Eclogite and omphacitite xenoliths of the Navajo Volcanic Field (NVF) provide a unique opportunity to study processes in an oceanic slab down to subarc depths. However, ambiguities remained about protolith origin, prograde metamorphic conditions, and metasomatic processes, which we address here with new geothermobarometric and in-situ U-Pb data. The earliest garnet (53.4 ± 4.8 Ma), omphacite, and phengite generations yielded conditions of 2.4–3.3 GPa, 400–540 °C. Zircon cores (87−37 Ma 206Pb/238U dates) yielded higher Th/U ratios (0.1–0.6) than zircon mantles (65−30 Ma and Th/U < 0.1). Rutile dates cluster at 32.3 ± 1.2 to 28.4 ± 1.9 Ma. The Th/U ratios suggest magmatic growth of the zircon cores, which we consider a strong argument that the NVF eclogites and omphacitites were at least partly derived from Cretaceous Farallon oceanic crust. Rare relic Proterozoic zircon can be explained by inheritance, or by derivation of some xenoliths from the Proterozoic North American lithosphere. Regardless of protolith origin, the rocks were brought to depth by the Farallon slab and resided at lawsonite eclogite facies conditions for a ca. 20–25 Myr interval bracketed by the garnet and rutile dates, during which they experienced two (likely ultra-high pressure) metasomatic events. Firstly, Na-Si-rich fluids, likely derived from metasedimentary rocks, caused growth of Na-rich omphacite. A second metasomatic episode through serpentinite-derived fluids happened just prior to 30 Ma xenolith exhumation as part of the “Great Hydration Event” that affected the Colorado Plateau.

Intrusions of the Navajo Volcanic Field (NVF) contain eclogite xenoliths that record processes related to the subduction of the Farallon plate beneath the Colorado Plateau. Previous geochronological work sparked controversies about their origin, especially whether they are derived from oceanic crust of the Farallon plate, or from older continental lithosphere, based on occasional Proterozoic zircon U-Pb ages. Moreover, the mechanisms and timescales of the recorded high-pressure processes, including several stages of fluid metasomatism, are largely unknown. We study the U-Pb systematics of garnet, zircon and rutile by LA-ICP-MS in order to achieve a refined petrochronologic interpretation of the NVF eclogites. The eclogite xenoliths are hosted by serpentinized ultramafic microbreccia (SUM) which intruded the Colorado Plateau at ~30 Ma as a consequence of extensive hydration of the lithospheric mantle by Farallon slab-derived fluids. In contrast to kimberlite-borne eclogite xenoliths, which often contain garnet and omphacite only, those of the NVF additionally contain ubiquitous rutile, and often pyrite, phengite, zoisite pseudomorphs after lawsonite (with rare lawsonite relics), accessory monazite, and rare coesite. Based on this assemblage, peak P-T conditions around 4 GPa and 600°C are estimated. Subsequent rapid uplift in the sub-solidus SUM is not thought to have caused significant further heating. Except for a few instances where a mid-ocean ridge basalt-like bulk chemical composition is essentially preserved, the xenoliths are strongly overprinted by several metasomatic events in the eclogite facies. Most notably, this involved interaction with a Na-Si-S-rich fluid, probably of crustal origin, and a later (just prior to exhumation) serpentinite-derived fluid. All our U-Pb rutile data as well as published U-Pb monazite data (~29 Ma) agree with the ~30 Ma SUM formation age. The majority of the U-Pb zircon analyses predate the rutile data by not more than several Myr, with a minority of older ages forming a continuum to the late Cretaceous. Unlike some earlier studies, we did not obtain any Proterozoic U-Pb zircon ages. The garnet U-Pb dates, which are mostly from pre-metasomatic zones, partly agree with the zircon dates within uncertainty, but sometimes predate the zircon dates from the same sample by up to several tens of Myr. Despite moderate peak metamorphic temperatures (~600°C), rutile remained an open system for Pb until exhumation. Garnet and zircon are resistant to Pb volume diffusion at these temperatures, however, the zircon ages appear to be largely reset during fluid metasomatism, as also indicated by often high common Pb contents. Due to the absence of preserved igneous zircon ages, the protolith origin remains uncertain. Garnet, which grew mostly before metasomatism, seems to provide robust ages of initial eclogitization, which may have been diachronous in our sample suite. Geochronological evidence implies that several tens of Myr passed between initial eclogitization and exhumation of the xenoliths.

