond 173, 916&amp;#8211;932&lt;/p&gt;