Northwest Africa 1000: a New Eucrite with Maskelynite, Unequilibrated Pyroxene Crisscrossed by Fayalite-Rich Veins, and Stannern-Like Geochemistry Paul H

Lunar and Planetary Science XXXIII (2002) 1147.pdf

Northwest Africa 1000: A new eucrite with maskelynite, unequilibrated pyroxene crisscrossed by fayalite-rich veins, and Stannern-like geochemistry Paul H. Warren Institute of Geophysics, UCLA, Los Angeles, CA 90095-1567, USA ([email protected]) I present here the first data and description for an (Fig. 2) leave no doubt that they are preterrestrial. extraordinary eucrite, NWA 1000. An allocation of The olivine-dominated central portions of the veins

20 g was generously supplied to UCLA by Dr. are typically about 1- 4 µm in width, but they are David Gregory, who reports that the total mass is lined by several µm-thick reaction zones, where the 1200 g. For this study, which only commenced a few olivine and (formerly more abundant) silica of the weeks ago, an ~200 mm2 thin section was produced, and a 0.50 g fragment was used for bulk analysis by INAA and fused bead electron-probe analysis [1]. This eucrite is subophitic and slightly variolitic (fan-spherulitic), with plagioclase laths up to 4 mm and pyroxenes up to 5 mm in length. The texture shows only fine-scale brecciation, but roughly half of the plagioclase has been shock-altered into iso- tropic glass (maskelynite). By eucrite standards, the pyroxenes are extraordinarily unequilibrated. The main zoning trend resembles that of Bluewing 001, Fig. 1

original vein material has reacted to yield pyroxenes with distinctive, ferroan yet very Ca-poor compositions (Fig. 1). In Fig. 2, essentially the whole view is Pasamonte, a handful of other unequilibrated HED pyroxene (brightness indicating composition), ex- basalts [1,2]. On the pyroxene quadrilateral (Fig. 1), cept for the white veins, which are Fo25 olivine. Fig. zonation starts from En68Wo4 and extends almost 3 shows x-ray maps for Mg and Si; the olivine and linearly toward En17Wo28. The pyroxene zonation is reacted pyroxene areas are clearly delineated. Else- not as extensive as in Bluewing 001, however, and where, at the tip of a zoned pyroxene, olivine as fer- the relatively high-Ca rims typically feature 1-µm roan as Fo16 has been found. scale (coarse enough to image, but too fine to be The plagioclase/maskelynite averages An83, with resolved in e-probe microanalysis) exsolution lamel- a range (29 analyses) of An75-86. With rare excep- lae. On the Takeda-Graham scale of eucrite meta- tions (which might reflect terrestrial weathering), the morphism [3], NWA 1000 is a 2, or maybe a 3; Fe-rich veins do not cross through plagioclase. A Bluewing 001 remains the single least metamor- typical eucritic proportion of silica is present, but phosed whole eucrite. mainly in the form of the main phase within ex- Larger pyroxenes are commonly crisscrossed by tremely fine-grained (glassy?) mesostasis patches. systems of injected veins of late-stage Fe-rich mate- The mesostasis has been most severely altered by the rials, which consist dominantly of fayalitic (e.g., weathering. Fo25) olivine. The rock is moderately to extensively Despite the weathering, the surviving composi- weathered, but faults extending through these veins tion shows few obvious differences from that of a

