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Characterization of the Continental Mantle in an Active Rift Zone: Kilbourne Hole, New Mexico

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Characterization of the Continental Mantle in an Active Rift Zone: Kilbourne Hole, New Mexico CHARACTERIZATION OF THE CONTINENTAL MANTLE IN AN ACTIVE RIFT ZONE: KILBOURNE HOLE, NEW MEXICO Eric Kappus1, Elizabeth Y. Anthony1 and Christopher Andronicos1 1Department of Geological Sciences, The University of Texas at El Paso, TX 79968, USA lower temperatures and pressures than coarse-grained, INTRODUCTION protogranular ones (Bussod and Williams, 1991). The ultramafic xenoliths at Kilbourne Hole, New TEXTURAL CHARACTERIZATION Mexico, have played a pivotal role in understanding continental mantle. This is true from the early studies There are three distinction textural variants in by Irving (1980), in which he characterized bulk rock Kilbourne Hole suite. They are 1) coarse, protogranular, and mineral chemistry and recognized that peridotites 2) fine-grained equigranular tabular, and 3) are wall-rocks to veins and dikes of pyroxenite, to porphyroclastic. The first two types were documented recent studies of Os isotopes (Burton and others, 1999; originally by Irving (1980), and the third type has not Meisel and others, 2001; Brandon and others, 2002), been discussed for the Kilbourne Hole suite. Within our which document low 187Os/188Os implying that sample suite, the coarse, protogranular and fine-grained continental mantle stabilization is as old or older than equigranular samples are exclusively fertile lherzolites. the age of overlying crustal rocks (Barnes and others, Also, significantly, composite xenoliths are restricted to 1999; Barnes and others, 2002). The peridotites the fine-grained, equigranular samples. Irving (1980) documented in these studies, with the exception of a made this same observation and interpreted the single analysis of harzburgite in Meisel and others, are lherzolites to have been chemically and texturally relatively fertile samples. They have, for instance, CaO modified by the injection of pyroxenite veins and dikes. contents of 1.9 to 3.5 weight percent and Al2O3 of 2.9 to 4.3 weight percent. They have been described as both The porphyroclastic textural group includes samples fine-grained, equigranular and coarse-grained, that are both fertile lherzolites and also the more protogranular in texture. We have collected a suite of depleted olivine-rich peridotite. Again, within our less fertile samples that are distinct from the more sample suite, the olivine-rich peridotite is essentially fertile samples in composition. They have, for instance, exclusively porphyroclastic. We have only a single 0.6 to 1.3 weight percent CaO and 0.9 to 1.8 weight sample whose texture may be described as a coarse, percent Al2O3 , and are characterized by a pronounced protogranular olivine-rich peridotite. porphyroclastic texture. Modal analyses of these two groups indicate that the more fertile ones are The three sample suites have textures that indicate they dominantly lherzolites. The less fertile group includes were deformed under varying conditions of strain rate, lherzolite, harzburgite, and dunite. In order to simplify differential stress and temperature. For instance, the nomenclature in this paper, we refer to this group as fine-grained equigranular samples have relatively olivine-rich peridotite. In addition to the lherzolite and uniform grain size. Many grains are polygonal tabular, olivine-rich peridotite are pyroxenites, which have been with curved grain boundaries. Both olivine and characterized by Irving (1980) and Wilshire and others pyroxene exhibit undulatory extinction, subgrains are (1988). common, and locally developed mantles of fine- grained neoblasts surround pyroxene. These textures The purpose of the present study is to characterize suggest that grain boundary coarsening was slow textures and thermometry for the various lithologies in relative to recrystallization suggesting relatively high order to determine their spatial distribution. Our strain rates for the ambient temperature conditions. In preliminary results suggest that the continental mantle contrast, the coarse protogranular samples have under Kilbourne Hole is chemically and rheologically strongly curved grain boundaries and subgrains are less layered. This observation is based on preliminary common within olivine. Additionally, mantled thermometry, which shows the lherzolites to be at lower porphyroclasts are much more common within this temperatures than the olivine-rich peridotites and earlier sample suite suggesting that subgrain rotation studies of Kilbourne Hole samples, which suggest that recrystallization may have been the dominate the fine-grained, equigranular lherzolites may have recrystallization mechanism. Finally, the olivine rich 8th International Kimberlite Conference Long Abstract 1 