WATSON - A NEW LINK IN THE IIE IRON CHAIN

Watson, which was found in 1972 in South Australia, contains the large st single silicate rock mass seen in any known iron meteorite. A compr ehensive study has been completed on this unusual meteorite: petrograp hy, metallography, analyses of the silicate inclusion (whole rock chem ical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) a nd mineral compositions (by electron microprobe and ion microprobe). T he whole rock has a composition of an H-chondrite minus the normal H-g roup metal and troilite content. The oxygen isotope composition is tha t of the silicates in the IIE iron meteorites and lies along an oxygen isotope fractionation line with the H-group chondrites. Trace element s in the metal confirm Watson is a new IIE iron. Whole rock Watson sil icate shows an enrichment in K and P (each almost-equal-to 2X H-chondr ites). The silicate inclusion has a highly equilibrated igneous (perid otite-like) texture with olivine largely poikilitic within low-Ca pyro xene: olivine (Fa20), opx (Fs17Wo3), capx (Fs9Wo41) (with very fine ex solution lamellae), antiperthite feldspar (An1-3Or5) with <1 mum exsol ution lamellae (An1-3Or>40), shocked feldspar with altered stoichiomet ry, minor whitlockite (also a poorly characterized interstitial phosph ate-rich phase) and chromite, and only traces of metal and troilite. T he individual silicate minerals have normal chondritic REE patterns, b ut whitlockite has a remarkable REE pattern. It is very enriched in li ght REE (La is 720X C1, and Lu is 90X C1, as opposed to usual chonditi c values of almost-equal-to 300X and 100-150X, respectively) with a ne gative Eu anomaly. The enrichment of whole rock K is expressed both in an unusually high mean modal Or content of the feldspar, Or13, and in the presence of antiperthite. Whole rock trace element data for the s ilicate mass support the petrography. Watson silicate was an H-chondri te engulfed by metal and melted at >1550-degrees-C. A flat refractory lithophile and flat REE pattern (at almost-equal-to 1x average H-chond rites)indicate that melting took place in a relatively closed system. Immiscible metal and sulfide were occluded into the surrounding metal host. Below 1100-degrees-C, the average cooling rate is estimated to h ave been almost-equal-to 1000-degrees-C/Ma; Widmanstatten structure fo rmed, any igneous zoning in the silicates was equilibrated, and feldsp ar and pyroxene exsolution took place. Cooling to below 300-degrees-C was completed by 3.5 Ga B. P. At 8 Ma, a shock event took place causin g some severe metal deformation and forming local melt pockets of schr eibersite/metal. This event likely caused the release of Watson into i nterplanetary space. The time of this event, 8Ma, corresponds to the p eak frequency of exposure ages of the H-chondrites. This further confi rms the link between IIE irons and the H-chondrites, a relationship al ready indicated by their common oxygen isotope source. Watson metal st ructures are very similar to those in Kodaikanal. Watson, Kodaikanal a nd Netschaevo form the young group of IIE meteorites (ages 3.7 +/- 0.2 Ga). They appear to represent steps in a chain of events that must ha ve taken place repeatedly on the IIE parent body or bodies from which they came: chondrite engulfed in metal (Netschaevo); chondrite melted within metal (Watson); and finally melted silicate undergoing strong f ractionation with the fractionated material emplaced as globules withi n metal (Kodaikanal). Watson fills an important gap in understanding t he sequence of events that took place in the evolution of the IIE-H pa rent body(ies). This association of H-chondrite with IIE metal suggest s a surface, or near surface process-a suggestion made by several othe r researchers.