Simultaneous analysis of carbon and nitrogen isotope ratios by SIMS was applied for the first‐time to a natural diamond from the Kelsey Lake kimberlite, State Line Distinct, Colorado (UWD‐1). This in situ procedure is faster, reduces sample size for analysis, and measures both isotope ratios from a single ~ 10 μm diameter pit, a critical advantage for zoned diamonds. The carbon isotope ratio (expressed as δ13CVPDB) of the bulk UWD‐1 crystal, determined by the conventional combustion method in the present study, is ‐5.9‰ ± 0.2‰ (VPDB, 2s). Nitrogen mass fraction ([N]) and isotope ratio (expressed as δ15NAir) were determined by stepwise combustion and gas‐source mass‐spectrometry, resulting in 553 ± 64 μg g‐1 and ‐6.7‰ ± 1.1‰ (Air, 2s), respectively. Secondary ions of 12C2‐, 12C13C‐, 12C14N‐, and 12C15N‐ were simultaneously measured by SIMS using three Faraday cups and one electron multiplier. The spot‐to‐spot reproducibility of δ13C and δ15N values for the UWD‐1 (178 spots on sixteen chips, 10 μm spots), were 0.3‰ and 1.6‰, respectively (2s). While 12C14N‐/12C2‐ ratios, which are an indicator for [N], varied up to 12% among these sixteen chips, such variation did not correlate with either δ13C or δ15N values. We propose that UWD‐1 is a suitable reference sample for microscale in situ analysis of δ13C and δ15N values in diamond samples.

