Ocean Drilling Project Hole 689B spherules and upper Eocene microtektite and clinopyroxene‐bearing spherule strewn fields

• Published in: Meteoritics & Planetary Science
• Main Authors: GLASS, B. P., KOEBERL, C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1999
• Subjects: Space and Planetary Science; Geophysics; Maud Rise; Pacific; Southern Ocean
• Online Access: http://dx.doi.org/10.1111/j.1945-5100.1999.tb01746.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1945-5100.1999.tb01746.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.1999.tb01746.x

Abstract— Montanari et al. (1993) reported a positive Ir anomaly in the upper Eocene sediments from Ocean Drilling Program Hole 689B on the Maud Rise, Southern Ocean. Vonhof (1998) described microtektites and clinopyroxene‐bearing (cpx) spherules associated with the Ir anomaly in Hole 689B and suggested that they belong to the North American and equatorial Pacific cpx strewn fields, respectively. We searched a suite of 27 samples taken through the spherule layer from Hole 689B, and we recovered 386 microtektites and 667 cpx spherules. We studied the petrography of the microtektites and cpx spherules and determined the major element compositions of 31 microtektites and 14 cpx spherules using energy dispersive x‐ray analysis. We also determined the minor element compositions of eight microtektites using instrumental neutron activation analysis. We found that the peak abundance of cpx spherules is ∼2 cm below the peak abundance of the microtektites (∼128.7 m below sea floor), which suggests that the cpx spherule layer may be slightly older (∼3–5 ka). The microtektites are mostly spherical and are generally transparent and colorless. They are similar to the North American microtektites in composition, the biggest differences being their generally lower Na 2 O and generally higher Zr, Ba, and Ir (up to 0.3 ppb) contents. We agree with Vonhof (1998) that the Hole 689B microtektites probably belong to the North American tektite strewn field. We calculate that the number of microtektites (>125 μm)/cm 2 at Hole 689B is 52. This number is close to the concentration predicted by extrapolation of the trend of concentration vs. distance from the Chesapeake Bay structure, based on data from other North American microtektite‐bearing sites. Thus, the North American strewn field may be at least four times larger than previously mapped. The Hole 689B cpx spherules range from translucent yellow to opaque black, but most are opaque tan to dark brown. They are generally spherical in shape and all are < 125 μm in ...