Meteoritics & Planetary Science, Volume 44, Number 1 (2009)http://hdl.handle.net/10150/6412282024-03-28T11:24:08Z2024-03-28T11:24:08ZApplication of a textural geospeedometer to the late-stage magmatic history of MIL 03346Hammer, J. E.http://hdl.handle.net/10150/6565322021-02-15T02:04:01Z2009-01-01T00:00:00ZApplication of a textural geospeedometer to the late-stage magmatic history of MIL 03346
Hammer, J. E.
Dynamic crystallization experiments performed on Fe-rich, Al-poor basalt are employed as a textural calibration set to quantify the late-stage igneous history of nakhlite Miller Range (MIL) 03346. The ratio of crystal-melt surface area to volume typifying morphologically distinct populations of Ca-pyroxene has been shown to vary as a strong function of cooling rate (Hammer 2006). Furthermore, a texture of phenocrysts surrounded by finer-grained groundmass crystals arises by sequential nucleation events during constant-rate cooling, but multiple populations nucleate only if the cooling rate is less than or equal to 72 degrees C h^(-1). Textural analysis of meteorite MIL 03346 reveals at least two distinct populations. The Ca-pyroxene phenocryst and microphenocryst three dimensional (3D) aspect ratios are 112 +/- 8.3 and 1530 +/- 160 mm^(-1), respectively. By comparison with the calibration set, the range of cooling rates consistent with 3D aspect ratios of both populations in MIL 03346 is ~20 degrees C h^(-1). An additional experiment was performed approximating a conductive heat transfer profile in order to interpret and apply results of constant-rate cooling experiments to the natural cooling of magma. Results suggest that the textures of constant-rate experiments parallel the initial period of rapid cooling in natural magma. Initial cooling rates of ~20 degrees C h^(-1) in the lava hosting MIL 03346 occur in conductively solidifying lava at depths of ~0.4 m, constraining the minimum total thickness to greater than or equal to 0.8 m. Crystal accumulation beginning in a subsurface reservoir and continuing after lava emplacement as an inflated pahoehoe sheet satisfies all textural constraints on the late-stage igneous history of MIL 03346.
2009-01-01T00:00:00ZPhysical distribution trends in Darwin glassHoward, Kieren T.http://hdl.handle.net/10150/6565302021-02-15T02:02:53Z2009-01-01T00:00:00ZPhysical distribution trends in Darwin glass
Howard, Kieren T.
Darwin glass formed by impact melting, probably during excavation of the 1.2 km diameter Darwin crater, Tasmania, Australia. The glass was ejected up to 20 km from the source crater and forms a strewn field of 400 km^2. There is at least 11,250 m^3 of glass in the strewn field and relative to the size of the crater this is the most abundant ejected impact glass on Earth. The glass population can be subdivided on the basis of shape (74% irregular, 20% ropy, 0.5% spheroid, 6% droplet, and 0.7% elongate) and color (53% dark green, 31% light green, 11% black, and 5% white). The white glasses contain up to 92 wt% SiO2 and are formed from melting of quartzite. Black glasses contain a minimum of 76 wt% SiO2 and formed from melting of shale. Systematic variations in the proportion of glasses falling into each of the color and shape classes relative to distance from the crater show: 1) a decrease in glass abundance away from the crater; 2) the largest fragments of glass are found closest to the crater; 3) small fragments (2 g) dominate finds close to the crater; 4) the proportion of white glass is greatest closest to the crater; 5) the proportion of black glass increases with distance from the crater and 6) the proportion of splashform glasses increases with distance from the crater. These distribution trends can only be explained by the molten glass having been ballistically ejected from Darwin crater during impact and are related to 1) the depth of excavation from the target rock stratigraphy and/or 2) viscosity contrasts between the high and low SiO2 melt. The high abundance and wide distribution of ejected melt is attributed to a volatile charged target stratigraphy produced by surface swamps that are indicated by the paleoclimate record.
2009-01-01T00:00:00ZGalactic cosmic ray-produced 129Xe and 131Xe excesses in troilites of the Cape York iron meteoriteMathew, K. J.Marti, K.http://hdl.handle.net/10150/6565292021-02-15T02:02:34Z2009-01-01T00:00:00ZGalactic cosmic ray-produced 129Xe and 131Xe excesses in troilites of the Cape York iron meteorite
Mathew, K. J.; Marti, K.
The flux of galactic cosmic rays (GCR) in the solar system appears to change with time. Based on the abundances in iron meteorites of cosmogenic nuclides of different half lives, Lavielle et al. (1999) found that the GCR flux increased in recent times (<100 Ma) by about 38% compared to average flux in the past 150 Ma to 700 Ma ago. A promising technique for calibrating the GCR flux during the past ~50 Ma, based on the 129I and 129Xe pair of nuclides, was discussed earlier (Marti 1986; Murty and Marti 1987). The 129I-129Xen chronometer provides a shielding-independent system as long as the exposure geometry remained fixed. It is especially suitable for large iron meteorites (Te-rich troilite) because of the effects by the GCR secondary neutron component. Although GCR-produced Xe components were identified in troilites, several issues require clarifications and improvements; some are reported here. We developed a procedure for achieving small Xe extraction blanks which are required to measure indigenous Xe in troilites. The 129Xe and 131Xe excesses (129Xen, 131Xen) due to neutron reactions in Te are correlated in a stepwise release run during the troilite decomposition. Our data show that indigenous Xe in troilite of Cape York has isotopic abundances consistent with ordinary chondritic Xe (OC-Xe), in contrast to a terrestrial signature which was reported earlier. Two methods are discussed which assess and correct for an interfering radiogenic 129Xer component from extinct 129I. The corrected 129Xen concentration in troilite D4 of Cape York yields a cosmic ray exposure (CRE) age of 82 +/- 7 Ma consistent, within uncertainties, with reported data (Murty and Marti 1987; Marti et al. 2004).
2009-01-01T00:00:00ZThe origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid-Ordovician limestone from the Gärde quarry (Jämtland, central Sweden)Alwmark, C.Schmitz, B.http://hdl.handle.net/10150/6565282021-02-15T02:02:09Z2009-01-01T00:00:00ZThe origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid-Ordovician limestone from the Gärde quarry (Jämtland, central Sweden)
Alwmark, C.; Schmitz, B.
The Brunflo fossil meteorite was found in the 1950s in mid-Ordovician marine limestone in the Grde quarry in Jmtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded >50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO2 content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L-chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule-size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent-body breakup. Chromite maximum diameters and well-defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO2 contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Grde quarry is relatively rich (about 0.45 grain kg^(-1)) in sediment-dispersed extraterrestrial chromite grains (63 m) with chemical composition similar to those in L chondrites and the limestone (1-3 grains kg^(-1)) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell.
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