&lt;p&gt;Large, single crystals (&gt; 1cm) are a familiar component of mantle xenolith suites of many kimberlites.&amp;#160; Confusion between different suites exists in the literature, however, which affects petrogenetic models, and some clarification is warranted.&amp;#160; Megacrysts of the Cr-poor suite&lt;sup&gt; &lt;/sup&gt;[1] are most common.&amp;#160; Cr-poor silicates (garnet, clinopyroxene, orthopyroxene, olivine) are characterized by lower Mg/(Mg+Fe) and Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and higher TiO&lt;sub&gt;2&lt;/sub&gt; values than typical mantle peridotite minerals.&amp;#160; Strong geochemical trends in most occurrences of Cr-poor megacryst suites (e.g., concomitant decrease in Mg/(Mg+Fe) and Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;) are interpreted by most authors as the result of fractional crystallization of a kimberlite, or kimberlite-like, magma.&amp;#160; &amp;#160;&lt;/p&gt; &lt;p&gt;The Cr-rich megacryst suite, comprising garnet, clinopyroxene, orthopyroxene and olivine, but not ilmenite, was described from the Sloan/Nix kimberlites in northern Colorado&lt;sup&gt; &lt;/sup&gt;[1].&amp;#160; Constituent minerals, all four of which are essential to the definition of the suite, are characterized, in part, by high and restricted values of Mg/(Mg+Fe) and wt% Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (e.g., 0.791 to 0.837 and 6.1 to 13.0, respectively, in garnet [2]).&amp;#160; Elsewhere, large crystals with Mg/(Mg+Fe) and Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; values higher than Cr-poor suite minerals do occur, but none correspond to the Sloan-Nix Cr-rich suite in paragenesis, size and/or composition&lt;sup&gt; &lt;/sup&gt;[2].&amp;#160; For example, almost no garnet megacrysts described as &amp;#8220;Cr-rich&amp;#8221; or &amp;#8220;high-Cr&amp;#8221; from other localities (e.g., refs 3-6) contain &gt;6 wt% Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and even garnets with &lt;2 wt% Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; are termed &amp;#8220;Cr-rich&amp;#8221; or &amp;#8220;high-Cr&amp;#8221;.&amp;#160; Most, or all, of these so-called &amp;#8220;Cr-rich garnet megacrysts&amp;#8221; are simply xenocrysts from coarse-grained peridotite.&amp;#160;&lt;/p&gt; &lt;p&gt;The &amp;#8220;Granny Smith&amp;#8221; suite, first described from Kimberley and Jagersfontein [7], is dominated by Cr-clinopyroxene associated with phlogopite (and ilmenite at Kimberley), with uncommon olivine or rutile.&amp;#160; Garnet and orthopyroxene do not occur in this suite, which is neither equivalent to nor a subset of the Cr-rich megacryst suite.&amp;#160; Other suites dominated by Cr-clinopyroxene, also not shown to coexist with garnet and orthopyroxene, have been described from Orapa and Bobbejaan [6] and Grib [8], though analogies have been drawn with the Cr-rich megacryst suite despite compositional and paragenetic differences.&amp;#160; A similar megacrystalline assemblage (Cr-cpx, ilmenite, phlogopite, olivine) has been described from Attawapiskat [9] and at Balmoral megacrysts of Cr-cpx occur with ilmenite, Nb-Cr rutile and zircon [10].&lt;/p&gt; &lt;p&gt;All of these suites of Cr-cpx +/- ilmenite, rutile, phlogopite, olivine, zircon (lacking garnet/opx), though varied, have more in common with each other than with the Cr-rich megacryst suite.&amp;#160; All might be best termed &amp;#8220;Granny Smith&amp;#8221;, and may have common origins.&amp;#160; The only feature they share with the Sloan-Nix Cr-rich megacryst suite is the presence of large chromian clinopyroxene.&amp;#160; Use of such populations as equivalents of the Sloan-Nix Cr-rich megacryst suite in mantle petrogenetic schemes can lead to faulty conclusions.&amp;#160;&lt;/p&gt; &lt;p&gt;References:&amp;#160; 1) Eggler et al. (1979) The Mantle Sample, 2) Schulze (2022) Goldschmidt Conf. Abstr., 3) Hunter and Taylor (1984) Am. Min., 4) Kopylova et al. (2009) Lithos, 5) Bussweiller et al. (2018) Min. Pet., 6) Nkere et al. (2021) Lithos, 7) Boyd et al. (1984) GCA, 8) Kargin et al. (2017) Lithos, 9) Hetman (1996) MSc., 10) Schulze, unpub. data.&amp;#160;&lt;/p&gt;

ABSTRACT Forty-one samples of diamond from the Jurassic 95-2 kimberlite pipe in the Lake Timiskaming Kimberlite Cluster, Superior Craton, Canada, were imaged using cathodoluminescence and analyzed by secondary ion mass spectrometry and Fourier-transform infrared absorbance spectrometry to determine carbon stable isotope composition, total nitrogen abundance, and nitrogen aggregation state. The carbon isotope compositions results (δ13CVPDB) range from –9.11 to –3.57‰, with a mean value of –5.8‰. Intra-stone variation is small (maximum ∼2.2‰, and in most individual diamond samples <1‰). Nitrogen contents range from 0.5 to 2040 ppm (mean of 483 ppm). The greatest range of values in a single stone is 825 ppm. The samples are poorly aggregated in terms of nitrogen. The samples are mostly type IaA or IaAB, with a few bordering on type Ib. Diamond growth was episodic, with nitrogen behaving highly compatibly (i.e., D = [N]diamond/[N]fluid >> 1). Precipitation was likely from a carbonate-rich fluid in a peridotitic (lherzolitic) environment within the mantle of the central Superior Craton. This generation of diamond growth is very similar to those reported from the Jurassic age Victor and U2 pipes of the Attawapiskat Kimberlite Cluster, and distinct from a possibly much older (>1.1 Ga) generation of diamond reported in other older host rocks (T1, Wawa, Lynx, and Renard). This older generation of diamond at these other localities is also predominantly of the peridotitic (harzburgitic) paragenesis but contains far less nitrogen (although typically more aggregated as B centers) and has higher δ13CVPDB. The younger generation of diamond formed after mantle heating during formation of the Mid-Continental Rift (ca. 1.1 Ga) destroyed any proximal prior generation(s) of diamond. Igneous activity after 1.1 Ga subsequently refertilized the cratonic mantle to a lherzolitic paragenesis in which the younger generation precipitated.

