May 21 (Reuters) - The Standard & Poor's 500 index ended trading on Wall Street slightly higher on Thursday after a volatile session amid falling oil prices and investor optimism about a possible Middle East peace deal, even as the United States and Iran diverged directly over Iran's uranium stockpile and Tehran's control of the Strait of Hormuz.

After falling in the morning, stocks gradually recovered during afternoon trading, while oil prices reversed course from higher to lower as investors monitored social media for news about progress in the peace process.

U.S. Secretary of State Marco Rubio told reporters there were "some good signs" about the talks, but indicated that a diplomatic agreement with Iran would not be possible if it implemented a transit fee system in the Strait of Hormuz.

Jason Pride, head of investment strategy and research at Glenmead, attributed the volatility seen in today's session to investors' reactions to speculation about the geopolitical situation.

“We are at high valuation levels driven in part by earnings,” Pride said. “That overshadowed concerns about Iran, but the earnings season is now largely over. We are not suddenly going to get any more pleasant surprises from earnings, which means that market attention has now returned to Iran. In the near term, the market will find its way based on rumors or actual announced deals regarding Iran.”

According to preliminary data, the Standard & Poor's 500 index rose 11.54 points, or 0.16 percent, to close at 7444.51, while the Nasdaq Composite index climbed 25.82 points, or 0.09 percent, to 26296.18.

The Dow Jones Industrial Average also rose 278.91 points, or 0.56 percent, to 50,288.26 points.