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Abstract Spongy clinopyroxene is common in most mantle-derived xenoliths and megacrysts of eclogitic and peridotitic parageneses. Its formation is commonly attributed to the partial melting of a primary clinopyroxene in response to various factors, including changes in pressure and temperature or infiltration of external melts or fluids. In order to study the mechanism of spongy clinopyroxene formation in detail, we selected six eclogitic clinopyroxene inclusions in diamonds with varying amounts of spongy clinopyroxene (from ~10 to 100%). We employed computed tomography, electron microprobe analysis, and Raman spectroscopy to study the textural characteristics, major element concentrations, and the types of volatiles present in both phases. We also used pMELTS to model the compositions of spongy clinopyroxene and associated melts produced by the melting of primary clinopyroxene over a range of pressures and temperatures. We compare these results with estimates from major element thermobarometry of the spongy clinopyroxene. We conclude that the studied spongy clinopyroxene is the solid product of partial melting that occurs upon decompression of the primary clinopyroxene within the diamond in a near-closed system. Melting of the primary clinopyroxene occurred continuously or in pulses at different depths during the diamond’s ascent to Earth’s surface and produced variable spongy clinopyroxene and melt compositions even within the same inclusion. This is possible due to relatively rapid kimberlite ascent. The degrees of melting are various and unexpectedly high for mantle melting (between <10 and 60% with an average of ~20–30%). The produced melts are highly silicic, phonolitic, and alkali-rich. pMELTS modelling shows the spongy clinopyroxene compositions can be reproduced at pressures between 0.5–2.7 GPa and temperatures of 850–1300°C, with the majority of them satisfying the P–T conditions of 1–2 GPa and 1100–1300°C. This indicates decompression melting of primary clinopyroxene at shallow upper mantle or lower crustal conditions. 

With the past decades of diamond inclusion research, it is now well established that the mantle-derived diamonds are originated either from the lithospheric mantle or sublithospheric mantle. The lithospheric diamonds can be further divided into mainly the peridotitic and eclogitic suites, which can be distinguished based on their inclusion chemistry, carbon, and nitrogen isotopic compositions (1, 2). The parental lithology of sublithospheric diamonds is less well established, partly due to their much lower occurrence relative to the lithospheric diamonds. But there has been growing isotopic evidence for the involvement of subducted materials in the source region of sublithospheric diamonds, such as carbon, boron, oxygen, and iron (3–6). Precipitation of diamonds in the Earth’s mantle has been thought to require the presence of a fluid phase. Being C-O-H, saline, carbonatitic, silicic, or metallic in composition, these fluids were released upon dehydration or partial melting of the parental lithology and migrate through the mantle until they reach diamond saturation point due to either the change in pressure-temperature, or redox conditions. Understanding the parental lithology and fluid composition of different diamonds has primarily relied on their carbon and nitrogen isotope compositions and major/trace element compositions of mineral/fluid inclusions. These tools have been shown to be powerful in many cases but each could have their own disadvantages. Nitrogen isotopes, for example, are less applicable to sublithospheric diamonds due to their low N concentration. Trace element compositions, on the other hand, can be easily manipulated by small mass fractions of low degree-melt that are enriched in incompatible elements. Understanding the diamond-forming fluids and their parental lithology require new tools that can provide a different perspective than the ones discussed above. In this presentation, we show recent developments in adapting Fe, Mg, and K isotope systems to diamond inclusion studies for a better understanding of their formation. These so-called “non-traditional” stable isotope systems were typically developed for large rocks that are not limited by sample amount. In order to adapt them to mineral inclusions tens to hundreds of micrometers in size, we’ve developed dedicated procedures to: 1) clean the diamond surface to remove contamination before extracting individual inclusions; 2) scale down the columns used for chemical purification to minimize blanks; and 3) improving sensitivity on the mass spectrometer to analyze small samples. With a Nu Plasma II at the Carnegie Institution for Science, we have shown to be able to analyze inclusion samples containing as little as 200 ng of Fe (6). With an upgraded Nu Plasma Sapphire at UCLA that is equipped with a collision cell, we are now able to analyze samples with >25 ng Fe. The same strategy has now been expanded to Mg and K isotope systems, for which a low sample limit of 25 ng and 300 ng has been achieved. With examples of Fe and Mg isotopic compositions of ferropericlase in sublithospheric diamond and K isotopic composition of fluid inclusions in fibrous diamonds, we show how isotopic compositions of major elements of mineral/fluid inclusions in diamond bring us new perspectives on their origin. Our tests show promising results to extend existing Mg and Fe protocols to silicate minerals and potentially applying similar strategies to silicon, calcium, and barium isotopes in the future. 

Abstract Chemical events involving deep carbon- and water-rich fluids impact the continental lithosphere over its history. Diamonds are a by-product of such episodic fluid infiltrations, and entrapment of these fluids as microinclusions in lithospheric diamonds provide unique opportunities to investigate their nature. However, until now, direct constraints on the timing of such events have not been available. Here we report three alteration events in the southwest Kaapvaal lithosphere using U-Th-He geochronology of fluid-bearing diamonds, and constrain the upper limit of He diffusivity (toD ≈ 1.8 × 10⁻¹⁹cm²s⁻¹), thus providing a means to directly place both upper and lower age limits on these alteration episodes. The youngest, during the Cretaceous, involved highly saline fluids, indicating a relationship with late-Mesozoic kimberlite eruptions. Remnants of two preceding events, by a Paleozoic silicic fluid and a Proterozoic carbonatitic fluid, are also encapsulated in Kaapvaal diamonds and are likely coeval with major surface tectonic events (e.g. the Damara and Namaqua–Natal orogenies).