The Colorado Plateau has undergone as much as 1.8 km of uplift over the past 80 Myr, but never underwent the pervasive deformation common in the neighboring tectonic provinces of the western USA. To understand the source, timing and distribution of mantle hydration, and its role in plateau uplift, garnets from four eclogite xenoliths of the Moses Rock diatreme (Navajo Volcanic Field, Utah, USA) were analyzed in situ for . delta O-18 by secondary ion mass spectrometry. These garnets have the largest reported intra-crystalline oxygen isotope zoning to date in mantle-derived xenoliths with core-to-rim variations of as much as 3 parts per thousand. All samples have core delta O-18 values greater than that of the pristine mantle (similar to 5.3 parts per thousand, mantle garnet as derived from mantle zircon in earlier work) consistent with an altered upper oceanic crust protolith. Oxygen isotope ratios decrease from core to rim, recording interaction with a low-delta O-18 fluid at high temperature, probably derived from serpentinite in the foundering Farallon slab. All zoned samples converge at a delta O-18 value of similar to 6 parts per thousand, regardless of core composition, suggesting that fluid infiltration was widely distributed. Constraints on the timing of this fluid influx, relative to diatreme emplacement, can be gained from diffusion modeling of major element zoning in garnet. Modeling using best estimates of peak metamorphic conditions (620 degrees C, 3.7 GPa) yields durations of <200 kyr, suggesting that fluid influx and diatreme emplacement were temporally linked. These eclogite xenoliths from the Colorado Plateau record extensive fluid influx, pointing to complex hydration-dehydration processes related to flat-slab subduction and foundering of the Farallon plate. Extensive hydration of the lithospheric mantle during this fluid influx may have contributed to buoyancy-driven uplift of the Colorado Plateau and melt-free emplacement of Navajo Volcanic Field diatremes.

ABSTRACT Xenoliths and xenocrysts of mantle material from kimberlite dikes located underground at the Certac Au mine, Québec, in the eastern Superior Craton, were studied in terms of the major element composition of their constituent minerals. The kimberlite was dated at 1151 ± 46 Ma by the U-Pb perovskite method. This suite thus provides a rare glimpse into the Mesoproterozoic mantle of the Superior Craton. Two parageneses of mantle material unrelated to the kimberlite magmatism occur: (1) an olivine + ilmenite ± magnetite association characterized by relatively Fe-rich olivine (Mg# = 0.68–0.84) and ilmenite enriched in Mg and Cr (4–13 wt.% MgO, Cr2O3 up to 3 wt.%), and (2) spinel peridotite characterized by Mg-rich olivine (Mg# = 0.91–0.94). The Fe-rich association is interpreted as a magmatic cumulate likely unrelated to the kimberlite. No mantle-derived garnet occurs in the xenoliths or as xenocrysts. The presence of Cr-rich spinel (Cr# = 0.84–0.98) in high temperature (860–953 °C) chromite peridotite indicates bulk compositions too depleted in Al for garnet to be stable, although geothermometry suggests they equilibrated at depths corresponding to garnet stability (90–131 km, depending on the geothermal gradient). Alternatively, the presence of phlogopite in two of the three high temperature (i.e., deepest) chromite peridotites suggests the absence of garnet and presence of low-Al chromite may have been caused by metasomatism from a K-rich fluid that replaced garnet with phlogopite + clinopyroxene ± chromite. Less depletion at shallower depths is indicated by a chromite (Cr# = 0.60) dunite that equilibrated at 831 °C and a low temperature (752 °C) Mg-Al-spinel lherzolite.

