PDF (Hi-Res) - Smithsonian Institution Libraries

More documents

Recommendations

Info

NUMBER 19 FIGURE 12.—Border zone of silicate inclusion 3, transmitted light. Coarse silicate grains occur next to the graphite zone (black, top) at the metal to silicate inclusion boundary, smaller silicate grains with graphite flakes (black) further inside (bottom). (Width of image = 1.5 mm.) As Ca and Al are located in diopside and feldspar in these inclusions, we tried to match the composition of chondrites more closely by the addition or subtraction of one or both of these minerals. A fit for Ca and Al should also result in correct values for Na and K, as these elements are found in feldspar only. Because the composition of the minerals in these inclusions is so close to that of chondrites, it is self-evident that a mixture of these minerals in the right proportions should also produce a good match to the bulk chemistry of chondrites. Nevertheless, it is interesting to see how much diopside or feldspar of the analyzed composition has to be added or subtracted to come close to a chondritic silicate composition. For this comparison FeO has again been omitted 119 from the calculations, as its content is much lower in El Taco olivines and pyroxenes than in ordinary chondrites. The results are shown in Figure 13: chondritic compositions can be matched quite well by addition or subtraction of small amounts of diopside and/or plagioclase for all inclusions. The correction is not sufficient, however, to match the Mg/Si ratio of normal chondrites completely. To achieve this, also the pyroxene/olivine ratio has to be increased: a ratio of 2:1 as in inclusion 4 would be appropriate. Thus it seems feasible to derive these inclusions from chondritic material (neglecting the FeO content, which is discussed further below), from which mainly feldspar and/or diopside was lost (the grains in inclusion 2 and 3 are so small that they may be neglected). A mechanism for this derivation would be partial melting and redistribution of the melt, as albite and diopside would be the first minerals to melt, at 1120° and 1390° C, respectively. It is important to note, however, that different inclusions need different amounts of diopside and feldspar and that these minerals are not lost in proportional amounts, as already evident from the highly variable Ca/Al ratio of these inclusions. It can be seen from the values of Table 1 that the relative amounts of olivine and pyroxene also vary by a factor of 2. These variations are probably not due to sampling errors, as the good agreement of chemical and calculated analysis of different areas of inclusion 1 (Table 8) shows. It may be argued, however, whether the lower melting silicates were entirely lost from the inclusion or may still be present in a small border zone towards the metal. A simple calculation shows that a surface layer 0.5 mm thick of an inclusion (Figure 12) with the dimensions 3x3x6 cm (like inclusion 3) would contain 8% of the material of this inclusion by volume. This border zone would then have to consist entirely of feldspar and/or diopside to account for the material which was lost (Figure 13). This is not observed in thin sections that are available from the metal-silicate interface of inclusions 2 and 3. Feldspar is somewhat enriched in this zone but it is still a minor phase in relation to pyroxene and olivine. Thus, it seems safe to conclude that the variations in chemical and mineralogical composition of these inclusions are due neither to inhomogeneities nor to nonrepresentative sampling.
2.0- 1.5- 1.0 0.8 : 0.6: 0.4- 0.2- 0.1 2.0- 1.5- 1.0: 0.8- 0.6- 0M- 0.2- 0.1: 2.0 1.5 1.0 0.8 0.6 0.4- 0.2- 0.1 - 120 I Si Mg Al Co No K Cr Q +5% diopside and +7% feldspar Inclusion 1 Si Mg Al Ca Na K Cr 7\ -3% diopside and +8% feldspar / Inclusion 3 / r- / / 171 Si Mg Al Ca Na K +7% diopside and+3% feldspar Inclusion 5 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES 2.0- 1.5- 1.0: 0.8- O.fr 0.4- 0.2- 0.1- zo- 1.5- 1.0 0.8- 0.6- 0.4- 0.2- 0.1- NWWWWW1 71 Si Mg Al Ca Na K Cr / +7% diopside and -2% feldspar Inclusion 2 Si Mg Al Ca Na K + 8% diopside and -2% feldspar Inclusion U FIGURE 13.—Open bars: Main element content of silicate portions of inclusions 1 to 5, normalized to average chondrite composition (from Table 9). Hatched bars: Composition after addition or subtraction of the indicated amounts of diopside and plagioclase feldspar of the composition measured in inclusions 1-5. 7 -
Page 1 and 2:
SMITHSONIAN CONTRIBUTIONS TO THE EA
Page 3 and 4:
Contents MINERALOGY COMPOSITION OF
Page 5 and 6:
Constituent SiO TiO Al 0 fl 2 3 Cr
Page 7 and 8:
PER CENT CBLCIUM FIGURE 3.—The ca
Page 9 and 10:
Constituent TiO,, 2 2 3 FeO* MnO Mg
Page 11 and 12:
20 Cfl 40 .60 SMITHSONIAN CONTRIBUT
Page 13 and 14:
35'50' I L 6.. 3.. 2.. r FOKT DEFIA
Page 15 and 16:
12 SMITHSONIAN CONTRIBUTIONS TO THE
Page 17 and 18:
14 20 SMITHSONIAN CONTRIBUTIONS TO
Page 19 and 20:
16 Constituent Si0 2 TiO2 A12O3 Cr
Page 21 and 22:
18 20 40 £0 SMITHSONIAN CONTRIBUTI
Page 23 and 24:
20 40 20 SMITHSONIAN CONTRIBUTIONS
Page 25 and 26:
Abyssal Basaltic Glasses as Indicat
Page 27 and 28:
24 Electron-M icroprobe Analysis An
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
30 Hill in the NE Atlantic, and fro
Page 35 and 36:
32 cores the arrangement is stratig
Page 37 and 38:
Page 39 and 40:
36 ATLANTIC OCEAN LAT. LONG. DEPTH
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
Page 67 and 68:
Page 69 and 70:
Page 71 and 72: 68 SMITHSONIAN CONTRIBUTIONS TO THE
Page 75 and 76: TABLE 1.—Chemical composition in
Page 85 and 86: 82 Literature Cited Bostrom, K., an
Page 87 and 88: Geochemical Differences among Compo
Page 99 and 100: The St. Mary's County, Maryland, Ch
Page 103 and 104: 100 SMITHSONIAN CONTRIBUTIONS TO TH
Page 107 and 108: Mineralogical and Chemical Composit
Page 111 and 112: 108 Sampling Inclusion 1 was taken
Page 115 and 116: 112 15 10 32 3.6 40 44 48 •fl-r O
Page 121: 118 SMITHSONIAN CONTRIBUTIONS TO TH
show all

PDF (Hi-Res) - Smithsonian Institution Libraries

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?