peridotites all contain a well- developed porphyroclastic samples having unsupported radiogenic isotopes. The texture. However, the average grain size is larger and elemental patterns of re-enrichment combined with old grain boundaries are commonly highly curved. but disparate Sr and Nd model ages require isolation of Additionally, adjacent pyroxene and olivine often form this mantle section at least 1 Ga ago, followed by amoeboid intergrowths. This suggests that phase enrichment during the pyroxenite injection event. With boundary migration was an important deformation the current data, it therefore appears that some process in this suite. Subgrains are least common within equigranular lherzolite and olivine-rich peridotite show this suite and many of the coarse olivine grains have the anti-correlation between metasomatism and melt polygonal 120o triple junctions suggesting that recovery depletion that has been documented in other regions kept pace with strain rate in these samples. The overall (e.g. McDonough and Frey, 1989). Given that the high temperature textures and coarse grain size within olivine-rich peridotites appear to lack pyroxenite veins, the olivine rich peridotites is consistent with the metasomatism in them is subtle and cryptic compared textures developing during melt extraction. to the pyroxenite-veined lherzolite. MAJOR/TRACE ELEMENT CHEMISTRY These patterns of depletion and re-enrichment are particularly intriguing in light of recent studies of Os isotopes for these xenoliths. Burton and others (1999) The olivine-rich peridotites were initially recognized on characterized mineral separates for a single, lherzolite the basis of their distinctive porphyroclastic texture and sample. They established that the majority of Os resides modal compositions. The olivine-rich modes imply that in the sulfides, 187Os/188Os is subchondritic, and the peridotites are less fertile; elemental chemistry of interstitial sulfide is in isotopic equilibrium with the whole-rock samples and minerals confirm the whole rock and silicate minerals. Sulfide inclusions and systematic differences produced by melt depletion silicates rich in these inclusions yield lower 187Os/188Os (Table 1). This table also highlights the distinctive than interstitial sulfide, indicating a multi-stage and compositions of the pyroxenites. ancient lithospheric history for this sample. A previous study of melt and CO2 inclusions for this sample Table 1: Geochemistry (Schiano and Clocchiatti, 1994) documented final melt temperatures of 1240o ±20o C, which is consistent with Lherzolite Ol-rich Pyroxenite liquidus temperatures for the trapped melts, and peridotite minimum pressures of 1.1 GPa. This pressure estimate CaO 1.9 – 3.5 0.6 – 1.3 16.1 – 18.1 is consistent with the studies by Bussod and our Al2O3 2.9 – 4.3 0.9 – 1.8 13.5 – 16.2 preliminary data, and supports the hypothesis that the TiO2 0.08 – 0.04 – 0.08 1.2 – 1.6 fertile mantle is shallow. Two-pyroxene thermometry 0.22 for the sample yielded 1120o±140o C, which agrees MgO 38.0 – 45.0 – 50.0 13.9 – 15.5 within error with our preliminary results discussed 41.9 below. Yb 0.28 –0.35 0.04 – 0.11 1.34 – 1.58 A second study by Meisel and others (2001) was Ni 1960- 2360 – 123 – 440 designed to refine the Os isotopic composition of 2220 2590 hypothetical primitive upper mantle. The study is based Cr 2310 - 2410 – 8 – 112 on samples from a number of localities, including 16 2350 2950 lherzolites (Al2O3 2.86 to 4.29 weight percent) and 1 Data from Irving (1980), Roden and others (1988), harzburgite (Al2O3 0.92 weight percent) from Long (1994). Kilbourne Hole and two samples (Al2O3 1.37 to 1.54 weight percent) from San Carlos, Arizona. They find High-precision neutron activation analyses for three of that all localities have a positive correlation for Al2O3 the olivine-rich peridotites (Long, 1994) indicate that vs. 187Os/188Os and that 187Os/188Os converges on a the olivine-rich peridotites have also been value of 0.1296±0.0008 for fertile compositions. For metasomatized, This pattern of re-enrichment was also the samples from Kilbourne Hole and San Carlos, TRD found in some tabular equigranular and composite = 1.55 Ga, which agrees quite well with recent Nd samples by Roden and others (1988). Their study model ages for crustal basement samples from southern documented that equigranular and composite samples New Mexico and west Texas (Barnes and others, 1999; were variably enriched, while protogranular ones were Barnes and others, 2002). The one harzburgite from not. Sr and Nd systematics were complex, with many Kilbourne Hole falls on the linear correlation of Al2O3 8th International Kimberlite Conference Long Abstract 2 vs. 187Os/188Os, implying that it is, in terms of its Os intruded into brittle fractures. Using temperature as a isotopic signature, continental lithosphere. This result is proxy for depth suggests
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