An empirical method has been developed to distinguish between crustal- and mantle-derived rutile using simple chemical screens based on the concentrations of minor and trace elements. This classification scheme is based on analyses of rutiles from 115 mantle-derived xenoliths (63 eclogites from Blaauwbosch, Roberts Victor, Kelsey Lake, Schaffer, metasomatic rutile-dominated nodules from Orapa, Balmoral, Jagersfontein; five MARID xenoliths from Kimberley) and 208 crustal rutiles selected from heavy mineral sands from six different localities (sands from Florida, Kerla (India), Arkansas, Athabasca Tar Sands (Alberta) and two Australian localities). Two variations of this scheme were derived using 1) concentrations of Cr, Mg, V, Al, Nb, and Si based on electron microprobe data and 2) concentrations of Cr, Sn, Mg, Zr, Hf, Mn, Mo, Si, and V from LA-ICP-MS data which correctly classify 98% and >99% of the analyzed rutiles, respectively.

Oxygen isotope geochemistry has provided useful insights into the origins of mantle eclogites that occur as xenoliths in kimberlite. Many workers now consider eclogites that have anomalously high or low oxygen isotope ratios, relative to typical mantle peridotite (δOVSMOW = +5.37 ± 0.36‰ in peridotite garnet; Mattey et al., 1994), as representing altered ocean lithosphere (e.g., MacGregor and Manton, 1986; Jacob et al., 1994). The number of eclogite xenolith suites that have been studied is relatively small, however, and fewer still have been investigated using high precision laser fluorination (LF) techniques. In order to broaden the oxygen isotope database for mantle eclogite xenoliths, we have begun a systematic study of eclogites from a variety of kimberlite occurrences, world-wide. Here we present our findings to date.

Mantle xenoliths provide an important glimpse of the characteristics of the continental lithospheric mantle. The picture is complicated by the fact that many xenoliths have experienced a number of events that have modified their chemical and mineralogical makeup. In some cases, these modifications have occurred by interaction with passing melts and fluids during residence in the mantle. Other chemical and mineralogical overprints within xenoliths (i.e. not confined to grain boundaries) have resulted from interaction with transporting magmas, both before and during ascent. Distinguishing which of the observed characteristics of a mantle xenolith reflect features that were present in the mantle versus those that were introduced as a part of the magmatism that brought the sample to the surface is not always straightforward. This step, however, is critical if the xenoliths are to be used to interpret the long-term chemical and thermal history of the continental lithospheric mantle.

Previous studies of diamonds and their mineral inclusions from the Guaniamo region of Venezuela have shown that the population is dominated by diamonds that belong to the eclogite-suite, and that most of the stones have unusually low carbon isotope ratios (Sobolev et al., 1998; Kaminsky et al., 2000). We have studied Guaniamo diamonds and their eclogite-suite mineral inclusions in situ in polished section by electron microprobe (EMP) analysis, cathodoluminescence (CL) and secondary ion mass spectrometry (SIMS), and have studied Guaniamo garnet xenocrysts by EMP and laser fluorination (LF).

An octahedral diamond crystal from Guaniamo, Venezuela shows a multitude of round radiocolouration spots that indicate a remarkable formation history. Spots always occur in pairs, with similar spacing and intensity ratio between the two spots of each pair. We interpret this pattern to be the result of long-term irradiation of the stone emanating from a multitude of radioactive point sources. At some point during the irradiation, the stone must have experienced a translational movement which shifted it ca. 50μm relative to the adjacent material [i.e., the (111) crystal face was a fault plane], after which irradiation continued. The Neoproterozoic age of the Guaniamo kimberlites and the high degree of radiation damage suggest that both of the two irradiation periods lasted over hundreds of millions of years. This interpretation is supported by results of He-irradiation experiments.

