Rocks from the mantle transition zone: majorite-bearing xenoliths from malaita, southwest pacific

Oceanic and super-deep continental diamonds share a transition zone origin and mantle plume transportation

Rare oceanic diamonds are believed to have a mantle transition zone origin like super-deep continental diamonds. However, oceanic diamonds have a homogeneous and organic-like light carbon isotope signature (δ¹³C − 28 to − 20‰) instead of the extremely variable organic to lithospheric mantle signature of super-deep continental diamonds (δ¹³C − 25‰ to + 3.5‰). Here, we show that with rare exceptions, oceanic diamonds and the isotopically lighter cores of super-deep continental diamonds share a common organic δ¹³C composition reflecting carbon brought down to the transition zone by subduction, whereas the rims of such super-deep continental diamonds have the same δ¹³C as peridotitic diamonds from the lithospheric mantle. Like lithospheric continental diamonds, almost all the known occurrences of oceanic diamonds are linked to plume-induced large igneous provinces or ocean islands, suggesting a common connection to mantle plumes. We argue that mantle plumes bring the transition zone diamonds to shallower levels, where only those emplaced at the base of the continental lithosphere might grow rims with lithospheric mantle carbon isotope signatures.

Recovery of an oxidized majorite inclusion from Earth's deep asthenosphere

Minerals recovered from the deep mantle provide a rare glimpse into deep Earth processes. We report the first discovery of ferric iron-rich majoritic garnet found as inclusions in a host garnet within an eclogite xenolith originating in the deep mantle. The composition of the host garnet indicates an ultrahigh-pressure metamorphic origin, probably at a depth of ~200 km. More importantly, the ferric iron-rich majoritic garnet inclusions show a much deeper origin, at least at a depth of 380 km. The majoritic nature of the inclusions is confirmed by mineral chemistry, x-ray diffraction, and Raman spectroscopy, and their depth of origin is constrained by a new experimental calibration. The unique relationship between the majoritic inclusions and their host garnet has important implications for mantle dynamics within the deep asthenosphere. The high ferric iron content of the inclusions provides insights into the oxidation state of the deep upper mantle.

Discovery of the Fe-analogue of akimotoite in the shocked Suizhou L6 chondrite

We report the first natural occurrence of the Fe-analogue of akimotoite, ilmenite-structured MgSiO₃, a missing phase among the predicted high-pressure polymorphs of Fe-pyroxene, with the composition (Fe²⁺_0.48Mg_0.37Ca_0.04Na_0.04Mn²⁺_0.03Al_0.03Cr³⁺_0.01)_Σ=1.00Si_1.00O₃. The new mineral was approved by the International Mineralogical Association (IMA 2016-085) and named hemleyite in honour of Russell J. Hemley. It was discovered in an unmelted portion of the heavily shocked L6 Suizhou chondrite closely associated to olivine, clinoenstatite and Fe-bearing pyroxene with a composition nearly identical to that of hemleyite. We also report the first single-crystal X-ray diffraction study of a Si-bearing, ilmenite-structured phase. The fact that hemleyite formed in a meteorite exposed to high pressures (<20 GPa) and temperatures (<2000 °C) during impact-induced shocks indicates that it could play a crucial role at the bottom of the Earth’s mantle transition zone and within the uppermost lower mantle.

Discovery of natural MgSiO3 tetragonal garnet in a shocked chondritic meteorite

MgSiO3 tetragonal garnet, which is the last of the missing phases of experimentally predicted high-pressure polymorphs of pyroxene, has been discovered in a shocked meteorite. The garnet is formed from low-Ca pyroxene in the host rock through a solid-state transformation at 17 to 20 GPa and 1900° to 2000°C. On the basis of the degree of cation ordering in its crystal structure, which can be deduced from electron diffraction intensities, the cooling rate of the shock-induced melt veins from ~2000°C was estimated to be higher than 10(3)°C/s. This cooling rate sets the upper bound for the shock-temperature increase in the bulk meteorite at ~900°C.

Rapid kimberlite ascent and the significance of Ar-Ar ages in xenolith phlogopites

Kimberlite eruptions bring exotic rock fragments and minerals, including diamonds, from deep within the mantle up to the surface. Such fragments are rapidly absorbed into the kimberlite magma so their appearance at the surface implies rapid transport from depth. High spatial resolution Ar-Ar age data on phlogopite grains in xenoliths from Malaita in the Solomon Islands, southwest Pacific, and Elovy Island in the Kola Peninsula, Russia, indicate transport times of hours to days depending upon the magma temperature. In addition, the data show that the phlogopite grains preserve Ar-Ar ages recorded at high temperature in the mantle, 700 degrees C above the conventional closure temperature.