Lunar and Planetary Science XXXIII (2002) 1147.pdf

ANOTHER COMPLEX, UNEQUILIBRATED NEW EUCRITE: P. H. Warrenr

relatively Ti- and REE-rich eucrite. Features such as Pasamonte, but still not as incompatible-rich as moderate Fe/Mn (38) and low-moderate Na/Al and Bluewing 001 and the two HED polymict breccia Ga/Al confirm the eucritic affinity. Our preliminary clasts [1,2] of Takeda-Graham metamorphic type 1. (fused-bead only) Ti result is 5.5 mg/g. The concen- The bulk mg ratio of 41 mol% (also implied by the tration of Sm (useful as a representative middle- pyroxene core compositions) is high, for a noncumu- REE) is 2.5 µg/g. The REE pattern has a normal late eucrite; as is (paradoxically) typical of incom- LREE/HREE enrichment (La/Lu = 1.38× chon- patible-rich eucrites (i.e., the “Stannern trend”). drites) with mild (−) Eu and marginal (−) Ce anoma- A strong correlation has emerged between the lies (the latter can only reflect terrestrial weather- minority of HED basalts “Stannern trend” geochem- ing). The incompatible element contents, especially istry and the (almost identical [1, this work]) minor- the Ti value, are uncommonly high, for a eucrite. ity that managed to avoid the otherwise ubiquitous In any event, the final composition of this rock thermal metamorphism [3]. This composition- may have already been “altered” on the HED aster- metamorphism correlation and other recent data oid, when some process (presumably shock) mobi- place important constraints on hypotheses for the lized late-stage materials and injected them into the thermal and magmatic evolution of HED asteroid. pyroxenes. The scale of this mobilization was most The burial hypothesis for eucrite metamorphism likely very short, however, based on the virtual ab- [5] postulates that the HED crust grew in a piece- sence of such veining within the plagioclase. meal way, by eruption of magmas formed by limited The best insight into the original parent melt partial melting of the mantle. Gradual melting of the

composition may come from a plot of mg [= mantle would inevitably deplete its incompatible Mg/(Mg+Fe)] vs. Ti or Ti/Al in pyroxene. In the elements, so incompatible-enriched materials (the simple system of the early (pre-ilmenite saturation) Stannern Trend, including Bluewing) would tend to stages of crystallization of a eucritic lava, Ti must be be among the first lavas to erupt. If so, they should essentially an incompatible element; it partitions just be among the most deeply buried and thus intensely enough into pigeonite, D = 0.20±0.04 [4], to yield a metamorphosed, rather than extraordinarily un- useful record of the parent melt evolution. For NWA metamorphosed. 1000, this plot (Fig. 4; excludes obvious products of Impact heating as the mechanism of metamorphism [6,7,3] is not favored, because it would gen- erally be scattered in both space and time, not compositionally and temporally systematic. The flow-to- flow baking model [8,3] has the advantage, vis-à-vis fossil 26Al and 53Mn evidence [9,10], of implying metamorphism soon after extrusion of the lavas. The flow-to-flow baking model suggests that the unequilibrated, incompatible-rich Stannern Trend eucrites formed later than the equilibrated, compositionally undistinguished Main Group. The earliest flows were probably more prone to flow-to-flow baking, as well as burial, as the heat source presumably de- cayed during the (brief) epoch of eucrite magma- Fig. 4 tism. reaction between fayalitic olivine and silica along References: [1] Warren P. H. & Kallemeyn G. W. the walls of the veins) shows the pyroxene has dis- (2001) LPS XXXII. [2] Takeda H. et al. (1994) EPSL tinctly higher Ti than that of Pasamonte, which is the 122, 183. [3] Takeda H. & Graham A.. L. (1991) Me- only example of an unequilibrated eucrite that has teoritics 26, 129. [4] Bartels K. S. & Grove T. L. (based on many literature analyses) a “normal” non- (1991) Proc. LPSC 21, 351. [5] Yamaguchi A. et al. cumulate composition in terms of incompatible ele- (1997) JGR 102, 13381. [6] Metzler K et al. (1995) ment concentrations. Planet. Space Sci. 43, 499. [7] Nyquist L. E. et al. On this basis (backed up by the bulk-rock data), (1986) JGR 91, 8137. [8] Warren P. H. (1997) MaPS 32, 945. [9] Srinivasan G. et al. (1999) Science 284, NWA 1000 appears to be Stannern-like, compared 1348. [10] Lugmair G. W. & Shokolyukov A. (1998) to compositionally “Main Group” eucrites such as GCA 62, 2863.