A hypabyssal kimberlite dike in southwestern Pennsylvania (USA), emplaced through Proterozoic basement and Phanerozic cover, contains a xenocryst and xenolith assemblage typical of material sampled within the subcontinental lithosphere, including xenocrysts of Cr-rich pyrope, magnesiochromite, Cr-rich diopside, and peridotite xenoliths. Temperatures and depths of equilibration of the clinopyroxene (840 oC and 130 km to 1350 oC and 170 km) indicate some sampling in the field of diamond stability. Diamonds have not been reported, however, and the chemistry of the garnet (lherzolite, Cr-poor megacryst, and Group II eclogite) and spinel (<56.0 wt.% Cr2O3) are consistent with diamond absence and the off-craton tectonic setting of the kimberlite. An unusual feature of this suite is that, unlike most mantle xenolith/xenocryst spinel, some of those from Masontown have an unusually high silica content (to 0.59 wt.% SiO2). The significance of the high silica content is unclear, but may be related to an ultrahigh-pressure precursor chromite polymorph with a calcium ferrite structure, which can accommodate Si in solid solution.

Stable isotope ratios of carbon (δ13C) and oxygen (δ18O) from tree ring cellulose can provide valuable paleoclimatic information at annual and subannual resolution, from time periods long before instrumental climate records. Recently, mummified (non-permineralized) wood was discovered within Canadian Subarctic kimberlites, inviting paleoclimatic investigations of the time in which the trees grew. In the mummified wood, polysaccharides (hemicellulose, α-cellulose) are frequently preferentially degraded, in the low-silica anaerobic burial environment of the kimberlites, resulting in a lignin-rich material. However, some samples from the Ekati Panda kimberlite pipe (ca. 53.3Ma) contain remnant cellulose, demonstrating the extraordinary preservation potential of kimberlites. Preservation of α-cellulose is important because it allows for the construction of stable isotopic proxy records of deep-time paleoclimates at annual to subannual resolution. Established α-cellulose extraction methods [i.e., using a 17.5% sodium hydroxide (NaOH) solution] were unsuitable for this material because all holocelluloses were dissolved. Therefore, we tested variants of two cellulose extraction methods [i.e., Brendel et al. (2000) and Jayme-Wise (Leavitt and Danzer 1993, Loader et al. 1997)] to optimize a procedure for extraction of a consistent yield of mummified wood cellulose for stable isotopic analysis. Stable carbon (δ13C) and oxygen (δ18O) isotopes were measured from cellulose from each extraction method, as well as Extractive-Free Wood and unextracted mummified wood. vitrinite reflectance, used to assess thermal alteration of the unextracted material to estimate post-burial temperatures within the kimberlite, suggested low post-burial peak temperatures (Tpeak=60°C). We detected no mineral contaminants (i.e., iron oxides) in the material using Energy-Dispersive X-ray Spectroscopy (EDX). Despite low cellulose yield (<5%), the Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) spectra of mummified cellulose samples strongly resembled modern α-cellulose. All cellulose treatments were similar in stable isotope values and ATR-FTIR peaks, but significantly different from unextracted wood and Extractive-Free Wood. As, reported by other sources (Anchukaitis et al. 2008, Brookman and Whittaker 2012), use of the Brendel method may cause cellulose acetylation, introducing an FTIR peak near 1720cm−1, thus complicating hemicellulose detection at a peak near 1725cm−1. For this reason, the Jayme-Wise method is recommended for detection of hemicelluloses in mummified wood. If hemicelluloses are not present in holocellulose, due to groundwater hydrolysis, the NaOH step may be omitted or reduced in concentration and still produce α-cellulose.